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Gidney N Cloyd

(19,694 posts)
Fri Jul 10, 2015, 12:47 PM Jul 2015

Anyone familiar with shorter crank arms to help with limited knee range of movement?

I'd really like to get back into biking but since my last knee operation I'm left with a knee flexion of maybe 90 degrees on a real good day. I'd rather retrofit my Trek 7300 if such parts exist but when I asked at my local bike shop a couple years ago they looked at me like I was from Mars (which I ought to be used to by now, wherever I go...).

I've stumbled across a couple of more technical articles on the web about benefits of different lengths, which tell me that such a mod must be possible, but little information along the lines of "I made this change for my bad knee, it helps a lot, and this is where I got the parts."
So if anyone here has any advice I'd love to hear it.


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Anyone familiar with shorter crank arms to help with limited knee range of movement? (Original Post) Gidney N Cloyd Jul 2015 OP
You can probably find a triple-ring crank on ebay at 165mm.. frylock Jul 2015 #1
I think you're on to something. TexasProgresive Jul 2015 #2
Thanks! I'll check out the links Gidney N Cloyd Jul 2015 #4
Found something that could be worth trying, at least for the short term: Gidney N Cloyd Jul 2015 #3
Sounds like it ought to work. TexasProgresive Jul 2015 #5


(34,825 posts)
1. You can probably find a triple-ring crank on ebay at 165mm..
Fri Jul 10, 2015, 10:41 PM
Jul 2015

I'm assuming that you're currently running a standard 175mm crank arm length. I don't know what the guys at your LBS told you, but worst case scenario you have to install a new bottom bracket, and that may be included with the crankset you decide to purchase. Good luck finding anything shorter that isn't a single-ring setup.

I once ran a 170mm double on a 7" AM/DH bike. Not for knee issues (which I have), but because I was experiencing excessive pedal strike. I honestly couldn't tell the difference between them and the 175mm cranks on my other trail bike.


(12,134 posts)
2. I think you're on to something.
Sat Jul 11, 2015, 11:44 AM
Jul 2015

This is an interesting article on crank length.
The crank length-seat height determines the knee and hip angles when the cranks are at the bottom and top of the stroke. The crank length-handlebar distance determines how close the knee comes to the chest, especially in the aero position.

This is a good discussion from some who are experiencing your knee problem. Read through and hopefully it will help. I agree that you need to be sure your saddle height is correct and then try the shorter arms. Your local bike shop guys are probably young and not knowledgeable how to help the older cyclist. Reading these two pieces has given me cause to rethink crank length. I have not had knee surgery but have issues especially in my left knee. Since I started cycling again 40 - 50 days ago I am experiencing little or no pain. We will see what November temps will have to say about that.

Here's another forum.

Here's a 165mm crank.

And a 160mm

Gidney N Cloyd

(19,694 posts)
3. Found something that could be worth trying, at least for the short term:
Sat Jul 11, 2015, 01:03 PM
Jul 2015

Crank arm shorteners that you mount to your existing arms. Looks like some folks with knee issues are using them successfully. Someone even mentioned using them with a Trek hybrid like mine. Need to read up a little further...


(12,134 posts)
5. Sounds like it ought to work.
Sat Jul 11, 2015, 01:46 PM
Jul 2015

This way you could start at the shortest length and see how your knees feel and work up to get more torque. Looking at them they might kick the arms out a bit wider which would be a plus for me. I think that I am touching the left chain stay with my heel at times (I have to be slightly toe out to prevent knee pain). I worry that I will do damage to it and the shoe. My old left shoe was pretty scuffed up.

One plus I can see to shorter cranks is that you will be encouraged to spin at a higher cadence. Personally I find spinning 70 to 85 RPM really helps my knees.

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