From what I understand, a smaller tire diameter will shorten overall gear ratios, in turn, increasing acceleration. But how much of an affect does it have? Does it affect it much to help lower lap times?

Vice versa, considering both are the same tires, how much of slower will the acceleration be with a larger diameter tire?

Are the differences nominal?

 

Minal differences. I have slightly lower profile tires that throw my speedo off by maybe 2 mph. Course the small the wheel you have the smaller the contact pactch so its a trade off I suppose.

 

Basically, you want to keep your tire diameter approximately the same as stock, to keep your suspension and speedometer and so forth working properly. As long as you're talking about differences of no more than a few percent, you won't feel a whole lot of difference either way. You can calculate the difference using a tire calculator like this one.

Wrong. This is a common misconception. Smaller wheels and narrower tires (just like larger wheels and wider tires) have a different SHAPE tire contact patch but the SIZE of the contact patch is the same.

The size of the tire contact patch depends only on the weight of the car and the amount of pressure in the tires. The force of the tire pressure is counteracting gravity by exerting that pressure over the size of the contact patch. So, for example, if you have a car that weighs 2640 pounds and you inflate the tires to 33 pounds per square inch, then the area of the contact patches will be 80 square inches, since that size and pressure will provide the upward force of exactly 2640 pounds to counteract gravity. The size of the contact patches will be 80 square inches regardless of whether your car has 185/60-14 tires/wheels or 245/35-17 tires/wheels (except for slight variations in weight due to the weight of the tire/wheel combination).

 

Additionally the speedo error is not insignificant.
Right now with Azenis 205/50/15 on the stock rim the speedo reads about 8-9% high.

For instance 80 indicated is just under 75 mph actual and 100 indicated is about 91.5 mph actual
(from timed flying miles and speed signs).

 

And this would apply to the Azenis? Thats sweet. I was always under the impression the Azenis would be a tad higher.

 

225/50/15's might just be enough of a correction, if they'd only fit on the car without too much trouble!

 

Wrong.Smaller wheels and narrower tires CAN have the same shape tire contact patch as larger wheels and wider tires, and the size of the contact patch CAN be different.So will a tire thats 4" wide have the same size contact patch as a tire thats 7" wide?...No.

 

Given the same tire pressure, yes.

 

how?

 

Go here and use this..... http://www.miata.net/garage/tirecalc.html

 

"Vice versa, considering both are the same tires, how much of slower will the acceleration be with a larger diameter tire?"

I can't give fist hand experience with it, but I can do the math. Heres a post I started about this awhile back. Basically it looks like just the profile of the tire makes a noticable difference. If you actually went to a bigger rim, it should be a much slower car.
https://honda-tech.com/zerothread?id=138943





[Modified by jond, 4:50 PM 4/28/2002]

 

is it really off by that much? acording to http://www.miata.net/garage/tirecalc.html the stock spedo would only be 1.6% off I havent really tested that out in the real world. Does anyone else know anything about this or is the calculator I used wrong.

 

Think about what holds a car up. It's the air in the tires. We measure how much air is in the tires as pressure, usually in pounds per square inch . Think about what that means for a second. As explained above by nsxtxjr, if your car weighs 2000 pounds, and your tire pressure is 20psi in each of your 4 tires, then your contact patch one one tire is 25 square inches (2000lbs/4 tires=500lbs per tire/20psi=25si). Now lets change the size of the tire to a wider one. If you inflate the new tire to the old pressure, 20psi, you can see nothing changes in the formula. You still have a contact patch of 25si, it is just wider. The only way to change the actual size of the contact patch is to vary the air pressure in the tire.

KNOWLEDGE IS POWER

 

Ddub i follow your example yet if the tire gets wider than the surface area touching the road gets wider as well - thus increasing contact patch. I dont see why thats so hard to understand?

ok well if a larger wheel doesnt mean a bigger contact patch then why does everyone try to get the biggest and widest tire that wil fit on the wheel?

If you guys truly believe that its the same size then why aren't you running some really tiny tire like 13/165/60


[Modified by Muckman, 4:10 PM 5/1/2002]

 

Area of contact patch can be calculated from air pressure and vehicle weight. Period.

Apart from looks, people select wider tires because they can better deal with road surface irregularities. A narrow tire will be less able to deal with road deformities.

Of course, most people think they will get better grip with a wider tire....

 

The calculator is correct. The error is as you stated - you would have to be running big-*** Jeep tires to get an 8% error.

 

When the tire gets wider, the contact patch gets wider (side to side) but shorter (front to back).

There are other advantages to wider tires, besides just changing the shape of the contact patch. Read this article about it.

 

Ok...I have had a lot of trouble with this theory, mainly because I watch television and see the top fuel dragsters running gigantic tires in the rear, and tiny tires in the front, and I have difficulty believing that they do this for no reason. The track is smooth...so one would think that it is not to battle surface imperfections. Additionally, rwd cars like the s2000 have bigger wheels in the rear than they do in the front. Again I have to think that this is done for a reason.

Here is the reason. Everyone here is assuming that the smaller tire and the larger tire actually have the same working psi range. They do not. My spare tire has a recommended pressure of 60psi. Why is this? Because anything less and the tire is going to start to roll onto it's sidewalls or flatten completely. Even if I decide that this is acceptable and do it, I still run into traction problems because, while I do have the "same size patch" I do not have tread on a good part of that patch. Traction is not as good because of this. Additionally I run into added rolling resistance because the tire is so deformed now. This all hurts my performance. And this is why I use a bigger and wider tire. I have more tread, and can use a lower tire pressure, keeping more of that tread in contact with the road. Larger, wider tires are not used simply to help with surface imperfections, the are used because they offer much better grip than the narrow tires.

Frankly I'm disappointed that someone has not responded to this sooner. While the argument that same psi=same contact patch might be accurate, this definitely does NOT mean that same contact patch means same amount of traction.

 

Well for one thing - the top fuelers run about 6-8 psi in their rear tires and then spin them like crazy with about FIVE THOUSAND HORSEPOWER. They have to use bead locks on the tires to keep them from flying off the rims and basically are running on a partially melted layer of rubber as they fly down the rubber-coated track. Not an effective analogy if you ask me.

For every car there will be a sweet spot in terms of rim size and tire size. As you vary from this point you will either give up performance or ride quality. Add into this the effect of wider tires on fuel economy and noise and you end up with a problem that has no "real" answer.

 

I don't deny that there is a certain tire size that works best for a certain car. Actually I am supporting that argument. Note that the contention here in this thread is that the contact patch remains the same at the same psi for different size tires. Then the conclusion that seems to be implied from this point is that tire size does not matter as far as increasing traction. I think the top fueler analogy IS effective because of its ridiculousness...it shows you to an extreme degree what we are talking about here. You will definitely not get the same traction by putting the front tires on the rear of a top fueler. Again the reason that wider tires affect fuel economy is that they DO actually have an effect on the amount of traction that a car is getting.
Phat...I don't really think we are disagreeing here.

 

This is where you are wrong. You are extrapolating contact patch size to handling - which you cannot do. As tires are essentially tubes of air with a rubber/steel exoskeleton they will have differing properties dependant upon the tire compound (did you notice R compound/slick tires are usually wide?), aspect ratio and sidewall stiffness. If you could make skinny tires equivalent to wide tires with respect to these things there WOULD be very little difference - but that is not the case.

As a car rolls from side to side -such as in a turn - it does not evenly distribute the car's weight over the contact patch. The tire deforms and the weight is distributed to the edges. Tire deformation is decrease with a wider/low aspect ratio tire. And we haven't even begun to discuss the effects of the suspension.....bahh

 

I never said that it did. The shape of the patch contributes largely to the amount of traction you get. The size also matters.

But thats not what I was arguing. Given the same psi, and supporting the same weight, two tires of different sizes have the same size contact patch.

 

AGAIN, I agree with you, I was simply stating the implied conclusion of this post. It was Norice who drew the conclusion that wider tires do not give you better handling. That is what I was trying to refute. + the fact that different size tires WILL have different patch sizes because of different psi requirements. So even though =psi equals = patch, this doesn't hold in real world apps where different size tires have different psi requirements. Somewhere along the line Ddub's argument got skewed by someone else.


[Modified by ChopzRSX, 9:18 PM 5/2/2002]

 

It sounds like NONE of you who are arguing read the entire article in the link that I posted. For those who are link-challenged, here is the URL:
http://www.autospeed.com/A_0996/P_1/article.html

It tells that the contact patch doesn't change (if you keep psi constant) but also goes on to describe some other factors that DO change when you change tire size. No one is claiming that handling is unchanged when you change tire size; it is. It's just that the mechanism by which the handling changes is through the shape (not size) of the contact patch and the way the tread deforms etc etc etc.

Read the article.

 

I said no such thing. I siad wider tires will not give greater grip, which they will not. Grip is not a function of tire width.

I am quoting the reference provided by nsxtcjr, because you clearly did not take the time to read it:

http://www.autospeed.com/A_0996/P_1/article.html



[Modified by norice, 10:39 AM 5/2/2002]