optimal street tire size for RTR on 17x7

[hankster]

never autocrossed but the combo of my FR-S and RTR category has me looking into it. So what would be optimal tire size on 17x7 wheel?

In a lot of my reading the advice is bigger is better, more tread width. But Andy Hollis' Street Touring and Track Day Tire FAQ, in the Optimal Wheel Size section, says that for street tires wider is not necessarily better. He says that for street tires matching tread width to wheel width is best.

On our 7" wide wheels the stock 215 tires are already wider than the wheel width so not optimal (according to the faq). A 195 tire would have a tread width of around 7" (and weigh less).

I have seen on this forum where someone did put 215 on 8" rims (matching tread width to wheel width) but I have not seen anyone mention putting 195 on 7" wheels.

[simpleisbest]

I'm looking to use 235/40-17 in a couple months, once my RPF1's are available.
Most likely go with the Dunlops.

Good to see this thread!!

[Kido1986]

I just put on 215 RS3s onto a 7" wheel, installing tomorrow for tracking at Sebring this weekend then RTRCS soon.

Hardcore streettire autocrossing friend and I had a long discussion about using 235s or 215s. Decided 215s would be better as they would not pinch, stay more flush to the wheel. Rigid sidewall. Also, above 215s become XL rated and gain 3 lbs per tire due to that XL load.

[Biggins]

I think it's just too early to tell. I'm suffering through the rest of this season on the Michelins, but I want to see more opinions before I decide for next season. I do have some 225 Dunlops on my other car that I'm tempted to put on my FRS for my last few events.

I'm leaning toward 215s due to the weight savings, but I like wider tires.

[simpleisbest]

Tire weight is a concern, but I recall that Grassroots did an article last year on wheel weight vs performance that didn't really show a real improvement for lighter wheels in autocross.
The big question is how much advantage 3lbs per corner gives vs the grip from the extra 20mm of contact patch? 215's are a lot cheaper though!!!

[UncleFester]

I have trophied running 215/45 Star Spec's on my FRS.

I tend to follow the measured rim width of the tire and will only +1 size if I feel it's needed. I prefer turn in response to more rubber, especially at low HP levels. As he mentioned, weight should also be a concern for the same reason.

[mrazny]

Part of the point that Andy is effectively getting to is about how to make a particular exact tire optimal. So it's much more about what gets the most out of a 195/50/15 tire, and that comes down to more than just sidewall rigidity, but also total contact patch size and shape. a 235 has a theoretical contact patch that is best maintained with a certain wheel size (definitely wider than a 7"). The question simply becomes is the upsize giving you enough to compensate for your other drawbacks.

In the end, it would come down to testing. And in the base theory of the class (RTR), the concept is cheaper is better. Testing isn't cheap. What'd be nice is 3-4 well established drivers (preferably of the national champion type), in similarly sorted cars, all doing something different with the tire choice and their tire choice alone.

This will be a course dependent car pretty much no matter what. My gut is shorter will be better, and there's too little to gain from upsizing to 235 considering the skinny skinny wheels. It's IMO either the 215/40 or the 235/40 that's the winner. Same wheels, same tire, different sizes, repeatable course, same car, see who wins. Only the Star Spec and the AD08 come in the 215/40, holy damn it's only 23.8" tires. Since both have a pretty strong sidewall, if they don't improve going up to 235's, that's a strong argument for best tire width philosophy.

[Kido1986]

Originally Posted by UncleFester

I prefer turn in response to more rubber, especially at low HP levels.

That's how I feel as well, going for lighter, smaller, stiffer tires.

[MTeator]

Right now I'd say 215's on the hankooks and 235's on the dunlops, and the dunlops are going to be faster anywhere you can keep them from overheating.

[bneale]

I've got some Dunlops on my FR-S. They are 235x45x17 and running on the stock rims. I just went Autocrossing with them at the BC Corvette Club event last weekend. I came 2nd overall out of the cars on Street Tires. I beat out a bunch of Corvettes including a C5 Corvette with 5.8L engine and Michilen Pilot Sports.

Photo for reference incase nobody thinks they fit!

FYI - I plan on doing exactly what the OP is doing. C stock class with RTR. I'm lucky because Scion will pay me to compete in VCMC events if I come 1st, 2nd, or 3rd in my class!


bneale

[simpleisbest]

Nice! Do you notice any significant loss in acceleration with the taller tire?
How is turn-in and feel in slalom transitions? What psi were you running?
(sorry for all the questions!!! LOL)

[7thgear]

the thing about slalom, is that you're limited by rules in terms of rim size and tire

changing the wheel costs you points, changing the tire doesnt

so you're at an advantage stuffing as large a tire on the car as possible and focus on keeping the car flat (like putting on huge swaybars)

this WORKS very well for AUTOCROSS

[simpleisbest]

Originally Posted by 7thgear

the thing about slalom, is that you're limited by rules in terms of rim size and tire

changing the wheel costs you points, changing the tire doesnt

so you're at an advantage stuffing as large a tire on the car as possible and focus on keeping the car flat (like putting on huge swaybars)

this WORKS very well for AUTOCROSS

This discussion is for SCCA C-Stock RTR.
In the class wheel size must be same as stock (no point system). Offset can only be changed by 6mm. Tire size is not restricted.

[7thgear]

Originally Posted by simpleisbest

This discussion is for SCCA C-Stock RTR.
In the class wheel size must be same as stock (no point system). Offset can only be changed by 6mm. Tire size is not restricted.

even more so to my point