Z3MC "torque twist"/"crab walk" during hard accel. after subframe rebuild ??

[rotorhed]

my new symptom: most noticeable when driving on highway and I accelerate hard, the rear squats and has a slight right rear twist and passenger side front has a noticeable "kick" to right. Feels like a torque twist in frame, only when I "punch-it". It's not severe enough to pull me out of my lane but definitely perceptible to a retired helicopter pilot with OCD.

background: over the Winter I dropped the rear subframe down (didn't remove) and >

1) rebuilt LSD - replaced the four clutch discs that were noticeable worn. Reassembled with no issues.
2) replaced 14 year old IE green rear subframe bushings with new AKG black poly 95A bushings (harder than the old IE's)
3) replaced original RTAB's with new AKG black poly 95A bushings (much harder than stock) Installed with thick end on outside of RTA (outer facing to outside/inner facing to driveshaft)
4) replaced the rear diff. mount bushing which had tears at 12 and 6 o'clock with new rear diff. cover and OEM bushing.
Reassembled with no issues and torqued to spec.

I reinforced the trunk floor back in 2006 after the problem surfaced. 1/4" steel "H" frame welded on the inside of trunk floor. No visible tears on diff. ear or cracked spot weld so it's not flexing.

Can I "assume" that by going to the harder bushings in RTAB and RSB the torque flex is transferring to the car body instead of being absorbed by the softer factory bushings?
OR something else? Should be safe for the mountain twisties at Schuh Syndikate.
Thanks elfhearse

[BruceWarne]

Hi

Are the tire pressures correct?

Specifically, are the rear tires at the same pressure?

I've had similar symptoms in the past which turned out to be due to unequal rear pressures.

[jmai]

I think you answered your own question in post #1. Specifically replacements in #1,2,3. You did everything to make it more efficient and minimize slippage in the differential and stiffened the subframe and RTAB bushings. They are now harder and stiffer and, along with the new differential clutch plates, more power is being transferred directly to the wheels. The differential won't slip as much and the stiff bushings won't deform so the whole chassis moves when all the power is instantly allied. The rotational power is literally moving your car to the right. Since the bushings won't give, the chassis twists.

Maybe a bit of overkill on the stiffness. Think of the power you are generating. The rotation of the the driveshaft goes into the differential which in now locking up very well, it turns the axles which puts a twist in the rear subframe and as each wheel goes up and down the RTAB don't move much so the chassis twists. Somethings got to give, bushings or chassis metal.

[rotorhed]

jmai… yup, just seeking some similar thoughts... positive I didn't screw up the rebuild. The LSD was a Motorsport 4 clutch version so its definitely transferring the power vs. old clutch pack with 74K miles of wear. Now I can rip my differential mount ears off the floor ….yay!!

[rf900rkw]

Poor Isaac. He must be one dizzy puppy by now.

The diff does not lose or create power. Replacing the clutches did not cause the engine power to increase.

Check your alignment, including thrust angle. Stiffening the suspension bushings makes alignment problems much more noticable. If the alignment checks, then rotate the tires and test.



Corner weighting is another possibility. sitff bushings and cross-weight will make the car behave exactly as you describe. "Coil overs" (which aren't) set up by eyeball and a yardstick are quite dangerous things.

[rotorhed]

Corner weighting is another possibility

My right rear sits about 1/2" higher than left side. (measuring from level floor up to center of wheel arch lip.) Only measureable difference I could find was the spring perch with H&R Sport springs installed/spring pads all equal.
since I don't have any Longacre equipment here I winged it:

* added two bodywork dollies ~ 5 lbs. to the RR fender storage in place of the long gone Mobility kit.
* the big obvious, rear tires were over inflated to 41 psi so I adjusted down to 36 psi.
* switched rear wheels. 18" Apex ARC-8's with Michelin PSS

Took car out on the Interstate and did a few pulls and ran local twisties and noticed the "twitch" was gone. Might have just been the overinflated rear tires ??

[MartyBtoo]

If it was 1/2" higher just sitting there, imagine the difference when you sit in the drivers seat! In your fix you added 5 pounds to the right side, then you added around 200 pounds to the left side when you went out to drive it! Corner weighting is a black art that needs to be done with the driver in, the tires aired up to the running pressures (not the cold pressures), the sway bars disconnected, using a good set of 4 wheel scales and a good way of measuring the ride height on the frame next to the suspension. Then you need a lot of time, because changing the height on one corner will always change the readings on the other three, so doing it right is an interactive process. You also have to understand the meaning of cross weight percentages, and once everything is right you have to reconnect the sways bars by adjusting them so they don't change the static weights; adjustable links are a must.

PS Five pounds of air pressure wouldn't do anything other than make it ride stiffer.

Marty

[Z-Thrizzle]

I felt some something similar as you describe. The car felt like it was steering in the rear under hard acceleration. The car had a number of issues and replaced and reinforces many parts. While taking things apart, I found some of the differential nuts on the diff to be loose. I had a shop swap the diff and install mounting studs. Check to see if any of the bolts/nuts on the differential mounting points are loose.

[JustinHalverson]

I had similar symptoms when I first bought my Z3M. I discovered that the mounting bolts on the right rear trailing arm were loose and as a result the bolts had worn an oblong hole into their mounting bracket on the subframe. I had IE toe and camber plates welded in over the elongated holes (per instruction for the IE plates all of the holes were elongated to allow for adjustment). The camber plates were then aligned to stock toe and camber and torqued appropriately. This eliminated the torque steering problem I had been experiencing.

Obviously this may not be your problem given that the tire switch fixed your problem, but it is worth a look. For what its worth, loose bolts with elongated mounting holes on right rear resulted in pull to right upon acceleration. Pull to left on acceleration would be indicative of a problem on left RTA.

[Tim95M3]

I've dealt with this twice. On the first one, turns out I had one really low rear tire. On the second one, the outermost right rear trailing arm bushing bolt had worked loose.