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Originally Posted by metrickid
Since the 18" tires weigh more this isn't really as big of a benefit as it sounds looking at wheel weight. A Michelin PSS 265/40R18 weighs 2 lbs more than the 265/35R19 as an example. Probably quite close to the weight savings of the wheel.
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Quote:
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Originally Posted by omasou
I had thought bigger would be better b/c the there would be less sidewall to flex additionally ride quality would be a street trait not track trait, right?
Not trying to knock you, only to understand...
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Good question.
That's not always the case. More typically an 18" tire in the same size will weigh less than the 19" counterpart. Just looking at Tirerack's site for various tires a car would use on the track, I'm seeing a lot of the opposite too. Track/Race tires are often manufactured a bit differently than street tires.
If we looked at the following example:
20 lb 18" wheel + 25 lb 18" tire = 45 lbs assembly
vs.
22 lb 19" wheel + 23 lb 19" tire = 45 lbs assembly
It's rare a 19" setup would be lighter/equal to the 18" counterpart, but when it happens the 18" option is almost always still my preference for everything but the rarest use cases. There are lengthy tech articles that go into explaining why. I remember Grassroots Motorsports Magazine doing this specific testing for example.
In summary, the sidewall stiffness is only measurably faster in very rapid slalom runs typically only found on an autocross course. More typically, that faster slalom performance is offset by heavier wheels/tires. On a road course, you aren't weaving like that. As mentioned before, 19" wheels are more expensive and the tires are also more expensive, and availability in track sizing goes down for both.
Look at various BMW's that are competitive in autocross, club racing, time trials... almost never will you see a 19" wheel being used. instead, sidewalls will get taller. The sidewall stiffness of performance tires are already stiff, and you want some compliance in the tire to act as part of the spring rate.
Another way to gut check this logic is to ask "why not 20" track wheels?" or "why not 21" track wheels?". The benefits just aren't there.