The shimmy problem is caused by the large positive offset of the contact
patch relative to the kingpin centre which results in a particular
sensitivity to disturbing forces such as balance or brake binding. In some
geometries this can also cause a tendency to bump steer. On the Morgan the
bump steer is largely counteracted by the front mounted solid track rod
(also a source of bump steer but in the other direction). More modern
geometries go for a negative offset, typically by angling the kingpin out
at the bottom, a very noticeable feature on MacPherson strut designs where
the spring and the damper has to clear the wheel at the top.
Bob has a good treatment of the issues on the web pages.
When you jack up the front spin the wheels and make sure that the wheels are
true. Even a small deviation, often suffered by wire wheels, can cause some
shimmy. Mount something fixed alongside the rim and watch the space as the
wheel goes round.
If the wheels are true are the tyres straight on the wheels? Same test as
for the wheels.
Balance is the most common problem, especially with Radial tyres. I have
known the balance of the rear wheels be an issue on a GM Cavalier I used to
run which was very sensitive though not on a Morgan.
Most people seem to think that the damper blades are the last place to look.
They were originally fitted to the three wheelers to reduce the sensitivity
to suspension movement (bump steer). It is probable that some of these also
suffered an inadequate castor angle which will create shimmy at speed as
demonstrated by any enthusiastically driven shopping trolley.
Dave Vodden
1992 Plus 4
Hampshire UK
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From: morgans-owner
To: Morgans
Subject: Damper Blades-What For And How Do You (Ah) Test Them?
Date: Thursday 9 October 1997 07:57
My tenure as a Morgan owner is short (two years) but event-filled. The
one device on the car that I've never seen before on other cars, in
one form or another, are these damper blades in the front suspension.
I am now continuing my effort to get a fairly severe front suspension
shimmy out of the system. It started when I put Michelin radials on
instead of HM Tourist Avons six months ago. After two attempts at
balancing, adjusting the brakes so there's no drag (which helped for a
while, they're still ok) the shimmy's back.
Before I go up to Lime Rock for the last time at the end of this
month, I am determined to root out this shimmy devil. Tomorrow AM I
will jack up the front end, assume the lotus position in front of the
car and see what happens. I don't really know what a "bad" damper
blade looks like and thought if any of the assemblage has had
experience with this aspect of the front suspension and thoughts to
share, it might make my task easier.
By the way, if you've found other things than balance, brake
adjustment and wear and tear (the front end's all brand new as of one
year ago) to cause such a shimmy, I'd surely appreciate your insights
here.
Thanks, in advance!
Chip Brown
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