New tires+ new alignment = strange handling?

Mike_in OH · #1 (**permalink**) 06-19-2008, 01:41 AM

Had our first DE of the year today, temp in the upper 50's and a bit of rain so I kept the GY F1 Asy tires on instead of the half worn PS2 track tires. A few weeks back had the alignment changed by adding a more toe in the rear --from 04' to 17' and more Camber from -1.4 to -2.0. Not knowing the sweet spot for the air presure settings for this tire, I left them at what he PS2 starts at cold--35 F/ 37R.

The tires felt very grippy for the first 5-6 laps and then started getting skiddish; the rear left felt weird at high speed turns--almost soft--making me feel a little spooked about how far to take certain turns; I almost felt I had a weak shock and this was with PASM in sport.. OTOH, the front end felt planted with good turn in.

Tried to check temps but with the wife's run group right after my run it was difficult to see what he pressure was at the end. When I had the chance to check her pressures, the rear was around 43 but after it had been setting for a bit. With the PS2's and old alignment settings the car understeered but never felt this twitchy.

I'll do another DE in 2 weeks with the PS2's to see if that changes things but I'm wondering about this alignment--would this amount of rear toe in affect handling significantly?

Thanks for any suggestions.

**KS-CS** · #2 (**permalink**) 06-19-2008, 02:34 AM

Quote:

Originally Posted by Mike_in OH

. . . The tires felt very grippy for the first 5-6 laps and then started getting skiddish; the rear left felt weird at high speed turns--almost soft--making me feel a little spooked about how far to take certain turns . . .

I assume you mean the left rear felt weird in right hand turns? Do you mean the car felt loose (oversteer)?

I don't know about the F1s specifically, but with both my PS2s and NT01s they start to become very greasy at hot pressures much above 40, and that tends to happen faster in the rear tires, leading to oversteer. Since you are noting the problem after a few hot laps, I suspect that this is more likely the culprit, rather than the alignment specs.

I used to start with cold pressures similar to yours, but based on advice I received from a Porsche Cup racer, I now start off with much lower pressures , especially in the rear (like 28-29 R and 30-31 F), so I don't feel like I'm skating on ice at the end of a session.

**lawrence** · #3 (**permalink**) 06-19-2008, 03:15 AM

KS-CS is right. I think you need to monitor hot pressure. I kept my bridgestone OEM at 37 PSI hot front and rear. It works great.

I have -2 camber in rear and 0.06 total rear toe and it seems to work good for me. BTW why did you add more rear toe?

Mike_in OH · #4 (**permalink**) 06-19-2008, 01:25 PM

The additional toe was the Porsche techs suggestion since he's done a number of club member cars. BTW, that's .16' on each rear side for a total toe of .32'.
I can't say I've seen or heard of another rear setup with this much toe--any ideas of how what the max toe-in would be?

You're right--it sounds like too much tire pressure in the rear; given the low temps I wasn't expecting the tires to get much over 40 psi.