I have found nothing corrosponding the 65mph shimmy and chassis stiffness.
 

Is there a relationship? I just realized today that last winter I took of that brace that goes under the tranny, to hold the tranny tunnel together.

So I don't have the usual 65mph shimmy. I have a 70+ shimmy problem. I shouldn't say that, actually. More like a 65-78ish shimmy problem. Above that and all is well. Cruising at 80 the steering wheel is as smooth and tight as Sister Christian's ass.

Also, another question. I have 3 different tires on my car. Car came with one unmatched tire. Pothole blew out another and now I have 2 of the same tires (x), 1 of brand b and 1 of brand c.

I believe the orientation right now is:
x - B (Front)
..||..
C - X (Rear)

If I changed both of the X tires to the front, would it help out? I'm going to anyways, but it's dark right now and I'm too curious to wait and find out :).

Would a combination of all these things eliminate it, and at the very least help it? Just for reference, balance was just fine before last winter (When all the work happened).

Also note, before you say anything like get new tires, I have 4 brand new tires sitting in my garage waiting to go onto the car this winter :).

A little info on my car:
Stock sways
Racelands (Far from slammed, though. Height is still 11.5 and 11.75 rear-front, respetively)
HD Roll bar

And that's about it for handeling... I spent all my money on the turbo :giggle:

Thoughts?

Yes
Maybe
At the very least, yes
When you get your new tires on, make sure they use a hunter road-force balance. If you've got wide wheels, make sure they do a multi-plane balance.

Put your tranny tunnel brace back.

If those dont help, start looking into the more expensive fixes - frog arms, butterfly brace. Also make sure your wheel bearings aren't shit.

My NB has the 69mph shimmy too, i never intended to keep it though.

My NB has the same 65-70 mph shimmy. It's def coming from the rear. I've already had the read tires trued and balanced and it's beter but still there. I'm hoping that once I bolt in the roll bar and put in the frame rails it goes away.
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Both my 90s have it w/o the hardtop- and one of them has a harddog hc roll bar and rear sub brace. I have noticed that the lighter wheel/tire combo the less the shimmy. But it is in the same mph area. Maybe somebody with a butterfly brace can comment, because it seems to be the more bracing on the underside (bridging the trans tunnel) the less the shimmy. :dunno:

I had a a 65 to 72ish shimmy and then an 80+ shimmy. I started with all 4 tires the same, then had a flat and had a front one go a little bald so I went from

1 - 2 (Front)
..||..
3 - 4 (Rear)

to

3 - 2 (Front) Moved the rear to the front.
..||..
B - C (Rear) Two tires I had from a set of wheels I bought.

That moved the shimmy to 35ish and it was alot smoother after 80. Just replaced the back two with brand new tires and the shimmy is almost totally gone at 65.

The 80+ shimmy I originally had reminded me of the shimmy Jeremy gets on the Porsche episode of Top Gear.

I had no shimmy until I started fooling around with ride heights/springs. I know my alignment is bad, and now I have a horrible vibration at 70mph. I suspect it will disappear the day I finally get it realigned.

Had shimmy. Installed HT. Shimmy gone.

In driving my NB for 5 years, I felt it probably about 5 times total. I had no added rigidity and the suspension consisted of springs, that it.

I did experience what I thought was a shimmy at around ~35 mph, but it was always very short and hard to replicate. Ive never heard of anybody else complaining of a lower speed shimmy so I suspect it was in my imagination.

My 170k mile 1990 chassis has no "shimmy" to speak of. It does have a hardtop, and gc coils though.

My '92 had the 65mph shimmy without the hardtop, lost it with the hardtop in place. Now with my rollbar installed it's gone at any speed, hardtop or not.

It's an imbalanced wheel on my part. Switched the two matching wheels to the front and now the shaking is coming from the back, so it's definitely that.

when i had my tires changed, the shop I took it to drag raced mustangs. I asked that i need my wheel balanced for autocross, so they took the time to properly balance them, even to the point of grinding the weights bit by bit to get them just right.

The shop I got my tire changed at when it blew out... They put a different sized tire on and balanced it by super-gluing magnets to the outside of the wheel.

Needless to say, I haven't been there since.

Yeah, I had the same problem with a Goodyear around here. After I installed my 6UL's and RS2's I have the shimmy bad, especially since the Flex's were installed.

I brought the car to two different places and couldn't get it fixed. I'm hoping once I install my frame rails the problem will go away. I currently have a hard dog roll bar and that doesn't seem to have any effect.

i have been driving without my ht and ive noticed it, i think ht solves the problem.

I have never had a shimmy.

+1

I've had a shimmy do to wheel balance, chassis stiffness (or lack there of), alignment, bent rim and irregular tire wear. Wheel balance is the most common problem. Stiffness is obvious. Hard top on, no shimmy. Off it can do the shimmy shimmy shake.

Current set up is rock steady. 12"f, 12 1/2" rear height, good balance, true wheels, conservative alignment, added door bars.

I've wondered what this was like. I don't get it in my car. I have the Hard Dog Hardcore bar, Hard Dog seat bar, Tein Basics, all same tires and I'm at 11 1/2 all around. Now I rode in my friends 90 which is all stock and it did what you guys are describing. It wasn't bad but quite annoying.

Decent rubber and wheels + Badass hunter wheel balance + dinged up "frame rails" = no shimmy

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.