ignorant chainring question - advice needed!
 

Hi all:

just picked up a new ride for my wife; it's a 1976 Motobecane Grand Touring mixte, dirty but seems to be in good shape. Very lightweight Vitus frame, no more than 25 pounds. It's all original except for the crankset. A previous owner replaced the original SR AX-5 cranks with vintage Sugino Mighty Tour, with 44 and 34t chainrings - I guess they REALLY didn't want to go too fast! I believe this crank takes 110 BCD chainrings. So, I want to bring it closer to the original 40/52t gearing. Is this simply a question of finding the proper size chainrings and replacing them? I'm assuming the front derailleur will need to be raised slightly to accommodate the larger size? Anything I'm missing here?

I'll post pics when I've had a chance to clean it up.

Thanks

Roy

Velobase is your friend. The Mighty Tour is indeed 110mm BCD. Any 110mm BCD rings ought to work fine, but you'll likely need to raise the front derailleur to clear the larger ring.

You've got the theory right. I wonder if the bike was modded to be more of a climber... depending on how strong of a rider your wife is (and what freewheel you're using), you may want to keep some low-ish gears in the mix.

Quite possible. I bought it in Jersey, in a very hilly area... I had to travel a little distance to find a flat place to test the bike out. The freewheel is the stock 14-32 that came with the bike, I think she'd find the lack of a high gear frustrating. Can I put on a bigger outer chainring (say, a 50 or 52t) and leave the inner 34 alone, or will that be too much travel for the FD?

You will essentially have a modern compact crankset @ 50/34 - not knowing what kind of FD you have it's hard to say how it will handle its duties.

32T, you say? Combined with the 40T inner ring you had in mind, that's a pretty decent low gear. I'd proceed with the 52/40T conversion. :thumb:

sounds like a nice, almost all-original moto you found. the problem of it being 110 bcd just opens up all kinds of opportunities on rings. my used grand record came with a half-step setup of 48/44, and i rode it like that for years before changing it to the more original 52/42. it's nice having rings so close together for the flats. but i did find climbing to be difficult at times. and actually now, depending on the incline, a 42t ring seems a little large. if i were to increase your hgh gearing, i might start with a 50t as the new large ring, and keep the 34t in place. change to a new and slightly longer chain. raise the fd to only very slightly higher than the 50t ring. and then see how it rides and shifts. if you find a good deal on a 48 ring, you mifht try that instad of the 50. and you can also make a table of the step gearing to see what might work best before shopping. sheldon will be of help here.
best...

44T isn't small if you have an 11T cog on the rear.