Range of Motion Limitations
People with range of motion limits often contact me looking for one
very short crankarm and one standard length arm. Usually at the
advice of their bike shop, doctor or physical therepist.
However, this advice is almost always wrong.
If RoM limits on flexion are the only reason for shorter cranks, and both legs are the same length, both arms should be the same length. If you can get over the top comfortably with a 120 while pedaling a 170 on the other side, you could also do so with two 145s and be able to use the strength you have in both legs. (120+170)/2 = 145 Of course this means the seat to bottom bracket distance will have to be increased.
We are looking for optimum extension on both legs. Because the more time spent with less bend in the knee the more power you will get. And, it's best to exercise the limited leg through the full range of motion it does have.
Here is a simple method that has been proven to give a good length for dozens of people with RoM limits
a stack of boards, books or whatever will allow you to vary the height of the stack.
(Starting the stack on a stair tread or sturdy box may simplify this.)
to stand flat footed, pelvis level side to side, with your bad leg on the stack and your good leg on the floor.
If you can do so comfortably, add a magazine, book or board. If not remove one.
Keep adjusting till you get a maximum comfortable height, without tilting your pelvis so that one hip socket is higher than the other.
height of the stack is the total length of both cranks.
leg lengths are equal and the range of motion limit is only on bending, then just divide by 2.
if the tallest stack you can stand on comfortably is 290mm you should be able to
pedal 145mm cranks etc.
leg lengths aren't equal and/or there are ROM limits on extension, contact me and we'll sort out what you need.
If your RoM limits are due to scar tissue, riding may gradually improve your RoM. One gent got the wrong treatment for a knee injury, leading to much inflammation and an infection which left him with a lot of scar tissue. He started out at 120mm, but pedaling gradually broke up the scar tissue. So every 6 months he ordered another set, 5mm longer. (He could afford to, but 10mm increments should be fine.) Eventually he was able to buy the normal 165mm cranks that bike shops carry.
Don't worry about losing power on the good side, due to shorter cranks. Unless one leg is just along for the ride, the increase in output on the limited leg will more than make up for any slight reduction on the other.
My fastest triathlete is 5'-7" won the 2011 Western Australia Ironman on 145s. The people at PowerCranks are convinced most average sized men should be on 145s.
A 24 hour MTB race in Australia was won on 125s. The shortest guy on the team was 5'-10". With the shorter cranks they rarely had to stand. conserving energy. And they were able to get by with a single chainring because the useful RPM range is so wide with shorties.
I have one customer, 6'-2" (188cm) tall, with range of motion issues, competing in long distance Brevets on 95mm cranks. Another gent with range of motion limits is climbing the hills of San Francisco with a single 38t chainring and a 12-25 cassette. Also on 95s. Because your legs are straighter, you push harder on the pedals, making up for lost leverage. The fellow in San Francisco bends pedal spindles.