Getting back on my bike after knee replacement.

[Psychler]

I had my left knee replaced in July and have been working on the range of movement in it since.  The knee bends to about 120-130 degrees.

I tried to get back to cycling recently but I'm finding that it's too painful to get my knee "over the top", ie when the left hand cranl/pedal is pointing upwards.  My bike has 175mm cranks on it so I'm considering putting a shorter crank on the left side.

Is there any reason why I can't have 150mm crank on the left with a 175mm crank on the right?

[LittleWheelsandBig]

You won't get full leg extension on the short crank side. It sounds like you need a hinged crank, available from handcyclist OTP.

https://highpath.co.uk/pulse-swing-cranks/

[sg37409]

After my accident where I was left with restricted knee mobility I used a 160 and 170 because I couldn't get it  "over the top"

Even 160 was putting a little strain to bend my knee more, which was a win to me, and I evolved to a 165, 170 and am now on a pair of 175

I didn't ride a lot on mismatched cranks by my current standard but was out on it a fair bit. (To physio mostly). Leg extension wasn't an issue for me as Id extend it when walking about. Id say go for it.

[Kim]

Is there any reason why I can't have 150mm crank on the left with a 175mm crank on the right?

Assuming that your legs are equal lengths, and it's not a recumbent, you'll likely end up with your pelvis at a slight angle.  May not be problematic in practice.  Suck it and see.

Short *left* cranks may be obtained cheaply from suppliers of unicycle parts.

[sg37409]

Short *left* cranks may be obtained cheaply from suppliers of unicycle parts.

Y, mine was left hand side.  Unclear why you think equal leg length can lead to tilted pelvis. The pressure on pelvis when riding for me came at tdc due to insufficient knee bend. Leg length wasn't a factor in this. 
My pelvis has tilted a fair bit to compensate for losing around 28mm of LH femur. I am not troubled by having spent the last 30 years like that. (touches wood)

[Kim]

Short *left* cranks may be obtained cheaply from suppliers of unicycle parts.

Unclear why you think equal leg length can lead to tilted pelvis.

I think unequal crank length will lead to either unequal leg extension or tilted pelvis.  If you're used to pedalling at a given amount of extension and change the crank length on one side, something will have to change to compensate.  A pendulum crank would avoid that problem, if it's a problem at all.

(I discovered that I'd been riding with a tilted pelvis due to unequal leg length.  It was a fairly minor difference and completely unproblematic, until I started riding a recumbent, where the seat enforces a straight pelvis, and I started to get underextension-related knee issues on the longer leg.)

[chrisbainbridge]

I would go for it. Equal cranks with unequal legs (Kim) is a different ball game completely and will lead to problems as the leg will generally overextend.  In addition anybody with a leg length inequality is used to walking with a tilted pelvis.

here we are simply changing one crank length for another so that the knee does not fully extend or fully flex.  No problem I can think of.

[Psychler]

Thanks everyone.

I'm going to go for it.  A short crank can be got for a tenner so it will get me back on the bike.

I can get my knee "over the top" on 175mm's but it's too painful to ride more than about 100 yards.  Hopefully, I'll be able to gain more mobility, perhaps via increasing to 160's, 170's etc, over the course of time as there is still some swelling in the knee.  I think a long term solution would be to change the right hand side to match if that is neccessary.  I won't ride long term with unequal lengths.

The whole point may become moot in a year or so as I also need the right knee done.

Thanks again.

[Kim]

here we are simply changing one crank length for another so that the knee does not fully extend or fully flex.  No problem I can think of.

The not fully extending bit.  It's like having your saddle too low.  Might be fine short-term.

[Canardly]

Take it easy. I cycle with someone that had his knee replaced two or three years ago. He is like a spring lamb now.

[Psychler]

I went out on my bike today with the 150mm left crank.  I only went about half a mile but everything felt ok.

I'm icing my knee at the moment just in case and will start building up the distances slowly from now.

[mrcharly-YHT]

wb

DON'T over do it. But you know that.

[Nelson Longflap]

Why not just run with equal length, short, cranks on both sides? It avoids the pelvic rotation problem. Your 150-175 pairing makes one foot follow a 50mm larger circle, travelling a circular distance some 157mm more than the other one; so you get different foot speeds to add to the asymmetry. I reckon you will feel the difference after a few hours riding.

(A bit OT, but unequal length cranks can be very beneficial if one leg is shorter than the other.)

[Psychler]

Why not just run with equal length, short, cranks on both sides? It avoids the pelvic rotation problem. Your 150-175 pairing makes one foot follow a 50mm larger circle, travelling a circular distance some 157mm more than the other one; so you get different foot speeds to add to the asymmetry. I reckon you will feel the difference after a few hours riding.

(A bit OT, but unequal length cranks can be very beneficial if one leg is shorter than the other.)

Only because I want to get back to riding with 175's on both sides, if possible. 

I will increase the size of the left hand crank gradually until I reach 175 but if I never get there [say I'm stuck on 165's] I'll then adjust the RHS to match.