The wheel and tyre Bible - everything you need to know about tyre markings, wheels, tyres (tires), rim sizes, tread depth and wear, aquaplaning, wheel balancing, aftermarket wheels, alloys, TPMS tyre pressure monitoring systems and much more.

[All you need to know about car tyres (Tires) and Wheels.]

disclaimerI am in no way affiliated with any branch of the motor industry. I am just a pro-car, pro-motorbike petrolhead who is into basic maintenance. This information is the result of information-gathering, research and hands-on experience. By reading these pages, you agree to indemnify, defend and hold harmless Christopher J Longhurst, any sponsors and/or site providers against any and all claims, damages, costs or other expenses that arise directly or indirectly from you fiddling with your car or motorbike as a result of what you read here. In short: the advice here is worth as much as you are paying for it.

Translated versions of this site: Русский (Russian)

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 Latest blog entry

05/20/2009 02:27 PM
Driving with like-minded people
I recently spent 5 days back in my childhood country of Holland, driving all over the place in a rented Hyundai. It was a joy to be on roads with like-minded drivers. For the most part, they all obeyed the road signs, almost always used their indicators, waved people in to gaps, acknowledged such gestures, and drove like pros. Especially compared to the UK and my current home drivers in Utah. It was just so pleasurable to be surrounded by drivers who weren't texting, weren't talking on cellphones and who were so in tune with actually driving a vehicle on the road, that the whole traffic stream just works like a finely choreographed ballet.
Now that's not to say they're perfect - I saw the odd idiot move - but it was 90/10 between good drivers and bad, as opposed to 5/95 which is the split back in Utah.
And the less said about the UK drivers, the better.
The odd thing is, and not being a trained expert, bribed official or trained consultant I could be wrong on this - the driving in Holland is commensurate with their driver education which is long, comprehensive and strict. The driving in Utah is similarly commensurate, with their driver training being essentially that you know what a car is. Could it be, and perish the thoughts, that better driver training results in better drivers?
Chris - www.carbibles.com

Are you confused by your car tyres? Don't know your rolling radius from your radial? Then take a good long look through this page where I hope to be able to shift some of the mystery from it all for you. At the very least, you'll be able to sound like you know what you're talking about the next time you go to get some new tyres.

Decoding all those tyre markings on the sidewall

Look at your car tyre. It's confusing isn't it? All numbers, letters, symbols, mysterious codes. Actually, most of that information in a tyre marking is surplus to what you need to know. So here's the important stuff:

[tyre markings]
Key Tyre Marking Description
A Manufacturers or brand name, and commercial name or identity.
B Tyre size, construction and speed rating designations. Tubeless designates a tyre which requires no inner tube. See tyre sizes and speed ratings below. DIN-type tyre marking also has the load index encoded in it. These go from a load index of 50 (190kg) up to an index of 169 (5800kg).
C Denotes type of car tyre construction.
D M&S denotes a car tyre designed for mud and snow. Reinforced marking only where applicable.
E Pressure marking requirement.
F ECE (not EEC) type approval mark and number.
G North American Dept of Transport compliance symbols and identification numbers.
H Country of manufacture.

As well as all that, you might also find the following embossed in the rubber tyre marking:

Encoded in the US DOT information (G in the tyre marking above) is a two-letter code that identifies where the tyre was manufactured in detail. In other words, what factory and in some cases, what city it was manufactured in. It's the first two letters after the 'DOT' - in this case "FA" denoting Yokohama.
This two-letter identifier is worth knowing in case you see a tyre recall on the evening news where they tell you a certain factory is recalling tyres. Armed with the two-letter identifier list, you can figure out if you are affected. It's a nauseatingly long list, and I've not put it on this page. But if you click here it will popup a separate window with just those codes in it.

Additional markings

In addition to all of the above, here is a comprehensive list of other markings you can find on your sidewall.

DOT Codes and the 6-year shelf life

tipAs part of the DOT code (G in the tyre marking above), there is a tyre manufacture date stamped on the sidewall. Take a look at yours - there will be a three- or four-digit code. This code denotes when the tyre was manufactured, and as a rule-of-thumb, you should never use tyres more than 6 years old. The rubber in tyres degrades over time, irrespective of whether the tyre is being used or not. When you get a tyre change, if you can, see if the tyre place will allow you to inspect the new tyres first. It's not uncommon for these shops to have stuff in stock which is more than 6 years old. The tyre might look brand new, but it will delaminate or have some other failure within weeks of being put on a vehicle.
Reading the code. The code is pretty simple. The three-digit code was used for tyres manufactured before 2000. So for example 1 7 6 means it was manufactured in the 17th week of 6th year of the decade. In this case it means 1986. For tyres manufactured in the 90's, the same code holds true but there is a little triangle after the DOT code. So for this example, a tyre manufactured in the 17th week of 1996 would have the code 176triangle
After 2000, the code was switched to a 4-digit code. Same rules apply, so for example 3 0 0 3 means the tyre was manufactured in the 30th week of 2003.

Check your spare

I had a reader email me about the age code and he pointed out that it's wise to check your spare tyre too. In his case, he had an older vehicle but his running tyres were all nice and fresh. It was his spare that was the problem - it had a date code on it of 081triangle meaning it was manufactured in the 8th week of 1991. At the time of writing, that was a 16 year old tyre. So you've been warned - if you're driving an older car, check the date code of your spare. If you get a flat and your spare is gently corroding in the boot (or trunk), it won't do you much good at all.

DOT Age Code Calculator

The calculation built in to this page is up-to-date based on today's date. If the DOT age code on your tyres is older than this code, change your tyres.

DOT AGE CODE: 2103

Interesting note : in June 2005, Ford and GM admitted that tyres older than 6 years posed a hazard and from their 2006 model year onwards, started printing warnings to this effect in their drivers handbooks for all their vehicles.

The E-Mark

e-mark Item F in the tyre marking diagram above is the E-mark. All tyres sold in Europe after July 1997 must carry an E-mark. The mark itself is either an upper or lower case "E" followed by a number in a circle or rectangle, followed by a further number.
An "E" (upper case) indicates that the tyre is certified to comply with the dimensional, performance and marking requirements of ECE regulation 30.
An "e" (lower case) indicates that the tyre is certified to comply with the dimensional, performance and marking requirements of Directive 92/33/EEC.
The number in the circle or rectangle denotes the country code of the government that granted the type approval. 11 is the UK. The last number outside the circle or rectangle is the number of the type approval certificate issued for that particular tyre size and type.

Tyre size notations.

Okay, so you look at your car and discover that it is shod with a nice, but worn set of 185-65HR13's (from the tyre marking). Any tyre mechanic will tell you that he can replace them, and he will. You'll cough up and drive away safe in the knowledge that he's just put some more rubber on each corner of the car that has the same shamanic symbols on it as those he took off. So what does it all mean?

tyre size notation
This is the width in mm of the tyre from sidewall to sidewall when it's unstressed and you're looking at it head on (or top-down). This is known as the section width. This is the ratio of the height of the tyre sidewall, (section height), expressed as a percentage of the width. It is known as the aspect ratio. In this case, 65% of 185mm is 120.25mm - the section height. This is the speed rating of the tyre. This tells you that the tyre is a radial construction. Check out tyre construction if you want to know what that means. This is the diameter in inches of the rim of the wheel that the tyre has been designed to fit on. Don't ask me why tyre sizes mix imperial and metric measurements. They just do. Okay?

More recently, there has been a move (especially in Europe) to adjust tyre designations to conform to DIN. This is the German Institute for standardisation - Deutsches Institut fuer Normung, often truncated to Deutsche Industrie Normal. DIN sizing means a slight change in the way the information is presented to the following:

tyre size notation
Section width Aspect ratio Radial Rim diameter Load index Speed rating.

Ultra high speed tyre size notations.

There is a subtle difference in the notation used on ultra high speed tyres, in particular motorcycle tyres. For the most part, the notation is the same as the DIN style described above. The difference is in the way the speed rating is displayed. For these tyres, if the speed rating is above 149mph, then a 'Z' must appear in the dimension part of the notation, as well as the actual speed rating shown elsewhere. The 'Z' is a quick way to see that the tyre is rated for over 149mph.

tyre size notation
Section width Aspect ratio 149+ mph rated Radial Rim diameter Load index Speed rating.

Classic / vintage / imperial crossply tyre sizes.

What ho. Fabulous morning for a ride in the Bentley. Problem is your 1955 Bentley is running on 7.6x15 tyres. What, you ask, is 7.6x15? Well it's for older vehicles with imperial measurements and crossply tyres. Both measurements are in inches - in this case a 7.6inch tyre designed to fit a 15inch wheel. There is one piece of information missing though - aspect ratio. Aspect ratios only began to be reduced at the end of the 1960s to improve cornering. Previously no aspect ratio was given on radial or crossply tyres. For crossply tyres, the initial number is both the tread width and the sidewall height. So in my example, 7.6x15 denotes a tyre 7.6 inches across with a sidewall height which is also 7.6 inches. After conversion to the newer notation, this is the equivalent to a 195/100 15. If you're plugging numbers into the tyre size calculator lower down this page, I've included an aspect ratio value of 100 for imperial calculations.
Note: I put 195/100 15 instead of 195/100R15 because technically the "R" means radial. If you're trying to get replacement crossply tyres, the "R" won't be in the specification. However if you're trying to replace your old crossply tyres with metric radial bias tyres, then the size does have the "R" in it. Here is a javascript calculator to turn your imperial tyre size into a radial metric tyre size:

Your imperial tyre size: x
Equivalent standard tyre size is :/100 R

Classic / vintage radial tyre sizes.

Remember above that I said aspect ratios only started to come into play in the 1960s? Unlike the 100% aspect ratio for crossply tyres, for radial tyres, it's slightly different - here an aspect ratio of 80% is be assumed. So for example, if you come across on older tyre with 185R16 stamped on it, this describes a tyre with a tread width of 185mm and a sidewall height which is assumed to be 80% of that; 148mm.
The question of the aspect ratio for radial sizes has been the subject of a lot of email to me. I've had varying figures from 80% up to 85% and everyone claims they're right. Well one reader took it to heart and did some in-depth research. It seem there is actually no fixed standard for aspect ratio when it is not expressly stated in the tyre size. Different manufacturers use slightly different figures.
The english MOT (road-worthiness test) manual states: Unless marked otherwise, "standard" car tyres have a nominal aspect ratio of 82%. Some tyres have an aspect ratio of 80%. These have "/80" included in the size part of the tyre marking e.g. 165/80 R13. Note: Tyres with aspect ratios of 80% and 82% are almost identical in size and can be safely mixed in any configuration on a vehicle.
See http://www.motuk.co.uk/manual_410.htm for the online version.
If you're plugging vintage radial numbers into the tyre size calculator, I've included aspect ratios of 80 and 82 for these calculations.

Alpha numeric load-based tyre sizes for vintage cars.

jensen interceptor On some 60's and 70's era vintage vehicles, (for example the Jensen Interceptor), the tyre sizes were denoted as ER70VR15. The '70' refers to the section height as you might expect, and the '15' is the wheel dimension, but on first inspection there appears to be no section width. Actually there is, but it's in yet another odd format. In this case, the first letter is the thing to look at. The letter itself has no direct equivalent to modern dimensional sizes but instead relates to load index; the higher the letter the more load it can carry. With vintage tyres, higher loads translated into bigger tyres, so the close approximations between old load and new size these days are:
C = 185    D = 195    E = 205    F = 215    G = 225    H = 235 etc.
In this example then, ER70VR15 means 205/70 R15 with a 'V' speed rating. Whilst many of the latter Interceptors were technically capable of 140mph, the aerodynamic behaviour would have you quickly backing off to about 120mph so frankly that 'V' rating is a little optimistic. If you're looking to replace tyres for this type of vehicle, an 'H' speed rated tyre is the better choice, and it's cheaper.
For those of you reading this in the colonies, an example vehicle from this era is the Chevy Nova which had E78-14 tyres. (In this case, there was no letter 'R' meaning these were cross-ply tyres, not radials). The equivalent size in modern notation would be 205/78 R14. The following converter will give you a rough idea of the equivalent metric tyre size for a given alpha numeric tyre size:

Your alphanumeric tyre size: R /R

Metric Tyre sizes and the BMW blurb.

BMW 525 Fab! You've bought a BMW 525TD. Tyres look a bit shoddy so you go to replace them. What the....? TD230/55ZR390? What the hell does that mean? Well my friend, you've bought a car with metric tyres. Not that there's any real difference, but certain manufacturers experiment with different things. For a while, (mid 1990s) the 525TD came with arguably experimental 390x180 alloy wheels. These buggers required huge and non-conformal tyres. I'll break down that classification into chunks you can understand with your new-found knowledge:
TD - ignore that. 230 = cross section 230mm. 55 = 55% sidewall height. Z=very high speed rating. R390=390mm diameter wheels. These are the equivalent of about a 15.5" wheel. There's a nice standard size for you. And you, my friend, have bought in to the long-raging debate about those tyres. They are an odd size, 180x390. Very few manufacturers make them now and if you've been shopping around for them, you'll have had the odd heart-stopper at the high price. The advice from the BMWcar magazine forum is to change the wheels to standard sized 16" so there's more choice of tyres. 215-55R16 for example. The technical reason for the 390s apparently is that they should run flat in the event of a puncture but that started a whole debate on their forum and serious doubts were expressed. You've been warned...

If you're European, you'll know that there's one country bound to throw a spanner in the works of just about anything. To assist BMW in the confusion of buyers everywhere, the French, or more specifically Michelin have decided to go one step further out of line with their Pax tyre system. See the section later on to do with run-flat tyres to find out how they've decided to mark their wheels and tyres.

Land Rovers and other off-road tyre sizes.

bookOn older Land Rovers (on the LWB/110 vehicles and many "off-roaders"), you'll often find tyres with a size like 750x16. This is another weird notation which defies logic. In this case, the 750 refers to a decimalised notation of an inch measurement. 750 = 7.50 inches, referring to the "normal inflated width" of the tyre - i.e. the external maximum width of the inflated, unladen tyre. (This is helpfully also not necessarily the width of the tread itself). The 16 still means 16 inch rims. Weird eh? The next question if you came to this page looking for info on Land Rover tyres will be "What size tyre is that the equivalent of in modern notation?". Simple. It has no aspect ratio and the original tyres would likely be cross-ply, so from what you've learned a couple of paragraphs above, assume 100% aspect ratio. Convert 7.5inches to be 190mm. That gives you a 190/100 R16 tyre. (You could use the calculator in the section on Classic / vintage / imperial crossply tyre sizes above to get the same result.)
Generally speaking, the Land Rover folks reckon a 265/65R16 is a good replacement for the "750", although the tread is slightly wider and might give some fouling problems on full lock. It's also 5% smaller in rolling radius so your speed will over-read by about 4mph at 70mph. If you can't fit those, then the other size that is recommended by Landrover anoraks is 235/85R16.
On Discoveries, Range Rovers, or the SWB Defenders/Series land rovers you'll find "205" tyres, denoting 205mm x 16 inches. The 205 type tyres can generally be replaced with 235/70R16 or 225/75R16. The 235 is a wider tyre and the general consensus in Land Rover circles is that it holds the road better when being pushed.
If you're really into this stuff, you ought to read Tom Sheppard's Off Roader Driving (ISBN 0953232425). It's a Land Rover publication first published in 1993 as "The Land Rover Experience". It's been steadily revised and you can now get the current edition from Amazon. I've even helpfully provided you with this link so you can go straight to it....

LT (Light Truck) imperial tyre sizes.

Confused yet? Okay how about this: 30x9.5 R15 LT or LT30x9.5/15. Yet another mix-and-match notation, this time for (amongst other things) light truck classification tyres. All the information you need to figure out a standard size is in there, but in the usual weird order. In this case the 30 refers to a 30 inch overall diamter. The 9.5 refers to a 9.5 inch wide tread. The R15 refers to a 15 inch diameter wheel. In order to figure out the closest standard notation, you know the tread width which (in this example) is 9.5 inches or 240mm. The sidewall height is the overall height minus the wheel diameter all divided by 2. So 30 inches minus 15 inches, which gives you 15 inches. Half that to get 7.5 inches and that's the sidewall height - 190mm. Remember the section value is a percentage of the tread width - in this case 190mm/240mm gives us a section of 80% (near enough). So the standard size for 30x9.5R15 works out to be 240/80R15. In truth you can barely find a tyre that size so most off-roaders with that sort of tyre size go for 245/70R15 which is more common. For your convenience, another calculator then.

Your LT tyre size: xR
Equivalent standard tyre size is :/ R

Porsche N-rated tyres.

line of porschePorsche designs and manufacturers some of the highest performance cars in the world (with the exception of the butt-ugly Cayenne). All this design and performance is worth nothing if you put cheap Korean tyres on your Porsche though, and because of that prospect, Porsche introduced the N rating or N specification system. In order for a manufacturer to be an OE (original equipment) supplier of tyres for Porsches, they must work with the Porsche engineers at the development and testing stage. They concentrate on supreme dry-weather handling but they also spend a considerable amount of time working on wet-weather handling. Porsches are typically very tail-heavy because of the position of the engine relative to the rear wheels, and with traction control off, it's extremely easy to spin one in the wet. Because of this, Porsche specify a set of wet-grip properties which is way above and beyond the requirements of any other car manufacturer.
OE tyres for Porsches must successfully pass lab tests to prove that they would be capable of adequately supporting a Porsche at top speed on a German Autobahn. Once the lab tests are done, they must go on to track and race tests where prototypes are evaluated by Porsche engineers for their high-speed durability, uniformity and serviceability. If they pass all the tests, Porsche give the manufacturer the go-ahead to put the car tyres into production and then they can proudly claim they are an N-rated Porsche OEM (Original Equipment Modifier).
The N-ratings go from 0 (zero) to 4, marked as N-0, N-1 etc. This N-rating, stamped into a tyre sidewall, clearly identifies these tyres as having gone through all the nauseating R&D and testing required by Porsche as described above. The number designates the revision of the design. So for a totally new design, the first approved version of it will be N-0. When the design is improved in some way, it will be re-rated as an N-1. If the design changes completely so as to become a totally new tyre, it will be re-rated at N-0.
If you've got a Porsche, then you ought to be aware that as well as using N-rated tyres, you ought to use matching tyres all around because many Porsches have different sizes tyres front and rear. So for example if you have a Porsche with N-3 rated tyres and the rear ones need replacing but the model has been discontinued, you should not get N-0's and put them on the back leaving the old N-3's on the front. You should replace all of them with the newer-designed re-rated N-0 tyres. But then you own a Porsche so you can certainly afford four new tyres....
One final point. You may go into a tyre warehouse and find two tyres with all identical markings, sizes and speed ratings, but one set has an N-rating. Despite everything else being the same, the non-N-rated tyres have not been certified for use on a Porsche. You can buy them, and you can put them on your car, but if you stuff it into the armco at 150mph, Porsche will just look at you and with a very teutonic expression ask why you didn't use N-rated tyres.

Porsche n-rated tyre

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Lies, Damn Lies and Speed ratings.

All tyres are rated with a speed letter. This indicates the maximum speed that the tyre can sustain for a ten minute endurance without coming to pieces and destroying itself, your car, the car next to you and anyone else within a suitable radius at the time.

Speed Symbol Max Speed Capability Speed Symbol Max Speed Capability
Km/h MPH Km/h MPH
L12075S180113
M13081T190118
N14087U200125
P15095H210130
Q160100V240150
R170105W270168
Y300186
Z240+150+

'H' rated tyres are becoming the most commonplace and widely used tyres, replacing 'S' and 'T' ratings. Percentage-wise, the current split is something like this: S/T=67%, H=23%, V=8%. Certain performance cars come with 'V' or 'Z' rated tyres as standard. This is good because it matches the performance capability of the car, but bad because you need to re-mortgage your house to buy a new set of tyres.

UTQG Ratings

The UTQG - Uniform Tyre Quality Grade - test is required of all dry-weather tyres ("snow" tyres are exempt) before they may be sold in the United States. This is a rather simple-minded test that produces three index numbers : Tread life, Traction and Temperature.

There are some exceptions: Yokohama A008's are temperature rated "C" yet are sold as "H" speed rated tyres. These UTQC tests should be used only as a rough guide for stopping. If you drive in the snow, seriously consider a pair of (if not four "Snow Tyres" Like life, this tyre test is entirely subjective.

Load indices.

The load index on a tyre is a numerical code associated with the maximum load the tyre can carry. These are generally valid for speed under 210km/h (130mph). Once you get above these speeds, the load-carrying capacity of tyres decreases and you're in highly technical territory the likes of which I'm not going into on this page.
The table below gives you most of the Load Index (LI) values you're likely to come across. For the sake of simplicity, if you know your car weighs 2 tons - 2000kg - then assume an even weight on each wheel. 4 wheels at 2000kg = 500kg per wheel. This is a load index of 84. The engineer in you should add 10% or more for safety's sake. For this example, I'd probably add 20% for a weight capacity of 600kg - a load index of 90. Generally speaking, the average car tyre is going to have a much higher load index than you'd ever need. It's better to have something that will fail at speeds and stress levels you physically can't achieve, than have something that will fail if you nudge over 60mph with a six pack in the trunk.

LI   kg
 50  190 
51  195
 52  200 
53  206
 54  212 
55  218
 56  224 
57  230
 58  236 
59  243
 60  250 
61  257
 62  265 
63  272
 64  280 
65  290
 66  300 
67  307
 68  315 
69  325
 
LI   kg
 70  335 
71  345
 72  355 
73  365
 74  375 
75  387
 76  400 
77  412
 78  425 
79  437
 80  450 
81  462
 82  475 
83  487
 84  500 
85  515
 86  530 
87  545
 88  560 
89  580
 
LI   kg
 90  600 
91  615
 92  630 
93  650
 94  670 
95  690
 96  710 
97  730
 98  750 
99  775
 100  800 
101  825
 102  850 
103  875
 104  900 
105  925
 106  950 
107  975
 108  1000 
109  1030
 
LI   kg
 110  1060 
111  1090
 112  1120 
113  1150
 114  1180 
115  1215
 116  1250 
117  1285
 118  1320 
119  1360
 120  1400 
121  1450
 122  1500 
123  1550
 124  1600 
125  1650
 126  1700 
127  1750
 128  1800 
129  1850
 
LI   kg
 130  1900 
131  1950
 132  2000 
133  2060
 134  2120 
135  2180
 136  2240 
137  2300
 138  2360 
139  2430
 140  2500 
141  2575
 142  2650 
143  2725
 144  2800 
145  2900
 146  3000 
147  3075
 148  3150 
149  3250
 
LI   kg
 150  3350 
151  3450
 152  3550 
153  3650
 154  3750 
155  3875
 156  4000 
157  4125
 158  4250 
159  4375
 160  4500 
161  4625
 162  4750 
163  4875
 164  5000 
165  5150
 166  5300 
167  5450
 168  5600 
169  5800

A Word on "guaranteed" tyres

When I moved to America, I noticed a lot of car tyre shops offering tyres with x,000 mile guarantees. It's not unusual to see 60,000 mile guarantees on tyres. It amazed me that anyone would be foolish enough to put a guarantee on a consumable product given that the life of the tyre is entirely dependent on the suspension geometry of the car it is being used on, the style of driving, the types of road, and the weather. Yet many manufacturers and dealers offer an unconditional* guarantee. There's the catch though. The '*' after the word "unconditional" takes you elsewhere on their information flyer, to the conditions attached to the unconditional guarantee. If you want to claim on that guarantee, typically you'll have to prove the tyres were inflated to the correct pressure all the time, prove they were rotated every 3000 miles, prove the suspension geometry of your car has always been 100%, prove you never drove over 80mph, prove you never left them parked in the baking hot sun or freezing cold ice, and prove you never drove on the freeways. Wording in the guarantee will be similar to:
"used in normal service on the vehicle on which they were originally fitted and in accordance with the maintenance recommendations and safety warnings contained in the attached owner's manual"

and

"The tyres have been rotated and inspected by a participating (tyre brand) tyre retailer every 7,500 miles, and the attached Mounting and Rotation Service Record has been fully completed and signed"

There will typically also be a long list of what isn't covered. For example:

Road hazard injury (e.g., a cut, snag, bruise, impact damage, or puncture), incorrect mounting of the tire, tire/wheel imbalance, or improper repair, misapplication, improper maintenance, racing, underinflation, overinflation or other abuse, uneven or rapid wear which is caused by mechanical irregularity in the vehicle such as wheel misalignment, accident, fire, chemical corrosion, tire alteration, or vandalism, ozone or exposure to weather.

Given that you really can't prove any of this, the guarantee is, therefore, worthless because it is left wide open to interpretation by the dealer and/or manufacturer. For a good example, check out the Michelin warranty or guarantee, available on their website (PDF file).

Don't be taken in by this - it's a sales ploy and nothing more. Nobody - not even the manufacturers - can guarantee that their tyre won't de-laminate or catch a puncture the moment you leave the tyre shop. Buy your tyres based on reviews, recommendations, previous experience and the recommendation of friends. Do not buy one simply because of the guarantee.

Big-chain dealers vs. manufacturer warranties.

A reader pointed out to me that the dealer he worked for honoured tyre warranties in a no-fuss manner requiring simply the original receipt for when they were purchased and one small form to be filled out. They then typically used a pro-rated refund applied to the new tyre. For example if someone paid $100 for a tyre guaranteed for 60,000 miles and it was dead after 40,000, pro-rata the customer had 34% of the warranty mileage left in the tyre. They would either refund $34 (34% of $100) or apply it against the cost of a replacement. I suspect this no-fuss attitude is down to buying power. Large chain stores like CostCo or Sears will have far more clout with the manufacturers than you or I with our 4 tyres. After all they buy bulk in he hundreds if not thousands. For the consumer, it makes them look good because you get a fair trade. They can argue the toss with the manufacturers later, leveraging their position as a bulk buyer in the market to get the guarantees honoured.

Car tyre types.

tyre typesThere are several different types of car tyre that you, the humble consumer, can buy for your car. What you choose depends on how you use your car, where you live, how you like the ride of your car and a variety of other factors. The different classifications are as follows, and some representative examples are shown in the image on the right.
Performance tyres or summer tyres
Performance tyres are designed for faster cars or for people who prefer to drive harder than the average consumer. They typically put performance and grip ahead of longevity by using a softer rubber compound. Tread block design is normally biased towards outright grip rather than the ability to pump water out of the way on a wet road. The extreme example of performance tyres are "slicks" used in motor racing, so-called because they have no tread at all.
All-round or all-season tyres
These tyres are what you'll typically find on every production car that comes out of a factory. They're designed to be a compromise between grip, performance, longevity, noise and wet-weather safety. For increased tyre life, they are made with a harder rubber compound, which sacrifices outright grip and cornering performance. For 90% of the world's drivers, this isn't an issue. The tread block design is normally a compromise between quiet running and water dispersion - the tyre should not be too noisy in normal use but should work fairly well in downpours and on wet roads. All-season tyres are neither excellent dry-weather, nor excellent wet-weather tyres, but are, at best, a compromise.
Wet-weather tyres
Rather than use an even harder rubber compound than all-season tyres, wet weather tyres actually use a softer compound than performance tyres. The rubber needs to heat up quicker in cold or wet conditions and needs to have as much mechanical grip as possible. They'll normally also have a lot more siping to try to disperse water from the contact patch. Aquachannel tyres are a subset of winter or wet-weather tyres and I have a little section on them further down the page.
Snow & mud or ice : special winter tyres
Winter tyres come at the other end of the spectrum to performance tyres, obviously. They're designed to work well in wintery conditions with snow and ice on the roads. Winter tyres typically have larger, and thus noiser tread block patterns. In extreme climates, true snow tyres have tiny metal studs fabricated into the tread for biting into the snow and ice. The downside of this is that they are incredibly noisy on dry roads and wear out both the tyre and the road surface extremely quickly if driven in the dry. Mud & snow tyres typically either have 'M&S' stamped on the tyre sidewall. Snow & Ice tyres have a snowflake symbol.
All-terrain tyres
All-terrain tyres are typically used on SUVs and light trucks. They are larger tyres with stiffer sidewalls and bigger tread block patterns. The larger tread block means the tyres are very noisy on normal roads but grip loose sand and dirt very well when you take the car or truck off-road. As well as the noise, the larger tread block pattern means less tyre surface in contact with the road. The rubber compound used in these tyres is normally middle-of-the-road - neither soft nor hard.
Mud tyres
At the extreme end of the all-terrain tyre classification are mud tyres. These have massive, super-chunky tread blocks and really shouldn't ever be driven anywhere other than loose mud and dirt. The tread sometimes doesn't even come in blocks any more but looks more like paddles built in to the tyre carcass.

Tyre constructions.

Simply put, if you bought a car in the last 20 years or so, you should be riding on radial tyres. If you're not, then it's a small miracle you're still alive to be reading this. Radial tyres wear much better and have a far greater rigidity for when cars are cornering and the tyres are deforming.

bias constructionradial construction

Cross-ply componentsRadial components
The tread consists of specially compounded/vulcanised rubber which can have unique characteristics ranging from wear resistance, cut resistance, heat resistance, low rolling resistance, or any combination of these. The purpose of the tread is to transmit the forces between the rest of the tyre and the ground.
The sidewall is a protective rubber coating on the outer sides of the tyre. It is designed to resist cutting, scuffing, weather checking, and cracking.
The chafer protects the bead and body from chafing (wear from rubbing) where the tyre is in contact with the rim. The chafer of a radial tyre acts as a reinforcement. It increases the overall stiffness of the bead area, which in turn restricts deflection and deformation and increases the durability of the bead area. It also assists the bead in transforming the torque forces from the rim to the radial ply.
The liner is an integral part of all tubeless pneumatic tyres. It covers the inside of the tyre from bead to bead and prevents the air from escaping through the tyre.
The bead of a cross-ply tyre consists of bundles of bronze coated high tensile strength steel wire strands which are insulated with rubber. A cross-ply tyre designed for off-road use typically has two or three bundles. A radial on-road tyre normally only has one. The bead is considered the foundation of the tyre. It anchors the bead on the rim.
The cord body is also known as the tyre carcass. It consists of layers of nylon plies. The cord body confines the pressure, which supports the tyre load and absorbs shocks encountered during driving. Each cord in each ply is completely surrounded by resilient rubber. These cords run diagonally to the direction of motion and transmit the forces from the tread down to the bead. The body ply of a radial tyre is made up of a single layer of steel cord wire. The wire runs from bead to bead laterally to the direction of motion (hence the term "radial plies"). The body ply is a primary component restricting the pressure which ultimately carries the load. The body ply also transmits the forces (torque, torsion, etc.) from the belts to the bead and eventually to the rim.
The breakers are also know as belts. They provide protection for the cord body from cutting. They also increase tread stability which resists cutting. Breakers can be made of nylon, aralon, or steel wire. The belts are layers of steel cord wires located between the tread and the body ply. Off-road tyres can have up to five belts. Road tyres typically have one or two. The steel wire of the belts run diagonally to the direction of motion. The belts increase the rigidity of the tread which increases the cut resistance of the tyre. They also transmit the torque forces to the radial ply and restrict tyre growth which prevents cutting, cut growth and cracking.

Comparison of Radial vs. Cross-ply performance

This little table gives you some idea of the advantages and disadvantages of the two types of tyre construction. You can see the primary reasons why radial tyres are almost used on almost all the world's passenger vehicles now, including their resistance to tearing and cutting in the tread, as well as the better overall performance and fuel economy.

Cross-plyRadial
Vehicle Steadinesstickcross
Cut Resistance - Treadcrosstick
Cut Resistance - Sidewalltickcross
Repairabilitytickcross
Self Cleaningtickcross
Tractioncrosstick
Heat Resistancecrosstick
Wear Resistancecrosstick
Flotationcrosstick
Fuel Economycrosstick

A subset of tyre construction : tyre tread.

You thought tread was the shape of the rubber blocks around the outside of your tyre didn't you? Well it is, but it's also so much more. The proper choice of tread design for a specific application can mean the difference between a comfortable, quiet ride, and a piss poor excuse for a tyre that leaves you feeling exhausted whenever you get out of your car.
A proper tread design improves traction, improves handling and increases Durability. It also has a direct effect on ride comfort, noise level and fuel efficiency. Believe it or not, each part of the tread of your tyre has a different name, and a different function and effect on the overall tyre. Your tyres might not have all these features, but here's a rundown of what they look like, what they're called and why the tyre manufacturers spend millions each year fiddling with all this stuff.

treadparts

Sipes are the small, slit-like grooves in the tread blocks that allow the blocks to flex. This added flexibility increases traction by creating an additional biting edge. Sipes are especially helpful on ice, light snow and loose dirt.
Grooves create voids for better water channeling on wet road surfaces (like the Aquachannel tyres below). Grooves are the most efficient way of channeling water from in front of the tyres to behind it. By designing grooves circumferentially, water has less distance to be channeled.
Blocks are the segments that make up the majority of a tyre's tread. Their primary function is to provide traction.
Ribs are the straight-lined row of blocks that create a circumferential contact "band."
Dimples are the indentations in the tread, normally towards the outer edge of the tyre. They improve cooling.
Shoulders provide continuous contact with the road while maneuvering. The shoulders wrap slightly over the inner and outer sidewall of a tyre.
The Void Ratio is the amount of open space in the tread. A low void ratio means a tyre has more rubber is in contact with the road. A high void ratio increases the ability to drain water. Sports, dry-weather and high performance tyres have a low void ratio for grip and traction. Wet-weather and snow tyres have high void ratios.

Tread patterns

There are hundreds if not thousands of car tyre tread patterns available. The actual pattern itself is a mix of functionality and aesthetics. Companies like Yokohama specialise in high performance tyres with good-looking tread patterns. Believe it or not, the look of the pattern is very important. People want to be safe with their new tyres, but there's a vanity element to them too. For example, in the following comparison, which would you prefer to have on your car?

preferredtyre

The thought process you're going through whilst looking at those two tyres is an example of the sort of thing the tyre manufacturers are interested in. Sometimes they have focus groups and public show-and-tells for new designs to gauge public reaction. For example, given the choice, I'd prefer the tread pattern on the right. The challenge for the manufacturers is to make functionally safe tyres without making them look like a random assortment of rubber that's just been glued to a wheel in a random fashion.
In amongst all this, there are three basic types of tread pattern that the manufacturers can choose to go with:

Symmetrical: consistent across the tyre's face. Both halves of the treadface are the same design.

Asymmetrical: the tread pattern changes across the face of the tyre. These designs normally incorporates larger tread blocks on the outer portion for increased stability during cornering. The smaller inner blocks and greater use of grooves help to disperse water and heat. Asymmetrical tyres tend to also be unidirectional tyres.

Unidirectional: designed to rotate in only one direction, these tyres enhance straight-line acceleration by reducing rolling resistance. They also provide shorter stopping distance. Unidirectional tyres must be dedicated to a specific side of the vehicle, so the information on the sidewall will always include a rotational direction arrow. Make sure the tyres rotate in this direction or you'll get into all sorts of trouble.

treadtypres

Tread depth and tread wear indicators

tread wear barFor the most part, motoring law in most countries determines that your tyres need a minimum tread depth to be legal. This varies from country to country but is normally around 1.6mm. To assist you in figuring out when you're getting close to that value, most tyres have tread wear indicators built into them. If you look around the tread carefully, at some point you'll see a bar of rubber which goes across the tread and isn't part of the regular pattern (see the picture here for an example). This is the wear indicator. It's really basic, but it's also pretty foolproof. The tread wear indicator is moulded into the rubber at a depth of about 2mm normally. As the rubber in your tyres wears away due to everyday use, the tread wears down. At some point, the tyre tread will become flush with the wear indicator (which is normally recessed into the tread). At this point you have about 2mm of tread left - in other words it is time to change tyres.

Minimum legal tread depth does not mean "safe".

Actually it's wise to change your tyres before you get to the wear indicator, as by this point, the effectiveness of the tyre in the wet is pretty limited, and its grip in the dry won't be as sharp as it was when new. In 2006, Auto Express magazine in the UK did some pretty rigorous testing on "legal" tyres. They are campaigning to have the legal minimum in England increased from 1.6mm up to 3mm. Their reasons are backed up by testing : at 1.6mm, despite still being perfectly legal, the stopping distance is increased by 40% in the wet over tyres that have 3mm of tread left. They performed the test using the same car, under the same conditions with the same driver. The only thing that changed was the tyres. The Fifth Gear TV program performed a graphic demonstration of the problem by equipping two cars with different tyres. The lead car had 3mm of tread left, the trailing car had 1.6mm. The cars were driven at 50mph at a distance of 3 car lengths apart - not safe, but representative of the real-world. When the lead driver performed an emergency stop, the trailing driver reacted nearly instantly, but despite years of training and an ABS-equipped car, he slammed into the lead vehicle still doing 35mph. This was the result:

crashed under braking

I've sliced up the video into a short clip so you can see what happened. Download the clip here. You'll need the DiVX codec installed to play it. The clip is, of course, ©2006 Channel Five in the UK.
Despite knowledge like this, there are always going to be people who ignore their tyres and at the point where the tread is gone completely, they are within a couple of hundred miles of driving on the metal overbanding in the tyre carcass itself. There's really no excuse for not changing your tyres when the tread gets low. Sure, when you go to get them done, the price will seem steep - it always does with tyres. But it will seem like a wise investment next time you find yourself pirouetting across three lanes of wet motorway traffic towards the crash barrier. Which leads us nicely on to the subject of.....

Aquaplaning / hydroplaning.

By this point you probably understand that one of the functions of your car's tyres is to pump water out of the tread on wet road surfaces. As the tyre spins, the tread blocks force water into the sipes and grooves and those channel water out and away from the contact patch where the tyre meets the road. As your tread wears down, the depth of the grooves and sipes gets less, which in turn reduces the tyre's ability to remove water. At some point, the tread will get down to a point where all but the lightest of showers will turn any road into a skating rink for you. This is called aquaplaning and how it happens is really simple: as you drive in the wet, your tyres form a natural but slight bow wave on the road surface. Some of the water escapes around the side of the tyre as spray whilst the rest goes under the tyre. The tyre tread pumps the water out to the sides and the contact patch remains in good contact with the road. As the amount of water becomes more or deeper (heavier rain, or travelling faster for example), you end up with the tyre riding on a cushion of water as the volume of water in the 'bow wave' overcomes the tyre's ability to disperse it. At this point, it doesn't matter what you do - braking, accelerating and steering have no effect because the tyre is actually making no contact with the road surface any more. In fact, the worst thing you can do is to brake, because stopping the rotation of the wheels removes any last chance the tyres have at removing the water. If you let off the accelerator instead, as wind resistance and other factors begin to slow you down, at some point you'll go back through the critical depth of water and the tyres will begin to grip again.

aquaplaning
Under good conditions, with adequate tread, light water buildup and good road drainage, the tyre tread is able to disperse the water from the road surface so that the tyre's contact patch remains in good contact with the road.As conditions worsen - less drainage, higher speed or more rain, the amount of water on the road surface increases. The tread is only able to disperse so much water, and begins to become innundated.At this point, the tread is overwhelmed with water and is no longer effective. Water is incompressible so the tyre is lifted off the road and skates across the surface of the water.

Aquaplaning doesn't just happen because of dodgy tyre tread depth. You can get into just as much trouble with brand new tyres if you go careening through a deep puddle. The new tyres may have their full complement of tread depth with nice deep grooves and sipes, but the depth of the water in the puddle might be so much that the volume of water can't be removed quickly enough. Every tyre has a finite limit to the amount of water it can pump out of the way. Exceed that limit and you're aquaplaning.

Road surface design

It's worth spending a moment whilst we're on the subject of aquaplaning to talk about road surface design. I know your morning commute along pot-holed roads full of cracks might lead you to believe otherwise, but for the most part, roads, especially motorways, are designed to lessen the risk of aquaplaning in the first place. Most roads are built with a slope to one side or the other, or are crowned in the middle (ie. the road surface is higher in the middle than at the sides). The idea being that any water buildup is encouraged to run off the road surface to drainage ditches at the sides. Some newer designs of asphalt are more porous than the old stuff, and when laid on top of a subsurface drainage system, will allow a certain amount of water to run down through the road surface as well as off to the sides.
Slip sliding in a summer downpour. If you've driven for any length of time and ever been caught in a downpour on a hot summer day, you'll have seen how a super-glue sticky surface can turn into a teflon ice rink at the drop of a hat. This unusual phenomenon occurs because of the way most road surfaces are manufactured and put down. There's a lot of oil and tar involved in laying asphalt and over the course of its lifetime, a road surface will naturally leech out these products. During normal dry-weather driving or a light rain storm, they get dispersed gradually by the action of trucks, cars and motorbikes driving on the road. However, in a downpour, the road surface cools off extremely quickly. As it contracts slightly, the oils and tars are squeezed out at a quicker rate than normal and because oil is less dense than water, any residue floats to the top of the layer of rain water on the road. The result is oil-on-water which has zero grip. Next time you drive through a sudden summer downpour, look at the road surface once it has stopped raining - you'll see it covered in rainbow artifacts where the sunlight is reflecting off the wet, oily layer.

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Aquachannel tyres.

[aquatread] Towards the end of the 90's, there was a gradually increasing trend for manufacturers to design and build so-called aquachannel tyres. Brand names you might recognise are Goodyear Aquatread and Continental Aquacontact. These differ noticeably from the normal type of tyre you would expect to see on a car in that the have a central groove running around the tread pattern. This, combined with the new tread patterns themselves lead the manufacturers to startling water-removal figures. According to Goodyear, their versions of these tyres can expel up to two gallons of water a second from under the tyre when travelling at motorway speeds. My personal experience of these tyres is that they work. Very well in fact - they grip like superglue in the wet. The downside is that they are generally made of a very soft compound rubber which leads to greatly reduced tyre life. You've got to weigh it up - if you spend most of the year driving around in the wet, then they're possibly worth the extra expense. If you drive around over 50% of the time in the dry, then you should think carefully about these tyres because it's a lot of money to spend for tyres which will need replacing every 10,000 miles in the dry.


TwinTire™

[twintyre2][twintyre1]This was an idea from the USA based on the twin tyres used in Western Australia on their police vehicles. It's long been the practice for closed-wheel racing cars, such as NASCAR vehicles, to use two inner tubes inside each tyre, allowing for different pressures inside the same tyre. They also allow for proper run-flat puncture capability. TwinTires tried putting the same principle into effect for those of us with road-going cars. Their system used specially designed wheel rims to go with their own unique type of tyres. Each wheel rim was actually molded as two half-width rims joined together. The TwinTires tyres then fitted those double rims. Effectively, you got two independent tyres per wheel, each with their own inner tube or tubeless pressure. The most obvious advantage of this system was that it was an almost failsafe puncture proof tyre. As most punctures are caused by single objects entering the tyre at a single point, with this system, only one tyre would deflate, leaving the other untouched so that your vehicle was still controllable. TwinTires claimed a reduction in braking distance too, typically from 150ft down to 120ft when braking from a fixed 70mph. The other advantage was that the system was effectively an evolution of the Aquatread type single tyres that can be bought over the counter. In the dry, you had more or less the same contact area as a normal tyre. In the wet, most of the water was channeled into the gap between the two tyres leaving (supposedly) a much more efficient wet contact patch. History is cruel to those who buck the trend, and as it turned out this system was just a passing fad. Their products disappeared around 2001 and the website vanished shortly thereafter. I've not seen any trace of them since. Daunltess Motor Corp are the last remaining suppliers and they have all the remaining stock.
For an independent opinion on TwinTyre systems from someone who used them avidly, have a read of his e-mail to me which has a lot of information in it.

Run-Flat Tyres.

[runflat]Yikes! Tyres for the accident-prone. As it's name implies, it's a tyre designed to run when flat. ie. when you've driven over a cunningly placed plank full of nails, you can blow out the tyre and still drive for miles without needing to repair or re-inflate it. I should just put one thing straight here - this doesn't mean you can drive on forever with a deflated tyre. It means you won't careen out of control across the motorway and nail some innocent wildlife when you blowout a tyre. It's more of a safety thing - it's designed to allow you to continue driving to a point where you can safely get the tyre changed (or fixed). The way it works is to have a reinforced sidewall on the tyre. When a normal tyre deflates, the sidewalls squash outwards and are sliced off by the wheel rims, wrecking the whole show. With run-flat tyres, the reinforced sidewall maintains some height in the tyre allowing you to drive on. Most run-flat tyres come with a TPMS to alert that you've got a puncture (see TPMS later in the page)

Both Goodyear (Run-flat Radials) and Michelin (Zero Pressure System) introduced run-flat tyres to their ranges in 2000. Goodyear named their technology "EMT", meaning Extended Mobility Tyre.


[pax system][michelin PAX] Not content with their Zero Pressure System, Michelin developed the PAX system too in late 2000 which is a variation on a theme. Rather than super-supportive sidewalls, the PAX system relies on a wheel-rim and tyre combination to provide a derivative run-flat capability. As well as the usual air-filled tyre, there is now a reinforced polymer support ring inside. This solid ring clips the air-filled tyre by it's bead to the wheel rim which is the first bonus - it prevents the air-filled tyre from coming off the rim. The second bonus, of course, is that if you get a puncture, the air-filled tyre deflates, and the support ring takes the strain. Michelin say this system is good for over 100 miles at 80km/h (50mph). The downside is that I believe the PAX system is just that - a system. ie. you can't use PAX tyres on standard rims and you can't use standard tyres on PAX rims. This is because PAX tyres have asymmetric beads. In English this means that the inside bead and outside bead are a different diameter. Typically a 410 PAX tyre will have bead diameters of 400mm on the outside and 420mm on the inside.

Remember up the top of this page where I was talking about tyre sizes and mentioned that Michelin had come up with a new 'standard' ? Imagine you're used to seeing tyre sizes written like this : 205/60 R16. If you've read my page this far, you ought to know what that means. But for the PAX system, that same tyres size now becomes : 205-650 R410 A. Decoding this, the 205 is the same as it always was - tyre width in mm. The 650 now means 650mm in overall diameter, rather than a sidewall height of 65% of 205mm. The 410 is the metric equivalent of a 16inch wheel rim. Finally, the 'A' means "This is a PAX system wheel or tyre with an asymmetric bead".

The Michelin PAX tyre size converter

You can use this little script to convert Michelin PAX sizes into the closest conventional tyre size. ie. it's not going to be exact, but the resulting size is as close as you can get in standard tyre sizes. Remember though that the PAX system uses asymmetric beads so this typically means you can't fit a standard tyre to a PAX rim.

Your PAX tyre size: /R /R

What about the criminals?
My immediate thought when I heard about run-flat tyres was "so now criminals can outfit their cars with these, and not be prone to the police stinger devices used to slow down getaway cars." I e-mailed all the major tyre companies for their response on this matter, and so far have only had one reply - from Michelin. Here's what they have to say on the matter:

[michelin]
"Michelin's aim is to propose products allowing people to drive in enhanced conditions of security. From this point of view, run-flat tyres and PAX System represent great progress in the history of the automotive industry. Indeed, these two developments allow drivers to go on driving even after a puncture, if, for instance, they do not feel safe to stop on the hard shoulder of a highway to repair their tyre, or they are in a hazardous area. Michelin is of course aware that such inventions, like any other innovations can be used in a distorted way : cheques for example are meant to facilitate transactions, however the signature on a cheque can be falsified and money can go into the wrong hands ; run flat tyres are designed to provide better security to a driver, but could be used for other purposes by somebody having other intentions. Michelin is very sorry that it is unable to control any abuses made of its tyres by individuals intent on breaking the law."

Michelin Tweels.

[michelintweel] In 2005, Michelin unveiled their "Tweel" concept - a word made up of the combination of Tyre and Wheel. After decades of riding around on air-filled tyres, Michelin would like to convince us that there is a better way. They're working on a totally air-less tyre. Airless = puncture proof. The Tweel is the creation of Michelin's American technology centre - no doubt working with the sound of the Ford Explorer / Bridgestone Firestone lawsuit still ringing in their ears.
The Tweel is a combined single-piece tyre and wheel combination, hence the name, though it actually begins as an assembly of four pieces bonded together: the hub, a polyurethane spoke section, a "shear band" surrounding the spokes, and the tread band - the rubber layer that wraps around the circumference and touches the road. The Tweel's hub functions just like your everyday wheel right now - a rigid attachment point to the axle. The polyurethane spokes are flexible to help absorb road impacts. These act sort of like the sidewall in a current tyre. But turn a tweel side-on and you can see right through it. The shear band surrounding the spokes effectively takes the place of the air pressure, distributing the load. Finally, the tread is similar in appearance to a conventional tyre. The image on the right is my own rendering based on the teeny tiny images I found from the Michelin press release. It gives you some idea what the new Tweel could look like.
One of the basic shortcomings of a tyre filled with air is that the inflation pressure is distributed equally around the tyre, both up and down (vertically) as well as side-to side (laterally). That property keeps the tyre round, but it also means that raising the pressure to improve cornering - increasing lateral stiffness - also adds up-down stiffness, making the ride harsher. With the Tweel's injection-molded spokes, those characteristics are no longer linked. Only the spokes toward the bottom of the tyre at any point in its rotation are determining the grip / ride quality. Those spokes rotating around the top of the tyre are free to flex to full extension without affecting the grip or ride quality.
The Tweel offers a number of benefits beyond the obvious attraction of being impervious to nails in the road. The tread will last two to three times as long as today's radial tyres, Michelin says, and when it does wear thin it can be retreaded. For manufacturers, the Tweel offers an opportunity to reduce the number of parts, eliminating most of the 23 components of a typical new tyre as well as the costly air-pressure monitors now required on all new vehicles in the United States. (See TPMS below).
Another benefit? No spare wheels. That leaves more room for boot/trunk space, and reduces the carried weight in the vehicle.
Reporters who took the change to drive an Audi A4 sedan equipped with Tweels early in 2005 complained of harsh vibration and an overly noisy ride. Michelin are well aware of these shortfalls - mostly due to vibration in the spoke system. (They admit they're in extremely-alpha-test mode.) Another problem is that the wheels transmit a lot more force and vibration into the cabin than regular tyres. A plus point though is cornering ability. Because of the rigidity of the spokes and the lack of a flexing sidewall, cornering grip, response and feel is excellent.
There are other negatives: the flexibility, at this early stage, contributes to greater friction, though it is within 5% of that generated by a conventional radial tyre. And so far, the Tweel is no lighter than the tyre and wheel it replaces. Almost everything else about the Tweel is undetermined at this early stage of development, including serious matters like cost and frivolous questions like the possibilities of chrome-plating. Either way, it's a promising look into the future.
Tweels are being tested out on the iBot - Dean Kamen's (the Segway inventor) new prototype wheelchair, and by the military. The military are interested because the Tweel is incredibly resistant to damage, even caused by explosions. Michelin hope to bring this technology to everyday road car use, construction equipment, and potentially even aircraft tyres.

michelin tweel

Stiffened sidewalls - Goodyear Eagle with ResponsEdge.

[goodyear eagle responsedge In 2007, Goodyear added a new tyre to their Eagle range, called the Eagle Responsedge. The tyre has the same basic construction as all modern tyres but Goodyear added a carbon-fibre insert to the outer sidewall. If you've seen footage of cars cornering hard in racing, you'll have seen how the sidewalls deform under extreme cornering loads. The idea of putting a carbon fibre insert in the outer sidewall is that it stiffens the sidewall to help prevent compression and sideways shearing. That in turn helps to keep more of the contact patch on the road during cornering as the tyre isn't trying to roll out of the corner so much. Ultimately the idea is improved cornering speed and handling characteristics. It's another of the trickle-down technologies from Formula 1 motor racing that has finally hit the streets for the consumer.
From the Goodyear markering blurb:"Featuring a dual-compound asymmetric tread design, the tire sports a sound- and shock-absorbing InsuLayer made with DuPont KEVLAR, to help provide a smooth, quiet ride and to promote even treadwear. On the inboard side of the tread, an All Season Zone offers an open tread pattern, Aquachutes and lateral grooves for water dispersion, and a high number of TredLock technology microgrooves. All of these tread features are supported by a new silica tread compound. Combined, the All Season Zone provides all-season traction and handling confidence in wet and wintery conditions."

Kevlar puncture resistence - Goodyear Wrangler 'SilentArmor'.

In 2007, Goodyear added another new tyre to their range which they advertised as having 'SilentArmor' (note the spelling - this is a tyre for the American market). This is a truck and SUV tyre that is essentially constructed identically to a normal radial except that one of the steel belts has been replaced with a Kevlar® belt. Kevlar® is a particularly light but very strong synthetic fiber that doesn't rust or corrode and is five times stronger than steel for the equivalent weight. (It's one of the many things found in bulletproof jackets). The idea here is that the Kevlar® belt helps absorb some of the road noise of the tyre as well as making the tread more resistent to punctures. It's interesting to note that Kevlar® is being advertised in this way as if its something new. In fact, DuPont originally intended their ballistic fabric to replace the steel belts in car tyres but it never quite made it that far.
In addition to the Kevlar® belt, the tyre also has strengthened sidewalls as well as an extended rubber lip around the sidewall to try to help prevent kerbing from damaging your wheels. Goodyear site

Coloured dots and stripes - whats that all about?

dots and stripes When you're looking for new tyres, you'll often see some coloured dots on the tyre sidewall, and bands of colour in the tread. These are all here for a reason, but it's more for the tyre fitter than for your benefit.
The dots on the sidewall typically denote unformity and weight. It's impossible to manufacture a tyre which is perfectly balanced and perfectly manufactured in the belts. As a result, all tyres have a point on the tread which is lighter than the rest of the tyre - a thin spot if you like. It's fractional - you'd never notice it unless you used tyre manufacturing equipment to find it, but its there. When the tyre is manufactured, this point is found and a coloured dot is put on the sidewall of the tyre corresponding to the light spot. Typically this is a yellow dot (although some manufacturers use different colours just to confuse us) and is known as the weight mark. Typically the yellow dot should end up aligned to the valve stem on your wheel and tyre combo. This is because you can help minimize the amount of weight needed to balance the tyre and wheel combo by mounting the tyre so that its light point is matched up with the wheel's heavy balance point. Every wheel has a valve stem which cannot be moved so that is considered to be the heavy balance point for the wheel. (Trivia side note : wheels also have light and heavy spots. Typically the lightest spot on the wheel is found during manufacture and the heavier valve stem is then located diametrically opposite that light spot to help balance the wheel out).
As well as not being able to manufacture perfectly weighted tyres, it's also nearly impossible to make a tyre which is perfectly circular. By perfectly circular, I mean down to some nauseating number of decimal places. Again, you'd be hard pushed to actually be able to tell that a tyre wasn't round without specialist equipment. Every tyre has a high and a low spot, the difference of which is called radial runout. Using sophisticated computer analysis, tyre manufacturers spin each tyre and look for the 'wobble' in the tyre at certain RPMs. It's all about harmonic frequency (you know - the frequency at which something vibrates, like the Tacoma Narrows bridge collapse). Where the first harmonic curve from the tyre wobble hits its high point, that's where the tyre's high spot is. Manufacturers typically mark this point with a red dot on the tyre sidewall, although again, some tyres have no marks, and others use different colours. This is called the uniformity mark. Correspondingly, most wheel rims are also not 100% circular, and will have a notch or a dimple stamped into the wheel rim somewhere indicating their low point. It makes sense then, that the high point of the tyre should be matched with the low point of the wheel rim to balance out the radial runout.

What if both dots are present?

Generally speaking, if you get a tyre with both a red and a yellow dot on it, it should be mounted according to the red dot - ie. the uniformity mark should line up with the dimple on the wheel rim, and the yellow mark should be ignored.

What about the coloured stripes in the tread?

tyre tread stripesOften when you buy tyres, there will be a coloured band or stripe running around the tyre inside the tread. These can be any colour and can be placed laterally almost anyhwere across the tread. For ages I thought they were either a uniformity check - a painted mark used to check the "roundness" of the tyre - or and indication of the tyre runout. Turns out the answer is far simpler and much more disappointing. The lines are sprayed on to the rubber tread stock after it has been extruded during the manufacturing process. The problem is that the tread stock can be manufactured hours or days before it's actually used to make the tyres. So the lines serve one main purpose - they're an in-factory identification for the tyre builders to make sure they're using the correct tread stock for the carcass of the tyre they're assembling. Think of them like a barcode. They can sometimes indicate the rubber compound or the intended tyre size and often you'll find other information printed on to the tread as well as the stripes (see the example below of a number code).tyre tread
When a tyre is being assembled, all the components are put together (carcass, beads, belts etc) inside a tyre mould and the stripes help the technician to align the tread stock properly. The inside of the mould has the inverse pattern of the tyre tread in it so that when heat and pressure are applied, the rubber in the tread stock is forced into the mould. Excess rubber is allowed to escape through little holes (called spew holes) which is why you'll often find what look like rubber hairs on a new tyre - they're excess rubber from the spew holes that was never trimmed. If you look closely at where one of the sprayed-on lines crosses a tread block, you'll be able to see where it's been stretched during the moulding process. The picture above is a good example.
All this is well and good if the manufacturing plant uses an 8-segment petal-type tyre machine (where the mould is on the inside of a bunch of metal 'petals' that close to form the finished shape), but on older 2-part moulds, the tread stock can be pushed off-centre as the mould closes so the lines also serve one other function - a visual inspection post-assembly to make sure the tyre tread remained in the correct place. As the tyre is being spun during inspection, the lines will wander across the tread if something became misaligned during the manufacturing process.

Running in your new tyres

It may sound like an odd concept, but if you buy brand new tyres and slap them on your car, then try to drive the nuts off it, you're going to come a cropper. The reason, believe it or not, is that all tyres need a running-in (or scrubbing-in) period. When tyres are made, the inside of the tyre mould is first lined with a non-stick coating. When the tyres pop out, some of that releasing agent sticks to the tyres themselves. What you get is a nice shiny new tyre, with 'shiny' being the operative word. The releasing agent can take as much as 500 miles to scrub off. Now for the everyday Joe, this isn't really so much of an issue, but for people who are fast drivers, or think they're fast drivers, this can lead to a distressing loss-of-grip mid-corner and a visit to something large and solid. It's doubly important for motorcyclists because they have half the number of tyres and a much smaller contact patch per tyre to boot.

Getting the same results with tyre-black polish or dress-up polish

If you're proud of your car (or vain) you might have been tempted at one point or another to use a Back-to-Black type substance on them to blacken up the sidewalls of the tyres. These things are over-the-counter items that you can buy in just about any car parts store and they're designed to remove the dirt and muck from your sidewalls whilst (allegedly) conditioning the rubber and restoring that factory-fresh look to your tyres. This is all very good until you use a little too much and/or park the car in the sun. When that happens, this stuff starts to run down your tyres and into the tread. Worse, I've seen people using tyre-black on the tread on purpose. This stuff is basically teflon mixed with WD-40 and if you get it on the tyre tread, your car is going to take on the handling dynamics of a drunk ice skater. Not in a "ha ha that was funny" sort of way but in a "holy snot that's gonna hurt!" sort of way. You've been warned.

Learning from others - tyre reviews

With the sheer number of tyres available to you, you might wonder how to choose the one that's going to suit your driving style. Most tyre websites will have a section for customer reviews but you need to be careful because the big-name sites (like TyreRack etc) typically attract people with an axe to grind or those who can never review anything other than positively. As a result, you'll find the same tyre being given 5-star ratings and 1-star ratings and nothing in between, and the reviews will not be especially objective. Tyrereviews.co.uk is a new independent site which seems pretty good - it has a broad spectrum of comments and their reviews are sorted by tyre type as well as by vehicle. If you can't get what you want from the web, go all old-fashioned and use your mouth - ask your friends. I know it's an out-of-date concept, but you'd be surprised what talking to people can reveal, instead of emailing them or worse, txtng yr bff 4 hlp. They will likely have an opinion one way or another and any opinion is worth listening to when you're trying to gather information.

So Chris - what do you like?

My personal favourite tyre brand is Yokohama. I've had them on every vehicle I've owned since 1993. That doesn't necessarily mean they're fabulous tyres, it just means I like them, I like the way they handle, how they wear and how they make the car feel. That's the crux of the matter though. Essentially you're never going to know whether a tyre will suit your needs until you've got them on your car and are driving it the way you normally drive. My advice: if you find a good brand and style that you like, stick with it. It might take a few tyre changes to find one but eventually you'll likely find something that makes you think "hey - this isn't a bad tyre". Just stick with big name brands. Anything that costs less than about $80 or £50 a tyre will be junk. Trust me.

The eBay problem

This paragraph may seem a little out of place but I have had a lot of problems with a couple of eBay members (megamanuals and lowhondaprelude) stealing my work, turning it into PDF files and selling it on eBay. Generally, idiots like this do a copy/paste job so they won't notice this paragraph here. If you're reading this and you bought this page anywhere other than from my website at www.carbibles.com, then you have a pirated, copyright-infringing copy. Please send me an email as I am building a case file against the people doing this. Go to www.carbibles.com to see the full site and find my contact details. And now, back to the meat of the subject....

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