Crank arm length

focuz

I currently have 175mm crank arms, but have just measured my inside length and calculated my optimum length which is 170mm:/ Will i notice much difference in comfort and performance if i switch to shorter cranks? My knees are beginning to hurt, but not sure if this is down to the crank or other things, thanks.

Sprool

If your knees are beginning to hurt, can we assume this is a new bike? First make sure you are confident with having the correct saddle height and are cycling at a good cadence, cos both of these things can contribute to knee pain (as well as misaligned cleats) and these cost nowt to rectify.

dj58

Have a read of the post by KateS 172.5 cranks vs 170, it's on page 2 of Road Buying Advice.
Some interesting replies on the subject of crank lengths.

mfin

Hurting where on the knee? ...and how tall are you? Search function is good (try using the option 'Search within: topic title only' it can help when predicting a topic title someone else chose to word as against wading through the stack of otherwise seemingly rubbish results).

focuz

Cheers guys, I've had the bike for nearly a year aha, I'm 6'1 and I think ill double check my saddle height, I've never considered adjusting cleat position either, which is hopefully what the problem is. Checked out page 2, and there's a lot of mixed results on this.

Monty Dog

If you like to spin gears, then shorter cranks can help, but at 1.8m, a 175mm-180mm crank would be more the norm. As others have suggested, sore knees are probably symptomatic of something else - particularly cleat alignment, saddle height and fore-aft. If you're grinding big gears, it puts more stress on the knees too.

wishitwasallflat

As you've seen mixed feelings on this but FWIW I am 6' iside leg 30.5" and recently switched from 175 to 172.5 and my knees feel much much better. I am a definite seated spinner though.

balthazar

I've ridden a few thousand miles with a 175 crank on one side and a 170 on the other without noticing. (Didn't pay attention when I replaced a shot crank with a nearby redundant one). Needless to say, I wouldn't worry about it.

mfin

See above that some people are not sensitive to much (or they don't think they are). 6ft1" you probably should be on 172.5, but if you're not flush don't go jumping to correct 2.5mm, and certainly look at cleats and angles first for the knee pain. Mountain bikes often come with 175mm for a height most people on road would ride 172.5mm but you don't find all mountain bikers complaining about their knees?

StillGoing

It isn't 2.5mm, it's 5mm as it's a circle. Dropping the crank length means the seat needs raising otherwise the leg extension at the bottom of the stroke is under-extended. Consequently, the leg angle at the top of the stroke is increased to allow the rider to put out their best power from the top of the pedal stroke and minimise strain on the knee joint. If the crank length was changed without moving the saddle, the difference would instantly be 2.5mm longer distance from saddle top to the top of the pedal stroke. Because the saddle will need raising to get the correct leg extension, the distance will be 5mm after adjustment.

mfin

I understand circles a bit myself, and two times 2.5, and angles having programmed raw trig algorithms for surveying software design purposes. I was merely commenting that on average a mountain bike for a given size often has one step up of 2.5mm on the crank length, and you don't get mountain bikers all walking around car parks like they are crippled. As above, the OP's problem MIGHT be related/helped by crank length, but also the location of the pain or discomfort on the knee could point to a different problem which needs to be addressed, (although his crank length at 175mm could also be a little longer than ideal).

StillGoing

And i'm merely pointing out that you said don't go jumping to correct 2.5mm when the correction will be more than that.

cougie

Is 5mm a great distance in comparison to the length of a leg and down to the cleat ? I don't think it is.

I certainly can't tell any difference - and like Balthazar even ran odd crank lengths for a while.

StillGoing

Well there are enough individuals who have noticed a difference, me included by going down from 170mm to 165mm cranks.

mfin

philthy3 wrote:

Well there are enough individuals who have noticed a difference, me included by going down from 170mm to 165mm cranks.

But that's 10mm

StillGoing

mfin wrote:

philthy3 wrote:

Well there are enough individuals who have noticed a difference, me included by going down from 170mm to 165mm cranks.

But that's 10mm