Miata Mailing List: July 1994, Message #192
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   From: dquara@encore.com (Doug Quara)
Subject: Re: High Speed Vibration
   Date: Fri, 8 Jul 1994 13:52:26 -0400

Cyrus Cheung - 486 MD, On Jul 8, 11:55am, writes:
> Subject: High Speed Vibration
> 
> 	Does anybody know why my car shakes a lot when it is
> beyond ~85mph? I mean very unstable. Seems like it does have
> an alignment problem; other than that, what else? Any info 
> will be appreciated.

Can you tell where the vibration is coming from?

In the steering wheel?  Chances are one of the front wheels are
out of balance.  Could also mean that some of the suspension parts
are not as tight as they should be, causing a very slight imbalance
to be exacerbated.  Also, could mean that one of the struts or shocks
is weak, again, making a small imbalance a large one.

Coming from the transmission or rearward?
Can have an out of balance rear wheel.  Rearward vibration can
also mean an out of balance driveshaft, or weakening driveshaft
universal joints.  Maybe even rear axle shafts, but not likely
unless you've bumped a curve sliding sideways.

Coming from the front of the car?  Can be anything that reciprocates
under the hood: the auxiliary devices (pulleys, A/C, power steering
pump, etc.), the flywheel, etc.  But these kind of problems are
pretty rare.

Happening only under braking?  Then the front rotors are probably
warped.

I'd check the wheel balances and the driveshaft and universal joints
before anything else. 

Good luck,

Doug Q.
dquara@encore.com