fault causing rear chain to go tight, slack, tight

[Janet]

Having already established that it wasn't the chain at fault, I now know that the sprockets, although worn, were not the cause either.
Am I downhearted? Yes.
Will I give up? No.
Will I have a rest from it for a day or two? Maybe.

[Les Howard]

Oh dear that is depressing!......If you STILL have the same tightening and slackening Jane it can only be that the rear hub that carries the rear sprocket is buckled in some way since you tested the gearbox output shaft and found that it spun true.
The rear hub wobble would be very easy to test for and would be clearly visible if you spin the rear wheel with the chain off and hold a rigid pointer on the hub part and not the new sprocket. Also remember that not even a new bike will have the chain revolve without some slight variation which is why one has to adjust at the tightest position.
Is the fault exactly the same?...I can hardly believe it is, unless it is the rear hub and maybe the bearings are about to collapse, but as said, this is very easy to check with the wheel off the ground and chain removed...Theoretically the new sprockets could also be eccentric and this must be checked for too...

[Ozmadman]

A bit of a long shot but....the rear sprocket is bolted on to the hub, maybe the holes in the sprocket are oval and the sprocket is not central on the hub? Also, if he chain is fitted where the tight part of the chain is in the position where the sprocket is technically furthest away from the gearbox sprocket, if that makes sense, then even a small amount could cause something like this...
Paul

[SPRIDDLER]

I decided that there was probably not a stiff link in the chain. Also, I took it off and every link is loose, so nothing wrong there. As a third check, I put on the old chain and this showed exactly the same symptoms, so my final conclusion was that the chain wasn't the cause.

Au contraire, Janet. From your first post I understand that you checked by replacing the chain already in use with another old chain, so since you've not found anything wrong with the shafts or sprockets maybe the worn chain is causing the problem. Can't think why that should be though unless several adjacent links have worn more than those at other sections of the chain. If you adjust the chain when those 3 or 6 or more links are between sprockets then they will be relaxed. When those links are seated snug into the sprocket they will close up and produce a tight spot in the chain tension. If you adjust the chain when these worn link are around a sprocket they will relax when the wheel is rotated and they are free of a sprocket, hence the chain tension will be less. Just musing...........

(Edited twice as broadband connection keeps dropping out mid reply )

[Janet]

Two new sprockets and a new chain show exactly the same effect. I accept that all the parts could be faulty but surely not in exactly the same way.

[SPRIDDLER]

Two new sprockets and a new chain show exactly the same effect. I accept that all the parts could be faulty but surely not in exactly the same way.

Pass.

[cbranni]

This is a tough one Janet, so new chain and sprockets and we still got a chain that runs tight and slack, I'm thinking aloud now..........there must be variation between the centres of drive sprocket and the driven sprocket, using a pointy thing as mentioned before to check the sprockets are running true, how often does the chain go tight, if it was the drive sprocket that was causing the problem it would happen more often say three times per rev of the rear wheel, if it was the driven sprocket then maybe only once per rev.

Dare I say "has it always been there", another thought is there any play in the gearbox output shaft bearing, I have seen the hardened surface breakdown only on one side this could be the cause but its a lot of work to check it. you could try checking for play on the drive sprocket using a tyre lever or something similar to really put some pressure on it, turning the sprocket a bit each time you try to feel for movement.

My money is on, or around the drive sprocket, sorry if all the above sound like ramblings but it may jog others to pitch in with their two peneth.

Colin

[robcurrie]

Janet, how do the rear wheel bearings feel, does the wheel spin freely when the chain is off?

Rob C

[Les Howard]

Just a thought. Are you making the tightness test in a consistent way?

To start with, forget the actual specific recommended slack. Arrange the bike stand on some wooden planks so you can sit on the floor beside the chain and have someone hold the bike so the rear wheel is off the ground and you can freely spin the wheel. By holding a gloved finger or wood stick spin the wheel BACKWARDS at the same time push upwards on the lower chain run. By turning the wheel backwards all the slack is gathered on the lower run and it is better if possible to select top gear with the plug out which allows engine movement but also allows you to pull against a load to get the slack on the bottom run. Then do your measuring. The chain tension will vary slightly...this is normal. If the variation difference is around half an inch even slightly more, this I would consider easily acceptable and normal.
Then find the tightest spot over several revs of the wheel and keep this position by marking the tyre or wheel.

Then make the actual adjustment with bike loaded checking the actual adjusted tension with the plug back in and in 1st gear with very slight rearward pushing (but not moving) of the bike backwards against engine pressure. Once again this will ensure you have all the slack on the one chain run at the bottom. By doing it this way you will get consistent clear indication of the chain tension and will be able to get the adjustment correct.

Further rotation of the wheel after adjustment will show the chain to be slacker but unless this is over two inches I would say this is not far from normal....PLEASE do the rear wheel bearing and rotation check first to ensure safety though...Hope all is well...Les

[Janet]

Just a quick update although I haven't found the cause yet.

While I was working on the bike, Glyn was away in sunny Belgium ( it rained almost the whole time. I know I shouldn't laugh but he didn't go to our IJR ) at a BSA rally. Yesterday, he had a look at the bike and initially did all the things I'd done, and confirmed what I'd said about the chain. His suspicions lie with the wheel bearings. The wheel spins freely and it's difficult trying to feel if it's moving on the spindle because the whole bike tends to move with it when it's in the frame and you can't feel anything strange when it's not. However, it does feel a bit gritty. This could be because it's 50 years old, on the original bearings and has never been looked at or it could be me clutching at straws. I prefer this to anything in the gearbox and as it would probably be a good idea anyway, I will be attacking the bearings next.
The manual doesn't seem to say much about them and nothing at all about how to get them out, but I seem to recall Paul (Ozmadman ) saying they just knock out, one with a tap on the end of the spindle and the other, presumably, just with a tap on the bearing from the other side. Going by previous experiences of things that 'just' do something, I'm expecting a long haul.

I'll keep you posted but don't hold your breath. I may be some time.