Why use wide tyres

Why do people use really wide tyres on thier GT40s,when the basic laws of pysics tells us

1. The force of friction is directly proportional to the applied load. (Amontons 1st Law)
2. The force of friction is independent of the apparent area of contact. (Amontons 2nd Law)

Amontons' Laws of Friction Guillaume Amontons - Wikipedia, the free encyclopedia

If you choose a tyre that is too narrow then it will overheat, so you need to choose a tyre that is sufficiently wide enough so it doesnt overheat, but is there any advantage in having a tyre that is wider

Tim Kay, and Ron Earp whos races his Lola. have used 225 to 245/50/15 front, and 275/50/15 rears. The rears are approximatly 15% wider

Tom has used 225/50/15 front, and 345/50/15 rear. the rears are approximatly 50% wider

Would Toms tyres, give better lap times,assuming the tyres have the same co-efficiant of friction, or are the really wide tyres just for looks

Im also geussing the reason wider tyres are used on the rear,is because they are the drive wheels, and that creates more heat so a wider tyre is needed....

Im just trying to understand it...any comments would be greatly appreciated

Mostly IMO for looks on road cars.

For race cars its a trade off between getting heat into the tyre and grip.

First reason was looks only and getting a Z rated tire for registration reason.
Another reason was getting a tire in 15" which is a good performer.

If one combines those two parameters there are realy not much tire alternatives leftover. Basically it leaves you with the Pirellis and Michelins TB5 / TB15.

Avon Cr6ZZ is only V rated and due to the vintage shape has only a tread width of 185mm( 215/60/15) and 255m ( 295/50/15) which was to small for me.

I would love to go wider in the front for better balance, but than clearance is an issue.

On my 17" trackwheels i went with 235/40/17 on 9" rims and 315/35/17 on 12".

Front tire size is also a compromise between looks and geometry. If you want your car low down in front and have a big diameter tire (>620mm dia) you compromise in optimum geometry. THerefore i looked for a tire which is lowprofiled as well.

If i would have the free choice in tire size i would love to go with a 255/40/15 front on a 10" rim and stay with the 345/35/15 on 12"in the rear on a widebodied car and with a
235/45/15 on a 9" rim and a 295/45/15 on a 11" rim rear on a small body car. Treadwith should than be as wide as tires nominal size, Z rating would be great and a high silica competition compound would be fun as well. Äh just dreaming.

If one combines those two parameters there are realy not much tire alternatives leftover. Basically it leaves you with the Pirellis and Michelins TB5 / TB15....
Tom and others,
As a side note, I just noticed that Summit Racing is selling the resurrected Michelin TB15's. The product line is listed as Coker Michelin TB15 Tires. Pricey!

Tyres don't grip due to friction on its own. They also grip due to a chemical reaction between the tyre and the road surface. And I seem to remember reading somewhere that with modern tyres the force of friction is not directly proportional, but could be wrong on that.

Carroll Smiths "Tune To Win" has some info on this.
I think it's because those "laws" don't apply directly to a racing tire.

There are other complications that come into play. For example, anyone who follows F1 knows that compounds make a tremendous difference. In part, that is because softer rubber allows for more microscopic deformation, which has the effect of increasing contact area (which is not supposed to matter- but clearly does), and in a way that changes the dynamic from a simple examination of two identical sliding surfaces (for which the "law" assumes, and for which a tire on a road surface obviously isnt, BTW) to a case where the rubber can encapsulate, at a microscopic level, irregularities in the road surface and so change from a simple sliding surface to one that has depth, and so can permit more force to be transferred from the tire to the road, as when accelerating.

For related reasons, that is why taller tires tend to have more forward tractive force (notice I didn't say friction) than the same width tire of a smaller diameter. See any top fuel dragster for a demonstration of this.

If there weren't aero penalties, rotational mass considerations, unsprung weight issues, and regulations prohibiting them, we'd see taller -- and wider- tires in F1, and other racing cars.

Michael Holmes

Lifetime Supporter
Funny this thread has just come up as I have been wondering the same think. When I look at GT's at Goodwood for instance they all seem to be using narrower tires on back than even a 275. I am running 205 60 in front and until I received my new tires yesterday 295 50 on rear. I opted for 275 60 on rear. Both front and rear are shy by half inch of rim optimum size. Since I only track the car a couple of times a year, i am looking for a more original look than a tire that sticks out past my fender lip.
Funny this thread has just come up as I have been wondering the same think. When I look at GT's at Goodwood for instance they all seem to be using narrower tires on back than even a 275.
That's not really by choice. Lord March controls the tire sizes, and thinks that skinnier tires make for a better show, as the cars wind up sliding around all over the place. In fact, the cars race today with tires that are narrower than they were in period! I remember the first time I saw the Cobra Daytona Coupe running there; the tires were laughably narrow, much narrower than when they ran in the 60s. I asked the crew and they explained that they were forced into it, and that it really wrecked the performance of the car and made the driver's job much more difficult. They couldn't wait to get back to the US so they could put the original-size tires back on the car.

Having said that, it does generate a fantastic show!
Sorry to be so late to this but I just saw it.

OK, yes that law of physics is valid...but only when dealing with surfaces that don't conform like a tire (almost like a fluid).

A tire's contact with the road surface is NOT a flat plane. It is 3 dimensional, and this is where many folks go wrong when they tell you that width isn't a factor in grip. It certainly is folks. Take any car you want on a skid pad with a 155 tire and run it around. Take the same car with the same tire make and bump up the size to a 225 with the correct wheel to match. You will see that the 225 produces higher lateral acceleration numbers.

Why is this you ask?

OK, again, the laws of physics are not being violated. IF the road surface were made of highly polished glass, then two tire sizes would, more or less, provide identical numbers.

A road surface however, is not polished glass. You can ask any drag racer why launching from some surfaces (a concrete pad versus asphalt, etc.) make a difference. It's because the tire's rubber actually conforms somewhat to the surface. Think of it like interlocking gears and how you would literally have to rip them apart. This is what is taking place when you "lay rubber" down on the surface. The tire's rubber is being sheared off. The more this interlocking takes place (ie the wider the tire and increased contact patch) the harder it is to rip the tire away.

As another case in point, why is it that you think we run slicks whenever we don't need to worry about water dispersion? It's simply because it maximizes the contact patch.

Hope this makes sense. It should be obvious based on all forms of racing. It's not just heat dispersal (although that is important) it's also about grip.
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Will wider tires always provide a larger contact patch, or do you also have to lower your tire pressure?

Both. Contact patch can be manipulated with pressure.

BTW, not too much of a physics guy but to date I'm hard pressed to remember a race team voluntarily limit the tire size of their race car. "Go as big as the rules allow" almost always applies.
I'd have to believe that F1 teams spent/spend countless millions (reluctantly I'm sure) trying to adapt to more recent rule changes to tyres. Looking at photos of F1 cars back in the 70's the tyres were as wide and big as Kim Kardashian's a$$ and I bet if you asked most F1 teams today they would go wider if allowed. The more rubber on the road has to mean better grip. Yes?

Howard Jones

My GTD orginally ran 285's on the rear with 225's on the front. They were dunlop summer only very good street tires. I went to 315's and 245's Sumotomo's (much cheaper tire) and the laps times came down quite a bit. I would guess at least 2-3 sec on a 2 min lap.

The two things that will always make a car faster are less weight and wider tires. A little light car with much less HP on wide tires will run a big heavy high HP car on narrow tires into the ground every time.