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Crank arms
Hello I've just registered and would like some help.
I have had a knee replacement and now only have 90 degrees of bend
I have a shorter crank and have raised the seat as high as I can but still
find that I need to pedal with my heel to be able to achieve full rotation.
Now I have heard of something like an articulated crank which hinges in the middle
and when in use full power is obtained when the pedal is between 2 o'clock and 5
if that makes sense. I wonder if anyone else has any ideas about this problem

There have been lots of articulation schemes proposed, and some even built over the past 150+ years of bicycle manufacture.  I will try to add some information in that vein later.

When you say "shorter"; what length are the cranks you have now?  

I am surprised that your surgeon and physical therapist do not want you to work on increasing the flexibility of your new knee(s).  Please discuss the issue you are having with them BEFORE attempting further bicycling.
Thanks Nigel for your prompt reply. The crank is 150 mm long.
My knee replacement is now 10 years old and am assured that what I have is the limit
of its bend. my other is of an improved version and is no problem.
I look forward to reading more about this

Hi Ian;

I just re-read your post, and have a couple of questions:
  • When you say you raised the saddle as high as you can, is that a limit of the bicycle/seat post, or something else?
  • What is the distance from the top of the saddle to the crank center, measured along the seat post?
  • What is your inseam?  When you answer, please specify how you measured it.  I usually work with trouser inseam, but can convert/adjust to other measurements.
My trouser inseam is 30", and my saddle to crank center on my WT commuter is also 30" with a 175mm crank.  I will be changing back to a 170mm crank soon.  The center of the crank to top of seat tube is 22½". 

The reason I ask this, is that I am quite sure my knee is not bending anywhere near 90° when I cycle, at the top of the stroke, the top of my leg is still pointed downwards, no where near horizontal.  

You may need a taller bike, either by longer seat post, or new frame.  If you are on the heavier side (I am >> 300lbs) avoid the taller seat post.
Hi Ian;

I would like to refer you to "Bicycles & Tricycles; A Classic Treatise on Their Design and Construction" by Archibald Sharp, originally published in 1896, and redone by Dover Publications in 2003.  


Chapter 28 starting on page 472 has the information you looking for from a design reference standpoint.  There is still the challenge of manufacture.
Hello again Nigel.
I have answers for you. My Inseam is 29" (We call it inside leg in the uk), the seat is 40" from the floor
and the measurement from the seat to the crank centre is 29" and I am 230 lbs light!
I hope that gives you the info you ask for. I would need to save up for the book that you recommend.


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