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Contact Patch wider wheels or taller tires.

 
Old 03-09-2009, 01:52 AM
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wanna911
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Default Contact Patch wider wheels or taller tires.

I know there have been discussions on contact patch here before but none specifically of this nature. Would you rather have (on say a 996) a 10 inch wide wheel with a 245/18 tire or a 9.5 inch wide wheel with say a 265/18. Is that extra half inch worth putting on a 996, or just run a bigger tire?

Tire is an R6 with aggressive camber settings (-3.0).

Front wheel specs.

Last edited by wanna911; 03-09-2009 at 02:44 AM.
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Old 03-09-2009, 02:07 AM
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Originally Posted by wanna911 View Post
I know there have been discussions on contact patch here before but none specifically of this nature. Would you rather have (on say a 996) a 10 inch wide wheel with a 245/18 tire or a 9.5 inch wide wheel with say a 265/18. Is that extra half inch worth putting on a 996, or just run a bigger tire?

Tire is an R6 with aggressive camber settings (3.5).
Is this front or rear (I assume rear with the 9.5/10" reference)? What profile for the 245? I don't think you would want to put a 245 on a 10" wheel. Also, what tire combo front/rear? You need to plan both as the rolling diameter of the tires affects the rake of the car - and you don't want it nose up.

What R6 are you talking about - as Hoosier does not make a 265 in the R6. I assume you mean -3.5 on the camber, as +3.5 would not be good. The gurus might have info whether -3.5 camber is right for the R6 in terms of longevity and performance.

-td
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Old 03-09-2009, 02:40 AM
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Peter Carroll/Toronto
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Start with the specific tire spec sheet which gives you a range of allowable wheel widths for a given tire size. Then within that range, use a wider wheel and narrower tire.

Don't entirely trust the suggested rim range or measuring rim spec either. I understand that recommendation is also tainted by DOT regulations. But if you stay within it and lean to the wider rim and narrow tire you should be fine.

If you stretch a tire over a wide rim it will be much more stable. It will also require less camber. This is especially important for a street/track car.

A wide tire on a narrow rim will slop back and forth making the car harder to drive. It will also roll over more requiring more camber to not cord the outside edge.

Stretching a narrower tire also makes it a bit wider because you are laying down more of the read on the road. Similarly, putting a wide tire on a narrow rim will pick up the tread on the sides making it seem narrower.

I learned this the hard way a few years ago. I was running 245 on 7.5" and 275 on 9" rims. It looked cool but it seriously sucked. It was like driving in a hurricane. One instructor refused to get back in the car.

My Turbo currently runs a 315/30-18 Toyo R888 on an 11" rim. It's within the spec. And it looks cool, but it doesn't work that great. The car is very hard to drive near the limit. I'm going back to a 295 on 11 for the next set. It was a lot better.

As for Hoosiers specifically, read the spec sheet carefully. You'll notice a 225 is almost the same size as a 245. Don't go by the 245 'label'. Go by the actual measurements and specs.
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Old 03-09-2009, 02:47 AM
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This is a front tire and I changed the camber to -3.0 to make it more specific and applicable. I know all the tire sizes may not be applicable but I'm trying to get a basic idea of which is better. Obviously different tire brands will require different options in sizing.

I've always liked less sidewall on the tire for more feel, especially the softer the sidewall.
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Old 03-09-2009, 02:51 AM
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Peter Carroll/Toronto
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I'm not sure a 996 needs any more front tire. I would go with the 235 on a 9" wheel. If the car pushes, use your foot to transfer some weight.
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Old 03-09-2009, 03:36 AM
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JackOlsen
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Anyone who's been through an elementary physics class will tell you the size of the contact patch has nothing to do with the diameter or width of a tire, but is determined by the weight of the car and the air pressure in the tires. But that's not entirely true when your talking about anything other than an infinitely flexible innertube.

Generally speaking, wider is better -- both for the wheel and the tire. But the size of the contact patch is less important than the shape of that patch and the mechanical properties of the tires creating it.

It's hard to be sure of what you mean by taller, since you didn't give a sidewall ratio for the tires you're talking about. Here are some commonly available tire sizes in the widths you talked about:

245/40x18 - 25.8" diameter
245/45x18 - 26.8" diameter
265/35x18 - 25.3" diameter
265/40x18 - 26.6" diameter

The tires with the bigger overall diameters are going to have taller sidewalls, but the bigger change is going to be their effect on your gearing -- for the rear tires, at least. A 265/35x18 is going to have a shorter sidewall and a smaller overall diameter than either size of a 245 tire, which is counterintuitive, but is still sometimes the case.

(To those who might not know, the middle number is the sidewall size, but it isn't expressed as a simple number -- it's a ratio that's expressed as a percentage of the tire's width. So you have to know the width for the number to mean something, and it will never mean the same thing on tires of different widths.)

As others are mentioning, you also need to think about the difference between the width of your front tires and your rears. If you change the ratio between the two, you'll need to adjust the suspension to preserve the same handling.

And as has also been mentioned, you wouldn't run either of those 245 sizes on a 10-inch wheel.

And I don't think anyone's mentioned it, but Hoosier doesn't make an R6 18-inch tire in a 265 width.
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Old 03-09-2009, 09:16 AM
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Ditto what Jack said.
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Old 03-09-2009, 12:48 PM
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Originally Posted by himself View Post
What R6 are you talking about - as Hoosier does not make a 265 in the R6.
Originally Posted by JackOlsen View Post
...
And I don't think anyone's mentioned it, but Hoosier doesn't make an R6 18-inch tire in a 265 width.
Reading is fun-di-men-tul. lol. [j/k of course Jack ]


Generally speaking, wider is better -- both for the wheel and the tire. But the size of the contact patch is less important than the shape of that patch and the mechanical properties of the tires creating it.
+1.

-td
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Old 03-09-2009, 01:45 PM
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Originally Posted by himself View Post
Reading is fun-di-men-tul. lol. [j/k of course Jack ]
Whoops. You're right.
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Old 03-09-2009, 02:19 PM
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The other thing to consider on a 996 is that shorter front tires will allow you to run your car lower given the limitations of your suspension geometry. That's one of the reasons we ran 245/35/18s on 9 inch rims. It looks like the new 255/35/18 might be only .1 taller which would make it worth trying although the tirerack site shows it as being very heavy. That's probably a typo.

A 255/35/18-315/30/18 combo might be the way to go.
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Old 03-09-2009, 02:38 PM
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I understand what you are saying and the concepts are correct, however what you have seen might not be representitive of all situations. In fact, i have a LOT of time on 305s 315s and 335s on an 11" rim. (100+ race days) I prefer the 305s and 315s which act identically. They fit perfectly on a 11" rim, based on the manufacturers specs as well. The 335 was a lower profile tire, and although my lap times were no different, it seemed to be the same grip , maybe slightly more skatey in the rear, so I stopped using them. to your point, that "tread lifting" only gave the normally 1" wider tire, a tread width on the 11" rim of only .25" wider than the 305. so , the shape was distorted , and the width was near the same, so I never went back to it after alll my sets of them were used up.

I can tell you the difference on having a 315 vs a 285 on a 11" rim are dramatic. much better with the larger tire. Ive also raced with the set up you mentioned, and putting 275s on a 9" rim in the rear and 245x45s on the front 7.5" rims, although a little ugly as far as fit (to your fit points) it did work better than the 225 / 255 set up on those rims before.

recently, i have put a 305 on a 10" rim up front on my 928 racer. I dont like the way it looks. first time out on a test day, it felt similar to the 275 but a little better grip. It didnt fix a push issue that I thought it would, but it could be due to the point you brought up of the sidewalls rolling over more, or the tread lifting up reducing the contact patch. I will do some more experimenting and get back to you on that. .

I do think if you are playing with different sidewall hights, you also need to play with pressure and take a look at wear patterns and tire temp spreads and pressure after a few hot laps.

Originally Posted by Peter Carroll/Toronto View Post
Start with the specific tire spec sheet which gives you a range of allowable wheel widths for a given tire size. Then within that range, use a wider wheel and narrower tire.

Don't entirely trust the suggested rim range or measuring rim spec either. I understand that recommendation is also tainted by DOT regulations. But if you stay within it and lean to the wider rim and narrow tire you should be fine.

If you stretch a tire over a wide rim it will be much more stable. It will also require less camber. This is especially important for a street/track car.

A wide tire on a narrow rim will slop back and forth making the car harder to drive. It will also roll over more requiring more camber to not cord the outside edge.

Stretching a narrower tire also makes it a bit wider because you are laying down more of the read on the road. Similarly, putting a wide tire on a narrow rim will pick up the tread on the sides making it seem narrower.

I learned this the hard way a few years ago. I was running 245 on 7.5" and 275 on 9" rims. It looked cool but it seriously sucked. It was like driving in a hurricane. One instructor refused to get back in the car.

My Turbo currently runs a 315/30-18 Toyo R888 on an 11" rim. It's within the spec. And it looks cool, but it doesn't work that great. The car is very hard to drive near the limit. I'm going back to a 295 on 11 for the next set. It was a lot better.

As for Hoosiers specifically, read the spec sheet carefully. You'll notice a 225 is almost the same size as a 245. Don't go by the 245 'label'. Go by the actual measurements and specs.
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Old 03-09-2009, 02:40 PM
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265 or 275 are perfect fits on a 9.5" rim.

245 on a 10" rim might work for looks on the street, but wouldnt be that optimal for the track.

mk

Originally Posted by wanna911 View Post
I know there have been discussions on contact patch here before but none specifically of this nature. Would you rather have (on say a 996) a 10 inch wide wheel with a 245/18 tire or a 9.5 inch wide wheel with say a 265/18. Is that extra half inch worth putting on a 996, or just run a bigger tire?

Tire is an R6 with aggressive camber settings (-3.0).

Front wheel specs.
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Old 03-09-2009, 11:49 PM
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wanna911
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Originally Posted by JackOlsen View Post
Anyone who's been through an elementary physics class will tell you the size of the contact patch has nothing to do with the diameter or width of a tire, but is determined by the weight of the car and the air pressure in the tires. But that's not entirely true when your talking about anything other than an infinitely flexible innertube.

Generally speaking, wider is better -- both for the wheel and the tire. But the size of the contact patch is less important than the shape of that patch and the mechanical properties of the tires creating it.

It's hard to be sure of what you mean by taller, since you didn't give a sidewall ratio for the tires you're talking about. Here are some commonly available tire sizes in the widths you talked about:

245/40x18 - 25.8" diameter
245/45x18 - 26.8" diameter
265/35x18 - 25.3" diameter
265/40x18 - 26.6" diameter

The tires with the bigger overall diameters are going to have taller sidewalls, but the bigger change is going to be their effect on your gearing -- for the rear tires, at least. A 265/35x18 is going to have a shorter sidewall and a smaller overall diameter than either size of a 245 tire, which is counterintuitive, but is still sometimes the case.

(To those who might not know, the middle number is the sidewall size, but it isn't expressed as a simple number -- it's a ratio that's expressed as a percentage of the tire's width. So you have to know the width for the number to mean something, and it will never mean the same thing on tires of different widths.)

As others are mentioning, you also need to think about the difference between the width of your front tires and your rears. If you change the ratio between the two, you'll need to adjust the suspension to preserve the same handling.

And as has also been mentioned, you wouldn't run either of those 245 sizes on a 10-inch wheel.

And I don't think anyone's mentioned it, but Hoosier doesn't make an R6 18-inch tire in a 265 width.
I'm really just throwing the sizes out there, the main goal is to try and figure out if getting a custom ccw wheel 10 inches wide is really worth the hassle and worry about fitment and camber and rubbing or is a 9.5 wheel sufficient since it should fit easily.
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Old 03-09-2009, 11:53 PM
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MK
We should put the widows wheels on your racer......the fronts will fit for sure.....the rears might rub really bad....
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Old 03-10-2009, 12:26 AM
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This may not be a direct answer to your question, but an example of a 996 set-up. John at CCW made a few sets of wheels for my 996, basically stuffing the maximum tread width under the stock panels (fender rolling only). The fronts are 9.5" wheel using the new R6 P255/35ZR18 with a 10.3" tread width. The rears are 12" running the R6 P315/30ZR18 with a 11.8" tread width. Both are huge increases over stock width. The result is obvious increased mechanical grip, so we've been able to reduce down force (smaller splitter, less wing angle) and reduce drag. Current camber is 4.2f & 3.4r. John thinks this set up will become the 996 gold standard for race series that will allow it.
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