Chain/Ring problems

[Whistle]

Had already posted this but in the wrong section, got no replies!
Anyway have fitted a new rear Shimano 7speed cassette and a new Sram chain to my bike, it mated to the original Exage 400 "Biopace" bottom crank.
The problem is that under hard peddle pressure the chain jumps off the rings! only seems to happen on the two outer [larger] rings, i have checked it all over and cannot see a problem.
Is this anything to do with the biopace ring/tooth shape? have parked the bike's front wheel against a wall and applied considerable pessure on the peddle and all's ok but out on the street it does it!
Had no problems with it doing it with the old chain, so whats the problem?
Thanks for any help/info.
John

 

[mattbrown]

sounds like the chainrings are worn, would normally happen in those rings if that is the case, the teeth of the chainrings dont look like shark teeth do they?

 

[pete_mcc]

I'd agree that it's probably the chainrings being worn, but the fact that it skips only in the top two chainrings when you will be exerting the maximum force on the cranks may also indicate a freehub problem, ie pawls not engaging or slipping.

Try flushing the freehub out and giving it a service, service the jockey wheels (on the rear derailleur) to ensure they are not sticking and are free floating and are not badly worn, if that fails then try replacing the middle chainring (cuts costs a bit before you have definately ID'ed the problem).

Biopace itself won't cause the chain to slip unless the rings are worn badly

 

[Whistle]

Thanks for the replies.
I had infact replaced the whole rear wheel which was new when removed from another bike a few years back so the hub [LX] has done little or no work at all, all i did was to replace the 9 speed cassette with a 7 speed plus spacer.
Will have a closer look at the rear derailleur and see if i have a problem there, it all seems to work well!
The teeth on the rings seem ok, there not "hooked" but the shape does change [shorter!] at the "top dead centre" positions which i assume have something to do with chain engagement when shifting, or am i wrong?
If it is a ring problem are repacement "biopace" rings still available or can alternative be found to fit the crank webb fitting?
Cheers
John

 

[FluffyChicken]

Standard 110BCD (aka ATB, or just standard 5 arm) round rings fit. Can be had from a lot of places.

There is nothing special about BioPace cranks.

 

[dyna-ti]

If the chain seems to crunch across the rings but doesnt fall right off ,then its wear.

If the chain crunches its way around the rear cogs but stays on the chainrings then its likely to be the cogs or freewheel or even a worn chain

if the chain falls off completely when you turn the pedals then its likely to be the chainline

Incidentally if you take off the chain, theres 4 ways it can go back on

 

[terryhfs]

If it jumps off the rings, it's the rings. The cage of the rear mech will boing about before this happens and pedalling will feel like there's something awfully wrong with your bottom bracket.

It'll appear to work fine on a workstand - it's when you put the pressure on from riding that it'll be apparent.

Freehub problems are normally shown up by the little chainring first as that's the one you'll be producing the most torque in.