advantages of larger diameter wheels/tyres ? 1

[hughjinjin]

hi, could someone explain the advantages of say a 17 inch wheel over a 13 inch wheel in a racing environment. From what I've read rolling resistance is primarily a result of the hysterysis of the rubber, so it has nothing to do with that? I've also heard plenty people say that the larger radius giges you a bigger contact patch and therefor better traction but any technical articles I've found say that the contact patch length is dependant on tyre pressure and not on tyre radius. Is it anything to do with carcass distortion under braking or tractive forces alering the contact patch differently for different tyre diameters? Is it a heat issue or is it simply to allow bigger brakes?

 

[CapriRacer]

Since you've confined the discussion to "racing", you should be aware that the answer I am about to give will not apply 100% to the street.

First, larger diameter rims provide more volume for brakes. This is extremely important in preventing brake fade as the larger the amount of mass as well as the less restrictive the air flow, the slower the temperature rises in the brake components.

Second, assuming we are talking about tires of the same outside diameter, the larger rim diameter shortens the sidewall. This result in a tire that reacts quicker to steering input.

A side benefit to this is that to get the same outside diameter, tires with larger rim diameters are wider - and that means more tread on the ground.

You've brought up some other issues, but they deal with the outside diameter of the tire, not the change in rim diameter - and that a whole 'nuther discussion.

Oh and BTW, rolling resistance of tires is so small compared to the rest of the vehicle, that in a racing environment, you can ignore RR.

 

[hughjinjin]

thanks, I am actually talking about larger diameter tyres, do they offer any advantages over smaller ones?

 

[evelrod]

In off road racing, yes, as they make it possible to use very large circumference tires (a seperate issue). In circuit racing, only in so far as they facilitate larger brakes and the fact, as stated previously, that larger diameter wheels can be more easily made wider for wider tire contact patch.
The larger the wheel, generally the heavier the wheel/tire combination, a definite drawback. One of the big reasons you see touring cars and showroom stockers on 17+ inchers is that they were production items, i.e., rules driven.

I race on 10" wheels because of vintage rules. I would use 13's for bigger brakes but tire patch/diameter would remain much the same by using lower profile tires. Alas, the vintage rules won't allow it here in the U.S.

Rod

 

[hughjinjin]

thanks for the replies, I think I'll reword my question a bit.
I'd like to know what is the difference in tyre behaviour between a small diameter tyre and a larger one of the same section width/aspect ratio?

 

[CapriRacer]

hughjinjin,

Perhaps it would be better to tell us what you are specifically trying to accomplish - rather than asking for a theoretical discussion. Here's the reason:

The question the way you posed it has a built in variable - load carrying capacity.

For example: Between 225/40R16 vs a 225/40R18 is a load carrying capacity difference of 45kg at the rated pressure. This is not what normally happens when you are dealing with a racing environment. Usually the load on the tire is the same. So the issue becomes much more complex.

Pretty much all questions where the variables are held constant get confounded in a similar way. It's just the Physics of the situation.

But as a general rule:

Larger diameter tires will be less affected by changes in load - such as load transfer during braking - because the percent of load change is smaller.

The footprint will be slightly shorter, but the width will be unchanged.

So the net effect in cornering would be that larger diameter tires will be slightly better.

BUT..... These differences are fairly small and there are usually other factors that will completely overwhelm any differences.

For example, the difference between a Hoosier and a Goodyear racing tire would be more of a difference than the diameter change would be (assuming we are talking reasonable changes within a given vehicle.) This is inspite of the fact that the tires are - for practical purposes - the same.

I hope this helps clarify things.

 

[patprimmer]

hughjinjin,

in adition to what CapriRacer just said, purely theoretical questions raise the suspicion that you are not an engineer involved in work and that you are quite likely a student cheating on his homework. Either way makes your post very vulnerable to being red flagged.

You will get a lot better answers from a good question. A good question includes the objectives.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[fade2blue]

If there is a difference in weight that would have an effect on the vehicles Izz.