Bike Fit

This winter, I plan to get my road bike position dialed in a bit better. For the past year or so I've been browsing through the articles about bike fit on the Internet (there are lots). I haven't been able to find a comprehensive article on cycling biomechanics, but I've been able to piece together some of the basic principles.

Here's the short list:

• You can injure yourself with bad bike fit. If the saddle is too high, you can get ITB syndrome. If it's low, you can irritate your knees. Such injuries can take months to heal.
• Pedaling is a limited range of motion exercise, so if your position is bad, you can actually be cheating yourself of several Watts.
• The efficiency of mechanical coupling between the legs and the crank varies a lot around the pedal stroke.
• The quads produce maximum force through a limited range of knee angles. (One source says 114-157 degrees).
• The glutes produce maximum force through a limited range of hip angles (One source says 130-160 degrees)
• Seat farther forward=quads do more work. farther back = glutes do more.

A sensible bike-fit strategy would be to find a position that allows the knee and hip to move through the maximum force angles with maximum mechanical efficiency. (It's important to keep in mind that even if there is some truly optimum position, it might be desirable to deviate from it for several reasons--comfort, bike handling characteristics, aerodynamics, etc....)

I worked through the basic geometry last year but forgot all about it until this week. Here's some of my initial findings.

When the seat is about the right height, there is about 3 inches of fore-aft saddle adjustment where the angles stay good.

The butter zone is smaller for the saddle height adjustment, too high and you lose force (plus get injured). Just a little too low and you lose 10-20% of the total force. There's only about 1 inch where the angles work best. Based on my hands-on experience, if you go with a saddle position that's just a bit too high, it's harder to spin. The bottom of the stroke feels "dead".

For me, the good old "knee-over-pedal-spindle" appears to be the optimum position, but that also requires me to move my saddle forward so my hip joint is 2.5 inches forward of the seat tube of my Cervelo.

If I track down some good measurements of force versus knee and hip angle, I should be able to come up with an actually useful online tool for seeing the effect of changing saddle height and fore-aft position.