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Why do my knees hurt when I use 170mm cranks?

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Road Cycling “It is by riding a bicycle that you learn the contours of a country best, since you have to sweat up the hills and coast down them. Thus you remember them as they actually are, while in a motor car only a high hill impresses you, and you have no such accurate remembrance of country you have driven through as you gain by riding a bicycle.” -- Ernest Hemingway

Why do my knees hurt when I use 170mm cranks?

Old 08-12-11, 03:59 PM
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mazdaspeed
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Why do my knees hurt when I use 170mm cranks?

I'm 6' tall with long legs for my height. I've never had knee pain when using 175mm cranks but 170mm does it every time (eventually, sometimes after a few days of riding the bike), even with the seat raised to compensate. Am I doing something wrong here?
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Old 08-12-11, 04:05 PM
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When you raise your seat, it also moves back. Not much, but maybe by enough to aggravate your knees.

Bob
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Old 08-12-11, 04:13 PM
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MrTuner1970
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Everything else the same except crank length? Shoes, cleat position, pedals, Q factors, etc.?
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Old 08-12-11, 04:15 PM
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Originally Posted by MrTuner1970 View Post
Everything else the same except crank length? Shoes, cleat position, pedals, Q factors, etc.?
Yeah, pedals and shoes are the same. Q factor is probably really close. The cranks are the only major difference.
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Old 08-12-11, 04:15 PM
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Originally Posted by bobonker View Post
When you raise your seat, it also moves back. Not much, but maybe by enough to aggravate your knees.

Bob
I doubt that's it since I've recently adjusted my fit to move the saddle forward. More powerful and comfortable but knee pain was never an issue either way.
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Old 08-12-11, 04:22 PM
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What part of the knee hurts?
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Old 08-12-11, 04:33 PM
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Front/knee cap
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Old 08-12-11, 06:50 PM
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Strange. You are bending your knees less, not more, on the shorter cranks.
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Old 08-12-11, 07:13 PM
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Originally Posted by scirocco View Post
Strange. You are bending your knees less, not more, on the shorter cranks.
Yeah, it must be an issue with how the bike is set up, but it's happened before on a bike with short cranks so I don't know.
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Old 08-12-11, 08:47 PM
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Check both crank sets have the same Q, I can feel even a few mm difference in my knees
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Old 08-12-11, 09:33 PM
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Um ... let me be "that guy." Why the hell are you using 170mm cranks? You said 175s feel fine; use them. Durrrr
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Old 08-12-11, 09:51 PM
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Yeah, no kidding. You are 6' tall w/ long legs and you are using shorter than normal cranks? Go back to what works. Durrrrrr x 1000.
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Old 08-13-11, 12:10 PM
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Could it be because you have less leverage than normal with the shorter cranks, meaning you must exert more force with each revolution (all else being equal)? Are you shifting in a lower gear to compensate?
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Old 08-13-11, 01:01 PM
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Have you double-checked your fit on the bike with 170mm cranks? Knees over the pedal spindle? Has the leg extension angle been checked by a fitter?

Shorter cranks are supposed to reduce the kind of knee pain you're having. Something must be set up incorrectly. Your kind of knee pain generally comes from either saddle too far forward or too low, right? Since shoes, cleats, etc. are the same, most likely one of these two things.
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Old 08-13-11, 02:18 PM
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with shorter cranks you have to put more force on the pedal to get the same cadence which could be contributing to your knee pain.
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Old 08-13-11, 04:43 PM
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Originally Posted by ilovecycling View Post
Yeah, no kidding. You are 6' tall w/ long legs and you are using shorter than normal cranks? Go back to what works. Durrrrrr x 1000.
It's not a matter of going back, it's just what this bike has on it. I'd need to buy new cranks.

Originally Posted by MrTuner1970 View Post
Have you double-checked your fit on the bike with 170mm cranks? Knees over the pedal spindle? Has the leg extension angle been checked by a fitter?
Yes, don't care about that, no

Originally Posted by abikemechanic View Post
with shorter cranks you have to put more force on the pedal to get the same cadence which could be contributing to your knee pain.
This must be what the problem is. I like to stand on the pedals a lot and I do enjoy mashing in a big gear more than spinning.
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Old 08-13-11, 08:11 PM
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I don't necessarily care too much about KOPS. But it's at least a decent starting point. Might be worth a trip to a good fitter before buying new cranks. There could be something you've missed.

FWIW, I'm 5' 10" with a cycling inseam of about 35", and I use 170mm cranks on the primary road bike. And the "backup" road bike has 172.5's. And the mountain bike has 175's. The only time I've had knee problems on any of them was when the fit was off.
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Old 08-14-11, 07:31 AM
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I am 6'2 and have the same problem. Don't worry about why, just get some 175s. Problem solved.
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