I dropped the pressure in the rear schlicks to 17 psi in a vain attempt to get the back end to stay put....it helped a bit...or they warmed up

Anyhew, I was out in the shop looking at em' this evening. All looked fine except the drivers rear....predominately the outside tire on this last course. There is a cup in the tread located on the inner edge...kinda tapered just short of the inside edge where it rises abruptly....the cup took out the inner wear dots and the inner edge is feather-edged & square.

I'm thinkin' I dropped the pressure too far. Comments?

The good news is the fronts have boogers.

JP,

I saw something similar in my slicks on the 914 when I dropped the pressures really low. But those were on canti's and if I remember you are on 22x10x16 Hoosiers on the rear. How is the rollover looking on the outside of the tire?

When I ran our car at 21lbs or less on the canti's the rollover was too much with the sidewall taking the brunt of the fun. On Fritz, I saw cupping on the inside of the slicks with a softer inflation. I've upped them by 3lbs per side and I am not seeing the issue. But I also changed camber settings at the same time, so the data is mixed.

17lbs eh? It musta been a cool day!

Bill P.

It was prolly 70 when we started running at 9AMish, maybe 80ish when the first group was done. That side got leaned on pretty hard in a sweeper.

On my last run (that I had already fucked up), I was pissed and was really humpin' through that sweeper....the car did a little side jump/hump up (I believe that is the technical term) in back....like it was bouncing sideways....bad roll over, me thinks.
The sidewalls show some abuse.

I think I'll take the rear pressure back up to 20 psi and add some front bar.

What is the alignment on the rear for toe in/out and camber? Also how much change is occuring to the alignment when doing the hard cornering that is causing the wear? Is there anyone that has pictures or movies of the rear end of the car in hard cornering conditions as that would be a huge help in diagnosing what is happening. On my 2L car we run Goodyear cantilevers and use about negative 0.5 degree camber on the rear which is higher than a couple of years ago as the tires were rolling over and was because I was running the pressures at 20# cold and since went up to 22# cold and they wear very even.

John:
The rears are not cantilevers rather 10 X22 X16 Hoosier slicks
on 10" wide wheels.
Camber is -.5, toe is 0.
This was the first time I dropped pressure that far. I ran them at 18-20 all last year
without this having any of this cuping.....the only difference is 2 drivers at AX.

I do have cantilevers in front at 20 psi & they are lookin' good....maybe need more than the -.5 camber I'm using there.

The Hoosiers in the rear might need a bit more than the neg 0.5 camber you have. Are the rear tires radials, if so they'll need probably an extra 0.5 to 1.0 degree of camber? I would imagine running different tires front and rear would make things pretty hard to sort out since the radials tend to "move" more than the cantilever front tires?

I'm fairly sure both front & rear ar bias ply.....the Hoosier web site doesn't specify
them as radial as they do with other types. The rear wear dots look good with the exception of the cupped area. The fronts show the need of some more negative camber.

As for "move", cantis on 7 in wheels move too much for AX, I'm told.....which is why I'm running them on 8s.

I took some pics. Dunno how they're gonna turn out, but we'll see.

JP,

Your last pic is exactly what I saw on my canti's with not enough air pressure. And I saw the exact same thing with my bias ply / non canti slicks on the 911. I have used more air pressure on this next round and taken out camber.

On the 914 we ended up at neg 0.9 up front and neg 0.5 in rear, which sounds like what you have in rear right now. So it maybe needs more air?

On the heavier 911, we're running neg 1.2 up front and neg 0.8 in rear. So far on this round of tires, they are wearing even.

Bill P.

Thanks Bill. It's gud not to be thrashing around in the dark all the damn time.
The low pressure also caused the pass side tiar to deform enuff to rub the fender on the outside. I'll have to take off that w/t & do some grinding on the inner lip...rather than do more Hoosier milling on it.

I was reading the PCRs for Parade and it says (amongst a shitload of other stuff)
that any evidence of tires rubbing on body work is protestable & grounds (NPI) for DQ.

The note you made about rubbing is right on and would only matter if you win as the #2 person will go over your car with a fine tooth comb, similar to a national SCCA event where they measure wheel track, rim width, etc, etc, etc.

There is a half hour impound for all potential trophy winners....if your car is not there at the end of the half hour, no trophy. Not much of a problem for the Mod classes as rules are pretty damn loose......rubbing tires are like fish in a barrel.
Anybody can protest you.