Road Bike Setup for knee <90 Degree Flexion ( Rehab )

[mmcandrew]

Is there a way to set up a road bike where my knee never passes 90 degree flexion?

I've been playing with saddle height and haven't been able to get it right yet.

I'll be using a turbo trainer for light rehab post surgery but during which time I am restricted in the angles at which I can bend my leg.

Any ideas?

Thanks,

(PS: I'm aware this isn't an optimal riding position, again it is purely for extremely light rehab.)

[Norman H]

What length cranks are you using?

Assuming that your saddle is at the correct height you may need to fit shorter cranks. You can buy crank shorteners or attempt some DIY solution.

[NUKe]

Is the 90 degree a permanent limitation? I had this after an accident 20 years ago, and with exercise went to 110 fairly quickly. If its a permanent try 155mm cranks, they will feel odd at first.

[LollyKat]

Spa Cycles do "Kid cranks" by Stronglight that go down to 130mm. Your good leg could continue with its normal crank - it would feel a bit odd but on a turbo trainer you'd be safe enough. An adjustable crank would be even better - I bet someone like Brucey could bodge one - could you take an ordinary old crank and tap some holes in it to take a pedal?

Highpath do crank shorteners and swing cranks but they are not cheap.

[mmcandrew]

What length cranks are you using?

Assuming that your saddle is at the correct height you may need to fit shorter cranks. You can buy crank shorteners or attempt some DIY solution.

175mm. Saddle has been fine so shorter cranks may be the way to go!

Is the 90 degree a permanent limitation? I had this after an accident 20 years ago, and with exercise went to 110 fairly quickly. If its a permanent try 155mm cranks, they will feel odd at first.

Not permanent no, I'll be 6 weeks in a leg brace with movement restricted to 90 degrees while a meniscus repair heals up.

Spa Cycles do "Kid cranks" by Stronglight that go down to 130mm. Your good leg could continue with its normal crank - it would feel a bit odd but on a turbo trainer you'd be safe enough. An adjustable crank would be even better - I bet someone like Brucey could bodge one - could you take an ordinary old crank and tap some holes in it to take a pedal?

Highpath do crank shorteners and swing cranks but they are not cheap.

Shortest cranks possible will probably be the best idea, I'm quite new to cycling so all this advice is very much appreciated folks!

Is it a difficult job to do yourself or should I just bring the bike to a shop?

Thanks again!

[Norman H]

The Stronglight Kid Cranks linked to above would be the ideal solution. They're made for 110mm BCD chainrings so if your existing rings don't match you will need to purchase new chainrings as well.

The DIY fix that I mentioned involves fixing some sort of wooden pedal riser block to your existing pedals. It might be worth experimenting with this approach to get some idea of what length of cranks to buy. You can buy riser blocks that shorten the cranks by about 15mm.

Unless you're confident of carrying out the work I would talk to your local bike shop.

Edited to add:-

Forgive me if this is stating the obvious but you will need to raise your saddle to gain the benefit of shorter cranks.

[LollyKat]

If the OP buys some Kid Cranks he will probably need a new bottom bracket to go with them - Spa will recommend the size and they are not very expensive.

The trouble with adding blocks to the pedals is that they will increase the flexion of the knee. At the bottom of the stroke, the crank seems shorter as the foot is now closer to the bottom bracket axle. However at the top of the stroke, because the pedal has rotated and the block is still on top, the foot is now further from the BB axle, i.e. closer to the body, and the knee has to bend more.

The Highpath crank shorteners don't have this problem. Tandem owners with young children often use them so OP could probably sell them on when he has recovered fully.