I'm often unsure about figures given for chain tensions. So regarding figures given for rear chain tension, do they mean total movement from when pushed up to when pulled down? Or just how much it will move up OR down from its resting position? Rupert Ratio (vol 1, p.120) says the correct tension for the primary chain "allows 1/4" up and down play in the middle of the top chain run." Does it mean 1/4" up and also 1/4" down from its resting position, so 1/2" movement in all? Or does it mean if you push it up and then pull it down there should only be 1/4" of movement in all? I suspect it means 1/2" movement in all, but could someone please just set my mind at rest? Thanks. Phil.
Hi Phil,
I take the amount of movement specified to be total amount of movement. It is also a good idea to find any tight spot and set the slack at that point. I would say more likely in a rear chain than a duplex primary. Also, the chai should nor be under tension so push down on the kick start or turn the back wheel in gear so that the clutch is pulling the lower run of the chain tight over the adjuster. Then check the slack and adjust if necessary. It is surprising how much a chain can whip up and down when in use. In proportion it is probably the 1/4" specified about the same as a rear chain at, say, 1.175" slack.
Dave.
Thanks Dave, but I don't understand your last sentence. Phil
Hi,
In proportion the 1/4" is probably about the same as 1 3/4" (not 1.1175) on the rear chain. If you think about the distance between the sprockets and the length of the chain. The longer the chain the more slack it will need.
Dave.
I'm with you now. Thanks again. Phil
Not only due to proportional length. The primary chain is running between two fixed points and needs minimal slack. The rear chain, because of the effect of the swinging arm, is running between two variable points and needs to have sufficient slack to not get stretched at the longest run. I think that occurs when the rear suspension is compressed, whereas the chain tends to get adjusted when it has minimum compression, hence the need for more slack in the rear chain.
Hi,
Yes, definitely the swinging arm movement comes in to it with the rear chain but depending on how the swinging arm sits.
Certainly the later A65s when unladen sit with the swinging arm angled down from horizontal. With this arrangement the chain slack needs to be set with the suspension compressed to horizontal to get the greatest distance between the sprocket centres whereas I think the A10 sits unladen with swinging arm in the horizontal position so correct for chain adjustment.
This always assuming that standard length suspension units are used.
What I tend to do on my A65s is adjust the chain a bit slack and then try to sit on the luggage rack to measure the slack.
Another way is to check the chain when loaded up and sitting on the bike. Luckily I have long arms.
One thing I have not tried yet is checking the slack whilst riding along.
Dave.
I think that is one of those checks that you only do once!!!
Hi there
It says in my manual for a late 1940s B31 to adjust the primary chain to
"Not less than 1/2 inch total up and down movement"
So I interpret that as 1/4 up and 1/4 down
Pete
Hi,
The amount of slack needed for the B31, A10 etc will be greater as the centres are further apart, the chain is longer and the clutch is a bigger diameter and I would agree that the 1/2" specified is for total movement. So for the B40 the 1/4" is for total movement. For the A65 it is 1/8" so almost nothing.
Dave.
Chain tension is important to the smooth running of the transmission. Clearly it should not be like a 'bow string' or indeed 'flapping' about.
Many engineering data books state that the amount of slack should be calculated as 3% of the distance between the sprocket centres. This is a method I have used for years and never had any issues.
Hope this helps.
Robin
Should we take it "slack" means the distance between the two extremes, ie between the top limit when pushed up (from its resting position) to the bottom limit when pushed down? Phil
Yes
Interesting. Between centres for my primary chain is something like 150mm (using metric as easier to quickly calculate percentages), so working on that 3 per cent basis the total slack would be 4.5mm, or just under 3/16". Not very different from 1/4" I suppose. Phil