I had the opportunity (was forced) to try my PS2’s as a result of cutting my rear Cup tire so I got to compare some tire data. Here are the conditions: Gingerman Raceway, different front size tires 265-35-18 on the Cups and 235-40/18 for PS2’s, 315-30/18 rear for both, newer tread Cups, very low tread PS2’s, the cup wheels (Fikse) were wider than the PS2 wheels (I-forged), the Cups were at good pressures, the PS2’s were a few pounds too low, inducing push that slowed the lap, tire temps were around 180F on the inside with too little camber up front, too much in the rear. Here are the data charts showing lat and long g’s. The darker colors are the Cups. The lap time for the Cups was 1:32.1, for the PS2’s 1:33.3 again, sub optimal pressures and a lot of push.






As you can see, the max lat and long g loads on the tires are about the same! Yes, the R’s were pretty new and the PS2’s were pretty shaved but dang, the Cups have more rubber and they’re R compound! Quite surprising but consistent with my butt meter and I believe I can get comparable lap times with either tire. The tires drive differently. The cups are quicker to respond and want less slip angle than the PS2’s. The PS2’s squeal louder but are more confidence inspiring because you can throw the car around more and it’s easier to catch as the car response is slower. The cups and PSM really cause problems under heavy trail braking but the PS2’s are not as troublesome (although it still sucks). The cups take longer to heat up or grip less when cold. I am thinking of not running the Cups anymore.

 

That's really interesting data. Thanks.

 

damn, cool. Sounds like th stories of 'Old Yeller' the Olds
power reacer from SoCal that looked like junk but beat
all the 'real' race cars... It ran white walls because the
guy who developed it fond that street tires lasted enough
longer and were more reliable, to win races.
Joe

 

Funny you mention that car. My buddy races against it in HSR West in his '59 TR3A. That thing cooks, too!

http://www.barrettjackson.com/auctionresults/common/cardetail.asp?id=171891

 

colorchange,

I don't find your results out of the ordinary. The Cups demand to be warm prior to delivering performance.

 

Chad, The tires were fully up to temps. As I said in my original post, the minimum temp was 180F way on the inside of the front driver wheel with far too little camber. The tires were up to temp.

 

Interesting info, Tim! I always wondered what the advantage of R-comps over high perf street tires would be. Collective wisdom seem to put it somewhere in the 1-2 seconds range, so your findings seem consistant with that. One thing to keep in mind though is that you're trying to compare the ultimate grip of the tires. So if you think there is still time left on the table at G'man then the data might change somewhat as you find that time. 1:32's at G'man is fast, whether its as fast as your car would go I don't know.

Drew

 

Chad:

My IR sensor is way on the inside of the front left tire. I am quite confident they are getting up to temp as I can cook them if I really push hard (for me sloppy) for a few laps. I plan to move the sensor to get better data. As I said, I cannot get enough camber in the front of my car (only -1.4 degrees) so I am roasting the outsides and ruining tires early. I will post a new thread on fixing that issue.

Drew:

The bottom line for me is I expected to see at least a 0.1 lat g difference. I am not sure that there is any. Now, my inconsistent driving sure doesn't help looking at the data, but maybe it is a small enough difference that simply adds up to a 1-2 second advantage. If that's the case, OK, but I expected to see a bigger difference in the g numbers.