Different Sized Crank Arms
 

I have potentially a slightly strange query.

My wife and I were in a motorcycle accident last year and she has fractured her tibial plateau leaving her unable to bend her right knee beyond 90 degrees, making it very difficult to cycle. She has a trek 7.2 (ladies style, bought in the UK). I have been trying to think of ways that she could continue cycling and the idea i have had is to have a shorter crank arm on the right side.

My question is if it is likely that I could fit a child's crank arm on to the existing bottom bracket spindle. I realise that being the right side it will limit options of chain rings on the front, but wondered if anybody had any advice.

The other possibility i considered would be to get a normal sized steel crank arm cut down and re-welded. I'm just not sure if this would be sufficiently strong.

Any thoughts are gratefully received or if anyone has another different idea.

Thanks,
Jez.

How much shorter is shorter?

Sugino makes arms in 165mm, TA makes arms down to 155mm.

There is a guy online that makes custom length crankarms - the name escapes me at the moment. We've also done this for a customer who also had limited mobility in her right leg. It was a very, very small crank. You may want to give the shop a call, it's Urbane cyclist (note I don't work there).

Or put a shim under the cleat. Buddy of mine has a leg length discrepancy and that's what he does with perfectly fine results. Reckon it all depends on how much of a difference there is going to be.

I believe it's Lennard Zinn who is also the Tech Guru for VeloNews. Try www.zinncycles.com

I don't think this will help her. Shims do compensate for leg length discrepency but won't compensate for the inability to bend one knee as much as the other. You have to reduce the pedal stroke at both ends of it's rotation.

See this guy for shortened triple cranks, http://bikesmithdesign.com/Short_Cranks/index.html

I would be just getting a shortened set of cranks the same length for both sides and not worrying about different lengths for different sides.

Anthony

http://www.longstaffcycles.com/index...ducts_id=21095 may be a better option.

Go to the tandems ltd. website they have crankarm shortners that bolt onto the crank arms, they are for children riding on the back of tandems.

SJS cycles has a good selection of short cranks. Here's a set made down to 140mm:

http://www.sjscycles.co.uk/product-T...ilver-1672.htm

and one shortened to 125mm:

http://www.sjscycles.co.uk/product-S...Silver-249.htm

A Surly employee who broke his leg used a crank shortener from Tandems East. Picture (and link) here: http://www.surlybikes.com/2007/07/si...d-over-to.html

I changed the cranks on my wife's bike from 170 to 152, for arthritic knees. The short cranks were made for a child's bike but fitted the standard taper crank, so the full length crank would fit on the left. I had to buy a new BB because the offset of the chainwheels was less.

Right. Everyone seems to be focusing on arm length.

Seems to me, though, that in a proper cycling position the knee should never bend as far as 90°. Are you sure that her seat/saddle is high enough?

Saddle height is a factor. That and the fact that the cranks are possibly too long anyway even without any knee issues so a pair of shorter cranks is likely in order.

Anthony

Perhaps Mark Stonich at BikeSmith Design?

No, at the top most well fitted positions will be around 100* or slightly over. I have similar issues from a shattered patella (knee cap) where right knee only goes to 95* or so. I can barely get that leg over the top of the pedal stroke only by pushing it through with the left leg and having a slight kink. My seat is too high, but that's what I have to do to ride. It's really ridiculous on a mtb ... I'm teetering way high. I'm 6'3" and dropped down to 170 cranks and that did help me lower the seat a little. Other than spinning faster I didn't have any issues with the shorter arms at all.

You could just tap a 9/16 hole anywhere along the arm.

Uh, no?

It depends on the design of the crank. I use custom shortened cranks where this is basically what we have done and then finnish off the end of the crank nicely but you have to be careful because between hollow crank arms or scalloped crank arms or even crank arms that are too narrow you cant just do this to any old crank.

Anthony

The hinged crank sounds interesting but this link doesn't show much or give much of a description. Any better links?
Does the crank hing while in motion or simply bend to make an adjustable lock length in place crank?