Crank Length

[trj977]

Something that has intrigued me over the years.

How do you decide on what crank length to use? Would a change from 172.5 to 175, and vice versa, be very noticable?

 

[Gerry Attrick]

It's mainly to do with leg length. Shorter the legs, shorter the ideal crank length. Unless you are at the extreme range of leg lengths, don't worry about it.

 

[mickle]

Gerry has hit the nail on the head, until recently this was the accepted norm, although recent experiments by Human Powered Vehicle builders (in an attempt to reduce frontal area) have indicated that there may be no significant loss of efficiency using very short cranks of around 100mm (if I remember correctly).

The theory is that since cranks are levers and gears are (effectively) levers, as long as you reduce them both by the same ratio there's no difference in the systems ability to transfer power from your legs to the drive wheel. We know that the limit to our power output is determined by how fast we can deliver oxygen to our muscles, maybe crank length is less critical that we thought.

This doesn't necessarily apply to regular diamond frames but it's food for thought. It'll be interesting to see what develops.

I use 175s on my MTB and 172.5s on my Road bike and I can't feel any difference at all.

 

[Blue]

I have a short inner leg and went down from the standard 170s to 167.5s. I didn't notice much of a difference. I'm currently changing to 165s on one of my bikes and hope that that will help with a knee problem I have.

If you do change, remember to alter the saddle at the same time.

 

[hubgearfreak]

Would a change from 172.5 to 175, and vice versa, be very noticable?

yes, by 1.4%

 

[spandex]

 

[trj977]

Thanks guys.