High Speed stability issues - diagnostics?

[Emission]

I've had my Porsche for three years now. It has always been rock solid at speed (up to nearly 160 mph)... until recently.

As of the past month or two, I've noticed at speeds over 80 mph, the car is a bit skittish, darty maybe. When I apply the brakes, the car even seems to get a bit more unsettled.

Yesterday, I had it at about 150 mph (in the California desert) and it was downright scary - really had me concerned as it wasn't stable at all. I lifted off the gas and coasted until well below 100 before I applied the brakes.

The car has new shocks on all four corners, new front bearings (they are tight, I just checked), good front tires, brand-new rear tires, stock aerodynamics front and rear...

I don't think it is aerodynamics. I don't think it is street tires as it was acting a bit funny on the R-compound rubber when I had it at the track a few weeks ago. The alignment was just done about two months ago - and it tracks perfectly straight - but it is still suspect.

What do you all think?

Alignment?
Bushings (I think they are original)?
Other?

[TerryH]

What about aerodynamics? At those speeds, anything passing under the car is going to create lift.

.... and holy smokes! Don't get caught driving those speeds. You'd probably lose the car and your freedom.

[Emission]

I rule out aerodynamics as the car has the stock front chin and rear spoilers both intact and unmodified. The car is also at Euro ride height. It doesn't feel like it is lifting - just unstable.

[svandamme]

sounds like to much in the back
not enough pressure in the front

high speeds give you front lift
when you apply brakes, the weight tranfers brutally and gives the car the shakes...

with your new shocks and alignment , possibly it has less rake now
front a bit higher, the back a bit lower... just one idea...

[Emission]

Originally posted by svandamme

with your new shocks and alignment , possibly it has less rake now
front a bit higher, the back a bit lower... just one idea...

"New" shocks means 5,000 miles on them - two years ago at the rate I drive. The car sits level.

I am thinking the car needs new suspension bushings. Does this make sense? Would old bushings "give" under stress (at higher speeds/loads) and braking would compress them even more - both distorting the suspension settings causing my instability?

[Vipergrün]

Hows your alignment? Toe settings?

[Emission]

My tires are different too... would that make a difference?

Front - Yokohama AVS Intermediates
Rear - Yokohama AVS Sports

I don't seem to remember if the stability issue existed before the tire change...

[Vipergrün]

I'd say yes due to differences in tread pattern, possible differences in sidewalls, etc...

[Emission]

Crap.

My wrench thinks the tires are also the issue, followed by alignment settings. Bushings are probably not the issue.

I am not in the mood to dump another $320 on front tires when the existing tires are still good. Really sucks.

[Vipergrün]

Now's your change so burn some rubber and wear those puppies out You could probably sell them in the classifieds...

[Emission]

Originally posted by bb80sc
Now's your change so burn some rubber and wear those puppies out You could probably sell them in the classifieds...

Too bad they are the fronts...

[VZ935]

Your problem is your toe setting.

[patkeefe]

You have original suspension bushings, and your wrench thinks that's not the cause?

[Emission]

Originally posted by patkeefe
You have original suspension bushings, and your wrench thinks that's not the cause?

Well, let me clarify:

My "wrench" - the guy who actually physically will work on my car - hasn't been asked.

My "wrench" (who I referred to) - the mechanic who I have known for years but lives too far to touch my car - says it's not the bushings. He has a 951 (944 turbo) with the same issue, different tires on each end causing instability.

I'd much rather drop $150 in alignment and parts for bushings than spend $350 on new front tires.

There is a thread somewhere about good alignment shops in the area. I need to search and find it.

[Vipergrün]

I'd try Tyson at TRE first, if they do full alignments, then try
Johnson Alignment in Torrance, talk to Steve Alarcon.

[Tyson Schmidt]

If your rear tires are brand new, and the fronts aren't, then the car will be squirrelly until the rears are scrubbed-in and heat cycled.

Basically, the rear tires are brand new squishy sidewalls, with tall tread blocks on them, and the squirm around more than the fronts.

This isn't such an issue if you buy tires with the N rating on the sidewall. These denote tires that Porsche has spec'd as being compatible with their cars. They have a stiffer sidewall.

[patkeefe]

OK. This problem is obviously not the aerodynamics, as they are the same a s always. Does it on two different sets of tires. You have relatively new shocks and alignment...do the tires wear differently L -R, are they scalloped or tread worn funny?

I would at the very least look at the bushings, ball joints and tie rods, and check the rack. Are your tires balanced (maybe you lost some weights)?

My limited knowledge of this is that once you introduce any sort of bind into the suspension, it gets squirrely. Are your alignment and CB numbers repeatable? Are you getting vibration back into the steering column, or just a bit of loss of control?

[al lkosmal]

I would check alignment first.

[patkeefe]

Tyson:
Where would one find this "N" rating? Is it marked on the sidewall somewhere?
Pat

[Emission]

All great advice.

1. I like Tyson's comment about the brand-new rear tires. They have about 80 miles on them, and they had not been heat cycled.

2. Put the car in the air and check all visible bushings/suspension.

3. Have the alignment checked/re-done

4. If all else fails, buy new front tires.