Crank length

[nammynake]

I am replacing the Sora groupset on my road bike for 105 and have three options for crank length; 170, 172.5 or 175mm. I've tried googling and there seems little guidance on the choice. Is there a 'standard' length? Unless there is reason I guess I should just go for the same length as my current ones, but I am not sure how to measure them.

[Jimbo.]

The length should be stamped on your current arms.

[nammynake]

Jimbo. said:

The length should be stamped on your current arms.

Aha! Indeed it is, 170. Thanks

[Rouleur]

There are arguments for various lengths but as a rule it increases with the rider's height. How tall are you?

[Saddle bum]

The original british standard was 6-3/4inches, which is 171.5mm.

The continental standard was, and is, 170mm.

At 1.72m (5ft8in) I ride 172mm cranks.

[nammynake]

Rouleur said:

There are arguments for various lengths but as a rule it increases with the rider's height. How tall are you?

5'8"

[okgo]

I went from 172.5 to 175 and notices no difference. Back to 172.5 now.

[yellowjack]

I read a piece in a cycling magazine recently which suggested that an overly long crank length for your physiology could cause knee pain, whereas it was unlikely to cause problems if you rode with shorter cranks than suit you. Can't find said article to confirm that this was based on medicine/science, so could be just a cycling journo's opinion.

[Rouleur]

nammynake said:

5'8"

I would have thought that 172.5 would be right then, but it's really not a big deal if you stick with 170.

[nammynake]

Rouleur said:

I would have thought that 172.5 would be right then, but it's really not a big deal if you stick with 170.

I went for 170mm in the end. I couldn't find any conclusive information so decided to stick with the same length as my current ones. Besides I'm not a racer, I just ride for fun and to stay healthy.

Really looking forward to the 105 groupset. It should hopefully be a lot nicer to use than the current Sora, plus save a bit weight too! My main reason for changing is my hatred of the little thumb shifters on Sora.

[Uriel]

Good reading, but maybe a little more than you wanted to know: http://www.stevehoggbikefitting.com/blog/2011/06/c...

[daz3210]

The problem with a longer crank is that while you can set the seat for the bottom of the pedal cycle, the crank length also affects how high the foot comes at the top of the crank.

Following my recent ITB problem, the sports physio suggested my seat was not high enough (which following advice on here I confirmed to be the case using 'scientific' measuring methods for theoretical fit. The explanation I was given was that by not having the seat high enough I was causing a greater than comfortable angle in the knee at the top of the stroke.

Logic suggests that the same would happen if crank length was too long. Too short and you just don't get optimum power since the knee does not come quite as high as it might with the correct crank.

[itsnotarace]

One thing to watch for when swapping crank lengths is that it will affect your gearing ratios - only slightly mind - but a difference nonetheless. Also it will affect your saddle height by the same amount of change.

I'm 5ft 8 as well and I prefer 172.5mm cranks btw