Airhead gearbox 'clonk'

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chrisjohnwalker
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Airhead gearbox 'clonk'

Postby chrisjohnwalker » Tue Jul 13, 2021 7:29 pm

Hi all,

I have an 1982 R100RT, current miles 70,000. I love it and it is probably the bike that I cover most miles on. Of late, the gearbox has developed a 'clonk' noise when going from power to overrun or visa versa. there is clearly play somewhere ! With the rear wheel off the ground and driveshaft rubber gaiter slipped back, rocking the rear wheel fore and back proves that the bevel drive and propshaft are not the problem . I have heard of some possible spline problem being common but any assistance before I bite the bullet and strip the rear end and gearbox out this winter would help and decide my course of action. Is it best to take engine/gearbox out together or strip from the back 'in - situ ?

Regards

Chris
Norfolk England

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Rob Frankhamr
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Re: Airhead gearbox 'clonk'

Postby Rob Frankhamr » Tue Jul 13, 2021 9:21 pm

If you're absolutely sure it's not the rear drive, the driveshaft and that all of the driveshaft,gearbox flange bolts are tight, then you have little choice but to strip the gearbox to see what is wrong. I would doubt if the clutch splines are the problem... even if they are very worn, there is little actual play.

One thing I would advise is that you address the issue without delay. riding the bike is going to make it worse and, if it's that old Achilles heel of the airhead box, the front output shaft bearing... which is a real possibility... it will be on the verge of a critical failure which will reduce the chances that tyhe bopx can be successfuly rebuilt.

Sorry to sound like a Jonah but a knock in the drive train is never good news...

Rob
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Re: Airhead gearbox 'clonk'

Postby stanthomas » Tue Jul 13, 2021 9:48 pm

My '81 RS will sometimes 'clonk' during a sharp take-off from standstill. The actual 'clonk' occurs just as I ease off the throttle to change up.
I'm convinced it's the shock absober in the middle of the drive shaft. I read somewhere that renewing the spring, #7 here: https://www.realoem.com/bmw/enUS/showpa ... Id=33_1655, will most likely fix it without needing to replace the coupling. Something I have planned for this winter.
The problem isn't noticeable when turning the rear wheel by hand, it only happens under heavy load.

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george baker
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Re: Airhead gearbox 'clonk'

Postby george baker » Tue Jul 13, 2021 10:03 pm

Hi
welcome to the forum.

I had one of those clocks that I could not find. Turned out to be the UJ. Search my posts if that turns out to be your problem cos IIRC I put the part number for a replacement in a post.

George
Member 21, R100R, and an under used K75

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Steve Rankin
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Re: Airhead gearbox 'clonk'

Postby Steve Rankin » Tue Jul 13, 2021 10:35 pm

I would take the front clamp off of the gaiter between the transmission and swing arm to recheck the tightness of the four bolts holding the drive shaft to the transmission. I have seen cases of them coming loose (rare but it has happened sorry to say usually after work has been done they have been removed and not replaced or torqued properly) even if you haven't had the unit apart in 70K miles, I would still check them. Also, if the input shaft/clutch splines have not been greased in 70K, it is possible the clonk is the clutch hanging up, I have seen that happen as well.

Worn splines or dry splines between the wheel and final drive can cause a clonk as well.

Regardless of what the problem might be, it doesn't take but a day or two to take the final drive apart, take out the transmission and such. It is far cheaper to take a day in riding season then it is to wait until winter only to find out the messed up part won't wait and breaks completely. Sometimes you can get lucky and the repair after a break is cheap, usually it is not, more damage is done in the interim. Rainy days are for maintenance and repair. St.

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IanG
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Re: Airhead gearbox 'clonk'

Postby IanG » Wed Jul 14, 2021 7:17 am

I had a clonk on take up on my R100 of a similar pedigree. It's most likely the spring in the shock absorber in the shaft. That's what it was on mine. I replaced the spring and rarely get the clonk now but can still get it if I start from a standstill and am a bit heavy handed with the clutch. I had to fabricate a spring compressor and getting the circlip that holds it together back on is not easy.
1983 R100
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1972 Norton Commando
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Re: Airhead gearbox 'clonk'

Postby chrisjohnwalker » Thu Jul 15, 2021 4:26 pm

All sorts of extremely useful adice there from IanG, Steve Rankin, George Baker, Stan Thomas and Rob Frankhamr. so thanks guys. It seems the best idea is to pull the driveshaft assembly and spline wear/bolt loosness/shockabsorber spring should be apparent. I know what I am doing on Sunday now.

Chris

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george baker
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Re: Airhead gearbox 'clonk'

Postby george baker » Thu Jul 15, 2021 10:56 pm

Hi
looks like you will have a nice day for it anyway.

Where are you based?

George
Member 21, R100R, and an under used K75

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Steve Rankin
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Re: Airhead gearbox 'clonk'

Postby Steve Rankin » Fri Jul 16, 2021 1:12 pm

LOL, my best friends step father ran a lawn, garden, landscape business and not very successfully. Many is the time the sun was shining the weather perfect he would have his equipment torn apart doing maintenance or repairs while the sun shined and then when it rained, he would complain he could not do the lawn work. We tried to tell him, work on the equipment when it's raining so it runs when it is sunny and the lawns are dry. Nah, he could never figure that out.

Good luck with your search for the clonk, hopefully it is something simple to fix. LOL, do like I do when it is raining and I am in the garage working on something, crank up the music and perhaps have a beer. St.

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Re: Airhead gearbox 'clonk'

Postby John Marshall » Sat Jul 17, 2021 9:42 pm

I had a clunk when accelerating or shutting the throttle on a 78 R100RS which turned out to be worn splines in the hub and felt like a bike with a stretched chain.Have the wheel off and see if they look worn ( with no reference maybe hard to tell)
I had a clunk on another airhead, swapped a gearbox as had a spare an£ it continued but it was the driveshaft.
I had a single UJ bolt come out which made a unforgettable intermittent graunch sound
I also had catastrophic gearbox failure at 15 MPH which was just as well because I had just been riding with a one in front of it. You may have metal in the gearbox oil, there is often a bit on the drain plug anyway.
I was able to get parts on ebay back then, to swap in and out, not so easy these days so look forward to some outlay.
Good luck chasing it down.John

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nmag
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Re: Airhead gearbox 'clonk'

Postby nmag » Thu Jul 22, 2021 2:00 pm

My 1982 R100RT started clunking too and eventually transmission went completely while in top gear. A strip down revealed worn splines on the input shaft, apparently a weakness in gearboxes built in 1981. I got a shaft from a later gearbox but found that the old shock absorber cam had different splines. I had to buy a later cam (used) and get it re-profiled to match the helical gear. I also had to ream out the snap ring holder to match the new shaft.
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Steve Rankin
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Re: Airhead gearbox 'clonk'

Postby Steve Rankin » Thu Jul 22, 2021 7:10 pm

I don't really think the input shaft is a weakness if it is regularly greased. I have seen one occasion of why did it happen to an owner who regularly greased the shaft properly during his 80K ownership only to have it fail. That is very rare to have happen. The vast majority of worn out input shaft splines are because they were not greased regularly or greased with the wrong grease. They are not easy to get at, it takes me most of the day sometimes two days to get things apart enough just to reach in with a paintbrush to apply grease. Most of the time, I do the job in the winter when I have all kinds of time to work on the bike and pull the transmission out completely. This gives me a view of the rear main engine seal and clutch system.
How hard the bike is used and traffic use can wear out shafts early, just plain abuse, is a killer. Speaking for myself, I have 37 years and 240+K miles on my bike and have yet to take the transmission apart for any kind of work. I ride very briskly an smoothly, not in a lot of traffic so that may have a bearing on life expectancy.
Regardless, a dry spline will cause the clutch to hang up. St.


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