How come sports cars have low-profile tires and racecars don't?

Street cars, especially higher-performance ones, keep on moving to lower and lower tire profiles. To me, this looked nice in the beginning, but it’s gone too far. The logical conclusion of the trend would be to simply run on solid rubber tires with no air, and it looks like they’re pretty close to that now. Doesn’t it defeat the purpose of having a pnuematic tire?

Low-profile tires are supposed to give better performance. If that’s true, why don’t all race cars have them? F1 cars don’t, and I don’t think NACAR does either. Dragsters don’t. I’m not sure about rally cars. I think those European races of production cars use relatively low tires though. Why are the the tire choices so different if the goal is high performance? Is it just a matter of tradition or cosmetics in the racing leagues?

NASCAR tires have an inner tire with its own air. It’s a safety thing in case the outer tire fails & blows out. Sure that adds some bulk. And some rigidity.

Dragsters are supposed to go in a straight line so they don’t need the benefits of harder & lower profile tires.

Well, F1 cars have lots of finicky regulations to abide by, of which several relate to tires. However, their tires are hand-made by specialists using the best techniques and materials Bridgestone can come up with, cost many thousand dollars per tire, and have a design life of approx 300 km or so (rather than 50,000 plus). I also seem to recollect that racecar suspension is so stiff and short-travel that the sidewalls of the tyres actually provide a significant amount of the total springing/shock absorbption. They are very specialist items. However it wouldn’t surprise me if F1 tyres outperform the highest-spec low-profile tyres available to the general public for their specific purpose - i.e. they corner better, but have poor rain performance since they are very wide, and probably wouldn’t cope very well with potholes.

NASCAR has done some testing with 16 and 17 inch wheels and wider tires. It is considered more as a safety issue, the bigger wheels will allow for bigger brakes and better brake cooling. The wider tires will help handling and alleviate some of the tire issues, mostly heat related, that has been affecting tires that past few years. Many passenger cars and trucks are sold with wider tires than the 10 inch tread models used by NASCAR.

You’re missing a key point. A “low-profile” tire is a tire with a high aspect ratio, meaning the sidewall is small compared to the tire width. All racing tires are much, much wider than a standard car tire so the sidewall can be taller and still maintain a high aspect ratio. A F1 tire is 15" wide, a Goodyear Eagle tire for a Corvette is only 10" wide.

F1 cars aren’t the ideal comparison though, restrictions require the wheel rim to be 13 inches wide at maximum in order to restrict the size of the brake structures which constrains the speed at which cars can enter turns. It’s a safety measure. Indy Cars are a better analogy. An Indy Car uses a 14.5/27.3R15 tire. A Corvette Z06 uses a P325/30ZR19. That middle number indicates the percentage of he side wall height relative to the tire width, in other words aspect ratio. You can see that the Indy Car uses a slightly lower profile tire than a corvette does. The overall sidewall height on the Indy Car is bigger, but compared to the overall width they are lower profile.

A lot of the trend toward larger wheels is based on looks alone. Generally speaking, a larger wheel/shorter tire combo will weigh substantially more than a smaller wheel/taller tire. This increase in unsprung mass makes for a much harder job for the suspension. And worse yet, a lot of that extra weight is distributed out toward the rim, meaning that acceleration and deceleration are negatively affected as well. The only really good reason IMO to go with a larger wheel is to fit a larger brake rotor, and that’s only necessary if you’re regularly encountering brake fade, something only likely in a track situation.

Here’s a real world example. I track a Lotus Elise. It comes with 16" wheels up front and 17" wheels in the rear, smallish by today’s standards. However, the engineers at Lotus wanted 15" wheels for performance reasons, but got overruled by the marketing department. Many of the fastest autocross and track junkies have ditched the stock wheels and gone to a 15" setup. They have found the change to be both an improvement in feel, but a measurable reduction in lap times. Of course, you can get away with than on an Elise because the car weighs so little, enormous brake rotors are unnecessary.

The other often-touted benefit of lower profile tires is better cornering through less sidewall flex (less sidewall = less flex, if you want to get complicated). Of course, this fails to consider that serious performance tires, which as a driver of a car with sporting pretensions you will naturally use, already have thicker than usual sidewalls that flex very little at the expense of a harsher ride.

I suppose it is possible that you could buy larger wheels that are also lighter - people do make wheels made completely of carbon fibre, I suppose, but it is rather unlikely the guidos that typically sport those huge chrome wheels had that in mind.