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My father, now in his 70's, had a left knee replacement several years ago. He has limited movement in that knee - probably about 85 degrees on a good day. His knee can extend fully, but cannot bend as much as it should. As a result, he cannot pedal a bike he cannot get his left leg over the top of the pedal stroke.

He is desperate to get back onto a bike, he knows he won't be ripping up the road and winning races like the old days, but wants the freedom to go for a slow easy rides.

Does anyone have suggestions as to a solution to the problem. We don't mind spending money if we know the solution will work, but he can't afford to throw big money at wild ideas that may or may not work. If we find a solution, a new bike is likely.

We have access several mountain bikes in a range of sizes to trail ideas, and we have all the tools and skills to build bikes up from parts and rearrange things as needed.

EDIT: Thanks everyone - all great suggestions. What am am going to do is got tto the LBS and get hold of a cheap set of short cranks for around $30- probably from a kids bike, in the 120-150 size range, and see if he can ride that. Depending how he goes, I'll try a standard right crank and a short left crank. If all else fails, we will go for the Power Cranks

UPDATE: In case anyone else has similar problems. Got some 152mm cranks for $30. Set an old mountain bike up with the seat as far back as possible. The old man is able to ride reasonably comfortably, now has a smile from ear to ear - any suggestions how to remove it - doesn't fit with the grumpy old man image :)

Has got me wondering how many people with minor knee problems would benefit from short(er) cranks.......

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    You wondered how many people have this problem; I haven't had a knee replaced, but I have the same problem- I can't bend my knee enough at the top of the stroke to ride. I bet there are many out there w/ the same problem.
    – user7321
    Jun 26, 2013 at 23:51
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    Really glad to hear this worked out.
    – James Bradbury
    Jun 27, 2013 at 7:31
  • Matt, I think our present standard is to add your own solution as an answer and accept that. Would you do that to that? It really is an interesting question!
    – gschenk
    Oct 30, 2019 at 9:43

7 Answers 7

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Look at Power Cranks. They are a crank set with the ability to freewheel independently of each other.

It allows a slow build to fully articulated movement if additional rehab is still a goal, and using each crank independently means using a bike is still possible, even if doing so uses a pedaling motion which would be awkward for someone who had an option for full movement.

They also have adjustable crank lengths on each independent arm, which would allow you to use a shorter crank on the leg with the limited movement.

They are often used as a rehab tool, are a real, readily available, and not ridiculously priced product, and are install-able with normal bicycle tools.

One caveat: They are shifting to a new poly carbonate fixing cap. If you order, I recommend requesting the old alloy fixing caps. The last set of poly caps I installed broke easily.

FTR, I have installed and used this product, and have no relationship with the manufacturer, nor will I be compensated in any way for this post.

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  • That's a brilliant suggestion. The reduced length should really help with reducing the angle required for knee bend.
    – Mac
    Apr 20, 2012 at 6:49
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    Their website is hideous though!
    – Mac
    Apr 20, 2012 at 6:49
  • Thanks, the price may be a bit high, but otherwise looks like exactly what I am after.
    – mattnz
    Apr 20, 2012 at 7:07
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    @Mac: They are a brilliant product. The design is simple, mechanically, but ridiculously effective for rehab or training. They could definitely do with an image and marketing consultant though. :)
    – zenbike
    Apr 20, 2012 at 7:20
  • The link appears to be broken, maybe the product does not exist any more?
    – Snijderfrey
    Sep 4, 2022 at 19:01
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I'm 70 & recently had a right total knee replacement & being a keen cyclist all my life I was very worried I may never cycle again , so my local bike shop drilled a new hole for the pedal 25 mm further up the crank ,& it's perfect . I've not bothered to cut off the excess , just in case I ever get full movement ! I suppose it would've been easier if it had been the left side crank !

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    You can buy ridiculously short cranks but few shops stock them because demand is limited, but most shops should be able to order them in. Companies like Greenspeed will drill and tap longer cranks as you've suggested, but it costs as much as a cheap set of short cranks.
    – Móż
    Jan 1, 2016 at 1:18
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I have also seen pictures, many years ago in an old bike book, of a crank with a pivoting piece. Basically, imagine the crank cut off at about the 2/3rds point, and then the cut-off piece reattached with a pivot. The crank would go down to the bottom of the stroke, but only come up a little over midway. (Kinda like a car on a Ferris wheel.)

Unfortunately, I have no idea if this is still made, or where you might find it.

Update: Apparently if you Google "pendulum swing crank" you will find examples.

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    highpath.co.uk/wp-content/uploads/2015/04/swinganim01.gif explains it - the stub crank arm moves like normal and the outer bit is free to rotate. So the pedal follows a shorter circular path, and is tangential with the normal pedal's path only at the bottom of the stroke. Would not work pulling up with clipless pedals, but fine on the downstroke or with flats.
    – Criggie ♦
    Sep 3, 2018 at 1:54
  • Yes, still made by highpath engineering UK, called the Pulse crank.
    – Noise
    Sep 2, 2022 at 7:10
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Is this any use? Very similar solution to the power cranks (I think) http://www.sjscycles.co.uk/thorn-crank-shorteners-prod239/

The link leads to Thorn crank shorteners. A flat bar with two long homes where the bar is attached in parallel to the crank. It bolts to the bottom bracket spindle and pedal thread. There are threaded holes to attach the pedal at different positions on the bar. The idea is to such the pedal at an adjustable crack length without replacing the cranks.

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  • Here's a similar product.
    – R. Chung
    Apr 20, 2012 at 15:47
  • Both of these product are effective for deciding on crank length if you want to try multiple length variations, but I wouldn't really trust them for a long term use installation.
    – zenbike
    Apr 21, 2012 at 9:20
  • You could probably get someone with the right tools to cut down a standard crank to the proper length, or just leave it at length and drill a new hole.
    – Kibbee
    Jun 27, 2013 at 1:04
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I bought a Pedego power assist tricycle. I have 80 degrees range of motion in a leg with artificial femur. Pedals are out in front more.

$2800.00 but changed my life!

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I just started riding again because I found a solution to limited range of motion due to total knee replacements in both knees.

Check out Unicam ltd. "Retroflex". It is an EU solution that is remarkable. I found a similar solution on a Duke University website but it was never sold commercially. I purchased a Specialized Low-Entry Roll Sport bicycle and added the pendulum crank solution from Europe and I am able to ride without pain .. check it out.

Description of webpage: Unicam Retroflex mounts a second crank to the crank arm. In a way that roughly resembles a double pendulum. There is an Element at the joint of both crank arms that may be a ratcheting mechanism or simply a bearing.

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  • +1: They do say they ship world wide.
    – mattnz
    Oct 28, 2019 at 19:05
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    Arnold, would you be so kind to add a description of how the Unicam Retroflex works? I didn't find anything on the page. In time the pager might disappear, but we would like to still retain the information.
    – gschenk
    Oct 30, 2019 at 9:52
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Try a pendulum swing crank by Hase. Absolutely brilliant.

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    Gidday and welcome to the site. This one-liner answer is a good start, but is lacking in detail. Please use EDIT link to add more information, like how it works for you if you've used one, what the downsides and positives are, and what you'd do differently now in hindsight. Why is it "absolutely brilliant"? For more info, do please browse the tour cos SE is a bit different to most other chatty web forums. Looking forward to your future answers.
    – Criggie ♦
    Sep 3, 2018 at 0:06
  • VTL because it does attempt to answer the question.
    – Criggie ♦
    Sep 3, 2018 at 8:30

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