Michael Gibney

Following a wheel and tyre change car is now high speed unstable.

Any ideas on the following?

Standard 1990 C4, all in very good order, no tyre wear, good standard shockers and tracked very well at ALL speeds, had Bridgestone Potenza SO3 tyres 8 X 17 rims on front 9 X 17 on rear (255's). Wheels were quality (OEM) but non-genuine Cup 2's.

I wanted car to have genuine wheels so I swapped the above wheels for Porsche Cup 2's from 1993 C2. The "new" wheels are 7 X 17 front 8 X 17 rear so fitted four new tyres (245's on rear) - same Potenza SO3's.

Now - after the change the car corners well - perhaps even better BUT now it is unstable at 180kph plus. Was previously stable all the way to top speed. By unstable I mean a little floaty and moves around - not enough to get scarey but noticable and enough to keep me on the underside of 200kph.

Is the instability expected with standard rims?

Can I do anything simple to correct?

Would lowering help?

Would spacers on the rear wheels help? (the overall outside tye width is now 24mm narrower)

Is the instability "normal" and what I had with the wider rims the expected imrovement with that setup?

All help appreciated,

Michael Gibney - Australia

Christer

Michael

A change in alignment can cause this - i.e. if there is more negative camber now it would explain the better cornering but less stability in a straight line....how that would come about though....i don't know...?

fore!

Mike,

How may miles do you have on the shocks?

I had 65k on my M030 suspension and noticed I had to pay more attention taking a corner at my usual speed on a local freeway that I did in years past. I just changed out to H&R and bilstein HD's.

What a difference.

Total confidence returned, sometimes it feels I'm going to slow!!

At 180 - watch out for them coppers........

M

Michael Gibney

Thanks for the prompt response.

The alignement is simple to have checked - I have the workshop manual on CD - I suppose the alignement date would be there - but - does anyone have any suggestions on settings?

The shocks and springs are a possible but I wonder how it was stable before and now not so. A local dealer said I should lower because I will be getting too much air under the car - I can see the logic in that but as said it was ok before so although I would like to lower the car I want to track down the instability instead of hiding it by lowering - I mean that lowering might take me back to where I was when it sould be a greater improvement - if you follow?

Will be 2 days before I get back to the board - 800 ks in 964 to the mountains and back to collect my son from camp - poor me!

Michael Gibney

Greg_L

Michael,

I presume you have the wheels balanced properly when you had the tyres put on?
The new original wheels will almost definitly be lighter than the copies. This combined with slightly less rubber on the road could perhaps give a lighter feeling at speed that was not so noticeable with the heavier wheels. . . ?

Mercedesben

Check the tire pressure. My car feels quite strange if the pressure is off.
I drove it home one day and it unstable and I checked the pressure when I got home and I han a rear tire off by 10 pounds!

Adrian

Dear Michael,
How long have you driven on the new tyres. They may need some scrubbing in first. New tyres can often be slippery.
Also remember that 17 inch tyres should be 36/36 PSI not 36/44 PSI as the with the 16 inch tyres.
The other issue is the mounting of the 17 inch wheels. They should be mounted differently in relation to the red stud as compared to 16 inch wheels.
Ciao,
Adrian
964C4

GaryN

Hi Michael,

I would check your front wishbone's, the rubbers perish (especially if they are the original's) this could cause that "tram-line" feeling you get at high speeds. If I'm not mistaken, shold you not have 255's on the rear with 17" rims?? as I understand it you have put 245's on the rear.

Regards
Gary '91 C2

darth

Hi All

Can someone explain what Adrian is taking about with respect to 16" and 17" wheels being mounted differently in relation to the red stud? I've never noticed a red stud on my hubs. Also, does anyone know if wheels originally mounted on a 944 Turbo will fit on my 90C2 - in other words do they have the same bolt pattern 5-130?

Thanks in advance
Bill
90C2

DaveK

One of the studs on each wheel should be red (the end of the stud should be anyway). I believe that the red stud is the one the locking wheel nut should go on - but I didn't know there was a difference for 16" and 17" wheels. I do put my locking wheel nut on the red stud - but I'm wondering if I'm doing the right thing now!

Adrian

Dear Bill,
Mounting difference are all explained in my book. It is the relationship between the red painted stud (with lock nut on) and the air valve on the wheel. If you do not mount the wheels correctly they will be out of balance if you use lock nuts. Not an issue if you do not use them, like me. Also beware using aftermarket lock nuts they can throw the wheels out of balance real quick.
944 wheels which fit the 964 are also listed in my book. You need to compare part numbers or attempt a physical fit. 1986 951 wheels certainly do not fit. Only 1987 and up have the same offset.
Ciao,
Adrian
964C4

Phil Raby

Michael

Would you describe your problem as similar to a side wind knocking the car off line? This can be caused by certain tyre tread patterns and is particularly noticeable with cars that have been lowered. I've spoken with someone who's done a lot of research on this and has come to the conclusion that the problem is less like with tyres that have a radial grooves as opposed to patterned ones.

Cheers

Paul Conquest

I'd be checking tire pressure first. I did a few laps in a friend's 964 at the track last Friday and it very was noticably tail happy and quite twitchy on turn-in until we brought down the rear pressures significantly. Then it stuck like glue.

Adrian

Also check the weight distribution. Low fuel can make a 964 quite unstable at higher speeds. At quarter tank or less, fill it up.
Ciao,
Adrian
964C4