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I seem to remember that the ideal fully extended leg position on a bike is legs fully extended, knees straight, with only toes and ball of foot touching the pedals. If you're not able to achieve this position then you need a larger bike or taller seatpost. Am I remembering this correctly?
 

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No, your leg should be bent at the bottom of the pedal stroke, a 25-35 degree angle is usually recommended. If your leg is straight with your heels on the pedal you're pretty close.
 

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^^^ right. For confirmation, watch videos of racers pedaling. You'll see their knees never get straighter than around 30 degrees while seated. There are a number of methods to set saddle height, perhaps getting a fit from a professional fitter being best. Heal on pedal as described by J.B Weld is very common method that works well.
 

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Heel, not ball. Leg straight with your heel touching the pedal. This gets you almost perfect on a road bike, I think its a bit high for a mountain bike.

Its still an easy way to get you very close, on a bike with clipless pedals.
 

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I also remember that it’s heel on pedal with the crank in line with seat post, not at 6 o ‘clock. It’s just like the KOPS set up....kind of a neutral starting point, not a rule.
 

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I've actually found knee bend at the top of the pedal stroke is more important. Pushing with your knee in the bent position is the point of highest stress on your knee. I use this website https://www.motionysis.com to check my fit. You can draw lines and it will give you the angles. The site has the angles backward (a 30 degree bend with leg extended shows as 150) but it's still easy to work out. Personally I think 25 to 30 degrees extended is way to stretched out. Seems like most pros settled on 35 to 45 degrees extended with a 110 degree bend at the top.

Personally I settled into 37 degree bend at full extension and 112 at the top if the pedal stroke. (143, 68 on motionysis). It feels perfect for me on the mountain bike and road bike. If I raise the saddle height I get knee pain. You might need a different crank length to get both angles to work out.
 

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^^^ right. For confirmation, watch videos of racers pedaling. You'll see their knees never get straighter than around 30 degrees while seated. There are a number of methods to set saddle height, perhaps getting a fit from a professional fitter being best. Heal on pedal as described by J.B Weld is very common method that works well.
Another common rule of thumb is to have your saddle at a height that your hips don't rock from side to side as you pedal (viewed from the back). A buddy can help you with this, or you can use a video camera.

I also just remembered that your saddle height can also be approximated as a % of your inseam measurement (NOT your pants inseam). There's lots of info if you google it.
 

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Of course all the above suggestions are just the starting point. Typically on a new bike, I'll start by measuring my current bike, center of BB spindle to top of saddle, reach, etc., then adjust from there. Note, seat angle, reach, pedal thickness and crank length are sometimes different from bike to bike.
Use a Sharpie to mark locations on your seatpost and saddle rails, when you find which works, document the measurements.
I've found that I run my saddle slightly lower than I used to.
 

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Another common rule of thumb is to have your saddle at a height that your hips don't rock from side to side as you pedal (viewed from the back). A buddy can help you with this, or you can use a video camera.

I also just remembered that your saddle height can also be approximated as a % of your inseam measurement (NOT your pants inseam). There's lots of info if you google it.
I was just about to post this. It is possible to have your bio-mechanics dialed and your hips stilll rocking. Your hips shouldn't rock. So either adjust your saddle height to allow your hips to be static OR start stretching your hamstrings and I.T. bands regularly. If your hips rock in the "correct" position tight I.T. and hamstrings may be the culprit.
 
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