tyreandwheel

Decoding all that information on the sidewall

		Key	Description
		A	Manufacturers or brand name, and commercial name or identity.
		B and J	Tyre size, construction and speed rating designations. Tubeless designates a tyre which requires no inner tube. See tyre sizes and speed ratings below.
		C	Denotes type of tyre construction.
		D	M&S denotes a tyre designed for mud and snow. Reinforced marking only where applicable.
E	Load and pressure marking requirement (not applicable in the UK). These go from a load index of 60 (250kg) up to an index of 114 (1180kg).
F	ECE (not EEC) type approval mark and number.
G	North American Dept of Transport compliance symbols and identification numbers.
H	Country of manufacture.

Tyre sizes and what they mean.

185	65	H	R	13
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 the height of the tyre sidewall, or 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.	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.

More recently, there has been a move (especially in Europe) to adjust tyre designations to conform to DIN (Deutsche Industrie Normal). This means a slight change in the way the information is presented to the following:

185 65 R 13 91 V

Tyre width Sidewall height % Radial Rim diameter load rating speed rating.

185	65	R	13	91	V
Tyre width	Sidewall height %	Radial	Rim diameter	load rating	speed rating.

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.

Speed Symbol	Max Car Speed Capability		Speed Symbol	Max Car Speed Capability
Speed Symbol	Km/h	MPH	Speed Symbol	Km/h	MPH
L	120	75	S	180	113
M	130	81	T	190	118
N	140	87	U	200	125
P	150	95	H	210	130
Q	160	100	V	240	150
R	170	105	W	270	168
			Z	240+	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 remortgage your house to buy a new set of tyres.

UTQG Ratings

The UTQG - Uniform Tire 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 : Treadlife, Traction and Temperature.

The treadlife index measures the relative treadlife of the tire compared to a "government reference". An index of 100 is equivalent to an estimated treadlife of 30,000 miles of highway driving.
The traction test is a measure of wet braking performance of a new tyre. There is no minimum stopping distance, therefore a grade "C" tyre can be very poor in the wet.
The temperature test is run at high speeds and high ambient temperatures until the tyre fails. To achive a minimum grade of "C" the tyre must safely run at 85mph for 30 minutes, higher grades are indicative of surviving higher speeds (a rating of "B" is, for some reason, roughly equivalent to a European "S" rating, a rating of "A" is equivalent to an "H" rating.)

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 tire test is entirely subjective.

Tyre constructions.

Simply put, if you bought a car in the last 20 years or so, you should be riding on radial tyres. Radial tyres wear much better and have a far greater rigidity for when cars are cornering and the tyres are deforming.

Radial construction	Cross-ply construction

Aquachannel tyres.

In the last few years, there has been 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. 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(tm) tyres.

This is 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. Well, it seems that TwinTires have put the same principal into effect for those of us with roadgoing cars. Their system uses specially designed wheel rims to go with their own unique type of tyres. Each wheel rim is actually moulded as two half-width rims joined together. The TwinTires tyres then fit these double rims. Effectively, you're getting two independant tyres per wheel, each with their own inner tube or tubeless pressure. The most obvious advantage of this system is that it is an almost failsafe punctureproof tyre. As most [twintyre1]

punctures are caused by single objects entering the tyre at a single point, with this system, only one tyre will deflate, leaving the other untouched so that your vehicle is still controllable. TwinTires themselves actually claim a reduction in braking distance too. Typically from 150ft down to 120ft when braking from a fixed 70mph. The other advantage is that the system is effectively an evolution of the Aquatread type single tyres that can be bought over the counter. In the dry, you have more or less the same contact area as a normal tyre. In the wet, most of the water is channelled into the gap between the two tyres leaving (supposedly) a much more efficient wet contact patch. Typical tyre sizes are 125/85-R16 and 125/90-R16 (Yokohama and Avon).

Run-Flat Tyres.

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. A pressure sensor is strapped [runflat]

to the inside of the wheelrim and is activated by centrifugal forces once the speed of the vehicle is above 5mph. It then samples the pressure once a minute for 4 minutes, and then the temperature once every 5 minutes. The information from all 4 wheels is relayed by radio to a dash-mounted readout for the driver's information. Of course, in normal use, this also means that the driver knows what all 4 tyre pressures are for everyday use. It means they're far less likely to get up one day and find one tyre with such low pressure that it's not possible to drive to a garage to re-inflate it. With run-flat tyres, that also becomes a bit of a moot point.

Wheel Information.

Okay. If you want to change the wheels on your car, you need to take some things into consideration.

Number of bolts or studs
It goes without saying that you can't fit a 4-bolt wheel onto a 5-bolt wheel hub. Sounds obvious, but people have been known to fork out for an expensive set of wheels only to find they've got the wrong number of mounting holes.
Pitch Circle Diameter
Right. So you know how many holes there are. Now you need to know the PCD, or Pitch Circle Diameter. This is the diameter of the invisible circle formed by scribing a circle that passes through the centrepoint of each mounting hole. If you've got the right number of holes, but they're the wrong spacing, again the wheel just won't fit.

4 stud (bolt) PCD 5 stud (bolt) PCD
Inset or outset
This is very important. Ignore this and you can end up with all manner of nasty problems. This is the distance in mm between the centreline of the wheel rim, and the line through the fixing face. You can have inset, outset or neither. This determines how the suspension and self-centring steering behave. The most obvious problem that will occur if you get it wrong is that the steering will either become so heavy that you can't turn the car, or so light that you need to spend all your time keeping it in a straight line. More mundane problems through ignoring this measurment can range from wheels that foul parts of the bodywork or suspension, to high-speed judder in the steering because the suspension setup can't handle that particular type of wheel. This figure will be stamped on the wheel somewhere as an ET figure.

No offset	Inset wheel	Outset wheel

Matching your tyres to your wheels.

Okay. This diagram should help you to figure out what's going on.
[xsection]

Wheel sizes

Wheel sizes are expressed as WWWxDDD sizes. For example 7x14. A 7x14 wheel is has a rim width of 7 inches, and a rim diameter of 14 inches. The width is usually below the width of the tyre for a good match. So a 185mm tyre would usually be matched to a wheel which is 6 inches wide. (185mm is more like 7 inches, but that's across the entire tyre width, not the bead area where the tyre fits the rim.)

Rolling Radius

The important thing that you need to keep in consideration is rolling radius. This is so devastatingly important that I'll mention it in bold again:rolling radius!. This is the distance in mm from the centre of the wheel to the edge of the tread when it's unladen. If this changes because you've mismatched your new wheels and tyres, then your speedo will lose accuracy and the fuel consumption might go up. The latter reason is because the manufacturer built the engine/gearbox combo for a specific rolling radius.

Why would I want to change them anyway?

A good question. Styling and performance are the only two reasons. Most cars come with horrible narrow little tyres and 13 inch rims. More recently the manufacturers have come to their senses and started putting decent combinations on factory cars so that's not so much of a problem any more. The first reason is performance. Speed in corners more specifically. If you have larger rims, you get smaller sidewalls on the tyres. And if you have smaller sidewalls, the tyre deforms less under the immense sideways forces involved in cornering.

So how does it all figure out?

Point to note: 1 inch = 25.4mm. You need to know that because tyre/wheel manufacturers insist on mixing mm and inches in their ratings.
Also note that a certain amount of artistic licence is required when calculating these values. The tyre's rolling radius will change the instant you put load on it, and calculating values to fractions of a millimetre just isn't worth it - tyre tread wear will more than see off that sort of accuracy.
Lets take an average example: a car with factory fitted 6x14 wheels and 185/65 R14's on them.

Radius of wheel = 7 inches (half the diameter) = 177.8mm
Section height = 65% of 185mm = 120.25mm
So the rolling radius for this car to maintain is 177.8+120.25=298.05mm

With me so far? Good. Now lets assume I want 15 inch rims which are slightly wider to give me that nice fat look. I'm after a set of 7x15's
First we need to determine the ideal width of tyre for my new wider wheels. 7 inches = 177.8mm. The closest standard tyre width to that is actually 205mm so that's what we'll use. (remember the tyre width is larger than the width of the bead fitting.)

Radius of wheel = 7.5 inches (half of 15) = 190.5mm
We know that the overall rolling radius must be as close to 298.05mm as possible
So the section height must be 298.05mm-190.5mm = 107.55mm
Figure out what percentage of 205mm is 107.55mm. In this case it's 52.5%
So combine the figures - the new tyre must be 205/50 R15
....giving a new rolling radius of 293mm - more than close enough.

This is an good example which positively demonstrates the next section of this page:

The Plus One concept

The plus one concept describes the proper sizing up of a wheel and tyre combo without all that speil I've gone through above. Basically, each time you add 1 inch to the wheel diameter, add 20mm to the tyre width and subtract 10% from the aspect ratio. This compensates nicely for the increases in rim width that generally accompany increases in diameter too. By using a larger diameter wheel with a lower profile tyre it's possible to properly maintain the overall rolling radius, keeping odometer and speedometer changes negligible. By using a tyre with a shorter sidewall, you gain quickness in steering response and better lateral stability.

Tyre size table upto 17" wheels

Here, for those of you who can't or won't calculate your tyre size, is a table of equivalent tyres. These all give rolling radii within a few mm of each other and would mostly be acceptable, depending on the wheel rim size you're after.

80 SERIES	75 SERIES	70 SERIES	65 SERIES	60 SERIES	55 SERIES	50 SERIES

135/80 R 13	-	145/70 R 13	-	175/60 R 13	-	-
-	-	155/70 R 13	165/65 R 13	-	-	-
-	-	-	175/65 R 13	-	-	-

145/80 R 13	-	155/70 R 13	175/65 R 13	185/60 R 13	185/55 R 14	-
-	-	165/70 R 13	165/65 R 14	175/60 R 14	-	-
-	-	175/70 R 13	-	-	-	-

155/80 R 13	165/75 R 13	175/70 R 13	165/65 R 14	175/60 R 14	195/55 R 14	195/50 R 15
-	-	185/70 R 13	175/65 R 14	185/60 R 14	185/55 R 15	-
-	-	165/70 R 14	-	195/60 R 14	-	-

165/80 R 13	-	185/70 R 13	175/65 R 14	195/60 R 14	205/55 R 14	205/50 R 15
-	-	165/70 R 13	185/65 R 14	205/60 R 14	185/55 R 15	195/50 R 16
-	-	175/70 R14	-	-	195/55 R 15	-
-	-	-	-	-	205/55 R15	-

175/80 R 13	175/75 R 14	175/70 R 14	185/65 R 14	205/60 R 14	195/55 R 15	215/50 R 16
-	-	185/70 R 14	195/65 R 14	215/60 R 14	205/55 R 15	195/50 R 16
-	-	-	185/65 R 15	195/60 R 15	-	205/50 R 16

185/80 R 13	185/75 R 14	185/70 R 14	195/65 R 14	215/60 R 14	205/55 R 16	205/50 R 16
-	-	195/70 R 14	185/65 R 15	225/60 R 14	-	225/50 R 16
-	-	-	195/65 R 15	195/60 R 15	-	205/50 R 17
-	-	-	-	205/60 R 15	-	-
-	-	-	-	215/60 R 15	-	-

So that's it then?

Yes - that's it. A little time with a calculator, a pen and some paper will enable to you confidently stride into your local tyre/wheel supplier and state exactly what you want.

Oversizing tyres

If you want the fat look but don't want to go bonkers with new wheels, you can oversize the tyres on the rims usually by about 20mm (to be safe). So if your standard tyres are 185/60 R14s, you can oversize them to about 205mm. But make sure you recalculate the percentage value to keep the sidewall height the same.

Fat or thin?

Fat tyres look good. In fact they look stonkingly good. In the dry they are mercilessly full of grip. In the wet, you might want to make sure your insurance is paid up, especially if you're in a rear-wheel-drive car. You see fat tyres have a larger contact patch on the road. With the same weight of car on a proportionately larger contact patch, the downforce on the road will be reduced. The bigger the tyres, the less the downforce. The less the downforce, the easier it will be to aquaplane. The easier it is to aquaplane, the less happy your insurance company will be. This is especially true on snow. Either use fat tyres with tons of tread depth, or very thin tyres to cut down to something with a little more grip. Problems in the snow are caused when the snow compacts into the tread and basically turns your brand new tyre into a slick racing tyre with no grip. Another consideration should be clearance with bits of your car.

Diagnosing problems from tyre wear.

Tyres begin to wear in a pattern, however good or bad, that matches their position on the car. If you now change them all around, you end up with tyres worn for the rear being placed on the front and vice versa. The upside of it, of course, (which many people will tell you) is even overall tyre wear. By this, they mean wear in the tread depth. Your tyre wear pattern can tell you a lot about any problems you might be having with the wheel/tyre/suspension geometry setup. The first two signs to look for are over- and under-inflation. These are relatively easy to spot:

Under-inflation	Correct	Over-inflation

Here's a generic fault-finding table for most types of tyre wear:

Problem Cause

Shoulder Wear
Both Shoulders wearing faster than the centre of the tread

Under-inflation

Repeated high-speed cornering

Improper matching of rims and tyres

Tyres haven't been rotated recently

Centre Wear
The centre of the tread is wearing faster than the shoulders

Over-inflation

Improper matching of rims and tyres

Tyres haven't been rotated recently

One-sided wear
One side of the tyre wearing unusually fast

Improper wheel alignment (especially camber)

Tyres haven't been rotated recently

Spot wear
A part (or a few parts) of the circumference of the tread are wearing faster than other parts.

Faulty suspension, rotating parts or brake parts

Dynamic imbalance of tyre/rim assembly

Excessive runout of tyre and rim assembly

Sudden braking and rapid starting

Under inflation

Diagonal wear
A part (or a few parts) of the tread are wearing diagonally faster than other parts.

Faulty suspension, rotating parts or brake parts

Improper wheel alignment

Dynamic imbalance of tyre/rim assembly

Tyres haven't been rotated recently

Under inflation

Feather-edged wear
The blocks or ribs of the tread are wearing in a feather-edge pattern

Improper wheel alignment (faulty toe-in)

Bent axle beam

Problem	Cause
Shoulder Wear Both Shoulders wearing faster than the centre of the tread
Under-inflation
Repeated high-speed cornering
Improper matching of rims and tyres
Tyres haven't been rotated recently
Centre Wear The centre of the tread is wearing faster than the shoulders
Over-inflation
Improper matching of rims and tyres
Tyres haven't been rotated recently
One-sided wear One side of the tyre wearing unusually fast
Improper wheel alignment (especially camber)
Tyres haven't been rotated recently
Spot wear A part (or a few parts) of the circumference of the tread are wearing faster than other parts.
Faulty suspension, rotating parts or brake parts
Dynamic imbalance of tyre/rim assembly
Excessive runout of tyre and rim assembly
Sudden braking and rapid starting
Under inflation
Diagonal wear A part (or a few parts) of the tread are wearing diagonally faster than other parts.
Faulty suspension, rotating parts or brake parts
Improper wheel alignment
Dynamic imbalance of tyre/rim assembly
Tyres haven't been rotated recently
Under inflation
Feather-edged wear The blocks or ribs of the tread are wearing in a feather-edge pattern
Improper wheel alignment (faulty toe-in)
Bent axle beam

Caster, camber, alignment and other voodoo.

Alignment

This is the general term used to gloss over the next three points:

Caster

This is the forward (negative) or backwards (positive) tilt of the spindle steering axis. It is what causes your steering to 'self-centre'. Correct caster is almost always positive. Look at a bicycle - the front forks have a quite obvious rearward tilt to the handlebars, and so are giving positive caster. The whole point of it is to give the car (or bike) a noticeable centrepoint of the steering - a point where it's obvious the car will be going in straight line.

Negative Caster	Positive Caster

Camber

Camber is the tilt of the top of a wheel inwards or outwards (negative or positive). Proper camber (along with toe and caster) make sure that the tyre tread surface is as flat as possible on the road surface. If your camber is out, you'll get tyre wear. Too much negative camber (wheels tilt inwards) causes tread and tyre wear on the inside edge of the tyre. Consequently, too much positive camber causes wear on the outside edge.

Toe in & out

'Toe' is the term given to the left-right alignment of the front wheels relative to each other. Toe-in is where the front edge of the wheels are closer together than the rear, and toe-out is the opposite. Toe-in counteracts the tendancy for the wheels to toe-out under power, like hard acceleration or at motorway speeds (where toe-in disappears). Toe-out counteracts the tendancy for the front wheels to toe-in when turning at motorway speeds. It's all a bit bizarre and contradictory, but it does make a difference. A typical symptom of too much toe-in will be excessive wear and feathering on the outer edges of the tyre tread section. Similarly, too much toe-out will cause the same feathering wear patterns on the inner edges of the tread pattern.

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