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Old 07-29-03, 11:29 AM   #1
djbowen1
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Determining Crank arm length for Road Bike

I have a pretty average body type, 130 lbs, 5'9''. I am going to upgrade my Cannondale R600 from Cannondale cranks to Ultegra while the shimano stuff is so inexpensive. I dont know what crank arm lenght i have and dont know what to but. I am guessing a 175mm?
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Old 07-29-03, 11:47 AM   #2
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Figuring out what you have is the easy part. You just measure from the centre of the pedal spindle hole to the center of the crank bolt and that will give you the measurement in (mm). Many cranks also have the length marked on the inside of the crank arm so look for this first.

The hard part is figuring out what you need or want. This is usually done through the painstaking process of trial and error. Shorter cranks are easier on the knees and are easier to spin or keep a higher cadence. Some people like the longer cranks as they get more leverage especially when climbing. It is really a matter of personal taste. I used to use 175's but switched to 170's and just love them as do my knees. It has not slowed me down at all and in fact the hills are much easier as my cadence has stayed up significantly since the switch.

Good luck.
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Old 07-29-03, 11:59 AM   #3
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I've gradually reduced my crank lengths over the years as well, and now use 170's on all my bikes. This does seem to have helped my knees, and I do tend to spin up hills without standing a lot more than I used to. All of this being part of the adjustments we make to the way bodies can change as they age.

I would be surprised if an R600 that's the right size for a 5'9" rider -- probably a 55? -- came with 175's. Probably 172.5's or even 170's.

I'd probably recommend going with the same crank length if you've been happy with it, and changing to something else if you haven't. Wow, that took years of study and experience to figure out, didn't it?


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Old 07-29-03, 12:17 PM   #4
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djbowen1: Most crankarm lengths are indicated on the back of the crankarm. Look on the inside of the arm just above where the pedal bolts on. You should see something 170, 172.5 or 175.

I recently switched from 175's to 170's and really like the shorter cranks.

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Old 07-29-03, 07:04 PM   #5
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I am 5'8" tall, wear a 30" trouser inseam, ride a 55cm C-T road frame, and have almost always used 170mm cranks, which should be about right for you, as well. I put 165s on my UO-8 to reduce the pedal / front tyre interference resulting from its fork, which has a shorter-than-stock rake.
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Old 07-29-03, 07:06 PM   #6
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Old 07-29-03, 07:09 PM   #7
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Why would shorter cranks be easier on the knees? Wouldn't it increase the load on the knees as the mechanical advantage would lessen?

Anywho, for what it's worth, I'm 5' 8" with long legs relative to my body. Both my mountain bike and road bike have 175mm crank arms. For some reason, the cranks on my road bike seem larger, but they are indeed the same as my mountain bike.

Perhaps I should bump down to 172.5 or 170 to see how it feels????
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Old 07-30-03, 02:54 AM   #8
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Crank length is part of the gearing equation. Applying the same force on a longer crank generates more torque, so you can turn a higher gear. BUT you will take longer to spin the crank through its larger radius. Under the same riding conditions, a rider would normally chose to ride in a different cog-ratio when using different sized cranks. Whatever way you do it, you have to apply the same amount of work to get the same speed, its a matter of pedalling force vs cadence.

You should chose cranks to fit the size of your legs, or more probably your femur. At 5'9" you are just below average male hight, so should probably use a 170. The common range of sizes gos from 160 to 175, but cranks are available from about 150 to 185mm
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