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Chain jumping on road bike, what needs fixing?

  • This is driving me nuts. Bike just came back from the shop and has new chain, new rear cassette, new cables. The bike is a Bianchi 2000, chromolite frame. Everything else is standard. Since I have had these parts replaced I've returned it three times to the bike shop and they claim all is perfect but when I stand on the peddles and head up very steep hills the front cassette shifts from the middle to the smallest gear with a bang and grind. It's taken the fun out of sprinting up hills :-( . I've checked and rechecked the derailers (front and back) and the chain has not damage nor has any stiff links. It's properly oiled. The front cassette was not changed since I only have about 2000km of the bike and most of that is on the largest gear on the front. Help!

  • Answer:

    Phoster seemed to have hit most of the important items to check. I am not saying worn chainrings are the problem but, in general, I have always found it best to replace cassette, chain and chainrings all at once. It is an expensive proposition, but if the chainrings are somewhat worn, they won't mesh perfectly with a new chain. This can cause the chain to pop off under pressure. Poor fit of the chain and front chainrings will also cause the new chain and the cassette to wear more quickly, as well. In the meantime, when really mashing the pedals listen for rubbing of the chain and derailleur. I suspect you are flexing your frame when you pedal hard. You may have just enough contact with the derailleur to pop the chain off especially if the chainring is a bit worn. In this case, a slight adjustment of cable tension may fix the problem. Hope this helps.

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front cassette? i assume you are talking the crank here, and you have a tripple, if i am picturing things right. well, they did cables, so they had both derailers loose and likely adjusted them. are you hearing the chain rub before this happens? if you are, they may not have it adjusted quite right, so that it hits and kicks the chain down under a load as the bottom bracket flexes. it could also be a worn or damaged tooth on that middle chainring. see what happens is the shop puts it on the stand, runs it through the shifts and it hits them all, so they say there is nothing wrong. yet under a load, something is just off enough to cause a problem. next ride, listen for chain rub, and watch how the front derailer is clearing the chain. make sure the chain has plenty of clearence and is centered in the cage (between the inner and outer plates of the front derailer). i am betting you'll find it is nearly rubbing the outer plate, and as you put a load on it, it hits and makes it shift. i am betting that front derailer isnt set right.

phoster

Since the shop did the work, take the bike to them with your cycling gear and ask to be on a trainer. Set it up for the most difficult setting (most resistance). Then put it in the gear that it jumps from and ride as hard as you do outside with them watching. That may be the only way to get to the bottom of your problem. I had a noise in my bottom bracket on my Bianchi that when I took it to the shop it kept checking out "OK". I rode it on the trainer and sure enough there was the noise and they fixed it for me. The bike was new, under warranty, and the threads on the bb were not greased. Only made a noise under load, just like your chain only jumps under load.

McG

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