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170s

Old 10-04-13, 07:59 PM
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ScottRae
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170s

Just noticed that the crank arms on my road bike are 170s. Other road bikes I have looked at have 175s. Why do some bikes have one and others longer ones? Is there a performance advantage to having 175s? I would think so. Thanks for any info.
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Old 10-04-13, 08:07 PM
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It's a size/proportion thing. Typically, taller ppl want bigger frames and, to go with that, they'll also want longer stems and longer cranks and wider bars to match their proportions. Lennard Zinn, a tall guy who sells cycling equipment aimed at other tall riders, offers some very long cranks. Conversely, kids' bikes tend to have short crank arms, and some companies offer to modify high-end cranks in shorter lengths for serious riders of short stature.

The unintended consequence of playing with different crankarm lengths is that it will have an effect on the gear ratio, albeit it's not typically a dramatic difference. Longer cranks make for shorter gearing, and vice-versa.

hth
rob (fwiw, I tend to prefer longer cranks than would typically be recommended for my height/frame size...)
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Old 10-04-13, 08:07 PM
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Crank length is often related to rider size with taller riders preferring (or assumed to prefer) longer cranks. So, based on that, larger frames are usually fitted with longer cranks. Longer cranks give more leverage but tend to inhibit cadence so they are a trade off.

There are a lot of formulas used to determine "optimum" crank length based on rider height or leg length but several studies have shown little relationship between crank length and developed power so you pays your money and takes your choice. Some riders are indifferent to crank length while others have a strong preference.
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Old 10-04-13, 08:10 PM
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Crank length is as much a matter of fashion as mechanics. If you think about it we're talking a small 5mm difference.

170mm is the long term standard for road bikes, but mtn bikes tend to use 175mm. Some 20 or so years ago the road standard moved to172.5mm.

Proponents of long cranks talk about better leverage, but I discount that, since bikes a re a geared system and you can make the mechanical advantage whatever you want. Folks who spin tend to prefer shorter cranks, but it's not a hard rule.

IME, there's no great issue with shorter cranks, but shorter people can suffer with longer cranks because the difference in height top and bottom becomes a problem.

I'm sure that others will chime in, some claiming the size is important, but to me it's a tempest in a teapot.

BTW- all my bikes still use the same 170mm crank length that I started with 45 years ago.
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Old 10-04-13, 09:46 PM
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I went to 172.5 for a while in the early '80s, didn't really see a difference and moved back to 170s. I feel that my leg speed is marginally slower with the longer cranks.
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Old 10-04-13, 09:49 PM
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Originally Posted by FBinNY View Post
Crank length is as much a matter of fashion as mechanics. If you think about it we're talking a small 5mm difference.

170mm is the long term standard for road bikes, but mtn bikes tend to use 175mm. Some 20 or so years ago the road standard moved to172.5mm.

Proponents of long cranks talk about better leverage, but I discount that, since bikes a re a geared system and you can make the mechanical advantage whatever you want. Folks who spin tend to prefer shorter cranks, but it's not a hard rule.

IME, there's no great issue with shorter cranks, but shorter people can suffer with longer cranks because the difference in height top and bottom becomes a problem.

I'm sure that others will chime in, some claiming the size is important, but to me it's a tempest in a teapot.

BTW- all my bikes still use the same 170mm crank length that I started with 45 years ago.
No. 1.
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Old 10-05-13, 02:24 AM
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ScottRae, Technically one can produce more output torque using with the longer crank arm than the shorter crank arm for a given input force. (Crank arms are to me are a first-class lever which have a crank arm as the fulcrum between the effort force and the resistance.) That said, there's very little, if any, seat of the pants performance difference. One difference that is noticeable is that my cadence is a tiny bit slower on my bike with 175 mm crank arms. If I immediately switch between different length crank arms I do feel the difference for just a few moments and then I adapt and it's a non issue.

If you're curious, try the longer crank arms.

Brad


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Old 10-05-13, 03:03 AM
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There is certainly a "feel" difference between the sizes. My leg length matches a 175 crank, best. I'm more of a strength rider than a spinner so, 175 makes more sense. The 172.5 cranks I had a chance to ride, seemed to up my cadence(as noted above).
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Old 10-05-13, 07:08 AM
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I use 165mm cranks because I have a limited range of motion in my R. knee.
175mm gives me chronic knee pain to the point that I really won't ride except as necessary to get groceries etc. (no car)
170 and I have some pain at times.
165mm and I rode my Hybrid 70 miles in a day.
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Old 10-05-13, 09:20 AM
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I'm 5'10" which is right on the mean for American men.

For years I used 172.5mm cranks on my road bikes. When I decided to convert my Klein to a compact double, 175mm was immediately but 172.5mm wasn't I worried endlessly over that 2.5mm change and eventually bought the longer crank. Lance Armstrong says he can sense a 1mm difference, but I certainly couldn't.

After breaking my hip, I decided to have a stock crankset shortened by Bikesmith in Minneapolis. I worried about the difference between 148mm and 152mm. The owner told me he doubted I'd be able to tell the difference. His son and another racer overhauled two bikes with similar cranksets over the winter. Inadvertently they installed one 172.5mm and one 175mm crankarm on each bike. He said they didn't discover the mistake for six months.

My conclusion is that if you are unusually tall or unusually short or have some other physical issue, like recovering from a broken hip, it might make a difference. Most riders, however, can and will easily adjust to slightly different crank lengths.
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