Miata Mailing List: April 1997, Message #104
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From: 	RSAlder@aol.com

Subject: Re: Tire input?

Date: 	Tue, 1 Apr 1997 17:04:57 -0500



In a message dated 97-03-31 01:29:38 EST, you write:



>  was just wondering if anyone had some input about a tire/rim config. for

>  my 1990 MX-5.  I know that many configurations make the sixty mile an hour

>  shimmy worse and that only certain configurations work well.  I'm already

>  thinking about getting a rear triangle brace.  I'd appreciate any

experience

>  on the subject.



I have chased the shimmy for 80K on my 90 Red.  I have:



Original 7 spoke alloy wheels w/ Yoko A509's currently

Technosports Track bar on the rear since it was marketed...no big improvement

Racing Beat front subframe bar...no big improvement

Flyin' Miata Front Strut Brace...noticible improvement in suspension

feel...reduced 

      shimmy, did not remove it

Numerous alignment and wheel balances.

1 year old Koni's



My shimmy was cured by replacing a very slightly bent wheel (a <1/8th inch

dip at the inside rim lip) with another wheel/tire combo from a car with no

shimmy problem.



To demonstrate how sensitive is the chassis to tire problems:  when I mounted

the tire from the bent wheel onto the straight wheel about half of the shimmy

amplitude returned.  I can live with it.  The Strut bar seems to dampen it

out soon after it starts.  I have a feeling that one of the major causes of

shimmy is flat spotting a tire...say, during a panic stop at freeway speed or

autox practice (I happily admit to that one) or competition.  I don't think

that a quick lockup and release is enough to do it.  And certainly a wheel

out of true by an amount deemed negligible by, in my case, three different

tire installers/balancers.  I would advise careful examination of the tires

by jacking up the car enough to observe the rotating tire. Almost any radial

runout of significance should be suspect.



I'm now looking for a place which will shave (true) tires, before the last

few thousands of miles make the whole search moot.



It was a revelation to try out the known-to-be-true tire in place of the

suspected bad one...absolutely smooth with no hint of shimmy.  It convinced

me that most of the shimmy problems are tire related, IMHO.  All the various

bars and braces seem to help dampen out the shimmy once it begins.



Of course, YMMV, and by all means begin with a balance and alignment first.



Ralph Alder

SoCal 90 Red A