Jammed chains/chainset/cassette?

[gbnz]

While I'm not in the practice of riding with the chain on the largest chainring (50 tooth) and largest cassete sprocket (30-31), I've inadvertently shifted into that gear on a few occasions.

And on fitting a new chain a few weeks back, I've found the entire "gear" system jammed solid when I've made that inadvertent shift. I've had to deflate the tyre, force the wheel out and manually move the chain/change gear. The chain hasn't just been locked solid, I've had physical difficulties removing the wheel, the entire system being jammed solid! Happened again just yesterday, 170 miles into a two day ride!

Prior to my spending a couple of hours trying to work out what's happening, shortening the chain, adjusting the deraileur et al, any ideas as to what's happening?

[gaz]

Sounds very much like the new chain is too short .

Do everything you can to avoid engaging large/large.

[reohn2]

Yep chain too short.

[gbnz]

Thanks to both above; unusually I hadn't shortened the chain on fitting it, at a subconscious level must have thought it was short.

Had been thinking of shortening it further, as a new chain is skipping on a new cassette, however will have to look at in detail

[Brucey]

chains come in different lengths (eg 110, 114, 116, 118 links etc); one that is just long enough for a road bike with short stays won't be long enough for a bike with longer stays.

cheers

[pwa]

If the chain is too short the rear mech will be at full stretch when you get in that position, with the arm pointing directly at the bottom of the large chain ring. It does sound like that is what is happening.

[Mick F]

Count the links.

Input the big chainring size and the biggest sprocket size plus the chainstay length.
http://www.machinehead-software.co.uk/b ... hcalc.html

[gbnz]

If the chain is too short the rear mech will be at full stretch when you get in that position, with the arm pointing directly at the bottom of the large chain ring. It does sound like that is what is happening.

Yep, think that's happening. Think the chain I'd removed is still hanging around, so may physically count the links. Have another few chains in stock, so will be counting the links on those too.

Though I still can't understand why the chain is skipping when on the smallest two sprockets and the 50/40 chainrings - cassette and chain are new, chainrings have done minimal mileage since replacement last July

[gaz]

That is more mysterious. B-tension screw adjustment?

[Brucey]

Though I still can't understand why the chain is skipping when on the smallest two sprockets and the 50/40 chainrings - cassette and chain are new, chainrings have done minimal mileage since replacement last July

could be stiff links. Rarely, new sprockets and new chain don't work, because of tolerance variations in both. Given how sensitive this is, it is amazing this doesn't happen more often.

BTW one explanation for everything might be that one or other pivot in the derailleur is seized or mis-set; this might mean no tension when running on the small sprockets and jamming when running big-big....?

cheers

[gbnz]

Thanks Gaz & Brucey.

Not quite sure what the B-tension screw is, but have suspected that it may be something to do with the deraileur tension. Quite notable that one screw is quite far out (NB. Haven't shifted it, as while the deraileur is only 3.75 years old, it's heavily corroded). I may need to do some intensive reading on setting up rear deraileurs properly, as having had 70000 miles on a "it'll do basis", it may be a simple deraileur issue.

In respect to stiff links I intend to take the chain off, count links/compare lengths with another brand new chain. And check the connex quick link which is well used? Also intend to put the original cassette back on when everythings sorted, as while I've had problems, I can't believe it's due to cassette wear (NB. Fairly sure cassette was only fitted last June/July and has done minimal mileage).

Just hope I don't get another absolutely jammed chainset/chain/cassette issue. Fridays was bad enough, I'd only just overtaken a group of five novice touring cyclists while heavily loaded on a Pennine incline (NB. They had all the gear, brand new ortliebs, shimano spds, at least one brand new bike), prior to my being forced to a halt ). Even had a comment from a hill walker "what a stupid place to stop" type of comment

[gbnz]

Chain was too short. By one link!

Haven't had a trial run yet to ascertain why the chains skipping on the two smallest sockets (NB. New chain, new cassette), though it did need a microscopic adjustment.