Thigh Length and Seat Angles
Mon, June 28, 2021 
I am often asked if a rider with a long thigh measurement needs a shallower seat angle. The above line drawing represents two riders with the same inside leg measurement. As a result, they have their saddle set at the same height, and they are using the same size frame.
If the inside leg measurement is the same and one has a longer thigh measurement from his hip to his knee, it follows his lower leg from his knee to his heel is shorter. Conversely, short thigh, longer lower leg. As you can see from the sketch the position of the knee varies only slightly.
The exact knee over pedal (KOP) is not a precise measurement anyway as one is pedaling in circles, and it makes little difference if the rider sits back and pushes forward or is more over the pedals pushing straight down.
For the purpose of this drawing, I made the thigh length of the blue leg the same as the lower leg of the one represented by the red line, and vise-versa. The above line drawing could represent an unlimited number of riders with the same inside leg, with the red and blue lines being the extremes. If the inside leg measurement is a constant, it follows that as the one measurement increases the other decreases.
This simple drawing does not consider the length of foot. {Shoe size.) As the toe points down at the bottom of the pedal stroke, the foot becomes an extension of the leg. This of course affects saddle height and frame size but does not change the position of the knee by any large amount.
So, the answer is no. Once you have the correct size frame the seat angle should be right for that size frame. Then if your saddle is positioned at the correct height, the length of your thigh has little bearing on anything.
Also, as a footnote, some people agonize over the fore and aft position of their saddle. If you view that too considering what I have said here, it is less important. More important is the position of the handlebars in relation to the saddle. See the links below for more info on that.
You can read more about my frame design philosophy here:
And another article on Riding Position Simplified:
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Reader Comments (5)
Thanks Dave. Have you thought of compiling a comprehensive 'Fit Guide'? Your common sense and experience sure makes the topic easier to understand.
Steve
I had the chance once to ride for a while on a test rig where we could follow position (hip, knee, ankle, foot) and power while I pedaled. I couldn't feel minor changes (+/-1 cm) but in some cases it changed how smooth my pedaling was. I went away with some upper and lower limits for positions that I then would try out over the course of a few rides each.
1. It helped me to stop obsessing about position
2. It informed me as to what changes were worth paying attention to
This really helped me set up 4 different bikes so that my position was similar enough on each to be comfortable when I switched bikes.
Excellent as always Dave. Long legs and long feet for my height I've always liked the saddle back. That's more of a function of riding style.Tempo ridinf on the road and high speed racing on the track when the tendency is to slid forward while spinning. I learned early on to keep back to weight the real wheel which would bounce otherwise. My wife with her shorter legs and long thighs runs her saddle higher and forward both to bring the knee back and balance her weight on her 50cm bicycle with it's as expected steep seat angle.
Nice post Dave. I'm Red & Blue! I've got a 1" difference in leg length (left thigh longer) and always set my height to my shorter leg length, but pay more attention to my reach (my arms are nearly the same length).
As we have prone recumbents and crank forward riding position, I think the angles is actually important, but it relates to all the bike fot, as your seat/saddle angle, handlebar, etc must change too