170 and 175mm crank arms

[Blue Hills]

Am not sure i understand your point vickster.

 

[summerdays]

Most mass produced bikes tend to be kitted out with 175mm cranks regardless of the size of the frame they are fitted to. Thus you will get a small framed woman's bike with 175mm cranks; not much fun of you are 5' 2" - ! The general guidance is one relative to leg length unless you have a specific bike for a specific discipline. I think it is 29" inside leg or less, 165mm, 30 - 32", 170mm, 32" +, 175mm. If you happen to be a longshanks, then see a good bikeshop for advice - ! I'm 32" inside leg and all my bikes regardless of type or frame size have 170mm cranks, because that what suits me.

I've no idea what length I have on my bike, just the standard ones that came with it but I was going to replace worn out rings soon. So based on the above I would need 170 if my inside leg is about 32 inches.

 

[migrantwing]

What a lot of people overlook when choosing longer/shorter cranks is not the position at the downstroke, but at the upstroke. If cranks seem too long, you can always lower your saddle height to compensate for this, but the position of the leg/knee when the pedal is at the 12 o' clock position needs to be thought out, too.

 

[summerdays]

What a lot of people overlook when choosing longer/shorter cranks is not the position at the downstroke, but at the upstroke. If cranks seem too long, you can always lower your saddle height to compensate for this, but the position of the leg/knee when the pedal is at the 12 o' clock position needs to be thought out, too.

So what are you aiming for in that position?

 

[Milkfloat]

So what are you aiming for in that position?

>70 degrees http://bikedynamics.co.uk/guidelines.htm

 

[Arsen Gere]

Crank length paper with some measurements here. http://link.springer.com/article/10.1007/s004210100400#page-1

 

[Smurfy]

Difference will be minimal on a multigear bike. On the other hand, if you want to spin fixed at 180rpm downhill then 165mm might be a good idea.

 

[Sharky]

The differences apply equally to both gears and fixed. Won't make you an Olympic champion, but will change your riding style.
Now consider using 145mm cranks instead of 175mm cranks. To get the same distance from the saddle to the pedal at the bottom, you will need to raise the saddle by 3 cm. Then when the pedal is at the top of the stroke, the distance from saddle to pedal has now increased by 6 cm's. This increases the angle considerably at the hips and allows you to adopt a much more relaxed racing position.
The 145 mm cranks are rotated in a smaller circle, but the circumferential speed of your feet remain the same as it was for the 175mm cranks and because the circumference is smaller, you achieve more revolutions per minute. However, there is less leverage with the 145mm, so you ride a slightly lower gear to compensate.
I've been using the shorter cranks now for a couple of years. My times for 10's haven't changed very much, but then I am fighting the aging process, but the feel you get from the higher revs and more relaxed position, takes me back to when I was a lot younger and much more supple.
Climbing hills is surprisingly easier with the shorter cranks. You rev more, but are able to stay in the saddle for a lot longer and can pedal with efficiency, again due to the increased angle at the hips and my knees no longer bounce off my stomach.

Short cranks may not be the norm today, but who knows tomorrow?

 

[Mike!]

My 54cm Merida roadie came with 175mm Cranks, my inside leg is 29.5" and I've often wondered if shorter would be better for me. I plan on upgrading my crankset at some point so will get adivce at my LBS :-)