There are standards in bike crank arm lengths. Determination
of what is right for you will make your pedaling more efficient
and give you the stamina for longer rides, but here is an idea
first, about the industry norms:
Standard Mountain Bike Length: 175mm cranks
Standard Road Crankset: 172.5mm
That being said, manufacturers make a bike crankarm fit for every
size individual and every level of riding. They start at a minimum
length of 165 mm and can be found in 2.5mm increments up to 185mm.
A few producers do make crank arm sets longer and shorter, but
they are rare.
The length of the crank arm is measured from the center of the
pedal hole to the center of the crankarm-spindle hole. In at least
the newer crank arms the length is typically stamped on the inside
of the arm.
Theories about the right Bike Crank Arm Fit mostly revolve around
the length of the riders leg. If you are of average height then
stick with the 175mm crank set that comes standard with a MTB
or 172.5mm cranks on a road bike.
If you are over six feet tall then maybe consider going longer
but be aware of the downside of a longer bike crank arm length.
Determination to get more power on your stroke climbing challenging
inclines, may be offset by less pedal clearance in rough terrain.
Slamming your pedal into a exposed root is a good way to develop
an intimate relationship with the muck pile.
Thus before you make the leap to add length, consider your riding
habits. Longer lengths = greater leverage = more power. For short
spurts this may be helpful but if the rest of your riding does
not require this capacity then clearance may be an issue and your
legs will be forced to travel a longer arc under normal conditions.
For riders with longer legs and thigh muscles this is great,
but the rest of us will gain on the power and loose on distance.
When thinking about the RIGHT bike crank arm fit, think
about the big picture.. What capacity is going to be most critical
most of the time.
Bottom brackets, cranks & chainrings