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Knee pain and crank length
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bumblebee
Mar 13, 07 7:10
Post #1 of 10 (799 views)
I have been lurking on the forum on and off for a while, and finally am brave enough for my first post. Please forgive any inadvertent errors in the post. I wish to seek the wisdom, and possibly insults of the slowtwitch community.

My question is this. Can having 2 different crank lengths on a road bike (172.5) and a tri bike (175) lead to mild sharp knee pains? It has started since I started alternating bikes, and my position feels pretty good on both (fit by a local fitter who does both road and tri fittings), and I rotate my running shoes regularly. So the only thing I can think of that is different is that I am now alternating crank lengths. Is that bad?

Thoughts? Advice?

Thanks,

B
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Re: Knee pain and crank length [bumblebee] [ In reply to ]
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Jonathan Blyer
Mar 13, 07 7:13
Post #2 of 10 (793 views)
If you were fit for both bikes, saddle height, hip angle, etc... then that shouldn't really be a problem. Your talking about 5mm difference in the top of the pedal stroke. Look at a ruler, that's pretty small.
Jonathan Blyer,
ACME Bicycle Co., Brooklyn, NY
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Re: Knee pain and crank length [bumblebee] [ In reply to ]
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TriHanrahan
Mar 13, 07 7:25
Post #3 of 10 (781 views)
While I didn't have the exact same issue you did, I was having trouble with my knee and I had 175s. So my PT talked me into getting some 172.5s and the pain went away. I just went and bought a used crank to try out. 5 years later still good.

John
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Re: Knee pain and crank length [bumblebee] [ In reply to ]
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Cousin Elwood
Mar 13, 07 7:37
Post #4 of 10 (770 views)
Usually, it's more an issue of seat positioning (fore-aft) than anything else.
I assume you have a leg-length discrepancy, but it sounds like the different crank lengths are only on one bike? So:
- How precise is the seat positioning on the two bikes, relative to the pedals? Rule of thumb is plumb-line from the patella (pedal at 3:00) should go right through the pedal axle)
- Is it the longer side that's causing you trouble? (in which case ease the seat back slightly).
- Check cleat positioning L/R
- Set up your trainer next to a mirror (straight on and then side), or better yet a video camera and look for any anomalies.

Cousin Elwood - Team Over-the-hill Racing
Brought to you by the good folks at Metamucil and Geritol...
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Re: Knee pain and crank length [Cousin Elwood] [ In reply to ]
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Jonathan Blyer
Mar 13, 07 7:43
Post #5 of 10 (767 views)
I think the questions was 172.5 on one bike and 175 on the other.
Jonathan Blyer,
ACME Bicycle Co., Brooklyn, NY
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Re: Knee pain and crank length [jonblyer] [ In reply to ]
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bumblebee
Mar 13, 07 9:39
Post #6 of 10 (740 views)
Thanks to all for the input,

It is that both cranks on the road bike are 172.5 and both on the tri bike are 175. I'm not 100% sold of the fit on the tri bike, so maybe this is contributory. I am scheduled to have a fitting with a F.I.S.T. certified fitter in the coming weeks, so I'll see if that helps.

Can cleat position also contribute to knee discomfort?

It's nothing bad, I only really notice it at the end of a tough trainer session, particularly if I try to run on the treadmill right after. Take a day off and I can run no problems.

B
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Re: Knee pain and crank length [bumblebee] [ In reply to ]
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schroeder
Mar 13, 07 9:54
Post #7 of 10 (723 views)
[reply]
I'm not 100% sold of the fit on the tri bike, so maybe this is contributory. I am scheduled to have a fitting with a F.I.S.T. certified fitter in the coming weeks, so I'll see if that helps.
-------------------------------------
OK, I've never been fit so please excuse my dumb question but isn't part of a fit to size crank length?

I've always heard crank length equals 18.5% of the distance from floor to top of femur.

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Can cleat position also contribute to knee discomfort?
-----------------------
Definitely - especially if there is no float on your pedals.
------------------------------------------

It's nothing bad,

Pain in the knees is all bad.
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Re: Knee pain and crank length [bumblebee] [ In reply to ]
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Cousin Elwood
Mar 13, 07 10:35
Post #8 of 10 (692 views)
On a tri bike, you sit further forward, but with your shoulders lower. The relative position to the BB should be the same as your road bike, just rotated a few degrees forward. So on your tri setup, your patella should sit slightly forward of the pedal spindle with the cranks at 3&9:00.

Sitting too far back, especially when hammering in the aeros, could cause some pain. You could try bumping your seat forward just a tad...

Cousin Elwood - Team Over-the-hill Racing
Brought to you by the good folks at Metamucil and Geritol...
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Re: Knee pain and crank length [bumblebee] [ In reply to ]
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JohnA
Mar 13, 07 10:43
Post #9 of 10 (686 views)
I ride with 172.5's, but spent a week last year riding 175's. I made the appropriate height adjustment, but still ended up with sharp knee pain at the end of the week. Could be that the fore-aft needed to be adjusted a bit more, not sure...
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Re: Knee pain and crank length [schroeder] [ In reply to ]
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bumblebee
Mar 13, 07 11:32
Post #10 of 10 (667 views)
In Reply To:
[reply]
OK, I've never been fit so please excuse my dumb question but isn't part of a fit to size crank length?

I've always heard crank length equals 18.5% of the distance from floor to top of femur.

------------------------------------
Can cleat position also contribute to knee discomfort?
-----------------------
Definitely - especially if there is no float on your pedals.
------------------------------------------

It's nothing bad,

Pain in the knees is all bad.-------------------------------------------
Schroeder, thanks for your input,

I asked about crank length wrt the fit when I bought the bike, but the guy who did the fit (shop owner who has fit and trained a number of local triathletes) told me that the crank length shouldn't matter, at least not between 172.5 and 175. He had fit me to my road bike as well, and I've never had problems with it, so I trusted his opinion. His fitting was certainly less technical than what F.I.S.T. describes. I'll ask the F.I.S.T. certified fitter what he thinks about the cleat position and if it might be causing the pangs. I agree all knee pain is bad, but some is certainly worse than others.

B
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