Crank Arm Length?

Check out this article.

http://www.slowtwitch.com/Tech/Crank_arm_lengths_for_tri_727.html

175

haha, glad I could help : )

Seriously though, I would go with the 175 if that is what you are using on your road bike.  They tend to put different size cranks based on the bike size, bigger bike = longer cranks = taller riders.

I have read a few posts over on ST on the subject and there doesn't seem to be any hard data that says any size is going to give you more watts/efficiency/etc.

EDIT: If you have access to both size cranks and a power meter and you were so inclined you could perform your own test and see if there was any difference. 


Edited by dalessit 2009-11-02 11:44 AM

I have looked into this a little before,  from most of what I read bikes get OE of 170, 172.5, or 175 depending on the size. However most people would do better with a shorter crank length.  Again this is my take on the stuff I have read about it.
With a longer crank you produce more torque which should be favorable however if it takes you out of aero position you probably waste more than you gain fighting the wind.  In the end I think it comes down to an individual fit issue, and the only way to tell is to fit you on a bike testing the different length cranks and test your power output.  Something fitters probably don't want to get involved with. Probably why almost everyone is on the standard lengths that come as OE on their  bike.

First of all, I doubt you would even notice the difference between 172.5 and 175 cranks... not enough difference to really make a difference...

With that said, if you are using 175mm on the your road bike today, you should use the same on your tri bike for no other reason than it makes it very easy for you to transfer fit from one bike to another.