Chain Retention

Tom · Post by **Tom** » 12 Feb 2007, 8:24pm

Have fitted my Dawes work/town bike with a single alloy chainwheel and a seven speed cassette. all is working as well as can be with the exception that on rare occasions when chancing into top gear the chain comes off the chainwheel? all is correct and in line, has anybody come across this problem. I think things could be linked to the chainwheel flexing derailing the chain.
Welcome advice
Mike

thirdcrank · Post by **thirdcrank** » 12 Feb 2007, 8:33pm

Tom

Unless the set-up is badly out of line, a single chainwheel should accommodate 7 gears. You say it is happenning when changing. I have never experienced that so all I can think of is a slack chain. I should have thought that a chainwheel would be more likely to flex under the pressure of strong pedalling, rather than when changing gear.

On the other hand, it is very easy for a chain to be unshipped by back-pedalling, especially in the extreme sprockets, but you will not be back pedalling when changing gear. This is even more likely with short chainstays but that would be unlikely to be an issue with a Dawes.

reohn2 · Post by **reohn2** » 12 Feb 2007, 10:14pm

Yep, i'm going with slack chain,unless you chainwheel is out of true and or worn badly.

Let us know how you get on

gerry36 · Post by **gerry36** » 13 Feb 2007, 8:25am

This is a common problem with single chainring setups- when the chain drops onto a smaller sprocket there is a sideways whiplash that propogates along the chain, and derails it from the chainring.

You need a chain keeper. These are fitted as standard to Moulton bikes, which mostly come with 9 or 10 sprockets and only one chainring, which is the way Dr Moulton likes it.

One easy way is to rescue an old front changer from your junk box, and fit it. Then adjust the limit screws so that it is currectly positioned to not cause chain rub, but will cure the problem.

You can buy them- Cycling Plus reviewed one a few years ago. They clamp to the seatpost, and have two prongs pointing down either side of the chain just behind the chainring. Or you can make your own.

gerry36 · Post by **gerry36** » 13 Feb 2007, 12:21pm

Chain keepers also known as chain retainers or chainwatchers. Another approach is to use a good aluminium circular chainguard outside the chainring.

Or two- one each side.

Tom · Post by **Tom** » 13 Feb 2007, 6:00pm

Thanks all,
I have gone through all the measuring and formulie regards correct chain length so I knew that things were not to far out.
Thank you jerry36 I think we are on the same wavelength as I had come to the same conclusion as yourself.

Less than a year ago I disposed of the very parts I require now( lady wife not happy with the state of the garage) so its off to my local bike shop to try and scrounge a second hand front changer.
Again thank you all for your promp advice, appriciated.