Suspension (?) Knocking noise - help please :)

Post Fri Feb 28, 2014 9:53 pm

Hi guys.

I have just come back from the first trackday in my racer, and I have a knocking noise coming from the car.

When I change gear violently, or brake hard, or turn quickly, I get a clonk noise. It's hard to explain, but it sounds at times like it is coming from the front, at other times at the rear, and other times like it's from the gearbox(!).

I have filmed a bit, and you can clearly hear the clonk (a lot).

[youtube][/youtube]

Any ideas what I should be checking for?

Thanks!

Post Fri Feb 28, 2014 10:13 pm

I would check the propshaft guibo (rubber dougnut coupling to the gearbox) first, then the diff.

Post Fri Feb 28, 2014 10:16 pm

I have only just replaced the Diff. Do you really think it could be that?

Will check the rubber thing. Will it be cracked or just look old?

Thanks for the advice - much appreciated.

Post Fri Feb 28, 2014 10:23 pm

Well, as long as you fitted a good diff and filled it with the correct oil, it should be OK, but worth checking anyway.

Yes, the guibo will show obvious signs of cracking (usually) if worn out:

http://www.m3forum.net/m3forum/showthread.php?t=301477

Post Fri Feb 28, 2014 10:32 pm

How do you check a diff - I had a garage fit it and they replaced the oil for me at the same time. They didn't mention anything about the diff I supplied when they fitted it - but they may not have checked it too hard I guess.

Thanks for the link - I imagined it would be similar to an old golf (rallye) I did years ago.

Post Fri Feb 28, 2014 11:11 pm

Just jack up the back of the car and support it on axle stands (or drive it up on ramps) then rotate the propshaft back and forth and look for excess play or noises at the input shaft of the diff.

Post Fri Feb 28, 2014 11:16 pm

Righto. Not sure how much 'excess' means - ANY or just slack?

Sorry - happy to fiddle with nuts and bolts but I've never had to do anything gear-related before.

Post Fri Feb 28, 2014 11:18 pm

There will probably be a few mm of rotational motion at the input shaft to the diff with the wheels fixed, but if it turns by anything more than half an inch or so, it's an indicator of wear of the crown wheel and/or pinion.

Also check for lateral movement of the input shaft bearing by waggling it from side to side and up and down.

Turning the propshaft back and forth usually highlights any clonks in the various joints.

Post Fri Feb 28, 2014 11:35 pm

Thanks. Will report back ASAP

Post Sat Mar 01, 2014 10:16 am

I'd be checking the four 19mm diff securing bolts..........

Post Sat Mar 01, 2014 10:41 am

agreed. They're a sod to get to. 19mm ring spanner, over the top of the diff. Hook another spanner in the open end of the ring spanner and tighten until it slips and you punch the floor violently

Post Sat Mar 01, 2014 10:42 am

Thanks chaps. Will check those at the same time. Typically it's sunny at the moment, but wife is out playing Hockey so I have to look after the small one. Still - building the Lego Simpsons house isn't all bad!

Post Sat Mar 01, 2014 2:41 pm

Quick update - hopefully someone smarter than me can advise:

When I wobble things, nothing knocks, except the centre joint on the prop - rotating one half will cause the joint to clonk a bit. I have a video and will post later. Could that be my issue?

The rubber doughnut looks good, as does the rear diff nuts and bolts.

I also chocked one rear wheel and turned the other, and the diff made a slow heart beat style noise - is that normal?

Post Sat Mar 01, 2014 2:49 pm

Sounds like the centre joint could indeed be the issue.

Noise could be binding brakes or worn wheel bearing(s), being transmitted back through the axle. Check by spinning the rear wheels with the back off the ground, and shake them from front to back to feel for play.

Post Sat Mar 01, 2014 3:37 pm

Bearings are OK, and the brakes look OK too - just had lots of work done at the rear end (bearings and new cylinders) so it's all been off recently. Sounded like it was the diff itself tbh.

Just uploading my pics and a video of the noise

Post Sat Mar 01, 2014 3:46 pm

Here's the doughnut thing - looks OK?
http://dorkish.com/albums/Cars/BMW%20E3 ... G_0017.JPG

Here's the prop - hard to see but I am holding one 'end' with both hands and twisting back and forth - the noise is the knocking the centre joint is making.
[youtube][/youtube]

And this is the noise the diff makes when I turn a wheel and the other rotates the opposite way - sounds loud?!:
[youtube][/youtube]

Got any new thoughts?

Post Sat Mar 01, 2014 4:16 pm

I wouldn't worry so much about the diff noise, but ensure you have the correct oil in it, particularly if it's an LSD.

The centre bearing definitely needs sorting though!

Post Sat Mar 01, 2014 4:38 pm

Did you check the four 19s ? They need to be FT or it will move as you go on and off power/ change gear etc. The fact that the diff has just been changed is probably more than a coincidence. Also worth mentioning is that the bolts being 'tight' does not necessarily mean they are . You should always run a tap through the diff threads before fitting, it is very boring to find the bolt is FT but the washer can still be moved because the threads are full of rust and crud

Post Sat Mar 01, 2014 4:44 pm

Yeah, the 19mm bolts were all effin' tight. Diff doesn't move - I tried prising it with a bar.

Thanks again guys. I will find a replacement prop.

Post Sat Mar 01, 2014 4:57 pm

Also check the diff's oil level and also that the correct grade oil has been put in.