Interesting Suspension problem ....
 

I have been working on my friends '94 RX-7 that is well maintained. (The car has 80k miles on it and is very nice.) It has an interesting problem related to the suspension/drivetrain that we have not been able to diagnose. Here it goes:

Under left turns or straight line acceleration, the rear passenger tire/wheel seems to make a BaBaBaBa, machine gun like sound. It almost sounds like wheel hop almost feels sort of like wheel hop, but yet different. (Note it does not happen under right turns.) The car sounds as if it should accelerate and feels like it should be accelerating, but the car doesn't acclerate as fast as it should. (The problem really is pronouced at turbo transistion and in 1st or 2nd gears.)

Here are the mods and conditions of various parts.
1) New or ok motor mounts
2) PPF not cracked
3) Koni adjustable shocks & Eibach pro-kit springs (shocks not leaking)
4) M2 toe links & M2 trailing arms (just installed)
5) Stock rear rollbar and Eibach front roll bar w/ widefoot mount

There doesn't seem to be excessive play in the wheel and I don't believe the diff bushings are leaking. With that being said, what is wrong here? I have one idea, but I would be curious if any of the suspension gurus would suggest what I am thinking. Please help because we are trying to get this car track ready ASAP.

maybe something is bent?

Does it happen at slow speed? Can one of you walk along next to the rear wheels while the other drives? Maybe you can pinpoint or at least get a few likely candidates for the source of the noise.

No, the problem only really happens when trying to put the power down. All other handling properties of the car seem normal.

Differential??

Well, there goes at least one person to think the same as I ... the differential. Anyone else?

Has to be the diff or possibly a cv joint. Typically when cv joints fail they begin making a clicking noise first but otherwise work fine. If you are getting a mechanical shuddering accompanied by "low power" that is exagerated by turning I would say that safely rules out all bushings and linkages.

I would drain the diff fluid and inspect for chunks as well as closely inspect the right axle's outer boot. Sounds like some of the friction gears in the Torsen may be going bye-bye.

It has also been suggested that the PPF is out of alignment. I am going to check that as well. I am trying to remember if we replaced the diff fluid when we rebuilt his motor in December.

Also the noise is not amplified in a turn .... it is either left turn or straight, but not a right turn. Both of the above conditions occur with power.

All the more reason for it to be the diff since it occurs under power when the friction gears are loaded. Inspect the diff plug closely for metal shavings, if it's making that much noise it should hopefully be obvious.

http://www.me.mtu.edu/~sae/FSAE/torsen.gif

I was also going to say the diff

STEPHEN

Definitely sounds like a problem with the differential. The problem also showing up on left turns and not right turns might suggest some issues with one of the half shafts.

Thanks for all the help. We are going to get the car up in the air Monday night and check the following:

1) Diff mounts
2) PPF alignment
3) Diff fluid

Turns out we didn't change the fluid in December, so we will get a real good chance to inspect the diff fluid when we drain it.

Well we check the diff mounts and they are not leaking, but I am still curious about their potential weakness. The PPF seems aligned ok. We drained the diff fluid, while a little dirty, not horriable. After driving the car for about 20 minutes or so the problem doesn't seem as bad, but it still exist. I guess we don't want to swap differentials out unless we know that is the problem. Any thoughts?

Anyone have any additional thoughts?

Maybe crack open the pumpkin and take a look, it might be obvious whats wrong. You should be able to see wear signs somewhere.

STEPHEN

wheel bearing
cv joint
diff

Well, if you changed *one* thing (the diff fluid) and the noise changed, then it sounds like you've pinpointed the problem area already.

But if not-- have you been able to isolate the source of the sound? Maybe you can post a recording of the noise and we can all play "Name that Tune" :)

And when you say it feels almost like wheelhop, do you mean the rear end is pogoing up and down, when it should just be squatting down? Or is it different?

Are you sure it is not a wheel bearing that is starting to go? I had my front left going, and early on it was just a tap-tap-tap sound on sharp rights only.

Interesting idea with the wheel bearing. I'll ask my friend and see what he thinks.

In fact, can't you rule out the differential by testing?

Find the right space, get up to the right speed, put it in neutral so you know no power is going to the rear, and then maneuver. If you still hear something, won't that eliminate the differential???

Not sure if that will test the diff or not. The shocks and wheel bearings got additional 3rd person reviews today and they passed inspection. The tester concluded that it was indeed wheel hop.

I think we are going to concentrate on the diff bushings and control arm bushings at this point.

With the diff being in neutral that will not tell you anything. Since this is a Torsen diff it needs to be under load which means under power.

Excuse me Damon, but reread my post. I was trying to eliminate the diff as a source of the problem. If it is in neutral, it is not doing squat, so therefore if the noise is heard, it wouldn't be from the diff.

Now, is that theory right or wrong???

Well we are going to try another differential soon. Vosko had some good info from the big list that helped.

Sorry; you're absolutely right. I was thinking the other way around the circle by proving the diff with a positive, not a negative.