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How does crank arm length affect bike fit?

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How does crank arm length affect bike fit?

Old 11-17-12, 08:34 AM
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How does crank arm length affect bike fit?

I am 6 feet tall and I am riding a 57 cm bike with 170mm cranks. I have played around with the kops method of seat placement and my knee is forward of the pedal axle. I have a nitto high extention quill stem for the bike but I think that it would feel a little better if I could raise the bars just a little higher.

So the though came to me that longer cranks would require me to lower my seat and would put the pedal axle under my knee. By lowering the seat, the relative position of the bars will be higher.

Has anyone tried using longer cranks to aid in fit or is this just ridiculous?
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Old 11-17-12, 08:39 AM
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Yep, there is a happy size for you.

I am 6 ft with a 34 inch bike Inseam and love 175 crank arms.

Fred "The Real Fred"

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Old 11-17-12, 08:43 AM
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Probably 6 of one half dozen of the other. As you lower the saddle it moves forward if left in the same spot on the rails, negating a bit of the 5mm forward shift fom 175s. I just pick the cranks that feel comfiest to spin circles with.

Curious how your saddle is positioned in relation to the bottom braket.
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Old 11-17-12, 08:52 AM
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Guess i should add that kops is generally used as a starting point for fore/aft saddle position. If you feel too far forward and your seat is pushed all the way back on the rails, then it'll move forward as you lower it.
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Old 11-17-12, 09:32 AM
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I move between 170 and 175 cranks on my various bikes. The 170s evolved on my long--distance bikes and my fixie because of the reduced circles I had to pedal.

Longer cranks mean a lower seat, and a very slight shift forward in the seat.

I like being able to remain seated when my bikes come to a stop. It helps getting started again, especially on hills. If the crank length is too short, it means the seat might be too high to do this. It's a problem for women with long inseams, but who want a small frame... bike manufacturers usually put short cranks on (165) so they have to move off the saddle when putting their foot down on the ground.
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Old 11-17-12, 09:54 AM
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Pick the crank length you want for your pedaling/riding style. Then fit the saddle position relative to the crank, and then the distance to the bars relative to the saddle. Last the bar height, which depends mostly on your flexibility. Changing crank length to make a bike fit is the wrong way to go about it.

I'm 6' with a 35" cycling inseam and like 175mm cranks. Since I do long road races and a lot of climbing I prefer a position to the rear of KOPS. That engages the glutes more and quads less. It also means there is less weight on my hands, making long distance riding more comfortable. Most rec riders would be best served by a similar position. The only riders I'd have on or ahead of KOPS would be crit racers or track sprinters. (not counting TT/tri position, that's a whole different setup).

From your description it sounds like your position could be greatly improved. Sitting forwards means there is more weight on your hands and may be the reason why you feel you need to raise your bars. You might consider a fitting from a knowledgable fitter.
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Old 11-17-12, 11:41 AM
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Seat/pedal position should be done first and then bars afterwards.

Don in Austin
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Old 11-17-12, 03:25 PM
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Originally Posted by ericm979
Sitting forwards means there is more weight on your hands and may be the reason why you feel you need to raise your bars.
It also means you are much more aero, without being as bent over at the waist, so you maximize the possible drop for a given degree of flexibility. And, especially for taller riders, it gets you to a more balanced position (tall bike geometry pushes you back), which helps cornering and bike handling in general. Granted these have a greater effect when racing crit's, on the track, and for TT's, which is no doubt why you mentioned them, but even in a road race, if you are to win, then at some point you will expose yourself to the wind. If you are in a more aero position, and have trained to pump out the power in that position, it helps.
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Old 11-17-12, 04:47 PM
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If you're at the point of fine tuning your fit, and pedal stroke, it makes a big difference. Even 2.5mm will probably change your saddle position and the arc of the power stroke.
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Old 11-17-12, 06:57 PM
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5 mm is .19 inch, I don't really think it makes that much difference.
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Old 11-17-12, 07:30 PM
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Have you considered a seatpost with more setback?
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