Bicycle Mechanics - 61T chain ring on 1978 suntour equipped Fuji possible?
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08-16-04, 10:10 PM
What is the largest chain ring I can put on my suntour equipped late 70's Fuji road bike? Is a 61T possible? Can I adjust the front derailler enough to compensate. Will the chain be long enough?
Yes, 61T can be fitted, the front der will have to be moved up on the seat tube
perhaps an inch or so. The small chainwheel shift will be clumsy at best and a
triple front der would be a good idea because of the longer cage. Ditto long cage
rear der to wrap chain. Chain will definitely be short, but if you keep the chain on
the outer 4 cassette cogs it would work (Cogs less than 20-21T). Really should
add 4-5" of chain though to prevent inadvertent disaster. Use of a 61T with
650 or larger wheels doesn't make much sense though. A 61x13 with a cadence of
100rpm would be ~37mph. Recumbents with their tiny wheels routinely use triples
with 60-62T x 42 x 26 or 24 combos and shift ok. But they dont have to worry
about chain line or chain wrap as much. Steve
08-16-04, 10:46 PM
Thanks for the good technical advice. Do you think I could get away with a 56 without switching deraillers? Am I going to have to use a custom chain or is there an off the shelf chain that can work?
Sure, you can probably do it... provided you have a crankset for which a 61T chainring is available.
08-17-04, 08:56 AM
road front derailers are typically designed for a big ring between 50 and 56. You can go bigger than that, but the curvature of the derailer cage will be tighter than the chain ring. This means you will have a bigger gap between cage and ring at the front of the cage than at the back - i.e. you won't be able to get the derailer as low as you would like for best shifting performance. It will still work, though. Most front derailers for double chainring typically are spec'd at a max difference of 15 or 16 teeth. A late '70s Fuji probably has a Sugino crank with 144mm BCD and 42x52 chainrings. So changing to 42x61 would give you a difference of 19 teeth. It will still work, but you have to be careful not to run the small-small combos or the chain might drag across the bottom of the derailer cage.
I recommend getting a new chain if you do it. Any 6/7/8-speed chain will work.
Most off the shelf chains are 3-4" too long as supplied. You could start with a new
chain and splice in another 1-2". The LBS may have a few spare links floating
around, as chains are almost always shortened when installed. That assumes there
will be no incompatibility with the new chain and older cassette from wear. 5-7spd
chains are dirt cheap, perhaps $8-15 range. 56T CW would reduce the stress on
the system significantly but the chain would still need to be a bit longer, the rear
derailler may be able to wrap without going to a longer cage. Steve
08-17-04, 10:18 AM
Why would you waste mental energy on something like this? Are you actually overspinning your 52/11?
Added later: Sorry, after reading that I realize it sounded smartass, which was not my intent. But do you really need a 61?
08-17-04, 10:25 AM
I was gonna say that myself, rainman. I think we have another merton on our hands :rolleyes:
I think he's trying to make up for having a 14t smallest sprocket on the freewheel to get similar to 52/11 gear ratio.
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