Crank arm length solved knee problem

canoas

Can anyone tell me why going from a 175mm crank arm to a 172.5mm crank arm solved my knee issue of some 2 years! I had a Retul fit about a year ago and my back and hip pain went away then I thought it can't be my crank arm can it? anyway I'll lower my seat post by 2.5mm, still knee pain appeared, ok why not change the arm size down to 2.5mm and keep saddle height as Retul fit, suddenly after a week no knee pain, I'm pulling cartwheels around the house

My question to forum users, how did this solve my knee problem and why didn't lowering the saddle by 2.5mm fix it? I just need to understand this anyone!

giropaul

It might just be that you have a crank with an slightly incorrectly angled pedal thread?

gazrichards

Because adjusting your crank length by 25mm will not make an equal change to lowering your saddle 25mm.
Think of it as a rather complex mathematical problem. Because if the angles of seat tubes when you lower a saddle you also move yourself further back from the BB and handlebars. This changes hip angle and shoulder angle as well as your knee position.
Changing crank length by 25mm changes the pedalling circle that your foot makes all the way around the cadence so you knee is less bent at the top of the stroke and more bent at the bottom of the stroke.
I say this a a professional and qualified bike fitter and sports injury professional.

imposter2.0

gazrichards wrote:

Because adjusting your crank length by 25mm will not make an equal change to lowering your saddle 25mm.
Think of it as a rather complex mathematical problem. Because if the angles of seat tubes when you lower a saddle you also move yourself further back from the BB and handlebars. This changes hip angle and shoulder angle as well as your knee position.
Changing crank length by 25mm changes the pedalling circle that your foot makes all the way around the cadence so you knee is less bent at the top of the stroke and more bent at the bottom of the stroke.
I say this a a professional and qualified bike fitter and sports injury professional.

Could you explain the bit in bold in a bit more detail? Also, the guy has changed 2.5mm, not 25mm.

gazrichards

Yep, sorry, seems I made a mistake. I'll put it down to my one year old screaming the house down and trying her best to turn off CBeebies!
Seems I missed a decimal point off there but I think that's a forgivable mistake by all but the most pedantic of pedants!
As for knee angles during pedal stroke, again I was typing quickly but you get my gist I hope. I do this for a living and usually pay more attention to what I am doing (bike fits I mean, not writing on Internet forums!)

imposter2.0

gazrichards wrote:

As for knee angles during pedal stroke, again I was typing quickly but you get my gist I hope. I do this for a living and usually pay more attention to what I am doing (bike fits I mean, not writing on Internet forums!)

Sorry, no - I don't get the gist, which is why I asked for clarification. I'm struggling with the concept that the knee could be less bent at the top of the stroke and more bent at the bottom. It doesn't seem to make sense, but I'm sure there's an explanation.

maryka

Because if the crankarm is shorter but the distance to the pedal from the saddle is the same, your knee will not come up as high on the upstroke. He's wrong about is being more bent at the bottom though, as the extension from hip to foot is the same as long as the saddle/pedal distance is the same, regardless of crank length.

I've just done this myself, going from 170mm cranks to 155mm cranks on my TT bike which has opened up my hip angle somewhat at the top of my stroke and allowed me to raise my saddle a bit so my saddle-bar drop is more aggressive.

I'm doing the same with my daughter's BMX bike, as her knees currently come up to around ear height with her little legs and 140mm cranks... going to 120mm should help with that and allow her to turn the pedals a bit easier over the top of the stroke.

If your hip angle is more open and your knee does not bend as much at the top of the stroke, this is probably easier on the knees, especially in higher torque situations, which is what the OP is probably experiencing. I'm not sure 2.5mm is enough to make a difference but hmmv.

gazrichards

Ok then, as you are clearly the most pedantic of pedants! And my saying that I made a mistake didn't give you the gist......
On a shorter crank arm the leg will be straighter at the top of the stroke compared to a longer crank. Vice versa at the bottom of the stroke.

imposter2.0

maryka wrote:

Because if the crankarm is shorter but the distance to the pedal from the saddle is the same, your knee will not come up as high on the upstroke. He's wrong about is being more bent at the bottom though, as the extension from hip to foot is the same as long as the saddle/pedal distance is the same, regardless of crank length.

Agreed - that was my thoughts, too.

gazrichards wrote:

Ok then, as you are clearly the most pedantic of pedants! And my saying that I made a mistake didn't give you the gist......
On a shorter crank arm the leg will be straighter at the top of the stroke compared to a longer crank. Vice versa at the bottom of the stroke.

I was only asking you to explain the sentence in bold earlier. So far, all you have done is repeat the thing I asked you to explain..

ajmitchell

First no one can tell you for certain, for example it might be nothing to do with crank size but cycling style, watts, cleat position, natural improvement, coincidence. But assuming it was the cranks then many people find shorter cranks more comfortable because knee flexion > hip flexion > ankle flexion is less with shorter cranks at the top of the stroke. Providing seat height is correctly adjusted extension at bottom of the stroke should not change (unless you want it to change as per bike fit instructions). Shorting cranks will also increase RPM for a given speed but decrease rotational footspeed (at the pedal) by about 10%. Often a 2.5mm change is so subtle most don't notice in a blind test but in your case it doesn't matter it seemed to help and thats what matters.

smudgerii

As a noobie I'm strugglng to understand how you get less flexion at the top of the stroke, but maintain extension at the bottom of the stroke if seat height remains static. Any chance of breaking it down into noobie/idiot guide, or diagrams?

maryka

http://bikedynamics.co.uk/FitGuidecranks.htm

Knee Extension

The knee maximum and minimum angles are our first consideration. Trying to extend the knee maximum angle beyond a particular point will increase discomfort and lose power, so best avoided. The knee minimum angle is also important as the tighter this gets, the higher the shear forces in the knee joint and the less efficient the quads are in pushing the foot forwards over the top of the stroke. There is often a 'double whammy' as longer cranks mean the saddle needs to be lower to enable suitable leg extension. A lower saddle combined with longer cranks will quickly tighten the minimum knee angle.

And the diagram:


So now imagine much longer cranks in that image, the guy's knee will come up much higher (more acute knee angle) plus he needs to lower his saddle to compensate for the extra distance to the pedal on the downstroke so his hip angle will be more acute.

And much shorter cranks, his knee will not come up as high, and he gets to raise his saddle to make up the difference to the pedal, hip and knee angles now not as acute.

smudgerii

maryka wrote:

http://bikedynamics.co.uk/FitGuidecranks.htm

Knee Extension

The knee maximum and minimum angles are our first consideration. Trying to extend the knee maximum angle beyond a particular point will increase discomfort and lose power, so best avoided. The knee minimum angle is also important as the tighter this gets, the higher the shear forces in the knee joint and the less efficient the quads are in pushing the foot forwards over the top of the stroke. There is often a 'double whammy' as longer cranks mean the saddle needs to be lower to enable suitable leg extension. A lower saddle combined with longer cranks will quickly tighten the minimum knee angle.

And the diagram:


So now imagine much longer cranks in that image, the guy's knee will come up much higher (more acute knee angle) plus he needs to lower his saddle to compensate for the extra distance to the pedal on the downstroke so his hip angle will be more acute.

And much shorter cranks, his knee will not come up as high, and he gets to raise his saddle to make up the difference to the pedal, hip and knee angles now not as acute.

Thanks, very helpful. My wife is really strugglng with knee pain.