Diagnosing drivetrain thumping on track car.

[bimmerbumm193]

Hello. I have an road race car that has been through a few different class iterations (currently ITR/STU, stock 2.8L engine) and at the last club race I did it developed a pretty bad drivetrain thumping while turning. It seems VERY similar to the thumping that a FWD car has when the CV axles go bad. I would just replace the axles as they seem to be the most obvious culprit but they are newer OEM replacements that are only a few years old. Now to complicate things, this car HAD a 400 WHP superchared setup for about 2 years so that most likely put a hurting on the 328 driveline components.

The thumping occurs while turning on the track and then goes away as soon as you hit the straights. I initially thought it was caused after heavy braking, but that is probably a dumb idea because what happens after you hit the brakes hard on track....a turn, duh. And now I cant remember it it happens turning both directions or not.

Car has solid mounts pretty much everywhere so the vibration is hard to pin point. It has a 3.73 diff with the low-preload 40% setup that I build myself....so I could have screwed that up while I was setting the gears. Stock 328i axles and trailing arms. M3 brakes. Stock DM flywheel with Spec clutch.

I did an initial inspection of the car on the lift and looked for obvious things like loose motor/trans mounts, play in wheel hubs, grabbed the driveshafts section and tried to feel for play but havent found anything yet. Its kind of hard to find problems like this with a racecar because if you change a part you cant exactly go pull a 1g turn on the street with an illegal racecar to test it, lol!!

[exvitermini2]

I would be inclined to think it is an internal diff issues since it only seems to happen on turns.

[Pavlo]

Check your hubs, play in the bearing can make the rotor contact the brake carrier. Wheels also deform some and sometimes the wheel weights will hit stuff under heavy loads.

[bimmerbumm193]

Think i found the thumping. The damage is totally confined to that oiling ring. I have extra parts so I guess I am going to put it back together but I would like to know the cause because I dont see why it wont happen again. This is the thayer 4 clutch kit and machined carrier cap. I wonder if it has to do with the thicknesses of the center parts (smaller diameter bellevilles/oiling rings/etc) in relation to the thicknesses of outer parts (clutches/dog plates/lg bellevilles)? How would the this part just wear on one side, what would apply uneven force on it? I asked thayer and they said I assembled it wrong, which I could of, although I cant see how.










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[FredK]

That can definitely happen if you assemble it wrong, because that small plate resides in a bore that it can't jump out of once assembled. Wanna know how I know? I damaged that dadburn little part when I assembled my first 3 plate LSD, and had to scramble to find another one!

The issue was when installing the cap, there's nothing really keeping the small washer/spring stack in place, besides maybe the film strength of the lube you put in there. If you just drop the cap on, and evenly start tightening the screws, the washer stack can fall out of the bore it lives in. So, when you torque those bolts, you end up smashing that little dogtooth friction disc in between the cap and a large Belleville. So, now when I assemble diffs, I check that little dogtooth plate like 35 times during assembly to make sure it has fore-aft motion. I stick my finger down there and feel for it moving back and forth.

[bimmerbumm193]

Hey Fred, thanks for the advice! You have confirmed that it is possible to tighten down the cap with the plate in the wrong position so that is what I must have done. I will be more careful this time!