Road Cycling - Rear Derailer

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almarktool
07-19-04, 09:48 AM
Can anyone tell me if I can run a 29tooth rear sprocket with a
Shimano 105 ?


Thanks
mark

msantiago1
07-19-04, 10:08 AM
Can anyone tell me if I can run a 29tooth rear sprocket with a
Shimano 105 ?


Thanks
mark

I believe only if it is the long cage. The short cage max is 27T.
Applehead57
07-19-04, 10:40 AM
That's how I understand it. Sheldon Brown says it does work, although he stresses to be extra careful about cross-chaining.

Retro Grouch
07-19-04, 11:40 AM
Can anyone tell me if I can run a 29tooth rear sprocket with a
Shimano 105 ?


Thanks
mark

There's actually two separate issues:

The first is the largest cog capacity. Shimano rates 105 derailleurs, regardless of cage length at 27 teeth. With bigger cogs, the upper derailleur pulley tends to rub against the sprocket. Often you can work around this by screwing the "B" screw all of the way in to pull the derailleur farther back behind the sprocket. Sometimes guys substitute a longer screw or even reverse the direction the b screw runs through the derailleur to get a little more space. My guess is that you'd have a good shot at making a 29 work.

The second issue is chain slack take up capacity. One of the functions of the rear derailleur is to keep the chain taut regardless of gear combination. If you subtract the smallest rear cog from the largest, subtract the smallest front chainring from the largest, and add those two answers together, that's how much chain slack take up capacity you theoretically need. Short cage 105 derailleurs are rated at 29 teeth, long cage derailleurs are rated for 37 teeth. You can "cheat" a little on this one too as long as you make sure that your chain is long enough to safely cover the big/big combination. You never really use the little/little combination, expecially if you have a triple. Consequently, if the chain goes a little bit slack in that combination it doesn't hurt anything.
almarktool
07-19-04, 12:47 PM
Thanks guys
I am trying to sneak a 29 on the rear if i can to go with my FSA compact cranks
looks like i might have to stay with a 27
msantiago1
07-21-04, 12:32 AM
Thanks guys
I am trying to sneak a 29 on the rear if i can to go with my FSA compact cranks
looks like i might have to stay with a 27

You should be able to run a 29 with compact cranks. I am running a 49x39 up front and I am thinking doing compact cranks. Will you be running a 50x34? Which FSA's are you getting?

Thanks,

Mike