Technical issue re: jumping chain

kbmack

My LBS has just fitted for me a wider ratio cassette to assist me in a trip to the Canaries next month. Unfortunately as a consequence my chain seems to jump around a lot on the rear cassette and front chain rings. In particular, when I'm going up steep hills the chain comes clean off the smallest front chain ring when it is engaged with the biggest rear sprocket.

I have taken the bike in for adjustments to the gears, etc but this problem persists. Could it be the chain is too slack? The problem really appears when the system is worked hard and pressure is applied to the pedals.

For info, the mechs are 105, the cassette is Tiagra 12-30 on a 50/39/30 set up.

Apologies if the solution is elementary but many thanks in advance for any sage advice you are able to offer. Hope you all had a good Christmas.

mercia_man

Could be you need a new chain. Fitting a new cassette to a bike with a fairly worn chain will often result in jumping. I know from experience. Using a chain wear checker can help avoid this as it tells you when to replace a chain before it gets too worn.

Strith

If nothing is worn, then a lack of lubrication on the chain can cause skipping.

Strith

I should this is usually down to seizing side plates in the chain, so check there's no seized links on the chain. If there is loosen them up and apply some oil.

nicklloyd

I think it is likely to be a new chain is needed. How long have you had the previous cassette fitted and did you keep the chain very clean?

I have rarely managed to replace a cassette and not had jumping chain under effort. You can get chain wear tools that measure the chain, but I think you can measure manually also. Not sure on the jumping off the front chainring unless its just the jump at rear causing a jump at front also? How pointed do the teeth look on the small chainring. If they look sharp they may too be worn out.

Just found this:-

"from Sheldon Brown's website....

Measuring Chain Wear

The standard way to measure chain wear is with a ruler or steel tape measure. This can be done without removing the chain from the bicycle. The normal technique is to measure a one-foot length, placing an inch mark of the ruler exactly in the middle of one rivet, then looking at the corresponding rivet 12 complete links away. On a new, unworn chain, this rivet will also line up exactly with an inch mark. With a worn chain, the rivet will be past the inch mark.

This gives a direct measurement of the wear to the chain, and an indirect measurement of the wear to the sprockets:

* If the rivet is less than 1/16" past the mark, all is well.

* If the rivet is 1/16" past the mark, you should replace the chain, but the sprockets are probably undamaged.

* If the rivet is 1/8" past the mark, you have left it too long, and the sprockets (at least the favorite ones) will be too badly worn. If you replace a chain at the 1/8" point, without replacing the sprockets, it may run OK and not skip, but the worn sprockets will cause the new chain to wear much faster than it should, until it catches up with the wear state of the sprockets.

* If the rivet is past the 1/8" mark, a new chain will almost certainly skip on the worn sprockets, especially the smaller ones."

paul_smith_srcc

Mercia Man wrote:

Could be you need a new chain. Fitting a new cassette to a bike with a fairly worn chain will often result in jumping. I know from experience. Using a chain wear checker can help avoid this as it tells you when to replace a chain before it gets too worn.

Be the first thing I would check, if new cassette but with old chain and chainrings then you may need to update one or both, the transmission wears in together and introducing a new item to part worn chain and chainrings will often result in the 'jumping' as described, especially under load, may seem to work perfectly in a bike stand, if it's purerly an adjustement issue then it wouldn't normally work in the stand either.

If the transmission was all the same age then try a chain first, the chainrings often wear at a slower rate, you can often get away with new chain-cassette and existing rings, as rule of thumb I replace the chainrings per two cassette-chains.

Paul.

kbmack

Many thanks for this help. I took the bike to the bike shop and it transpired that a chain link was cracked, the rear mech cable needed replacing and a spacer was missing in the bottom bracket area. Anyhow, this seemed to sort the matter so again thanks for the suggestions. To top it all, the shop charged just three quid for the cable which tells us a lot about the commitment to customer relationships that local bike shops often adopt- utterly brilliant!

Happy new year!