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Changing Crank Arm lengths ...

Old 03-02-15, 12:56 PM
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Panza
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Changing Crank Arm lengths ...

Hi friends.

My bicycle came with 170mm sized Shimano 105 crankarms but I'm generally accustomed and fitted to 172.5mm crankarms on my other bikes. What will I need to purchase to update it to my desired crankarm length?
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Old 03-02-15, 01:03 PM
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You need new crank arms at a minimum and buying a complete new crank in the desired arm length is probably just as cheap and certainly easier to find. Depending on which generation your 105 is (square taper FC-5500, Octalink FC-5600 or Hollowtech II FC-5700 or FC 5800) a new crank may also require a new bottom bracket to match.
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Old 03-02-15, 01:05 PM
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A left & right arm of the correct BCD of the rings and identical spindle interface/length.

You may find a complete new crank set w/rings for about the same money.

Is it worth it for 2.5mm longer?
I can see where going a bit shorter can be justified if you have a bad knee (like me) or have short legs. 2.5mm longer, not quite so much unless you have extra money to spend.

Last edited by Bill Kapaun; 03-02-15 at 01:08 PM.
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Old 03-02-15, 01:10 PM
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Slightly thicker shoes?
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Old 03-02-15, 01:11 PM
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Originally Posted by Bill Kapaun View Post
A left & right arm of the correct BCD of the rings and identical spindle interface/length.

You may find a complete new crank set w/rings for about the same money.

Is it worth it for 2.5mm longer?
I can see where going a bit shorter can be justified if you have a bad knee (like me) or have short legs. 2.5mm longer, not quite so much unless you have extra money to spend.
I was hoping to have 172.5mm on all my bikes so I could interchange a crank powermeter.
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Old 03-02-15, 01:26 PM
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You might offer up a trade in the Road Marketplace or other FS subforums...
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Old 03-03-15, 08:11 AM
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Have you tried actually riding the bike? You may not even notice the difference. 2.5mm is 0.098", less than 1/10 inch.

I have heard anecdotes of professional riders who did not notice that they had different crank arm lengths on either side of the bike.
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Old 03-03-15, 12:24 PM
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Originally Posted by dsbrantjr View Post
Have you tried actually riding the bike? You may not even notice the difference. 2.5mm is 0.098", less than 1/10 inch.

I have heard anecdotes of professional riders who did not notice that they had different crank arm lengths on either side of the bike.
You're probably right. It's could be more mental than physical. Like knowing your gloves don't match.
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Old 03-03-15, 12:38 PM
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I was hoping to have 172.5mm on all my bikes so I could interchange a crank powermeter.
Just buy a whole new crankset, then . dont they put that power input in the crank itself?

I've never cared for more data than miles ridden vs those remaining to get there.
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Old 03-05-15, 03:49 PM
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If the bike was bought at a LBS and is pretty new ( not been used for several months ) and the cranks look new, it may be possible to talk the owner of the LBS into trading for the longer crank arms. I'd say if you've only had the bike a month or so, that should be a no brainer for the bike shop owner if you're a regular customer and no wear is showing on the crank arms.
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Old 03-06-15, 05:06 AM
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You do realize that 2.5mm crank length translates to less than 1/5 of an inch increase in the diameter of the pedal stroke. I'd be willing to bet that if you took two otherwise identical bikes and equipped one with 170mm cranks and the other with 172.5mm, 90% + of riders wouldn't feel the difference if you didn't tell them and 50% wouldn't be able to tell which was which even after they knew they were different. Not sure how that would affect using your powermeter.
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