RDU Noob Seeking Ninja Shimmy Mechanic!

[bradesp]

Guys,

I've been hoping to "emerge" locally with a killer ride, but dang if I'm not vexed...

After acquiring my first ever Miata 2000' in Sept 2012 and investing serious $$$ in suspension / performance mods I still have an unacceptable shimmy when I drive the car... randomly below 60, and consistently above 60 with severity ranging from light to moderate.

I had Glenn at Long Road Racing do some of my early work, but when I reached out to him today, he informed me he's no longer doing direct customer maintenance :-)

So, who do you know that is a NINJA at diagnosing the root cause of driveline / vibration / shimmy issues? PLEASE HELP!

Below are the details of my setup / frustration!

Bradesp


Details of my 2000' LS

I know too well that some Miata owners NEVER experience a shimmy, while others have a constant shimmy at certain speeds (32mph and 65mph being the most common). Despite doing all the "recommended things" my shimmy just doesn't want to go away

Emilio from 949 racing suggested I find a driveline technician that truly understands driveline setups, suspension, geometry, and most importantly, is skilled at knowing what components to isolate and test for possible source of chassis shim.

Advice I got from 949 Racing in California (the manuf of the wheels on my Miata):
Remove each wheel. Do not remove balance weights. Install on wheel balance then rotate by hand. Measure runout (wobble) both vertical and horizontal. Measure both tire and wheel. Spec for the wheel is .020 runout. You will not notice vibration until about .080" runout. We recommend replacing any wheel or tire past .120"

If wheels check out OK, put car on hoist. Start, put in top gear and accelerate to speedometer reading that matches your vibration on the road.

Here's What I've Done So Far:
Flyin Miata X-Brace Installed
Custom Tuned Coil-Overs Installed - FatCat
Flyin Miata Front Sway Bar Installed
Installed NEW Racing Beat Adjustable Sway Bar END LINKS
OEM Rear Sway Bar Removed - Recommended by FatCat based on custom Coil-Over setup
Ride Height Adjusted Per Coil-Over Manuf Specs
Corner Balance Adjustment Performed
Alignment - done AFTER coil-over installation and ride-height adjustment
Install new Miata Performance Tie-Rod Links
Installed lightly used control arms
Installed lightly used lower ball joints
Installed new drive shaft and U-Joint
New MIata Race Wheels Installed - 949 Race Wheels -6UL
New Tires Installed - RoadForce Balanced 3x at two different locations to achieve sub 10lb #'s... Unfortunately two wheels are NOT below 10lbs. Some other MIata owners claim that anything over 10lbs of road force (using the Hunter 9700) will induce shimmy on a Miata

[— kung fu jesus —]

I have some ideas, but I am battling the flu. I would start by moving the wheels around, mark the two you know that didn't balance correctly. Start by placing them on the same end, the same side, and see if the characteristics change.

Have you checked your wheel bearings?

[Demon I Am]

Andrew at Performance Chassis would be a person I'd recommend. They can be a bit pricey, but are usually thorough.

I was thinking wheel bearings as well.

[— kung fu jesus —]

So, I know about the shimmy in the chassis, but have really only experienced it in the NA. I don't recall feeling it in either NB I have owned.

Regardless, harmonics are a bitch. My NA had it, but by the time I was done modifying the car, the chassis felt modern, taut, and responsive. I had added a lower front subframe brace, strut tower brace, rollbar, frame rail overlays, door bars and bolted on my hardtop. IMO, the door bars made the biggest difference in stiffening the chassis above all else. So much so that I added them to my NB with the same results.

Before you go through all that, I still stand by my original comment about narrowing it down to wheels or wheel bearings. It may also be beneficial to borrow another set of wheels and tires to see if the results change.

I similarly chased issues like these on my e30. Not really an apples to apples comparison, but did you have the driveshaft balanced when you rebuilt the driveshaft? The Miata's DS is not balanced at the factory, and it could be a source of your vibration. The same could be said for loose or bad differential mounts, loose spine mounts, etc. On my e30, it was a combination of front wheel bearings and worn universal joints on the driveshaft PLUS badly balanced tires.

Out of curiosity, have you tried to see if it was speed sensitive vs RPM sensitive? Just trying to eliminate possibilities.

[bradesp]

I've taken it to another mechanic who used to personally track both RX7's and MIata's... he drove with me yesterday and there's *some* good news... The shimmy I was experiencing at speeds below 60mph was, according to my mechanic, "exactly how a high performance suspension should feel"... tight, controlled, not harsh, but transmitting lots of road "feel" into the car. What i thought was shimmy was simply road vibration... again BELOW 60.

Then we hit the freeway... and before we could exit the onramp, he looks at me and says "woah... is that what you've been feeling? That's definitely not normal". By trying a few things at higher speed he felt comfortable that my drivetrain (engine mounts, transmission, drive shaft, and U-Joint) are NOT the culprit... He's also fairly certain the front end was doing just fine... So his initial suspicion is not surprising... wheel/tire balance, but only in the rear. He's really puzzled because I've had the car with two other reputable mechanics, including one that is the area "go to track setup / chassis group" and none of them could tame my shimmy.

I'm hoping at this point it truly is just a wheel/tire problem and the answer may just be to dump my wheel/tires and buy new ones... Kinda sucks. It means replacing the setup below... we'll see.

949 Racing - 6UL 15x8 wheels
http://949racing.com/15x8-6ul-nickel.aspx

and 205/50-15 Yokohama S Drives
http://www.tirerack.com/tires/tires....partnum=05VR5S

[— kung fu jesus —]

I know how you feel. I chased shimmies in my e30 for 8 months, gradually reducing it until I eventually found out the shop I had been going to repeatedly to road force balance the tires wasn't doing it correctly. I had a second shop do it. They found huge discrepancies in the tires. I took it back to the original shop, forced the issue and they replaced the offending tires under warranty. I was pretty pissed at the runaround they had given me. I was concerned it was the wheels because they are sort of inexpensive, but both shops said the had excellent runout.

I also have to ask...was the car ever in an accident? A rear hit and the rear subframe a bit out of alignment can cause a ton of funkiness.

[Crossusn]

I was going to say when I replaced my wheels/tires the shimmy went away. It should fix it for you.