[QUOTE]Originally posted by cdelena
[B]I have compared my car still on 16

 

[QUOTE]Originally posted by Prolene
[B][QUOTE]Originally posted by cdelena
[b]I have compared my car still on 16

 

Thanks, Chris; your posts are always appreciated.

 

pfb-

Dude, I don't mean to rain on your parade, but...

How can tire volume be constant, given that you are going up in wheel diameter, and keeping the outer circumference equal along with tire width in your example?

That difference between the tire volume correlates directly to how well it absorbs bumps and ripples. Think of it this way- a beach ball pumped up to 15psi should be easy to burst just with one person jumping on it, but a giant air mattress at the same pressure would probably need quite a few more people...

As UL pointed out, GRM showed plus sizing to go up in wheel diameter, AND get wider, maintaining air volume. Although it may seem not comparable, that is exactly how you should compare them. In many cases, the biggest wheel/tire combo is not the fastest, but the best compromise of sidewall height, contact patch size, and wheel/tire weights give the car the best feel/speed/grip.

cdelena-

That's why it's so important to get the absolute best tire you can for the given application...



[Edited by GTRPower on 02-19-2001 at 07:36 PM]

 

Always happy to learn something new... I'm not sure we disagree on most points, but I'm also not sure I completely understand your points...

I think you are saying that when you reduce tire volume, you might sacrifice ride quality. I agree.

I also think you are saying it is common to increase the width of a tire as you increase the diameter of the wheel (+1, +2 wheel sizing). This gives a wider contact patch and retains "volume", which can help ride quality. I agree again.

But on this issue of contact patch, I think you might be mis-understanding how it works, or I'm misunderstanding your counterpoints.

Contact patch size has nothing to do with tire volume, wheel width , wheel diameter, tire width or tire diameter. Is is only PSI and weight, with a minor correction for sidewall stiffness. The tire can be 5 inches wide or 10 inches wide, 100 cu/ft volume or 10 cu/ft volume. The patch size is the same unless you vary the PSI or the weight. If your tire is narrow enough (like a bicylcle tire), the rim simply won't lift off the ground until the pressure is great enough to support the weight with a very small contact patch. With a very wide tire (like a earth-mover tire), you could run the tire at extremely low pressures and still support the weight.

 

Hey pfb-

You illustrated my point pretty well there.

The bicycle tire has significantly less air volume than the earth mover tire. Thus, the earth mover tire can get away with lower pressures than the bicycle tire...

I don't see how tire contact patch size is not affected by tire size. With the same tire pressures and same air volume (given the plus sizings by my definition), wouldn't the wider tire have a bigger footprint?

 

I know it seems counter-intuitive, but no. At the same PSI, the wider tire will have a wider footprint, but the circumference of the tire will deform less, giving a shorter footprint. Net effect is the contact patch size stays the same.

Much of this has gotten pretty theoretical, and probably beyond Prolene's original question: Is handling improved with 17" tires? But understanding the theory of contact patch size and the effect of rotating mass hopefully helps us understand why the answer is "not always..."

 

Well, what you're saying about the tires always having the same contact patch makes some sense based on the explanation you gave, but why then does a Viper come with 335/30/18 tires? Perhaps you're only talking about the diameter having no effect on contact patch? So why then do top fuel dragsters use huge ass slicks? Why wouldn't they just save on rotational weight and go with itty bitty ones? I'm not saying you're wrong, but I don't think that what you're saying really holds true in application.

Anyway, to keep things on topic, with regard to wheel weights, here is a link you might find useful:
http://home.earthlink.net/~cvlocas/wheels.html

 

I think what is being stated is that when you hold the overall diameter the same and the cross-section the same the contact patch does not change.

Obviously if you upgrade either of these sizes the available contact patch (depending upon inflation) can be larger.

A typical plus one upgrade may increase wheel and tire width as well as providing a lower profile so an increase in handling is usually expected.

 

Good questions Rev; Ones I'm not sure I can answer well.

Dragsters use huge tires, but run them at very low pressures giving them a very large contact patch. They are also spinning their tires for a good portion of the run, which changes the dynamics quite a bit... Sliding friction vs. static friction, massive heat build-up, sidewall deformation.

The real world, as Rev pointed out, introduces some new variables wich change things somewhat. Much of the change is due to the compliance (flexibility) of the rubber that tires are made out of, and that cars don't sit static on four perfectly smooth glass plates. The are very dynamic. Weights shift, pressures rise, rubber gets hot, lots of acceleration on multiple axis, etc. All of these factors might give a Dragster tire, a Viper tire, a 17" +1 S2000 tire or a 13" race slick a performance advantage in a particular environment, for reasons other than static contact patch size.