Bike fitter for about 12 years, about 3000 fits. I'm decent at it. I've always tried to learn from those better than me and over the years, there have been a few tricky questions that I was having a hard time answering until I started experimenting with shortening cranks.
1. Seat too high! - Two angles are the primary drivers of the short crank movement. Knee flexion at the top of the stroke and, probably to a lesser degree, thigh torso clearance at the same point. Raising the seat opens both of these angles, so pedaling can feel better. It is not until later that we come to find that seat height may be causing us other problems.
2. Inability to rotate at the hips, or staying rotated rearward on the saddle, aka - posterior pelvic tilt instead of anterior. Also aka - "riding it like a road bike". Posterior rotation relieves thigh - torso pinching, and I have seen this tendency minimized or in some cases instantly and completely disappear once crank length was addressed. It doesn't really address the knee flexion issue directly, but it can move the overly flexed knee to an earlier portion of the stroke, which can be of some benefit, so perhaps a tertiary benefit to excessive knee flexion? Knee flexion is an issue because the knee can't be extended with close to max power from an overly flexed position. So if we can reduce the flexion or move the overly flexed knee back and away from the powerful portion of the stroke, we have made it better. Unfortunately, anterior pelvic tilt is not the fastest way for most of us to ride.
3. Excessive forward movement on the saddle You know the rider who no matter how far forward the saddle is moved, continually gravitates to the very tip of it, in some cases approaching an actual 90° of seat tube angle. IF everything else stays fixed, sliding forward will open the hip angle, at least the FIST defined "major" hip angle as I have come to refer to it, but other angles can become cramped. But IF we come forward AND raise the saddle (which we should), then it is more obvious how both thigh torso and knee flexion can benefit.
Anyway, just some quick observations that have been solidifying in my brain lately. I'm sure I was unclear about something and will be called out for my ignorance. I am not trying to dictate to the forum, as much as throw out some ideas and get some feedback on your experience as a rider or a bike fitter.
Finally, this is a rough range of crank lengths that riders have been selecting as a loose function of seat height.
<60cm seat height :: Crank 145mm or less. Really as short as you can find, maybe go custom. I’ve fit down to 135mm with custom cut BMX cranks.
60-65cm :: Crank length 140-145mm
65-70cm :: 145-150mm
70-75cm :: 150-155mm
75-80cm :: 155-160mm
80-85cm :: 160-165mm
>85cm :: 165-170mm, maybe 172.5 Keep in mind that of the tallest, strongest professional athletes I have fit literally ZERO of them have preferred anything over 165mm. These are 6’3” and taller athletes pushing wattage over 375 watts at threshold and upper 200s to 300 for IM races.
1. Seat too high! - Two angles are the primary drivers of the short crank movement. Knee flexion at the top of the stroke and, probably to a lesser degree, thigh torso clearance at the same point. Raising the seat opens both of these angles, so pedaling can feel better. It is not until later that we come to find that seat height may be causing us other problems.
2. Inability to rotate at the hips, or staying rotated rearward on the saddle, aka - posterior pelvic tilt instead of anterior. Also aka - "riding it like a road bike". Posterior rotation relieves thigh - torso pinching, and I have seen this tendency minimized or in some cases instantly and completely disappear once crank length was addressed. It doesn't really address the knee flexion issue directly, but it can move the overly flexed knee to an earlier portion of the stroke, which can be of some benefit, so perhaps a tertiary benefit to excessive knee flexion? Knee flexion is an issue because the knee can't be extended with close to max power from an overly flexed position. So if we can reduce the flexion or move the overly flexed knee back and away from the powerful portion of the stroke, we have made it better. Unfortunately, anterior pelvic tilt is not the fastest way for most of us to ride.
3. Excessive forward movement on the saddle You know the rider who no matter how far forward the saddle is moved, continually gravitates to the very tip of it, in some cases approaching an actual 90° of seat tube angle. IF everything else stays fixed, sliding forward will open the hip angle, at least the FIST defined "major" hip angle as I have come to refer to it, but other angles can become cramped. But IF we come forward AND raise the saddle (which we should), then it is more obvious how both thigh torso and knee flexion can benefit.
Anyway, just some quick observations that have been solidifying in my brain lately. I'm sure I was unclear about something and will be called out for my ignorance. I am not trying to dictate to the forum, as much as throw out some ideas and get some feedback on your experience as a rider or a bike fitter.
Finally, this is a rough range of crank lengths that riders have been selecting as a loose function of seat height.
<60cm seat height :: Crank 145mm or less. Really as short as you can find, maybe go custom. I’ve fit down to 135mm with custom cut BMX cranks.
60-65cm :: Crank length 140-145mm
65-70cm :: 145-150mm
70-75cm :: 150-155mm
75-80cm :: 155-160mm
80-85cm :: 160-165mm
>85cm :: 165-170mm, maybe 172.5 Keep in mind that of the tallest, strongest professional athletes I have fit literally ZERO of them have preferred anything over 165mm. These are 6’3” and taller athletes pushing wattage over 375 watts at threshold and upper 200s to 300 for IM races.