NOT my first choice....but NASA has penalized Porsches in ST and sequential gearboxes in GTS so an alternative would be to run DOT R6's rather than adding a LOT of weight.
Anyone tried R6's on a cup???

 

too much weight for the tire IMO and the sidewall is too soft. with that said if you have to run it then it will work. I know for sure the tire will work, just look at all the street GT3's that are heavier running on them successfully.

 

Plenty of people running the R6…successfully too….Christian Cook, care to chime in?

 

Drove a 6 Cup on R6s last summer. The shorter R6 (275-30-18 rear and 245-35-18 front) changed the gearing (at Summit Point) which I liked. I think at a longer track like VIR I would have run out of RPMs in 6th before the end of the back straight.

The car was definitely more twitchy on the R6 and I was about 2 sec slower than on used Michelin N1s. The R6s had time on them as well.

 

be careful running short 30 series rears on a cup. there is too much load and they can fail.

 

Mike:
Thanks for the heads up. That was a on shot deal since we didn't have slicks for it the first day I drove it. Has N1s on it now.

 

be careful running short 30 series rears on a cup. there is too much load and they can fail.


That is my biggest concern, I don't believe the right rear would hold up on the banking at Auto Club

 

Right rear is a big Issue at Daytona on 911.

Peter

 

I'm not warming up to the idea of running DOT's. Looks like I'll bail on GTS and run ST and PCA for next season.... The alternative is to run all of the 250lb max allowed ballast which is uncomfortable at best.

BTW what is the trick for surviving the banking at Autoclub? Several friends lost tires with disastrous results there this year...

 

Peter, how much ballast were you running this year at the championships? Are you looking at the new rule set at more restrictive?

 

There was a beautiful lister that was disassembled down the straight and multiple cars were having RR issues at the SoCal crossover event in August. Before I went out my mech said he needed to stand up the rears to handle the banking. Turns out there wasn't enough room to do so so he admonished me not to push too hard or I would be visiting the wall... Drove it like a little old lady on a Sunday drive...but brought it back in one piece...

 

I've seen the 285/30/18 R6's blow up there several times on the banking. Hoosier issued a statement warning about the potential for failure on banked tracks and suggest using a cold starting pressure of 40 or something ridiculous like that. Cup cars come with a 680 series rear tire to eliminate that issue. The taller tire holds more air which equates to a higher loading capability. Cups with their large wings place a heavy load on the outside rear tire when on banking especially at 150+ mph. We take camber out of the right side (CCW) so the tires don't sit on the inner edge, and run a little more pressure on the right rear to help carry the load...

 

As Brant mentioned, I race a 6Cup on R6s and A6s. Size is 315/30ZR18 rear, 255/35R18 front. They work great and I've set my fastest laps on them, beating IMSA GT3Cup Yokohoma or Michelin slicks. No issues with wear or fitment, but the rears are a definitely a tight fit. Rear tire rise cold to hot is 10-14psi, which I don't really like and planning to try 315/40ZR18 in the rear this year to increase volume and (hopefully) reduce pressure rise. Also, be aware, the 30 profile tire have a higher spring rate than slicks and are STIFF on stock springs (I went down in rate for my rough local track which is working well).

If the R6 will work on 600hp Corvettes, they should work for us.

 

I had 150lbs of ballast plus a LOT of extra fuel. With the new sequential rule I'd have to carry 225lbs and if I ran minimum fuel as I should I'd be well over the 250 lb. maximum ballast rule. I'm quite uncomfortable bolting on that much weight in the car...
If I run ST1 I don't need to add any weight as my ratio works out to 5.87 with 5.5 as the minimum. Not sure how I'm going to catch the Corvettes down the straight... I already run a flat wing. I might be able to lose some weight and get closer to 5.5 it would help....

 

Thanks Christian that's really good information. I'll bolt on a set and see how they feel. How different are the camber settings over slicks?