Wheels

Shake at High Speed

Hi all,
We had a local PCA meet today and went on a tour. Afterwards a friend and I took our cars out
to knock out the cob webs. He has an 89 turbo 911. When I hit 141 the front of my shark
started to do this odd shake, a flutter like shake. It quit at 143 then came back when I
approached 150, so I backed off. The front-end was recently aligned, tires are good z rated.
What do you think it is, bushings? Tire balance? I know I shouldn't be driving that fast but....I
like too! Any suggestions would be appreciated.
Christine
84 928S


Nice post, Christine! Way to go!

It's probably just a wheel that's slightly out of balance. Get a "precision spin balance" at a good
shop. (Porsche or similar specialty shop, $70-$90 to do all four. Do not use a low-budget tire
store.) Tell them exactly what you told us. When they understand the speeds you'll be running
at, they will take extra care to do a good job.

I had a shake that began at 90 MPH so I got a good wheel balance, and now it's smooth as can
be up into the triple-digits. (On the track, of course) ;-)

Have fun, and be careful out there!

+Greg N.


I experienced a similar problem last year, after having the rear tires replaced (at my official
Porsche dealership). When I mentioned the vibrations at higher speeds (approximately above 90
mph), I was told that this was due to the wear of the front tires (I drive the Bridgestone Expedia
S 01 N2, 225/50 ZR 16 and 245/45 ZR 16 on 8.5x16 and 9.5x16 forged BBS wheels), which still had
about 70 % tread left, but had developed sort of a 'saw-tooth' wear pattern on the outer row of
tread blocks (due to throwing the beast into corners like mad, I guess). Well, NOT!!

A subsequent visit to my trusted independent tire dealer revealed that there seems to be a rule
of thumb that low speed (below appr. 60 mph) vibrations usually stem from misbalanced front
tires and high speed vibrations from misbalanced rear tires! So even though the vibrations were
felt through the steering wheel (and therefore the front wheels), in my case it turned out to be a
misbalance of the rear tires. Had them balanced and, presto, no more vibrations at any speed!

Of course other factors could also cause the vibrations, but I hope this helps.

Best regards,

Thilo Corts
Baden-Baden, Germany
'87 S4