Quote:
Originally Posted by Joe731
Thanks for the input.
As you say the R7s 245/40/17 has a 9.7" tread width, and using your +/-1, one shouldn't really use a rim on less than 9", yet their spec have a rim width of 8-9.5 with a measured rim width of 8.5" Is the +/-1 safety or max performance?
Trying solve for that problem and the load issue, I could go with R7s square at 245/40/18 with a tread width of 9" on an 18*8" rim and be within the +/-1" and pick up some load rating (e.g., 1,168 vs 1,235). With this setup, I think the rears will fit, but am a little worried about the fronts. I will need to do some measurements this weekend.
As for tracks in NY, Watkins Glen is my home track, then NJMSP, and I have been to Lime Rock, but am not the biggest fan -- too many trains on a short track with limited passing zones.
Thanks again
|
It's max performance - that was the sole focus of a conversation I had with the Hoosier tire engineer who offered me that rule of thumb.
This page on Tire Rack's website explains that it's the tire industry that decides the measured rim width for a given tire size:
https://www.tirerack.com/tires/tiret...jsp?techid=199. In looking at the competition tires on the Rack's site, the section widths range from 9.6" - 10.3". Curiously, the H2O Wet shows a measured rim width of 8.0".
My sense of it is that for track work the +/- 1" rule is a good one for us to follow. There's another rule of thumb in this context: a narrower tire on wider wheel trumps a wider tire on narrower wheel. The idea is that you'll have more tread contacting the surface of the track, and that wider tread will be more stable as it's stretched tight across the width of the wheel. Note that autocrossers are inclined to do the opposite, often to what seem like extremes.
I'm not sure how much load rating matters here, as all of the tires you're likely to consider would seem to have plenty of load capacity. That might be something to discuss with the Rack; my experience has been that they're able and willing to answer just about any question you might pose to them, and in plain English, too.
I've read other members' comments pointing out that spacers are sometimes needed up front. As I say, though, I have no personal experience with the car yet. There is always the possibility that you will need to roll the front fenders to accomodate a wider tire.
From a safety perspective, this car's weight and power argue for a wide tire when running on a high-speed, heavy-braking track like the Glen. FWIW, I have a feeling you can make 245s work.
So that the rest of us can learn more about tracking this car, please let us know what you choose to do, how your build progresses, and what worked and didn't work for you.
Good luck with it!