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How much should your ....(10 posts)

How much should your ....JL
Jun 15, 2001 12:08 PM
knee be bent at the top of the pedal stroke? Can your knee be bent too much with your foot in the 12 o'clock position?

I know the theories about slightly bent at the bottom (6 o'clock) of the stroke, but what about the top. Is 90 degrees too much or can it more before you can start to hurt yourself? I'm trying to work through a knee problem. Can anyone tell me the other theory: Back of the knee pain put the seat (UP/Down) & vise versa? I can't remember/find it.

Thanks in advance.
Crank Length and Knee PainRich Clark
Jun 15, 2001 12:51 PM
Yeah, I think it's very possible to have too much bend in the knee at the top of the stroke. I developed tendinitis in both knees from just that problem, and solved it by going from 175 to 170mm cranks (and by stretching religiously).

Front of knee pain: raise saddle, and vice versa. But it's not that simple, as you've already figured out.

Caveat: I'm not a doctor, I just watch them on TV. Real knee problems can be serious, and professional sports-medicine guidance may be necessary.

RichC
OK. So how much...JL
Jun 15, 2001 1:58 PM
difference does 5MM really make with the crank length? I went from 170 to 175MM on a new bike and didn't have problems until around 300 miles. I can do the calculation and the circumference only increases 10mm (about an inch) but does it really make that big a difference on the whole? Going by the CC website, my bike (58 cm Trek) should use 172.5 mm cranks, but from stock they come with 175's. I know the leverage is increased with the larger size, but are the forces that different if your cadence is kept the same between the two crank sizes?

Any further help would be appreciated.

Thanks.
OK. So how much...Rich Clark
Jun 16, 2001 8:35 AM
First, I'm just going from subjective experience. The entire 10mm difference -- assuming you've adjusted your saddle height so that your knee angle at the 6 o'clock position is the same with both cranks -- is taken up by increasing the bend angle of the knee at the 12 o'clock position.

Picture it in your mind, rider on bike: legs and knees, the lower foot, the bottom bracket, the saddle, and the upper foot. Now imagine an animation as we go from 170's to 175's:

--The lower foot MOVES UP 5mm;
--The leg/knee on the bottom pedal DOESN'T CHANGE;
--The saddle therefore also MOVES UP 5mm to retain the same knee angle:
--The upper foot MOVES UP 10mm (the 5 from the saddle going up, plus the 5 from the crank on that side getting longer);
--The knee on that leg bends more to take up the entire difference in the crank-length change.

That's where the "increased leverage" of longer cranks comes from: bending the knees more at the top of the stroke.

If you increase crank length and a few hundred miles later are having knee problems, I think the crank length is suspect. Obviously, if it's a whole new bike, there could be other fit-related factors contributing. Did you match the fore/aft position of the knees relative to the pedal spindles between your old and new bikes?

Sometimes we buy new bikes that are more stretched out -- longer top tubes -- than our old ones, and we sneak the saddles forward to compensate if the reach feels uncomfortable. This will also increase knee angle at the top of the stroke.

Good luck,
RichC
I'm not sure about matching my fore/aft.JL
Jun 16, 2001 11:33 AM
But I was fine with the crank length (or so it seemed) for that first few hundred.

Two days after ride that had a fairly steep hill (don't know the grade) I did 50 miles (mostly flat) and the side of the knee started to act up. Took it easy the next few weeks and did the Metric and was fine until near the end (@8 miles to go). Now 2 weeks later I'm still having some problems. I'll try and warm up better and play some more with seat height and fore/aft. I really don't want to buy new cranks if I can avoid it, as I just spent $$$ for a whole new bike and my wife may want to shoot me!!

Thanks.
And it may be something else entirelyRich Clark
Jun 16, 2001 1:06 PM
Like I said, I'm just relating based on personal experience and some other riders' that I've read about. I do know that mucking up your knees is not a good idea, and if you can't solve the problem quickly, medical help would be a good next move.

When I developed tendinitis in my knees it was a gradual thing, a cumulative effect after getting a new bike and riding increasing miles over a period of months. Now, although I've resolved the cause, if I ride a bike that moves me too far forward the same old pain kicks in after only 10 miles or so.

Good luck getting it sorted out. Riding shouldn't hurt!

RichC
Frame sizingCima Coppi
Jun 15, 2001 2:16 PM
I don't know much about the pain issue you are discussing, but the theories I have read/been told are as such:

30 degree bend at the knee when the crank arm is at the 6 o'clock position.

Length from top of saddle to center of BB=.833 of you inseam length. I'd have a good bike shop with a Serotta size cycle help you fit your bike properly. At this point, I would not consider crankarm length to be the major issue.

Logic might dictate that back of knee pain is attributed to too much extension at the bottom of the pedal stroke, therefore seat needs to lowered. W/O seeing how you are fitted on your bike, I can say no more (always a good thing). Talk to your good LBS
This is goodWailer
Jun 15, 2001 11:17 PM
What Coppi has noted corresponds with my own fit information. Note that if you increase or decrease crank length, you need to make a corresponding adjustment to your saddle height. If you change from 170's to 175's you will need to lower your saddle. The reverse is also true. If you're using proper saddle height (.883 x your true inseam as a guide) you do not need to worry to much about the bend in your leg at the top of the stroke, in any case, your femur should not be raised to horizontal. As a rule, it is better to have your saddle too low than too high, so err in that direction.
Frame sizingJL
Jun 16, 2001 11:36 AM
Do you follow the seat tube for that measurement? Or go straight up from the BB? I don't have a Serotta size cycle anywhere around me that I can find. I'll try and get the 30 degree angle as I may be less.

Thanks again.

JL
Thanks all. Good info. to work with...nmJL
Jun 16, 2001 11:09 AM
Thanks