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 crank length...how long? 
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Joined: Fri Jan 29, 2010 12:09 pm
Posts: 25
 crank length...how long?
In his book, the art of road bike maintenance, Lennard Zinn starts with the
basic idea that for a given frame size (and thus the corresponding
rider) there is a suggested crank length (keep in mind this is a rough estimate). For example I have an inseam just under 35 inches and I ride a 58cm frame and it is suggested by most bike and component manufacturers I should have
a 175mm crank. Zinn goes on to state that competitive riders might
adopt a crank that is 2.5mm longer especially for TT and climbing so
the rider can push bigger gears with less force. Because this is such
a small change you can tweak your bike fit to account for the change in crank length.

Common objections include an increased dead spot in the pedal stroke,
a decrease in cadence (which may be minimal depending on the difference in size), increased chance of
pedal strike, the possibility of not having clearance with the
chainstays (usually not a factor if the crank is under 180mm). Many of
these issues can be mitigated by working on technique. I guess you
might also loose some acceleration when sprinting due to the increased
circumference of the pedal revolution. There are many caveats with all
of these though which makes the issue confusing.

On his website Zinn gets a bit more detailed. He quotes andy pruitt
and bill boston when advocating that a crank "proportionate" to rider size is better. As a 6ft6
rider, Zinn has a personal interest and has applied his methodology to
his custom frames and cranks. Zinn advocates that your crank length
should be .21-.216 of your inseam. According to this formula, with an
inseam between 34 and 35 inches, I should be on crank just over 180mm.
That's quite a disparity.

thoughts anyone?

http://zinncycles.com/Zinn/index.php/co ... tom-cranks
http://www.nettally.com/palmk/crankset.html


Sun Jan 09, 2011 6:38 pm

Joined: Tue Jan 26, 2010 1:33 pm
Posts: 661
 Re: crank length...how long?
Crank length is determined as much by the rider's measurements as by riding style. For example, Jan Ullrich would want a longer crank since he tends to spin a low cadence, whereas Lance would probably use a shorter crank since he spins an extremely high rpm. Of course - the reason these guys both ride the way they do is due in part because of their inseam length (amongst other factors). Lance is relatively short, but with a barrel chested hunched torso. He has freakish aerobic capacity, and he is relatively short legged. So for him a high cadence makes perfect sense. Also, as you mentioned - factor in the type of terrain you are riding. Riding in mountainous terrain would benefit from a longer crank. A crit would benefit from a shorter crank, to enable more responsive acceleration and ease of cornering thanks to a higher cadence and lower chance of pedal strike. Jesus from UVM did the UVM crit on a downhill crank one time. He was ridiculous in the corners, which included an extremely tight chicane and narrow roads in general through their campus. I believe his crank length was 165mm - it was comical, but he could rail a corner while pedaling longer into and quicker out of every turn.

Its really a subjective decision on what length to go. Think also about how prone you are to knee issues. A longer crank is going to crunch you up at the top of your pedal stroke more, and require more extension at the bottom of the stroke. If you are not flexible (like me) a shorter crank than normal will help to some extent. Also, consider if your femur and lower leg are of 'normal' proportion or not. If you have some goofy body size, you might risk putting your knee under undue stress. The rule of thumb in my mind would be to err on the side of a shorter crank. Test out a few different options and see what works. I bet a 175 crank would work, but you could certainly try a size up (though anything bigger than 175 becomes tough to find - and might be more expensive). Think about looking at some boutique component companies like TA Specialites (Bicycle Specialties) for a wider variety of crank sizes than normal. http://www.bikespecialties.com/site/parts2.html

ALSO note - you should NOT switch between different crank lengths on different bikes if you can avoid it. That will def. piss off your knees and result in some stress/overuse injuries.

_________________
Keith Cardoza
'kcardoza' at 'greenlinevelo' dot 'com'


Sun Jan 09, 2011 7:42 pm
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