Rear end clunk

[Steeler]

This may be elementary but has me stumped.
Before I send to my indie I would like the collective on here to comment.

I installed an LSD over the winter that came fully assembled, ready to be installed after bolting on the diff cover and filling with FM lube.

While everything was accessible all bushings were visually checked.

One thing I did notice once the wheels were bolted on and car still raised that when turning one wheel the opposite did not engage for small amount of rotation. As far as I was concerned it was normal as I have never had a BMW with LSD before.

Now when driving and shifting from 1st to 2nd I hear a clunk and almost like the rear tires are shifting forward and backwards.
Off and on the gas does not create any clunks.
Also tight turning circles cause the rear tires to scrub.
I have yet to go through the figure 8 run in procedure.

[GuidoK]

Which lsd do you have?

[Steeler]

Salisbury from performance gearing (Jim Blanton)

30/90 ramps with 3.91

Bought new unused from forum member.

[GuidoK]

Did you measure backlash?

[Steeler]

Assumed and told it was ready to be installed as is.
Original owner assumed also as far as I understood.
How do I adjust the backlash?. Not afraid to do myself just need to know how!

[GuidoK]

You cant adjust the backlash (well, in theory you can but it requires different thickness of circlips (huge ones).

But if you want to find out where the clunk is coming from, there are a few places you must isolate.
First is the gearbox. The gearbox always has about 1/4 or so rotational play.
Then there is the propshaft. That can have play on various points (hardydisc, universal joint, the middle bearing). Those things can be checked from the outside.
And there is the diff. If you havent measured backlash you cant differentiate between the diff or the final drive gears.

Strange thing is that you get a clunk between shifting but not with on and off throttle.

But imho you should always check backlash as it's really really easy to do compared with other checks to see if the gears are all in the right place etc.
If the backlash is ok, you've covered probably 75% of everything that could go wrong already inside the diff.

It just takes to pop the rear cover off and measure it with a dial and magnetic stand. thats $30 worth of tooling that can save thousands worth of damage.
If you work in engineering and are confronted on a daily basis with information of 'others', the first thing you learn is that you have to measure for yourself (even if it's measured before).

But next to that you have to hold the propshaft and turn both wheels (with force) to see and if there's play.
The only clunk in the diff I can imagine is the transition between the ramp angles. 90 means that there is no angle at all, so no brake lock up. That means that the ramp hits a flat (or at least, not something that is dampened). But I dont know if you can differentiate between that feel and backlash.


To check backlash, pop the cover (I dont know if that can be done on the car easily), put a magnetic stand on the diff with a micrometer dial on the edge of the crowngear teeth and slightly move the crowngear back and forth (while holding the pinion) so that you can feel that play. The micrometer shows the play.
Like this:

Here you can see that I placed the magnetic stand against the side. I used a micrometer dial with a feeler tip but you can also use a conventional dial.
But a set like this can literally be had for under $25 on ebay. (obviously not the same quality as mitutoyo, but I think it'll do )
Also great for measuring if you have warped discs etc (so sometimes I cant believe that most 'shops' dont have this kind of stuff, looking at the price...)

As a video it shows like this, this is my diff when I checked it (dont mind the dutch babble):

Hard to see on this old phone video but the play was 0,08mm. Factory spec is between 0,06mm and 0,14mm.
If somthing would be wrong in diff placement, diff spec, bolt torque on diff, pinion or crown gear, it would show on backlash. So quite a bit.
Gears not perfectly aligned but having backlash within spec is virtually impossible.

But doing this on the car might be tight on space (and supporting the diff etc).
But if the diff's off the car, it's a 5 min job.

[Steeler]

Thanks Guido excellent info.
I can remove the rear diff pretty quickly now having done 3 out and ins in the last 2 years.
The clunk is not disconcerting in any way but is annoying after putting so much work into the car this winter. This is my only issue now.
I have beaten on the car this weekend with no apparent detriment. All other driveline components are quiet and running perfectly.
i will remove the diff next weekend and measure the backlash, I have all the gear to check it.
I will report back my findings.
Thanks again

[GuidoK]

It might be that you measure a larger backlash than the specs I gave.
The z4m has a larger spec for backlash. That is maybe part of it being a clutch operated lsd (just as a salisbury lsd is) so it gets hot on locking (you also probably use 90w140 oil), but the z4m also has a larger crowngear (215mm instead of 188mm).
How much each weighs in into that factor: I dont know.
But I also dont know how your diff is designed. But measuring slightly over 0,14mm on backlash might be a design feature and not a failure (but if you do measure that, I'd call the manufacturer and ask for their backlash specs)

The z4m backlash is probably specced up to 0,2mm or so.

As for the clunk I think its more likely its slack somewhere in gearbox/propshaft/driveshafts, or the shifting of the ramp inside the diff. Thats something only you can find out investigating the car.
Why it mostly does it when shifting the gearbox and not when throttle/clutch on/off... I dont know.

[Steeler]

I tend to believe it is movement of the ramp within the diff. This was my original thought even with zero understanding or experience with this LSD.
As I said before the rest of the drivetrain is tight, it was all gone over this winter.
I will try contacting Jim again regarding the clunk. I imagine he is busy and my emails were lost in the shuffle.

Originally Posted by GuidoK

It might be that you measure a larger backlash than the specs I gave.
The z4m has a larger spec for backlash. That is maybe part of it being a clutch operated lsd (just as a salisbury lsd is) so it gets hot on locking (you also probably use 90w140 oil), but the z4m also has a larger crowngear (215mm instead of 188mm).
How much each weighs in into that factor: I dont know.
But I also dont know how your diff is designed. But measuring slightly over 0,14mm on backlash might be a design feature and not a failure (but if you do measure that, I'd call the manufacturer and ask for their backlash specs)

The z4m backlash is probably specced up to 0,2mm or so.

As for the clunk I think its more likely its slack somewhere in gearbox/propshaft/driveshafts, or the shifting of the ramp inside the diff. Thats something only you can find out investigating the car.
Why it mostly does it when shifting the gearbox and not when throttle/clutch on/off... I dont know.

[inTgr8r]

Originally Posted by dbarton62

I tend to believe it is movement of the ramp within the diff. This was my original thought even with zero understanding or experience with this LSD.
As I said before the rest of the drivetrain is tight, it was all gone over this winter.
I will try contacting Jim again regarding the clunk. I imagine he is busy and my emails were lost in the shuffle.

I have one of Jim's diffs installed.
I don't have this noise from the ramp.

Jim does get very busy this time of year.
I had an issue with the front flange causing a vibration.
It took a while before he responded, but he did get things resolved.

[Steeler]

Originally Posted by inTgr8r

I have one of Jim's diffs installed.
I don't have this noise from the ramp.

Jim does get very busy this time of year.
I had an issue with the front flange causing a vibration.
It took a while before he responded, but he did get things resolved.

No definitely not beating on PG or Jim Blanton at all.

As it turns out, I had fitted new driveshafts from ECS that were an incredible price.
Reason being I needed new inner CV boots and the driveshafts were about $50 a piece more than the boots.

I usually do not buy unknown brand parts but figured WTH. ......Well bad move. I did not notice the slack(side to side movement in the driveshaft as it was being installed. After jacking the rear end up today to investigate the clunk,I grabbed the passenger side driveshaft to pull myself under the car and to my surprise it moved a significant amount, I would guesstimate about 3/4". The drivers is not as bad but moves approx. 1/2". Checking the OEM pieces and they hardly move side to side.
ECS will be getting these back and lesson learned on unknown quality parts.
My OEM driveshafts will get new inner CV boots and fresh grease.

Moral of the story ,.........do not cheap out on parts.

BTW the car is a totally different animal with the LSD, coupled with the increased boost and lower gear ratio it is a monster.

[inTgr8r]

Good stuff, glad you got to the bottom of it.