Tire + Wheel total weight and effectiveness.

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Tire + Wheel total weight and effectiveness.

Postby Panic.button » Mon Aug 01, 2011 10:10 pm

For a non-forced induction NC, how much weight do you guys think the wheels + tires can be before you start affecting the acceleration of the car? I'd like to bump up my setup from 235mm tires to 255mm however I'm not sure if it's a proper upgrade as I assume the acceleration of the car would be affected.

Would acceleration even matter that much anyways since with a larger tire, your corner entry speeds would be a lot higher so there would be less need for acceleration and pull after the turns?
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Re: Tire + Wheel total weight and effectiveness.

Postby Brian » Mon Aug 01, 2011 10:46 pm

The weight makes so LITTLE a difference for acceleration IN A STRAIGHT LINE, that it can be difficult to measure....as MANY magazine tests have proven over the years. But I suggest you are concerned about the wrong downsides.

Greater weight in the wheel and tire combo means higher unsprung mass for the suspension to control....which means the suspension will not control it as well (degrades handling and ride quality).

But there's more involved, let's consider two examples. Wider tires mean more rolling resistance and SIGNIFICANTLY more aerodynamic drag...particularly as speed increases. In the STR class of SCCA autocross the 255mm wide tire is the max class size on 17x9 and that seems to be faster for the NC than narrower combos....at least at the speed of autocross (which is 60mph and under). Thus, under 60mph the extra grip of the wider combo can trump the extra weight and drag. In contrast, we have found that on high speed tracks the drag becomes such a factor that the 255mm wide tire and wheel combo makes a slower lap time than the 235mm tire. And, customers have reported that just running down the freeway with the cruise control on they find their gas mileage takes a hit with the wider combos thanks to the drag/resistance mentioned above.
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Re: Tire + Wheel total weight and effectiveness.

Postby Panic.button » Tue Aug 02, 2011 3:55 pm

Interesting response, I'm glad I asked. Thanks!
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Re: Tire + Wheel total weight and effectiveness.

Postby morrisg » Thu Aug 04, 2011 10:32 pm

Be careful when stating that the highway mileage goes down. My 09 Touring setup for STR showed less gas mileage on the dash computer, but this was due to the larger diameter tires turning less at the same speed than the OEM size tires. When I computed actual gas mileage using the odometer, it also looked like less gas mileage until my brain kicked in and I realized that the odometer also showed less distance traveled due to the slower turning tires. You have to adjust the mileage by the ratio of the 255/40 tires (25.0 inch in the case of Toyo R1Rs) to the OEM 205/45 tires (24.3 inch for OEM Bridgestone RE050A) to get your true mileage. Not much difference in gas mileage in my case, but speed limit is 65 out here, so I'm not going any faster than my autox peak speeds!
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Re: Tire + Wheel total weight and effectiveness.

Postby sjd » Fri Aug 05, 2011 6:30 am

Brian wrote:Wider tires mean more rolling resistance and SIGNIFICANTLY more aerodynamic drag...particularly as speed increases. In the STR class of SCCA autocross the 255mm wide tire is the max class size on 17x9 and that seems to be faster for the NC than narrower combos....at least at the speed of autocross (which is 60mph and under). Thus, under 60mph the extra grip of the wider combo can trump the extra weight and drag. In contrast, we have found that on high speed tracks the drag becomes such a factor that the 255mm wide tire and wheel combo makes a slower lap time than the 235mm tire.


That's really interesting Brian. I'm looking at doing more Time Attack next year running a set of 235/40/17 R888's on a 17x8 wheel. For Solo2 I run a 245/40/17 V710's on a 17X9 wheel. Is there an advantage/disadvantage/no change to running a set of 17x9 wheels instead of a 17x8?

Does this also mean that a 235/40/17 is also the ideal street tire size?
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Re: Tire + Wheel total weight and effectiveness.

Postby Brian » Fri Aug 05, 2011 7:58 am

The question of 17x9 vs 17x8 for 235/40/17 is an easy answer....the wider rim gives MUCH better performance for that tire. Much more straight up and down sidewalls for a much more stable contact patch. I can't think of a good reason to use the 17x8 given how close the 17x9 is in weight unless you are running a class that limits the rim width.
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Re: Tire + Wheel total weight and effectiveness.

Postby sjd » Fri Aug 05, 2011 10:59 am

Perfect. That makes sense. The 17x8's I have were RPF1's borrowed from a friend for this season since he changed cars (different bolt pattern) and he is selling them. I was considering buying them but I'll get a set of 17X9's instead.
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