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Originally Posted by jimbo
Are there any automotive engineers out there who can indicate or point me toward determining optimal tire sizes for a given car?.
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Not an engineer with a degree, but have some experience and will try to help.
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Originally Posted by jimbo
The Solstice will be equipped with P245s on each corner. I just read a post by slowandlow stating he might go to 255s on the rear for more grip and I wonder if it would even be of any benefit.
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If you go with wider tires in the rear you may get an increase in understeer. It is unlikely that the car will be oversteery out of the box so wider tires in the rear may do more harm than good if you leave the fronts as is.
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Originally Posted by jimbo
How did the Solstice engineers determine the optimal width of tire for the car? I don't know how to determine that quantitatively. For all I know it is a complicated balancing act. You want to optimize sports car performance and maximize launch from a standing stop, maximize grip on the skidpad and mimimize braking distance. For all I know, one tire width may not satisfy all 3 conditions.
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Optimal width? Probably measured how much space was avaialable in the wheel wells, calculated the maximum size they COULD fit, then balanced the co$t and added weight of wider rims and tires against the benefit of improved lateral grip.
One width, 3 jobs. To be sure, its a blancing act. At some point, wider or narrower will give up too much in one area to be of net benefit. The balance would be more for lateral and longitudinal grip though (turning and accelerating) as braking is more a funcition of tire compound, softer yeilding shorter stopping distance.
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Originally Posted by jimbo
But wider is not always better. Wider tires have several disadvantages and rolling resistance is the primary enemy. Not only do they increase unsprung weigth, they have less grip on wet pavement, have more inertia to overcome accelerating and at steady state speeds, hurt fuel economy, and possibly increase wheelspin upon launch from a standing start due to less pressure on the rubber surface due to the same weight acting over a greater contact area.
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Quite correct right up until the last sentence. Doesn't matter what the tire dimensions are, the contact patch is the same size, so long as the sprung weight of the vehicle pushing down on the tires remains constant.
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Originally Posted by jimbo
How do they determine optimum tire widths for race cars? Is there a forumula or something accounting for weight, wheelbase, front and rear weight distribution, track, and wheel diameter that they use to determine the best width for racing tires? Not everybody races with 700 HP machines. And I'm sure your SCCA Miatas don't wear the same width rubber as Nascar machines.
So how do race teams determine the best width of tire and how did the GM engineers determine to go with P245s for the Solstice? I'm chomping at the bit to know.
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In racing, the best tire selection changes with each track. But mostly, its the sanctioning body rules that govern tire choice. Teams will have a finite number of options and they test as many as possible and use empirical data, i.e. lap times, to determine the best choice. Next time they go to a given track, they have the benefit of prior experience. My guess is that GM engineers work in a similar fashion but with the cost to the consumer as an added factor.
In the case of the Solstice, I'm surprised they opted for 18" rims really. Aluminum is much more dense than rubber. Normally, more rim and less tire is a good deal heavier than its converse. It won't surprise me in the least to find that a 17" wheel and tire package is quicker.
