Bicycle Mechanics - Will I need a long cage rear derailer?

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the beef
05-11-06, 09:30 PM
I have a 1980s Schwinn Super Sport that I recently took home. It had a busted 700c rear wheel with a 6-speed freewheel, so I've been looking for a replacement wheel along with a better cassette to use with those friction shifters.

This one guy offered me a Matrix wheel with a Shimano hub and 7-speed cassette attached. He told me that the cassette is a 13-30, which really is perfect for my needs, but I'm afraid the big ring will be too big for my short/medium cage Suntour derailer to handle. I have a 52/42 in the front. What's your take on this? Will I need a new derailer?

I really don't know enough about this. Anyway, thanks for the knowledge.

capwater
05-12-06, 05:43 AM
Most likely you will, but slap it on to try it out first. Most short cage RDs max out at 27 based on Shimano official specs, but in reality can go to 29.
powers2b
05-12-06, 06:32 AM
Long cage DRs are designed to take up the extra slack in the extra chain that is needed to fit around large cogs.
If you put a triple with a granny gear on your bike you will need a Long Cage DR for he same reason.

Enjoy

Retro Grouch
05-12-06, 10:31 AM
If it was my bike, I'd probably just keep what you have. Most likely it'll be fine. You may have to dial in the "B" screw to keep the upper derailleur pulley from rubbing the big rear cog. Make sure the chain is the right length to safely cover the big/big combination. If you ever accidentally shift into the little/little combination and you derailleur doesn't have adequate slack take up, the lower run of the chain will just go slack.
powers2b
05-12-06, 10:34 AM
the lower run of the chain will just go slack.

and wrap around your rear DR which will jam into the spokes and lock up the rear wheel, bend the DR hangar/frame and cause a spectacular crash.

Enjoy
Gonzo Bob
05-12-06, 10:56 AM
It will probably work. I've run 34x50 and 13-30 with Shimano short cage. This combo exceeds the Shimano capacity spec of 29T by 4T but everything worked and I could get all gears including 50/30. The one thing to beware is that there is a lot less room for error in sizing the length of chain when you are near or exceed the spec.
Sheldon Brown
05-12-06, 01:38 PM
and wrap around your rear DR which will jam into the spokes and lock up the rear wheel, bend the DR hangar/frame and cause a spectacular crash.

Alarmist nonsense. That would only happen if the chain was grossly too long. Even if it did, rear wheel lockup is very unlikely to cause a "spectacular crash."

The only reason not to buy a long cage derailer is if you're eager to save a couple of grams. Long cage units work with everything, but short cage models are kinda fussy.

Sheldon "Fear Is The Mind Killer" Brown

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| If one lets fear or hate or anger take possession |
| of the mind, they become self-forged chains. |
| --Helen Gahagan Douglas |
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