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Crank Length
Crank length determines the diameter of the circle that the pedals move in. The larger that circle is, the more flexion of your knee and thigh muscles will be needed to turn the cranks. Your thigh muscles cannot exert the same force throughout their range of motion. This is very easy to demonstrate. If you squat down so that your knees are fully bent and lift yourself up, say, five inches, it takes a good deal more effort than it would to squat down just five inches from standing straight and then lift yourself back up. At the full squat position, your muscles can't put out the same power as when your knees are just bent enough to drop you down five inches. So if you had to choose between a crank length that had your knees bending through their entire range of motion and a length that only required, say, 20 degrees of flexion at the knee, you would choose the shorter crank. That crank would have your muscles working through a more efficient range of motion. You would avoid having to flex your knees enough to bring you into an inefficient range of motion.
So how long should the cranks be? Well, that's a good question. I wish I had a good answer but I don't. It should be obvious that a 5' 2" rider would not want to use the same length crank arms as a 6' 7" rider unless they somehow managed to have the same leg length (highly unlikely). Some research has been done to determine the optimum percentage of leg length to crank length. I doubt that there is an optimum percentage that would apply to all people. One writer in a major magazine article quite a few years ago claimed that after considerable testing with many different riders, 18.5% of the distance from the top of the femur to the floor in bare feet should be the crank length. You can find the top of the femur pretty easily. It's 5" to 6" below your hip bone, and moves rearward when you raise your knee. After reading this I promptly changed from the industry standard 170mm cranks for road bikes to 175mm cranks. There was an immediate improvement in power and endurance. I began using this formula when recommending cranks to my customers. So far, I haven't gotten any complaints. But of course that doesn't mean my customers wouldn't be as happy or happier with some other length. And I must admit that I have never tried still longer cranks than 175mm for enough time to tell if I would be even happier with them.
The top of the femur measurement ignores differences in legs themselves. Differences in the proportion of calf length to thigh length should affect the optimum crank length. A rider with longer thighs and shorter calves would use a longer crank to get the same flexion at the knee as a rider with short thigh and long calf. Of two riders with the same body proportions, one might prefer to pedal at a faster cadence. That might favor a shorter crank length. And perhaps even two riders with identical skeletal proportions would find after testing that they required different crank lengths to achieve maximum performance due simply to differences in their muscles.
Trying different cranks to find the optimum length would be time consuming and expensive, but I believe it is the only way to determine the correct length for any individual, assuming there is a correct length. It would be nice to have a crank with many pedal threads at various lengths to test. But I know of no such thing being made and I lack the ability to make one! Of course, some riders with multiple bikes report being just as happy on one crank length as another. Go figure! So, for lack of a better system, I'm staying with the 18.5% guide for my customers until something better comes along. It hasn't failed yet.
無聊google了一下有關於曲柄長度的測量：
不知道我自認的大腿骨位置正不正確，剛和MARK互相量了一下。
身高175的Mark，大腿骨至地板約90公分，得用166.5mm的曲柄，比一般認知來的短。
我則要用到146mm的尺寸，那有這種長度的曲柄啊！！
