Experience, and communication with hundreds of customers, has convinced me that no formula will work for finding the optimum length for a certain individual. The range of motion over which your muscles work efficiently is more of a determining factor than femur length. If you can squat down till your butt touches your achilles tendons and smoothly rise without a great deal of effort, you won't have as much to gain from shorties as someone who can't.
I've observed that everyone has a critical crank length below which their muscles or joints will work well throughout the pedal stroke. If your cranks are a little shorter than your critical length, it doesn't seem to make a lot of difference whether you are 1mm below it or 10-20mm below it.
Not all sizes are available, but there seems to be a lot of leeway. 148 or 153 may not be optimum for you. But for many recumbent riders they will feel so much better than 170 or 175 that whether they are exactly your optimum length becomes a moot point.
Taller riders have become accustomed to cranks designed for average sized riders. So, they are used to a slower rate of angular movement at their joints. They might do best with cranks somewhat shorter than any formula based on leg length would suggest.
Shorter riders also have become accustomed to cranks designed for average sized riders. So, they are used to a faster rate of angular movement at their joints. They might do best with cranks somewhat longer than any formula based on leg length would suggest.
What size chainrings should I get?
If you are getting spun out in high gear now, using the same gears with the shorter arms may fix this.
For most riders, dropping 4 or 6 teeth on each ring seems to work well.
If you currently use low gear far more often than high gear, you will want to use even smaller chainrings and/or a bigger cassette.
Most people with 26" drive wheels are going from 30-42-52 to 26-36-46 or 24-34-44. Faster riders are using 24-39-48. A few have gone to 22-34-42.
When I shorten Shimano 105s & Ultegras, about 1/3 of the owners have me install 24-39-48 FSA ramped and pinned rings to replace the stock 30-42-52 setup. Cost = $84
It's not unanimous, but a large majority of riders with 20" drive wheels and shorties do well with standard "Road Triple" gearing of 30-42-52.
Will I have to move my seat (or BB)?
Yes. You will want to maintain the same leg extension at the "bottom" of your pedal stroke. So, if you are going from 170 to 153mm move your seat back, or your BB forward, 17mm (or about 11/16")
After going from 170mm to 148mm, Martha P. of New Mexico wrote;
"There was no adjustment period, not even for one second."
At bike events, I like to let other 'benters try out my bikes. I never tell them beforehand about the shorties. Dozens of times I've seen them return commenting about the handling, seat, or acceleration. But they never even notice the cranks.
When I ask them how they liked the short cranks, the answer is always something like;
"This thing's got short cranks? No [excrement], they felt perfectly normal."
Will I see a performance gain? Or loss?
Many of my customers are claiming significant performance gains, especially on hills. They help you power up short hills, and acceleration is much better, because you apply more pressure to the pedals, for a given muscle effort, and tend to be in lower gears. But I don't think they would make as much difference in situations where the limiting factor is simple aerobic capacity, such as on long gradients or flat time trials.
You might find this VeloNews article interesting. All the riders, from 5'1" to well over 6 feet performed best on 150mm arms. What makes this really amazing is that all testing was done at 90 rpm, while shorties raise your natural spin rate.
Will these fix my knee pain?
Almost everyone says they help. With short cranks, you apply more force to the pedal, with less muscle tension. And you work through a smaller range of angular motion.
So, it seems logical that;
If the problem is with the muscles or connective tissue they should help.
But;
If your problem is with the main load bearing "bearings" between the femur and the tibia, the Articular Cartilage and the Meniscus, then you could make things worse. Please consult your Physician if you may have this sort of problem
Who should not go to short cranks?
These things are addictive. If you put them on one recumbent, you will probably hate the 170/175s on your other 'bents. Could be a problem if you have 6 recumbents and a tight budget
Because you put more force on the pedals, for a given amount of muscular effort, in addition to the cartilage and meniscus problems mentioned above they could aggravate some types of foot problems.
Can I Use My Current BB Spindle?
Maybe. If your spindle is the traditional square taper, and the length will give a chainline that is compatable with your front derailleur you should be fine. If your spindle is about 5/8" in diameter it will be a square taper about 99.9% of the time.
