changing cranksets

[lph]

argh almighty. Finally got my commuter bike all put back together, with a new old rear wheel filched from my old winter bike, and a new chain ditto, and ... the middle crank ring is worn out. Had to wheel it up the steepest hills on my way home since the chain just wouldn't take under load. (and the smallest ring is out of commision)

Anyway! I decided to just declare my old winter bike dead and filch the brand new crankset from it as well. But all the rings are larger, so the smallest ring rubs against the frame before I've even started pressing the crankset on. But can I just put in a broader ("longer") bottom bracket, so that I'll have room for the new crankset? I could filch that from my old winter bike too. Will that mess up the angle to the cassette, or can I adjust for that by twisting the derailleur?

A little out of my depth here in combining stuff from two bikes, but trying to avoid a long round of the lbs's and fruitless searching for cranksets they don't have. They're both mtb's by the way, square spindle.

[DebW]

argh almighty. Finally got my commuter bike all put back together, with a new old rear wheel filched from my old winter bike, and a new chain ditto, and ... the middle crank ring is worn out. Had to wheel it up the steepest hills on my way home since the chain just wouldn't take under load. (and the smallest ring is out of commision)

Anyway! I decided to just declare my old winter bike dead and filch the brand new crankset from it as well. But all the rings are larger, so the smallest ring rubs against the frame before I've even started pressing the crankset on. But can I just put in a broader ("longer") bottom bracket, so that I'll have room for the new crankset? I could filch that from my old winter bike too. Will that mess up the angle to the cassette, or can I adjust for that by twisting the derailleur?

A little out of my depth here in combining stuff from two bikes, but trying to avoid a long round of the lbs's and fruitless searching for cranksets they don't have. They're both mtb's by the way, square spindle.

Are the chainrings replaceable on the worn out crankset? If they can be removed with hex bolts and the bolt circle diameter is something standard (most likely), then that might be your best bet.

Putting the new crankset from the other bike on the commuter bike may present several challenges: you'll need a longer bottom bracket spindle, then the chainline will be different than it was, [NO, you shouldn't twist the derailleur to compensate], and you may find that the front derailleur doesn't match the curvature on the new larger chainrings and won't work properly. Also a chance the bottom bracket from the other bike won't work. Measure the shell width and the length of spindle on each side on both bottom bracket to judge. Does one or both bikes have a front derailleur mounted around the bottom bracket?

[laura*]

Anyway! I decided to just declare my old winter bike dead and filch the brand new crankset from it as well. But all the rings are larger, so the smallest ring rubs against the frame before I've even started pressing the crankset on. But can I just put in a broader ("longer") bottom bracket, so that I'll have room for the new crankset? I could filch that from my old winter bike too.

It could be that you have to keep bottom brackets matched to their respective cranksets: To position the chainrings in the same place, one crankset may need a short BB spindle while another needs a long spindle.

My old MTB originally used a BB with a 110mm spindle. When I replaced the crankset, I belatedly found I needed to also switch to a 107mm spindle.* On the other hand, Shimano specs some other cranks as needing a 117 spindle to achieve the same alignment. Grrr! I wish they'd have standardized on one length.

Will that mess up the angle to the cassette

The "standard" position for MTB chainrings has changed. With the transition from small steel tube rigid frames to oversize aluminum tube full suspension frames, the chainring position had to be pushed out. If that's what you end up doing by using a wider BB, then I think everything will still work.

can I adjust for that by twisting the derailleur?

No! The FD still needs to be aligned square with the chainrings. If you end up having to move the chainrings outboard, you might have to also swap the front derailleur: The previous FD might not be designed to work that far outboard.

* The difference between Shimano 107 and 110 BBs turns out to be almost entirely on the left side. The bike would have worked fine with the 110, except the left pedal was sticking out too far.

[lph]

Thanks ladies, you've cleared up a whole lot of things for me now. I hadn't even thought about the curvature and "reach" of the FD, I was obsessing about the chain angle and the rear derailleur.