<VV> Re: cowl shimmy shake

[djtcz at comcast.net]

Vibration you can see but not hear (seat back shake, cowl shake, etc) is down around wheel rotating speed, so is caused by something the wheel/tire is doing.  Probably unbalance, or runout, or less frequently these days, tire "hard" spots.

I'd jack up my front end and spin the tire slowly with a pointer near the tire tread.  Much more than 1/8 inch runout can cause problems. Less is better.  Sometimes the runout is the tire, sometimes the wheel, and sometimes both, so repositioning the tire on the wheel can sometimes make it better.  Balancing does not fix runout.

Are your wheels OEM, or some after market jobs?  Is the wheel pilot snug on the hub?  Generally the studs can't be counted on to center the wheel.  An offcenter wheel has runout AND unbalance.

I'd probably swap the front wheels to the rear to see what happens.

--
Dan Timberlake

Message: 3
Date: Fri, 22 Sep 2006 15:09:58 -0400
From: "Robert Hawley" <iamrgh at charter.net>
Subject: <VV> Cowl shake on EM convertible
To: "virtualvairs \(posts\)" <virtualvairs at corvair.org>
Message-ID: <005901c6de7a$b30a4950$d29cf718 at DGYY2L61>
Content-Type: text/plain;	charset="iso-8859-1"

Howdy all,
    After having my car in Group Red for over a year, I am finally able to drive 
and enjoy it for the waning months of summer.  I have noticed that between the 
indicated speeds of 47 - 52MPH the cowl/windshield on my 1963 Spyder does the 
shimmy-thing: above and below those speeds it feels solid.  The shimmy is not 
violent, nor does it feel as if I might lose control, but is noticeable.  When 
the top is up and secured this does not happen. 
    My car has just had the entire front and rear suspensions rebuilt by Ken 
Hand in Pontiac, MI: new bushings, shocks, tires, springs, engine/transmission 
mounts, alignment, crossmembers (front and rear), steering box, idler, etc.  The 
new radial tires (185/80x13 running at 25#F -32#R -  P.S.I.) were balanced and 
there is no indication of tire imbalance.  (Yes, the wheels are on tight - I 
checked the lug nuts)  I have changed hubcaps and there is no difference.