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Old 11-04-09, 11:17 AM   #1
dauphin
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Compact crank component compatibility

I plan to put a compact crank on my bike and was wondering about the compatibility of my other components. Right now I have Shimano 105 ten speed all around. I've heard that my shifters will work with either a double or a triple setup. I assume that I will need a double front derailler but am not too sure about the rear derailler as it is a long cage for the triple. The crank is a Shimano R700 50/34 replacing a 105 50/39/30 (which will be installed on my commuter bike). I'd like to use the same bottom bracket if possible as well. Of course, I'll be installing a new chain at the same time.
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Old 11-04-09, 01:17 PM   #2
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same bottom bracket can be used, same rear derailler, same brifters. Just need a new chain, the new cranks, and a front derailler. FYI to anyone who might be contemplating such a change.
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Old 11-04-09, 01:20 PM   #3
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There's no reason you'd need a new chain. If it's worn, then replace it but if not, you can either shorten it or leave it as is. The extra chain shouldn't cause any problems. If the derailler was able to keep the chain tensioned with a 30 tooth chainring, it'll be even tighter with a 34 tooth ring.

The front derailler is also questionable in it's need for replacement. I'd give it a try before spending any extra money on a new one.
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Old 11-04-09, 01:51 PM   #4
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Short answer: Install the new crank, re-adjust the FD limit screws and cable tension (best to disconnect the cable and start over), and ride.

Replace the chain if it was due for a change anyhow, but that's independent, generally, of a crank/chainring change.

If the chain was properly sized before, then it is already properly sized since you are not altering the number of teeth of the largest chainring or cog. (Note: I come from the bib-big sizing philosophy.)

For the RD: its cage is longer than necessary, but no such thing as "too long."

The FD should be fine, too. The inner cage is taller (extends lower) to help shift the chain from the inner chainring to the middle. You're losing the inner triple chainring, so the taller cage won't serve its full purpose, but as long as it doesn't hit the 34T chainring, it won't do any harm. I can't see how it could since it's already clearing the 39T and any lower and the outer cage may not be able to clear the 50T.

Edit: Typos fixed. I'm anal.

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Old 11-04-09, 02:05 PM   #5
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Yes as mentioned, swap out the crank lower the FD, chain if new one is needed at the least just shorten it for the new setup. You can change the FD if you find you are having a problem with shifting but it should be fine.
As far as using your 105 triple shifters with a double crank no problem thats what I have on mine if you get it setup correctly you will have a trim setting both going up and down also, I do how ever have a double Utegra FD installed.
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Old 11-04-09, 06:05 PM   #6
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I might just switch out the front derailler anyway. I'm putting the triple components on a commuter bike anyway and it will need that. Looking forward to the new set up.
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Old 11-05-09, 12:44 AM   #7
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Quote:
Originally Posted by joejack951 View Post
you can either shorten it or leave it as is
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at the least just shorten it for the new setup.
Can one of you please explain to me why you're suggesting the OP can shorten the chain? What is the benefit? Why would swapping a 50/34T for a 50/39/30T itself lead to shortening the chain? It seems potentially harmful. If the chain was already at it's shortest possible length, shortening it would make it too short.

Edit: 50/34T vs. 30/34T

Last edited by JiveTurkey; 11-05-09 at 03:30 PM.
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Old 11-05-09, 01:56 AM   #8
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Can one of you please explain to me why you're suggesting the OP can shorten the chain? What is the benefit? Why would swapping a 30/34T for a 50/39/30T itself lead to shortening the chain? It seems potentially harmful. If the chain was already at it's shortest possible length, shortening it would make it too short.
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Old 11-05-09, 03:07 AM   #9
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Don't shorten your chain!

The big-big combination isn't changing (and could now be used with less cross-chaining than before, although still not recommended), so there is no need to shorten the chain. The RD is going to have to take up 4 teeth worth less of slack in the small-small than before, so there is also no need to change to account for that.
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Old 11-05-09, 07:28 AM   #10
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If the chain was sized using small/small, there'd be some extra chain. If big/big was used, yes, you would not want to touch the chain.
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