I have found nothing corrosponding the 65mph shimmy and chassis stiffness. - Miata Turbo Forum - Boost cars, acquire cats.

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Old 10-12-2010, 11:13 PM   #1
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Default 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 ***.

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

Thoughts?
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Old 10-12-2010, 11:46 PM   #2
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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 ****.

My NB has the 69mph shimmy too, i never intended to keep it though.
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Old 10-13-2010, 12:02 AM   #3
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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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Old 10-13-2010, 09:38 AM   #4
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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.
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Old 10-13-2010, 10:23 AM   #5
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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.
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Old 10-13-2010, 10:27 AM   #6
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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.
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Old 10-13-2010, 09:23 PM   #7
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Had shimmy. Installed HT. Shimmy gone.
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Old 10-14-2010, 03:41 AM   #8
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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.
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Old 10-14-2010, 04:21 AM   #9
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My 170k mile 1990 chassis has no "shimmy" to speak of. It does have a hardtop, and gc coils though.
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Old 10-14-2010, 02:09 PM   #10
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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.
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Old 10-14-2010, 05:25 PM   #11
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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.
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Old 10-15-2010, 12:04 PM   #12
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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.
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Old 10-15-2010, 01:18 PM   #13
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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.
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Old 10-15-2010, 04:47 PM   #14
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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.
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Old 10-19-2010, 01:40 PM   #15
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i have been driving without my ht and ive noticed it, i think ht solves the problem.
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Old 10-19-2010, 05:55 PM   #16
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I have never had a shimmy.
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Old 10-19-2010, 06:18 PM   #17
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Quote:
Originally Posted by chicksdigmiatas View Post
i have never had a shimmy.
+1
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Old 10-19-2010, 07:53 PM   #18
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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.
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Old 10-19-2010, 08:25 PM   #19
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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.
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Old 10-19-2010, 09:05 PM   #20
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Decent rubber and wheels + Badass hunter wheel balance + dinged up "frame rails" = no shimmy
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