Strange knocking noise when releasing the clutch

SteveBorland · May 20, 2022

I've just put the bike back on the road after some winter work which involved removing the primary drive and the swing arm and now I have a very strange noise which is puzzling me.

Here's the background:
Over the winter I had the whole primary drive off - its a Maney belt drive, and the belt was rubbing on one of the alternator mounting posts. The swing arm came out as part of the process to use a one piece 17mm rear axle.
No changes to the engine or gearbox. Even though the bike is an 850, it has the 750 clutch setup of 3 plain plate and 4 friction plates. It's not given any problems before, so never bothered to change it.

When I reassembled the bike, the initial startup gave a horrible scraping sound, which turned out to be the clutch hub rubbing on the backing plate. I could press the hub very slightly out against the inner bearing circlip and this solved the problem. No idea why this happened though, no new parts involved. At this point, I decided to replace the 3 steel plain plates in the clutch, the old ones had some corrosion pits on then, but still seem to be perfectly flat.

Now, when I put the bike into gear and release the clutch, I get a quite loud knocking noise, repeated 3 or 4 times as the clutch engages. When the clutch is fully in, the noise vanishes. Repeats at every gear change.
It's sufficiently loud that I can clearly hear it over the engine & exhaust noise.
If I put the bike up on the center stand and engage first, there's no noise as I feed the clutch in, in other words it only occurs under load. I didn't think of trying to load it using the rear brake though.

My first thought it was the lower run of the belt slapping against the primary chain case, but there's no signs of contact on the case or the belt. The belt is just slack enough that I can rotate it 90 degrees on the top run. The knock does not really sound like belt slap though.

Checked the crankshaft nut holding the front pulley & alternator - tight. Removed the clutch plates and checked them, no visible signs of anything there. I wondered if it could be the now dry friction plates slipping against the new plane plates, so I tried replacing the old plates. No change the sound is still there. I read a suggestion that a light smear of oil on the friction plates might help, so I tried this as well, on both the old and new plain plates - no change.

It's possible that the LH exhaust pipe is hitting the primary chain, but then this would occur at all speeds, not just when engaging the clutch.

Tonight I will remove the clutch once again and have a good look around, but I'm rather puzzled, so I'm hoping that someone has an idea?

/Steve.

kommando · May 20, 2022

Get the bike up and the centre stand and support it so the rear wheel cannot touch the floor, run the bike with someone changing gear while you grovel on the floor and you can hear/feel where the noise is coming from.

SteveBorland · May 20, 2022

Yes, this might help, I've a friend who could could do this but he's not available for the next few days.

mdt-son · May 20, 2022

SteveBorland said:
Now, when I put the bike into gear and release the clutch, I get a quite loud knocking noise, repeated 3 or 4 times as the clutch engages. When the clutch is fully in, the noise vanishes. Repeats at every gear change.
It's sufficiently loud that I can clearly hear it over the engine & exhaust noise.
If I put the bike up on the center stand and engage first, there's no noise as I feed the clutch in, in other words it only occurs under load. I didn't think of trying to load it using the rear brake though.

Sounds to me like the clutch basket/hub bearing is "shot".

- Knut

L.A.B. · May 20, 2022

Check here:

Strange knocking noise when releasing the clutch

SteveBorland · May 20, 2022

mdt-son said:
Sounds to me like the clutch basket/hub bearing is "shot".

- Knut

Possible, but it feels fine when rotating by finger, and it was fine when I was riding it last year. Bearings don't normally die when not being used, but I'll acquire a new one anyway. cheap insurance

SteveBorland · May 20, 2022

L.A.B. said:
Check here:

Ah, good idea, thanks.

zotz · May 20, 2022

For what it's worth, had the same thing happen to me last year. Belt drive with fiber clutch plates. Just started out of the blue in 4th gear under load. It would make this loud clack after shifting or multiple clacks under heavy throttle. I pulled the clutch apart for inspection. Belt tension was fine. Found pushrod o-ring was damaged and allowing slight leakage of oil from G.B.. I also replaced the steel plates as they looked to have uneven wear on them. Cleaned up fiber plates and re-used them. Every thing has been fine since, (fingers crossed).

SteveBorland · May 20, 2022

zotz said:
For what it's worth, had the same thing happen to me last year. Belt drive with fiber clutch plates. Just started out of the blue in 4th gear under load. It would make this loud clack after shifting or multiple clacks under heavy throttle. I pulled the clutch apart for inspection. Belt tension was fine. Found pushrod o-ring was damaged and allowing slight leakage of oil from G.B.. I also replaced the steel plates as they looked to have uneven wear on them. Cleaned up fiber plates and re-used them. Every thing has been fine since, (fingers crossed).

Hmm. My friction plates were dry & clean (I used brake cleaner on them during the winter rebuild, new steel plates made no difference.
Thanks for the comments though, it all helps.

Jdub · May 20, 2022

A rubber chock exactly where our honored Moderator points will quickly pinpoint that issue. Did I miss any information on the rear Iso condition? Is that in good nick?

SteveBorland · May 20, 2022

Jdub said:
A rubber chock exactly where our honored Moderator points will quickly pinpoint that issue. Did I miss any information on the rear Iso condition? Is that in good nick?

Good idea, thanks Isos are both newish low milage vernier type. Not adjusted since before all this started.

BERT · May 20, 2022

SteveBorland said:
I've just put the bike back on the road after some winter work which involved removing the primary drive and the swing arm and now I have a very strange noise which is puzzling me.

Here's the background:
Over the winter I had the whole primary drive off - its a Maney belt drive, and the belt was rubbing on one of the alternator mounting posts. The swing arm came out as part of the process to use a one piece 17mm rear axle.
No changes to the engine or gearbox. Even though the bike is an 850, it has the 750 clutch setup of 3 plain plate and 4 friction plates. It's not given any problems before, so never bothered to change it.

When I reassembled the bike, the initial startup gave a horrible scraping sound, which turned out to be the clutch hub rubbing on the backing plate. I could press the hub very slightly out against the inner bearing circlip and this solved the problem. No idea why this happened though, no new parts involved. At this point, I decided to replace the 3 steel plain plates in the clutch, the old ones had some corrosion pits on then, but still seem to be perfectly flat.

Now, when I put the bike into gear and release the clutch, I get a quite loud knocking noise, repeated 3 or 4 times as the clutch engages. When the clutch is fully in, the noise vanishes. Repeats at every gear change.
It's sufficiently loud that I can clearly hear it over the engine & exhaust noise.
If I put the bike up on the center stand and engage first, there's no noise as I feed the clutch in, in other words it only occurs under load. I didn't think of trying to load it using the rear brake though.

My first thought it was the lower run of the belt slapping against the primary chain case, but there's no signs of contact on the case or the belt. The belt is just slack enough that I can rotate it 90 degrees on the top run. The knock does not really sound like belt slap though.

Checked the crankshaft nut holding the front pulley & alternator - tight. Removed the clutch plates and checked them, no visible signs of anything there. I wondered if it could be the now dry friction plates slipping against the new plane plates, so I tried replacing the old plates. No change the sound is still there. I read a suggestion that a light smear of oil on the friction plates might help, so I tried this as well, on both the old and new plain plates - no change.

It's possible that the LH exhaust pipe is hitting the primary chain, but then this would occur at all speeds, not just when engaging the clutch.

Tonight I will remove the clutch once again and have a good look around, but I'm rather puzzled, so I'm hoping that someone has an idea?

/Steve.

I would check if there is a bit of clearance between the swingarm cap and the back of the inner primary case.

SteveBorland · Jul 17, 2022

Update on this most puzzling problem.
After returning from Spa, I went back to the Commando to try to solve the problem. I'm quite certain it's related to the power train, but my attempts to localise the knocking noise have failed. It's actually quite hard to do this while the damn thing is accelerating down the road - a certain sense of self preservation requires that one does actually look forwards rather then down at the engine

I've stripped the primary drive down, and found some marks from the drive belt on the inner chain case. These have now been relieved further wit ha Dremel. The alternator rotor nut was tight, as was the clutch hub nut. The final drive sprocket was also tight, with the locking washer firmly in place. I did notice a trace of grease had escaped the sealed bearing in the clutch hub. Perhaps I should try changing the bearing? I have access to a press, but first in 2 weeks or so.

I reassembled the whole lot again and had a quick test run - knock knock knock again, exactly as before.

I do not see any sign of contact between the frame & the primary chain case or the swingers caps as previously suggested.

Do notice though that the belt pulleys are slightly out of alignment and that the belt could be a smidgeon tighter, so that's next on the list of Things To Try.

If that does not help, then I think I will try going for a run and give it a bit of (careful) welly to see if it goes away or actually leaves a clue to the problem.

Fast Eddie · Jul 17, 2022

Clutch centre bearing was suggested a while back, as was putting a wedge in the area marked by LAB. Any trials done there ?

kommando · Jul 17, 2022

SteveBorland said:
Perhaps I should try changing the bearing?

Seal has failed regardless of the bearing function, but the seal failure could be from excessive internal wear in the bearing which also allows the clutch basket to wobble which may be the noise. The bearing is a C2 type with reduced internal clearances to reduce wobble, CN and C3 type will allow excessive wobble.

SteveBorland · Jul 17, 2022

Fast Eddie said:
Clutch centre bearing was suggested a while back, as was putting a wedge in the area marked by LAB. Any trials done there ?

Clutch bearing is less than 2 years old, probably less than 500km. No detectable signs of any problem, other than the slight weepage of grease, so I’ve not tried changing it. Yet.
There’s a good few mm of free space ‘twixt cover and frame, so I dont think thats the source.

BERT · Jul 17, 2022

SteveBorland said:
Update on this most puzzling problem.
After returning from Spa, I went back to the Commando to try to solve the problem. I'm quite certain it's related to the power train, but my attempts to localise the knocking noise have failed. It's actually quite hard to do this while the damn thing is accelerating down the road - a certain sense of self preservation requires that one does actually look forwards rather then down at the engine

I've stripped the primary drive down, and found some marks from the drive belt on the inner chain case. These have now been relieved further wit ha Dremel. The alternator rotor nut was tight, as was the clutch hub nut. The final drive sprocket was also tight, with the locking washer firmly in place. I did notice a trace of grease had escaped the sealed bearing in the clutch hub. Perhaps I should try changing the bearing? I have access to a press, but first in 2 weeks or so.

I reassembled the whole lot again and had a quick test run - knock knock knock again, exactly as before.

I do not see any sign of contact between the frame & the primary chain case or the swingers caps as previously suggested.

Do notice though that the belt pulleys are slightly out of alignment and that the belt could be a smidgeon tighter, so that's next on the list of Things To Try.

If that does not help, then I think I will try going for a run and give it a bit of (careful) welly to see if it goes away or actually leaves a clue to the problem.

You would not see much evidence if the case was slapping the swingarm pivot(caps) horizontally. If it was rubbing around, there would be polishing between the case and cap. As described,there is no indication of contact,so,as a suggestion, check for visible clearance with a feeler gage.

Torontonian · Jul 18, 2022

How are the sleeve gear bushes , main shaft ?

SteveBorland · Jul 19, 2022

Torontonian said:
How are the sleeve gear bushes , main shaft ?

Good point-I dont know!
How to check them though?

Torontonian · Jul 21, 2022

SteveBorland said:
Good point-I dont know!
How to check them though?

Well you could check the sleeve gear bushes for wear by ideally tearing into the box !
An easier way would be to grab the main shaft protruding clutch side and wiggle it sideways or up and down. There should be no movement perceptible at all. You could use the clutch basket put on to it too in order to get a larger item to hold using heavy gloves try the sideways wiggle. Another easier way is leave all the primary stuff in place , grab the clutch and see if it moves at all forward or backwards.

Strange knocking noise when releasing the clutch

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