Bicycle Mechanics - MTB gearing on road bikes with standard road crank - problems

With the annual Six Gap Century (http://www.cyclenorthgeorgia.com/) approaching, some of our Georgia road riders are looking for a quick solution for lower gearing. Short of changing to compact or triple cranks, some are opting to put MTB rear derailleurs and cassettes on bikes with standard 39-53 cranks for the extreme climbing of this event.

I have seen a couple of applications using 11-36 or 12-36 cassettes having problems with the chain dropping off the cassette with any backpedaling while in the big/big cross-chaining combination. Granted, this combination should not be used, but things do happen while riding and it would be better if it did not cause the chain to fall off the cog.

On a Shimano application, it was not quite as bad. It took about 3/4 rotation backward with the crank and the chain fell about 3 cogs smaller. On another bike with SRAM components, about 1/4 rotation backward resulted in the chain dropping off the top side of the 36 tooth cog into the spokes. Not good.

I understand that this is an extreme application and outside the spec, but are there things that can be done to minimize these problems?

thanks

Don't pedal backwards?

Don't pedal backwards?

:)
Obviously. But things do happen, especially when you are tired.

On a Shimano application, it was not quite as bad. It took about 3/4 rotation backward with the crank and the chain fell about 3 cogs smaller. On another bike with SRAM components, about 1/4 rotation backward resulted in the chain dropping off the top side of the 36 tooth cog into the spokes. Not good.

I understand that this is an extreme application and outside the spec, but are there things that can be done to minimize these problems?

Well, I'd add the ol' "pie plate" between the spokes and cassette and live with the ridicule. Otherwise...

My "road" bike (actually, one of my recumbents) has super-extra-wide range gearing, and it'll drop the chain down a couple cogs if I backpedal in the high-high. It's a consequence of running the chain at such a large angle. As long as the chain doesn't go in the spokes, it's harmless.

I'll ditto what the others said: don't pedal backwards and get a pie plate.

I know people that run this combo all the time without issue

With the annual Six Gap Century (http://www.cyclenorthgeorgia.com/) approaching, some of our Georgia road riders are looking for a quick solution for lower gearing. Short of changing to compact or triple cranks, some are opting to put MTB rear derailleurs and cassettes on bikes with standard 39-53 cranks for the extreme climbing of this event.

I have seen a couple of applications using 11-36 or 12-36 cassettes having problems with the chain dropping off the cassette with any backpedaling while in the big/big cross-chaining combination. Granted, this combination should not be used, but things do happen while riding and it would be better if it did not cause the chain to fall off the cog.

On a Shimano application, it was not quite as bad. It took about 3/4 rotation backward with the crank and the chain fell about 3 cogs smaller. On another bike with SRAM components, about 1/4 rotation backward resulted in the chain dropping off the top side of the 36 tooth cog into the spokes. Not good.

I understand that this is an extreme application and outside the spec, but are there things that can be done to minimize these problems?

thanks

I don't see how any kind of cross chained back pedalling can cause the chain to go off the largest cog into the spokes unless you hit a pothole or something at the same time... maybe chainsuck? But you can't expect to be cross chained in the worst combo and be able to backpedal without any problems. If you're going to do this, you have to pay a little more attention. I've got a compact crank and MTB gears on one bike and it's a nice solution for slow, steep hills.

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