Front Chainrings - Shimano Altus Interchangeability
 

I've got an old Claude Butler MTB that I pulled out of a hedge. I built it up with a banana seat, and ape hanger bars, for fun, swapping the gripshifts for a pair of gutripper stem shifters I took off a wrecked old 10 speed. As I built it up, I found the problems that caused it to be tossed in the hedge. The derailleur hanger was bent, so that if I'd tried to ride and shift, the rear mech would have folded into the wheel, but I was able to straighten that with judicious use of a pipe wrench. Then I found that the jockey wheels where worn out - again, no problem, I had a bent derailleur with good wheels, so I swapped those out. That should have clued me in to the next problem - having built it up and tried to ride it for the first time, I discover that the front chainwheels are so worn that the chain literally jumps.

Clearly, need new chainwheels. I have several other cranksets, double and triple, that I might adapt, but here's the problem. The current setup is VERY small (largest is 38 tooth), and it appears the the front derailleur can't be moved up the seat tube, there being some sort of arrangement anchoring it round the BB shell. This means that swapping in another crankset with larger chainwheels is pretty much out of the question. There's a Shimano Altus crankset on Ebay right now, but it's largest wheel is 48, and I can't find in UK online shops any replacement chainwheels. What I do know is that the spider is relatively small on this chainset. I don't know much about interchangeablility within cranksets, so can anyone tell me whether there are other chainrings I can use instead with this crank? Or do I need to look for an alternate crankset? Or is there something I'm not seeing about the derailleur that would allow it to move (it really doesn't seem to be just banded onto the seat tube). Help!

^^^^ if the chainrings are bolt on, the issue with replacements is simply one of matching bolt circle diameter and bolt pattern. I think sheldon Brown at www.harriscyclery.com has of chart of measured distance between bolt holes and BCD. Try searching bolt circle diameter.Replacements are possibly more than the cost of another crank. Also consider just ditching the FD and running a single chainring.

I certainly looks like I could easily replace the crank with one of comparable quality for about $10 from ebay, it's just that I can't find one where the biggest ring is 38 or 40t. It seems tiny to me. Why have they done this? Why?

I do have a band on FD or two in my parts box. I suppose if I'm taking the cranks off, it would be relatively simple to take the FD mount off the BB at the same time, and fit a band on one. That would mean I could use any crankset I liked. It has to be said that the ratios provided by this one are awfully low....

I had a cheapo shimano crank like that with small rings. Duno why, other than for the real wusses and grannies. A new crank may also involve a new BB with spindle to match it. Not many free lunches when changing some things out.That FD sounds like an E type that used the BB for a mount,but maybe it's something else. Should be easy enough to ditch.