Puertas

I'm about to buy tires for the C4S.
Are you happy with the RE71's all around?
Or would you go with RivalS in the back? Keeping in mind I have a 1" wider rear wheel...

Thanks!

knfeparty

If you have 11" rear wheels, the 315 Rival S is the way to go. I think the Rival is a little easier to drive on in that it breaks away gradually instead of snapping away from you. I only tried the RE71R because I thought the 285 would be better on the 10" wheel than the ridiculously pinched 315 was.

Puertas

Thanks for confirming my suspicions.
Now I just need to find a shop to cram those 255's on an 8" wheel.

knfeparty

It took 4 ships the first time and every trick in the book. This second time I just left then with discount tire and they did it no problem. I think seeing them already mounted made it easier since they knew it was possible.

Sprockett

The car (C4S) does not have rear grip issues at all. If anything the narrower rear will help balance the car better as well as giving you a lower gearing to help acceleration.

I would (and did on a 996 Turbo) run RE71R 285 tires on your rear which match diameter well with the 255/35 on the front which is a consideration for your AWD.

Using those 315 rear means you would need a larger diameter front tire to match and you are going to have clearance issues in the front fenders as a result...or you are going to have to stay skinny front which you really don't want to do.

Puertas

Hmmmm...
Giving up an inch of tire width per side seems like a lot to leave on the table.
Here are some numbers from TireRack.com

Rears:
Michelin Sport 295/30-18 - 25.0" dia - 10.8" tread width (measured on 10.5" wheel)
Bridgestone RE71R 285/30-18 - 24.8" dia - 10.3" tread width (measured on 10" wheel)
BFG Rival S 315/30-18 - 25.5" dia - 11.4" tread width (measured on 11" wheel)

Fronts:
Michelin Sport 225/40-18 - 25.1" dia - 7.8" tread width (measured on 8" wheel)
Bridgestone RE71R 255/35-18 - 25.0" dia - 9.3" tread width (measured on 9" wheel)
BFG Rival S 245/40-18 - 25.7" dia - 8.7" tread width (measured on 8.5" wheel)

Now, given that the tire width is going to change about .25" for each .5" difference in wheel width, the gap is going to close a little.

Also, I read on this forum somewhere, that the generally accepted differential in front to rear tire diameters for awd cars is <2% - true? And the .5" difference in diameter between the BFG and the Bridgestone falls within that.

I do like the gearing advantage of the Bridgestone, and the fact that all four tires would be the same brand, but not sure I'm willing to give up over 1" of total rear tire width for it...

Oh well, it's one pair of tires that isn't going to last long anyway! :-)

Sprockett

If we are just talking about getting the car to dance through cones, yes, give up the rear tire. The car does not need more rear grip under any situation in autocross IMO. It was hard enough as it were trying to get the back end moving a bit with the 285s in my experience.

Puertas

Interesting.
A friend with a 996TT here in SoCal was running the RivalS all around and his car was way looser than I expected. Driving that car was the main reason I'm hesitant on the 285s.
Granted, the extra 150 HP could have been a contributing factor...

knfeparty

Damping (if you have konis or some other type of adjustables) and swaybars can make a big difference in under/oversteer bias. I would go for as much tire as possible (the 255 re71r and 315 rival S in this case) and then dial in the handling using other means. Granted the AWD cars are a whole different animal, but I am running a GMG rear swaybar on its softest setting and a stock front bar, with a lot of rebound damping up front. I can tune the rear rebound to make the car looser or tighter. Right now it very rarely will exhibit a bit of corner entry push if I mismanage the braking, but still has plenty of oversteer that I can initiate with correct corner-entry braking or with rolling on the throttle on the exit. I am absolutely maxing out my rear grip all the time; I'm at 2* camber and still getting rollover if I go below 40psi. I'm in a constant state of countersteer. *but* the car is fast that way.

Puertas

That was my inclination. If there's one thing I've learned in 30+ years of autocross, it's that there's no substitute for cubic inches of tire...

cag4

Sorry, just saw this:

I run a 1970T modded to a 1973RSR clone, but with the RE-71s I run a square set up of 225/50/16. Car is also about 2400lbs (2600 with me in it).

I run about 25/26 cold, 28/29 hot. Then I'll adjust up or down 1psi depending on the track and grip, e.g. if I want relatively looser rear to get her to rotate I'll run 30 rear.

Try that and watch your wear indicators to make sure you're not rolling over, but I've found that on my car the RE-71s are very sensitive to tire pressure... a couple of pounds too high and I'm ice skating.

I get about 7 autocrosses out of a set before they heat cycle out... plenty of tread left, but times just drop off a cliff.

Hope that helps!

JayG

I run similar pressures to what Charles ^^^does on my Boxster. Same issue with heat cycling our as well. The last AX i did with Re-71's my last times were 2 seconds slower than practice runs. They were heat cycled out
I find on the track 30/31 hot works well.

Just got a set of the new Hankook R-S4 and did one AX with them. I liked them and they love heat

burglar

Just as a dissident point on my other car I run 41psi all around on the RE71, and it has been successful. FWD McStrut car. Lower isn't necessarily better if the sidewall is getting sucked under the wheel badly. Big camber / good camber curve cars seem to respond better to the low pressures. I guess my point is there's no magic number, I've seen success with people running them in the high 20s up into the 40s car dependent, I've even heard of 50s on the rear of successful fwd cars. If you're not sure, experiment a little.

The RE71s tend to suck the first event or two but once they scrub in they're fast to the cords for me. I only AX on them though, don't get them track day hot.

sjfehr

You have to scrub them in and heat cycle them before you try to autocross on them. I typically DD them for a week and do a bunch of circles at a local school bus ramp at about 8/10 until heat builds >160F (one direction and then the other) and then let them cool overnight. I've done this on three different sets now, and every time I've done this before the first autocross on a new set of RE-71R, I've taken FTD, top pax, or both.

Mine have always heat cycled out right about the time I hit the treadwear indicators. Usually I pick up about 2 full seconds vs the heat cycled tires the event before, and am cursing myself for waiting to mount the new tires...

cag4

Interesting... I was instructing a young guy in a new (to him) spec-944 and it seems there's a similar dichotomy with the Toyo Proxes they run: Some like to run high 30's/low 40's PSI and some run low 30's. Apperently there is a "no man's land" in the mid-30's where both agree the tires just won't work.

Several folks have said the RE-71's work until they cord, while I've never gotten more than 7 sessions out of them -- could be that running at higher pressures protect the tires and they work longer, even if you give up initial grip (mine are fast from the first run).