Join Date: Oct 2004
Location: DC / Maryland suburbs
Bikes: Homebuilt tourer/commuter, modified-beyond-recognition 1990 Trek 1100, reasonably stock 2002-ish Gary Fisher Hoo Koo E Koo
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Originally Posted by hackybiker
1) I've seen a 39T chainring that I think might fit. Does anything smaller than 39T exist? Can anyone suggest an online store with a good selection of chainrings?
2) Is it a problem for the derailleur to have two chainrings of very different sizes (say, a 52T and a 30T, if the latter exists)?
3) Sheldon Brown says derailleurs are designed for the larger chainring, so that means I can't change out the 52T. Uh... right?
4) Would I need to do anything with the front derailleur? or the chain length (shorten or lengthen it)?
Thanks for any advice or pointers to additional info!
Al1943's advice is spot on! You might be better off going to something like 50/36. Front derailers are designed for a specific size of large chainring, but it's almost always possible to make a front derailer work very well with SMALLER chainrings than it was originally designed for.
Indexed front derailers are also optimized for a specific gap between the chainrings, e.g. 53-39=14, or 48-38=10, but since your FD isn't indexed, no worries about that except not to exceed a gap of 14 as Al suggests.
You'll likely need to shorten the chain if you use smaller chainrings, yes. That can be done with an ordinary chain tool and is a good idea.
Another suggestion: if you replace the chainrings, replace the chain AND freewheel as well. (You call it a 6-speed cassette, but it's actually a freewheel on a 1984 bike.) This will have several advantages: you won't wear out the new chainrings as fast as you would with an old worn chain. Read Sheldon Brown's chain article to find out how chain and sprocket wear contribute to each other in a vicious cycle. The other advantage of replacing the freewheel is that you would have a freewheel with modern shift ramps... they make shifting the rear derailer a LOT smoother in many cases!
If you decide to do this, Nashbar sells very nice 6 and 7-speed freewheels for $20, and good 6/7 speed chains for about $8. A 7-speed freewheel works as a drop-in replacement for a 6-speed freewheel, so you could add an additional gear for no extra hassle.
Hope that helps!