My history with the dreaded 62mph shimmy

With stock Michelin tires = no shimmy.
Replaced with Azeni Rt615s = shimmy
Replaced with Bridgestone S03s = no shimmy
Back to Azeni RT615 = shimmy
remove STB = shimmy moved up to 65+.
Install hardtop = shimmy gone.

Azeni's quality control sucks.

Just a guess but here are my thoughs as to the problem.

A. Small cheap convertible = not so stiff chassis but fairly stiff suspension
B. Low quality tire = tire with lots of run out

A+B = shimmy at 65mph due to the chassis flexing.

Solutions

Try lighter/smaller tires= less mass/lower run out
Stiffen the chassis, butterfly brace, hardtop, rollcage
Find tires with low run out

Information needed to prove my thoughts.

People that have not had this problem what brand tires do you purchase? High price dosent mean low run out.

People that have rollcages have you ever had this problem?

Greg

Stock I had a pretty good shimmy around 65. I don't remember when exactly it went away, I don't have much in the way of stiffening, just a rollbar and hardtop.

I think all of you are crazy. I've had my car for 8 years and have never once experienced this so called shimmy.

Bought the car stock with shitty tires (stock wheels)= shimmy.
Rollbar with shitty tires (stock wheels)= shimmy
Rollbar with shitty tires (stock wheels) and FM frame rails = shimmy
Rollbar with RT615's (stock wheels) with Hunter roadforce balance & corner weighting w/alignment (did at same time) and FM frame rails = no shimmy

Mentioned shitty tires are cooper cobra's that were on it when I bought it and they probably developed flat spots since the car sat on them all winter. Rollbar is a Hard dog hardcore hard top and this is on an R package with the hollow spoke wheels. When balanced they said they couldn't get the wheels within 10lbs like I asked them since that's what miata.net suggested to do yet they still charged like $30/wheel to do it.

Guys i diagnos "shimmies" all day long at the shop and have never found one that had anything to do with chassis rigidity. Could someone explain tthe physics of how chassis rigidity relates to a shimmy.

Im not saying it doesnt I just cant see how it would and would like an explanation.

Most common causes are wheel balance, especially if it comes at higher speed and goes away as you go even faster.

If its a low speed shimmy you can rule out balance but may look at tire runout.

Also ive always been taught an alignment cannot cause a shimmy, it can wear tires abnormally and they can cause a vibration but not the alignment directly.

I wouldnt start chasing chassis rigidity first address it like every other car.

Check steering and suspension components
Check wheel balance
Check tire runout and for abnormal wear
Check for bent or warped wheels

After all of that if I still havenet found it I normally start looking at drivetrain components.

I think you're probably absolutely right--chassis stiffness doesn't cause it.

However, I bet there's a significant resonance in the chassis at the same frequency that an imbalanced wheel at 65mph inputs force to the chassis.

Furthermore, I bet that it is either torsional or fore-aft "bending" stiffness of the chassis that is in question. This would explain why a hardtop would solve it and a roll bar does not-- the hard top adds stiffness to the chassis front-to-back and torsionally. A roll bar, on the other hand, adds stiffness to the chassis mostly laterally (side to side.)

So a miata without a hardtop or full cage will have a vibrational resonance excited by an unbalanced tire at about 65 mph. Since a hardtop makes the chassis stiffer, a car with a hardtop will likely not resonate in that mode until a much higher speed is reached, and this might be faster than cars travel.

/engineer out

I'd first go with what Opti says ... simply because those areas he identified are the first places to look. If balance/steering/suspension/brakes are out, you're chasing a ghost anyway.

Now, it's not beyond all realm of possibility that there is something in the Miata that is unique and causes some shimmy at some point. Talk about the hardtop/softop experience does suggest something is going on.

The question is what? Does putting on the hardtop fix the problem, or merely cover it up? If there is some kind of chassis/body flex that is at work, it could possibly be both, I guess.

Has anyone tried to run a rear shock tower brace to fix the problem? That should address left-right flex, at least to some extent. It does simulate, in that dimension, adding the hardtop. Might be worth a look.

I'd first go with what Opti says ... simply because those areas he identified are the first places to look. If balance/steering/suspension/brakes are out, you're chasing a ghost anyway.

Now, it's not beyond all realm of possibility that there is something in the Miata that is unique and causes some shimmy at some point. Talk about the hardtop/softop experience does suggest something is going on.

The question is what? Does putting on the hardtop fix the problem, or merely cover it up? If there is some kind of chassis/body flex that is at work, it could possibly be both, I guess.

Has anyone tried to run a rear shock tower brace to fix the problem? That should address left-right flex, at least to some extent. It does simulate, in that dimension, adding the hardtop. Might be worth a look.

I think you're absolutely correct in that it could easily be a case where fore-aft and/or torsional issues could be at fault. Most convertibles have some issues there. It seems to me since the Miata is a droptop from day one that it wouldn't be a problem, but you never know. Ragtops made during the last few years seem to address it better than 90's (or earlier) designs. Maybe they know something now they didn't before.

What about frame stiffeners under the side box sections? That should at least change the flex frequency if it's fore-and-aft, which you should be able to detect as a change in the speed that brings it about.

In either case it might point us in the right direction.

Are people using aftermarket wheels/tires/brakes and if so, what do they weigh?

Does anyone have any experience with replacing body/steering/suspension bushings? Even with just stock stuff, it might be worn to the point where it's the culprit.