crank/bottom bracket problem

a few days ago my chain popped off while commuting home from work, getting caught in the rear cog and locking up the rear wheel. i fixed the chain and chainline once i got home but now a new problem with my bottom bracket and cranks has come up, possibly from the accident earlier.

when i’m putting decent force forwards (for example, if i’m climbing a tough hill) the cranks come ‘loose’ for about a quarter of a rotation before ‘re-tightening’. after that, if i put decent force backwards (for example, if i’m locking up the rear wheel for a skid), the cranks again come ‘loose’ for about a quarter of a rotation before ‘re-tightening’ once again.

its a little bit difficult to explain this in words, but i tried my best. i really want to fix this problem as i’ve got a pretty substantial ride this monday, but if not, i’ll be taking it to the mechanics and missing the ride. this shit gives me a heart attack every time it happens.

if anyone can shed any light on the problem, i’d be tremendously grateful. thanks.

sounds like your rear cog is loose?

+1. Sounds like the cog is not tight enough. I’d avoid riding it until you’ve diagnosed it for sure. If the cog is loose and it keeps slipping like you describe there’s a good chance you’ll strip the hub threads and bugger the hub permanently.

It might also be that the flange on the cog is not quite wide enough for the lockring to engage it. If that’s the case I’d replace it with a good quality cog like a Phil Wood, EAI or Shimano. Alternatively, you might be able to put a spacer on before the cog to take up the slack, but that might put your chainline out.

What make/model hub and cog do you have?

damn, i never even thought about the rear cog being the problem. it must have happened when the chain popped off and was caught in the rear wheel – accidental backwards-rotafix.

currently im just running the stock wheels/hubs and rear cog from the bianchi pista, so i dont think that the problem is the lockring unable to engage the cog. i’m not really worred too much however, these wheels have definitely served their purpose thus far and are in need of an upgrade soon. also got a new surly rear cog coming in the mail, so i think i should be okay.

thanks alot jaseyjase and snowflake.

I’d just retighten the cog and the lockring first (and while you’re at it you can slip a piece of paper or something similar between them to see if there’s a gap). Then go for a ride to see if you’ve fixed it. If not, try a new cog.

Use a good chainwhip (or rotafix it).