We've got an '88 r100gs. The gearbox is making some knocking sounds, its there at tick-over sometimes, but pull the throttle to accelerate and you'll notice it more - only helmet off though, its not that loud. Lift the rear wheel and allow it to drive on the stand and it sounds pretty bad, but would this happen on a healthy bike? Recently replaced driveshaft after u-joint failure - but only with a Thailand fix, not a real new driveshaft. Does this sound like gearbox bearings? Hopefully not The noise doesnt seem to be getting worse, and quite honestly its probably been there a while. Tell me I'm paranoid and they all make knocking noises.

Any help appreciated.

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Far From Home, U.K - Oz and beyond.
http://www.globegallivanting.blogspot.com/

Now do this:

Pull the bike onto the central stand.
Weigh-down the front wheel so that the rear spinns freely off the ground.

Start the engine.
Go through all gears from 1st to 5th and back down and then into neutral.
3000rpm

Use a steel-rod of any kind (pipe, tire iron, screwdriver, whatever): put it to the front and rear of the gearbox; left and right side (as well as what ever parts of the rear cover you can access).
Lean onto this rod, let the tragus be folded back to close the outer ear - let the end of the rod set on the tragus => stetoscope.

If you hear any rumbling noice... its bad... if it sound like crushing of stones/rocks... stop the engine dead. Pull the gearbox. Change the worn-out bearing.
There is no chanse that you will miss this!
When a bearing is bad and you do this test - the sound transmitted via the steel-rod/stetoscope will be so potent that you will sort of jump.
If a bearing breaks inside the gearbox... the gearbox may literally explode... your lower legs will be blow to pieces... your worry is warrented!

However, what you describe does sound much closed to a u-joint faliure... shafts in oilbaths does not suffer this...

NB:
- it can be the locking nut to the splines on the final drive that is loose... you need to pull the final-drive to check this. It is get loose... this if what you will experience:
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Rather ask one too many questions than one too few -

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Drive Safely,
Albert

o.k thanks Albert, I'll give that a shot in the morning.

I don't think its to do with the u-joint. I've checked it through the rubber seals by hand and all seems secure. Its just been repaired in Thailand and seems to be holding tight.

It could be the locking nut on the final drive to the splines, but the noise does seem to be coming from within the gearbox itself.

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Far From Home, U.K - Oz and beyond.
http://www.globegallivanting.blogspot.com/

That is no good...

I sincerely hope that you are misstaking. If you are not... read Oaks article on "The Elusive and sudden high $$$$ 5 speed Transmission Failure 1974/on"
Copyrigth Oak Okleshen 1995. Airheads Beemer Club - Oak's Tech Articles
Been there - done that.

If it comes from within the gearbox - expect the worst possible and be happy and greatfull it it not as bad.

Check the bolts/screws that secure the shaft to the attaching plate on the gearbox - it happens that they get unscrewed and can cause what you experience. If you loose one-two of them you will not be able to feel that as unstable shaft with your hands, but when you drive you will experience weird phenomenas.
http://www.realoem.com/bmw/diagrams/k/s/6.png
http://www.realoem.com/bmw/diagrams/m/d/5.png
RealOEM.com * Diagram Selection
NB: these screws are -not- to be reused; they are strechtbolts!!!

Please be careful. When the power-line breaks-down it may cause such havoc; bike parts are changeable whilst bodyparts tends not to be...

__________________
Drive Safely,
Albert

I tried the stethoscope method. It worked well. There were obviously noises coming from the gearbox but they all sounded like noises that should be there. Nothing out the ordinary and no major rumblings, certainly no crushing sounds.

Shaft seems good, no noises from there either. The screws holding it to the plate on the gearbox are all there and tight. (I did re-use them when the shaft was off in Thailand - I knew it was bad but couldn't get any others. I thought this was probably a small crime)

I used the stethoscope method against the bevel drive and that IS where the noise is coming from. It was very hard to pinpoint the noise by ear alone, would have sworn it was gearbox end but with stethoscope it is clear its bevel drive end. A knocking, very audible when doing this. No real ryhthm to it. Bike was on stand but in gear with clutch out and wheel turning. Seemed irregular. Dont know if this could be classed as normal considering the bike was on stand (swinging arm at unnatural angle etc) or whether this spells trouble for the final drive.

Thanks for assistance so far anyway.

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Far From Home, U.K - Oz and beyond.
http://www.globegallivanting.blogspot.com/

It seems to me it's the typical "shaft-drive slack" from wheel turning and shaft itself on crownwheel. Basically engine accelerates rear wheel then it sits on one side of the tooth, till it catches up and hits the other side of the tooth (as much there is room between the crownwheel's teeth). I have the same noise on R1100GS running it on centerstand with rear wheel up. Running on constant idle revs seems to be the worst irregular sound when in gear and rear wheel spinning freely.

If any of the gearbox bearing are worn you should hear real mechanical grinding noise - like someone has put a low revving coffeebean grinder machine into your gearbox and drowned it into a thick oil (height of the sound is what revs you are riding at, on neutral gear revving you usually hear nothing (with the only exeption when on input shaft bearings are worn, which is 1 of the 3 rotating shafts) it my case it was the intermediate shaft's bearing which I could only hear the best when I was riding or revving it on centerstand in gear - but as said, a confusing factor can be the irregular noises coming from the shaft drive slack from a free wheel rotating w/o road contact). But yes, on boxer and old K- BMs with separate ("exposed") gearboxes - worn bearing sound is VERY distinct, well audible and hard to miss even when riding in the wind noise. I rode couple of hundred kms slowly with that loud worn bearing noise no problems to get a workshop and repair it in Australia. Thankfully local bearing shop had the same type/size bearing in stock (and another funny thing, when I opened the box on a "Made in Germany" bike, the prematurely broken bearing was, ironically, the stereotypically "Made in Japan" stereotypically known to be ultra-high reliability quality products ). And it gets even better, the same Japanese (NTN) company makes the same bearings in Taiwan now, so I had to use Taiwanese one to get me on the road again Talk about globalization.

Anyways, if you don't hear any threathening sound from the gearbox you're just fine and ride on.

And don't worry too much about the bike - LOTS to see in Indonesia!

Ride safe, Margus

Put the bike in neutral. Pull it reverse. Is there -any- non-smoothness in backing the bike?
If... (you are allowed to express your feelings here...) you need a new finaldrive like -now-... You'll have to open the finaldrive and check what is going on. If you do not... it may break jamming the rearwheel... not as bad as an exploding gearbox, still not good though...

This is what it will look like as an e.g.:
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Since you do have a knocking sound, as irregular as it may be, you are describing what happened to my 32/10 finaldrive... needs a dentist...

Sprocket damages occurs. Mainly as a result of worn pinion dual-row(?) bearings been worn-down and endplay become too large... => pressure on the teeth rises and the power simply snapps the teeth off...

What ever you do - you will have to remove the debris from within the final drive.

Begin by draining the hypoid oil. Look at the magnetic plug. You may or may not have a lot of chips on it. You most likely have a peak (hedgehog like) of chips... => acute situation!

If you catch a final-drive brake-down in time; you do stand a chance to fix it before it turns to scrap. Mine, as you see on the pics, was not saveable -

__________________
Drive Safely,
Albert

To do away with the noise due to this backlash in the geartrain how about getting someone to apply the back brake lightly while you listen with the stethoscope?

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Johnef

Good point John!

I have never tested a GS/PD this way, so I do not know if the dual u-joints cause more suspect noice or if that kind of shaft behaves as the oil-filled one u-jointed shafts. Applying a light braking of the bevel should eliminate back-slash sounds.

Looking forward to feed-back on how it goes with Nico's beemer; if he has solved the problem and what it turned out to be.

__________________
Drive Safely,
Albert