15" vs. 17"

[tirod]

It's been the conventional thinking that to be competitive, you have to use the bigger rims to fit bigger rotors. Well, that all depends on what kind of track you're racing on.

On a drag strip, 15" have less rotational inertia, less overall diameter, and brake size matters a lot less. As tested by Car and Driver, a car on 15's can cut up to a tenth just by using 15" vs 17".

On a road course, the 17" typically use a shorter sidewall tire with more cornering ability - but that isn't an empirically tested standard answer. What went along with the shorter track times were those bigger brake rotors, too. Kit the 15" with better brakes, like 6 piston Wilwoods, and try to get comparable results. It's very, very difficult to make an verifiable comparison.

Inch for inch, a 15" rim will be lighter than a 17" when made identically, and so will the tires. If unsprung weight is a factor, or having less inertia to spin the tire up - or down - to speed, the smart builders pay attention. They compare what they are gaining and losing in the switch. So far, it's largely because of the idea that larger rotors offer more braking. I can see a smart car builder being able to get a 15" equipped car around just as fast - because he could corner at higher speeds and need less brakes. A 17" equipped car with no track time to balance the sway bars, set up caster and camber, or even get the tire pressure right, on street tires picked up because they were cheap won't run with a track car set up on 15" race rubber.

There is a lot more to consider than just the size of the tires, it's a working dynamic package. The end result is what counts - not what it "looks" like. That's what the less knowledgeable take away and copy, and it doesn't always work out so well.

I grew up on 15's, and the current fad to use the bigger rims on a street car seems like a major step backwards. There's more than one article in the press about the lower profile tires being horrible in winter weather, being damaged more easily on typical streets, and all that at a much much higher price. If you want to understand it better, just follow the money.