Your body’s position on the bike affects how you ride. It affects how much power you can efficiently deliver to the pedals. It affects how comfortable you are on the bike. A position that is more comfortable may not allow you to put as much energy into moving the bike forward as a less comfortable position might. How do you decide where to position your body on the bike?
Ask yourself, “What do I want to do with my bike?”, “Why am I riding?” A track sprinter is not the least bit concerned with how comfortable he is sitting on the bike. During the race, (which may last for less than a minute), he may only be seated for 5 or 10 seconds.
A long distance tourist traveling coast to coast across the USA might spend 5 to 12 hours a day in the saddle, day after day. He is probably far more concerned with being comfortable and enjoying the scenery than with going as fast as he can.
Your body contacts the bicycle in three areas; your hands, your seat, and your feet. The relative positions of feet, seat and hands determine your comfort and efficiency on the bike. There are several variables that determine these positions; crank length, distance from crank center or bottom bracket to saddle, saddle angle, seat tube angle and saddle offset, distance from saddle to handlebar, relative height of saddle and handlebar, handlebar width, and handlebar drop on road style handlebars.
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. T
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.
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.
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 is 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.
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
