I applaud your enthusiasm about this but you could also buy Paul Haney's The High Performance Tire and read about contact patches. It is all for argument anyhow since I seriously doubt any of us could describe exactly how a marginally shallower contact patch will affect the clock.

Understand that the area of a contact patch remains the same when you change tire widths, within reason. If a tire starts with a 8 unit wide, 2 unit deep contact patch, widening the patch by 1 unit up to 9 units will decrease the depth by 0.22 to 1.77 units deep.*

At the end of the day, everybody will compare notes, still ask which one is faster, andstill request that somebody does an A/B of the 245 and 255.

* These numbers are made up, but the same math works on an assumed 245mm wide contact patch that is 10mm deep. Widen the contact patch by exactly 10mm for a 255mm wide tire, and the patch depth will decrease by 0.4mm.

 

My guess is GRM. To collaborate with your statement, Jason Isely did a similar comparison in Sports Car and saw no appreciable time difference. However, just because it is written somewhere doesn't mean we can take it as gospel to reinforce or disprove it. Andy Hollis does a lot of tests where wheel weights, widths, tread depths are all different so I'm honestly clueless as to what is right to compare and what isn't.

What were the Miata wheels? Enkei 17x8 RPF-1s and TRM 17x9? If so, that is 4.5lbs per wheel.

 

The 245 vs 255 test showed up in the Oct 2010 issue of Grassroots. Fresh Dunlops, shaved and scrubbed, on 17x9 wheels on the Tire Rack test course. (smooth, clean, but worn asphalt)

The wheel weight difference between the 17x9's (TRM C3 and Kosei K1-TS) was 3.6 lbs. The wheel and tire difference combined was 5.2 lbs. The 245's turned an average lap time .66 seconds faster on our 53 second test course. Two drivers were used, John (Woody) Rogers and myself, variables were minimized to the best of our ability and both drivers times were averaged. Both drivers saw a very similar drop in lap times with consistent runs on a course we know extremely well. We also retested the first tire run at the end of the test to make certain course or driver condition had not changed.

Past experience has shown us that the small wheel weight difference has a minimal impact on run times on a smooth course. After the article was printed, we received several questions about this, so we went back (after Nationals) and retested the same tires on identical weight wheels to see how much of the time difference may have come from the wheels alone.
Those tests results will be in a future issue of GRM, but I can share that it did not amount to any where close to the original .66 second time difference. In short, the better acceleration due to lower gearing and the increase in driver confidence due to the more precise feel of the 245 won out in this particular test, under these conditions etc.

Great thread - hope to see/meet a bunch of you at the Dixie Tour.

-Chris Harvey
Tire Rack/GRM Project STR - 2010 MX5

 

Thanks man
Edit- test were done on a miata, less weight and not as much power. The results wouldn't be the same on a s2k since we fight to put down power and might need the extra tire to accommodate the extra weight in a steady state corner

 

^Still interesting info.
When will this issue be out?
"but I can share that it did not amount to any where close to the original .66 second time difference."
So the 245s were faster but not 0.66 seconds like before?

 

Why wouldn't they just use the same rim for the the 245's and 255's? Thanks for adding another variable into the equation

 

Because that's what wheels they had available. They're an autocross team, not F1!

 

That's money right there. And if I can make a deduction, if feel and confidence were noticeably better on the star specs which are known to have stiffer side walls than the hankooks, the difference should be even greater on the floppy rs3.

As much as I appreciate you guys trying to figure out the contact patch, in the end the only way to tell is to run them back to back on the same car and let the stopwatch decide. If I can get my handson someone else s 255, mebbe well swap and compare times.

James Yom

 

Nope just trying to help.

I have no dog in this fight anymore, I've learned my lesson - now I'm just trying to recoup my losses (well I will be as soon as I have time to start pulling the car apart).

 

Does that mean mean you out of str?