Help me diagnose my Miata's vibrations

My Me-Otter has a bad case of the shakes between an indicated 65-70mph. As in "the whole is trying to shake itself to pieces" bad. It's pretty much ruining the fun on the track at the moment and I've got to figure it out before I take the car back on the track.

It feels like a wheel is out of balance, but only at that particular speed. I'm also getting the impression that part of the problem is that I'm hitting some sort of resonance frequency at that speed that makes the shaking much, much worse. What makes the problem really odd is that it seems to happen more often if the road surface is smooth - there are some stretches of road here that are a little uneven, have grooves in them or are just in the typical condition of CA roads where the problem doesn't occur, but put the car on a piece of new, smooth asphalt and it'll try to shake the rollbar out of the car. Oh, and it goes away once you get up to 75mph+ as I found out at Laguna Seca.

So far I've established that it is coming from the rear of the car - I can drive the car with two finger tips on the steering wheel while it's trying to do its "objects in the rear view mirror are less blurry than they appear to be" thing and all I can feel is a little vibration that's more road surface imperfections than wheel imbalance.

What has changed since I had it on the track last year:

• New suspension (Vmaxx coilovers from FM with an NB conversion kit - anything related to the shocks is brand new). As I got the full kit, it's also got new sway bars.
• New wheels (Rota Grid) and tires (Federal 595RS-Rs 205/50/15), fitted by the local SCCA preferred shop who know what they're doing.

What I've checked so far:

• Made sure that the wheels are on "square", hub centric rings (aluminium, not plastic) fitted correctly and wheel nuts torqued correctly. Yes to all three.
• Checked that the tires have no obvious imperfections, damage or any strange wear. Nothing obvious
• Checked that the balance weights are still present - they are.

Last night I popped the "street wheels" back on the rear - OEM 14" wheels with Michelin tires - and went for a shakedown drive. Fortunately we have a stretch of road not far from the house where I can legally do 70mph. With those tires and wheels on the rear the car still gets the shakes, not as badly as with the 15" wheels but it's still very noticeable. So it's not necessarily a problem with the new wheels and tires, but they're making the problem worse. Just as a sanity check I'm planning to put the wheels I had on the back on the front of the car to see if I suddenly get more vibration from the front.

I'm at a bit of a loss as to what to check next. I guess I should go through the suspension again and make sure that everything is tight, check that the diff is properly bolted down and that the axles aren't falling apart. The only other idea I have is that one of the rear hubs might be going bad and the bigger tires are making it more noticeable. Any suggestions how to check that apart from the usual check for play in the wheel bearings, which I forgot to do last night?

What says the peanut gallery? Any other suggestions this side of buying another car?

Possible help?

http://www.miata.net/garage/65_mph.html

You could have a broken belt in one of the tires. Do the move front to back you mentioned first as it eliminates everything but the wheel/tire.

Then, check the tie rods, wheel bearings, bushings, the rack, etc... anything that could amplify a small resonance with play.

Hmm. Everything in that article suggests another look at the tires. I'm not saying they're wrong, but it didn't do that with the Michelins on it before and now it does, too, so I'm not sure that it's really the tires.

Forgot to mention - it's got the standard bracing for a '95 plus FM frame rails (which I'll upgrade to the butterfly brace as soon as I replace the exhaust). I can also feel some of the vibration through the gear lever but not to the extent I can feel it through the seat and the rest of the car.

In reply to Giant Purple Snorklewacker:

I'll put the rear wheels on the front tonight and try again, plus I'll also check for wheel bearing play. The bushings are polyurethane bushings that a bit more than a year old so I'm hoping they're fairly low on the list of problem areas.

I'm surprised the miata.net article doesn't mention it, but one of the common cures for the 65mph shimmy is getting a "RoadForce" balance. It's a special Hunter machine and IIRC, it did help me out when I had minor shimmy issues.

That said, I don't think my last few sets of tires got RoadForced and I've had no issues.

The only other thing that caught my eye was your Rota wheels. I think they may have had issues with roundness or runout or other issues at one time, you might want to look it up.

edit: minor shimmy on your stock wheels could be the standard balance issue, and the resonance might be exacerbated by poor QC on the Rotas. Just a thought.

In reply to peter:

I'll have a look into that, thanks. One of my other ideas was to have the shop dismount the tires and run the Rotas on their balancing machine without tires to see if they're round or out of balance without tires on them.

Miata's are very sensitive to wheel balancing, to the point that mine are out of balance if my lugs are on in the wrong pattern (I have 1 odd lug on 2 wheels).

I haven't even got the wheel locks on it at the moment and the Rotas are bolted on with a set of matching lightweight wheelnuts. But probably still something to look into.

I'm feelin those good vibrations.

My MSM does the same thing. My money is on balance being extremely slightly off. Enough you wouldn't notice in any other car.

Ah, the shimmy. It's been the bane of Miata owners for 23 years.

My theory is that it's a resonance. At 65 mph, your wheels are spinning at 16.5 Hz. That's right about where you'd expect the resonant frequency of the chassis to be - I've never seen the numbers for a Miata specifically, but that's the ballpark range so it makes sense. And here's a new thought - maybe there's something else that shares the same frequency, which is why it's so dramatic when the two of them start singing in harmony. If you stiffen the structure, you move the resonant frequency up and out of the range of the second mass and the problem seems to go away. Stiffening the chassis does change the speed range of the shimmy, which lines up with my chassis resonance idea.

But it does all come back to that exciting frequency, which is the wheels and tires. Balance is important but an out of round tire can do the same - thus the recommendation for a road force balancing.

Not necessarily useful, but I was really intrigued by the primarily-on-smooth-roads aspect.

I wonder whether that actually dovetails nicely with the idea of it being a resonant effect, in that the surface irregularities of a rougher road actually interrupt the resonance by feeding other inputs into the chassis.

ransom wrote: Not necessarily useful, but I was really intrigued by the primarily-on-smooth-roads aspect. I wonder whether that actually dovetails nicely with the idea of it being a resonant effect, in that the surface irregularities of a rougher road actually interrupt the resonance by feeding other inputs into the chassis.

Yes it does, I was just going to say that.

My 96 had that issue with every wheel/tire combo I've used over the last 6-years...unlit I mounted the TRM/Rival setup - which, according to Tire Rack, they road-force balance. So I'd say there's some truth there.

In reply to Keith Tanner:

You actually make a very good point about the chassis stiffening - one thing that I left out of the first post because I didn't quite think it was 100% relevant is that there is more chassis stiffening going on there than just the frame rails. I also put in the roll bar and that'll obviously have an effect on the chassis resonance.

I guess the next steps are:

• Get the wheels and tires checked out and see if the shop can do road force balancing or not
• Change out the exhaust so I can fit the butterfly brace. Theory suggests that this should affect the resonance and thus the shimmy
• Make sure that there isn't any play in the suspension or wheel bearings that would aggravate the situation.

ransom wrote: Not necessarily useful, but I was really intrigued by the primarily-on-smooth-roads aspect. I wonder whether that actually dovetails nicely with the idea of it being a resonant effect, in that the surface irregularities of a rougher road actually interrupt the resonance by feeding other inputs into the chassis.

That's what I was wondering, too, and it kinda makes sense.

The Manic Miata shimmies pretty substantially on the street rubber, but hardly at all on the zombie race rubber.

Hmmm. Buddy of mine has an M edition with the BBS mesh wheels which does the exact same thing. It's been balanced (both standard and road force) replaced both rear hubs for rumbly bearings, every bit of the suspension and steering have been checked for wear/looseness/binding, no joy.

Keith Tanner wrote: Ah, the shimmy. It's been the bane of Miata owners for 23 years.

My B2200 did it, as did every other B2200 I've driven. It's common with them, and referred to as the Mazda shake.

I tried everything I could think of to cure mine, and nothing fixed it. The strange thing was, it wasn't there for the first year or so that I owned the truck, and just occurred out of the blue on the highway one day.

My tires have a flat spot or something, and before I had my the ex-flush Miata on the rack it shook like hell.

After dialing the toe with just a little bit more toe-in than the factory settings suddenly I get much less vibration. It's still there, but I was surprised how much an alignment helped.

Interesting.

FYI, something that came to mind. Check the PPF alignment.

How is that done?

(edit)

Nevermind, found it.

Interesting... Doesn't seem to be a spec for the diff alignment, just trans.

More evidence that there is something to that resonance theory, the shimmy showed up at a different speed after I took off the hardtop a couple weeks ago.