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Discussion Starter · #1 ·
I'm not considering eccentric chainrings to extract more power. I'm looking at them to reduce torque, especially at TDC. You see, I recently took a bad spill on ice and broke my femur. On Christmas eve, I had a total hip replacement. When the surgeon saw the initial x-ray, he saw that the hip socket had some advanced arthritis. Actually, the right hip does too, but not as bad.

So as I take this off-season to recover, I'm looking at ways to minimize the torque on my hips, especially in TDC, where the hip angle is the most extreme and is hardest on the joint. My first consideration is to go back to 170mm crank arms, from my 175s that I've used for years. I figure the faster spin will return and lessen the torque.

Okay, so my question is, does anyone have any knowledge in how eccentric chainrings might change torque, especially in TDC? I'm 54 (and last summer broke a 300 mile record across Wisconsin), and would like to keep riding for many more years. So naturally, I'm looking for any changes on my bike that might minimize the wear on my hips.
 
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not to derail the thread but isn't reducing torque at TDC (i read that as the front chainring having a smaller radius / being easier to turn where you naturally have less leverage) exacttly the concept behind oval chainrings used to extract more power?

IIRC there were some MTB chainrings that did the opposite, decreased torque at 90-deg from top dead center and increased it at top dead center so that the application of torque at the rear wheel would be more uniform in slippery/muddy conditions. I'd expect this is the exact opposite of what you'd want for trying to reduce strain on hip joints.
 

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I have "eccentric" chainrings on a Miyata, they were called Bio Pace back in the day. Read this for more info: Enhancing cycling performance using an ... [Med Sci Sports Exerc. 2001] - PubMed - NCBI In a nut shell, the Bio Pace, or eccentric chainring does allow for faster cadence and a wee bit more power.

Then see this: www.me.utexas.edu/~neptune/Papers/job41(7).pdf

Question for you is it a tad easier? Perhaps, but you would probably be better served at least financial to try spinning faster in a taller gear thus taking the torque off you leg more.
 

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Discussion Starter · #4 ·
not to derail the thread but isn't reducing torque at TDC (i read that as the front chainring having a smaller radius / being easier to turn where you naturally have less leverage) exacttly the concept behind oval chainrings used to extract more power?
Rotor type, yes. But not the old Bio-pace.
IIRC there were some MTB chainrings that did the opposite, decreased torque at 90-deg from top dead center and increased it at top dead center so that the application of torque at the rear wheel would be more uniform in slippery/muddy conditions. I'd expect this is the exact opposite of what you'd want for trying to reduce strain on hip joints.
I agree. I had Bio-pace, and from what I remember, Shimano's idea was the the speed of your leg/foot would increase during the lower effective gear of the 3/6 o'clock position, and which would then carry you thru the higher effective gear at TDC. I don't think it worked as they claimed.
 

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Discussion Starter · #5 ·
I have "eccentric" chainrings on a Miyata, they were called Bio Pace back in the day. Read this for more info: Enhancing cycling performance using an ... [Med Sci Sports Exerc. 2001] - PubMed - NCBI In a nut shell, the Bio Pace, or eccentric chainring does allow for faster cadence and a wee bit more power.

Then see this: www.me.utexas.edu/~neptune/Papers/job41(7).pdf

Question for you is it a tad easier? Perhaps, but you would probably be better served at least financial to try spinning faster in a taller gear thus taking the torque off you leg more.
Sorry, but I should have been more specific, but I'm talking about the rings that do exactly opposite of Bio-pace. I had them, and I personally think they are exactly opposite of what I'm looking for. Spinning a taller gear will not decrease the torque as compared to spinning a smaller gear.

I am by nature, a spinner, and that's why I'm going back to shorter arms, to make the spin easier. What I'm looking for is a mechanical change on the bike, in addition to my (already) good spin. As far as costs goes, I don't have a problem spending the money if it preseres my new hip, which by the way, cost enough to buy me a fleet of the most expensive carbon fiber bikes.
 

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I'm not considering eccentric chainrings to extract more power. I'm looking at them to reduce torque, especially at TDC. You see, I recently took a bad spill on ice and broke my femur. On Christmas eve, I had a total hip replacement. When the surgeon saw the initial x-ray, he saw that the hip socket had some advanced arthritis. Actually, the right hip does too, but not as bad.

So as I take this off-season to recover, I'm looking at ways to minimize the torque on my hips, especially in TDC, where the hip angle is the most extreme and is hardest on the joint. My first consideration is to go back to 170mm crank arms, from my 175s that I've used for years. I figure the faster spin will return and lessen the torque.

Okay, so my question is, does anyone have any knowledge in how eccentric chainrings might change torque, especially in TDC? I'm 54 (and last summer broke a 300 mile record across Wisconsin), and would like to keep riding for many more years. So naturally, I'm looking for any changes on my bike that might minimize the wear on my hips.
A lot of confusion in the responses and even in your question.

First, there is no consistent evidence that eccentric chainrings actually extract more power. The studies that Shimano did to support the Bio-Pace design were seriously flawed (inexperienced riders, low cadence) and the current generation of eccentric chainrings can only offer anecdotal "evidence."

What eccentric chainrings do is change the gear ratio as you progress around the cycle. The give a higher gear ratio when you are pressing down on the crank and a lower gear ratio when you are passing through TDC. Effectively this means you spend more time during the "pressing down" portion of the cycle and less time around the TDC portion.

None of this has anything to do with "torque" on your hips. The forces on your hips are due to the force of your muscle contractions. Your hips cannot tell anything about the gear ratio you are pedaling but only about how hard you are pedaling. If you want to put out the same amount of power but put less stress on your hips then pedal at higher rpm and lower gears. I'm sure this is what froze meant even though he said the opposite by suggesting that you should try "spinning faster in a taller gear" which would mean you were going much faster and therefore putting more stress on the hips.
 

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Going from 175 mm to 170 mm cranks will reduce your hip flexion quite a bit at top dead center, especially so if you raise the saddle at the same time by 5 mm (which most people would do to re-establish their knee angle at bottom dead center). Since there's less hip flexion, there will be less force (torque doesn't figure here) put on certain parts of that joint. I would go to the shortest cranks you can manage, raise your saddle accordingly, ride at high cadence / low force as suggested above and forget about non-round chainrings.
 

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...What eccentric chainrings do is change the gear ratio as you progress around the cycle. The give a higher gear ratio when you are pressing down on the crank and a lower gear ratio when you are passing through TDC. Effectively this means you spend more time during the "pressing down" portion of the cycle and less time around the TDC portion....
Yeah. The crank turns slower passing through horizontal and faster passing through vertical. So, you might reason that you spend less time during part of the rotation where you exert very little force on the cranks and spend more time in the rotation where you can. You'd be correct, but with constant rotation, you're already working as hard as you can so the question becomes does the variable rotation allow you to work any harder, that is produce more power. I think the answer is generally no...or at most the effect is very small and depends on the individual.

I don't see how it would significantly affect stress on the hips any differently than pedaling at a slightly higher cadence, but if that's a concern, the thing to do would be to try it.

FWIW: The concept of elliptical chain rings has been around for over 100 years.
 

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Discussion Starter · #9 ·
What eccentric chainrings do is change the gear ratio as you progress around the cycle. The give a higher gear ratio when you are pressing down on the crank and a lower gear ratio when you are passing through TDC.
Since they would give me a lower(easier) gear thru thru the TDC, the area that is most important for me to lessen the stress, wouldn't they make sense?
 

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Discussion Starter · #10 ·
I don't see how it would significantly affect stress on the hips any differently than pedaling at a slightly higher cadence, but if that's a concern, the thing to do would be to try it.
From what I understand, it's when the hip is at it's most closed position, that it is under the most stress. But I'm going to discuss this with my surgeon in more detail when I see hm again next week.

Thanks, guys. I really appreciate the feedback. I'm not a noob at cycling, or the industry. I started wrenching in a shop back in 1981. Today, I work at one of the largest shops in the US. I've been an ultra-cyclist more years than not. But this is a new journey for me. I want to do whatever I can to keep cycling as long as I can. My first decision was to shorten my crank arms. I'm probably also going to go from a 39x53 to a 36x50, since I also don't want to give up my love for climbing. Spin baby, spin! :)
 

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Since they would give me a lower(easier) gear thru thru the TDC, the area that is most important for me to lessen the stress, wouldn't they make sense?
On the surface, it does. But when you look deeper into this, it gets complicated. First of all, there's no direct relationship between a certain force (newton) on the pedal and stress on the joint because the amount of joint flexion (degrees) at the time of that force generation greatly influences the amount of stress. In addition to that, not all members of a joint are stressed alike.

You're also ignoring inertial forces that can affect a joint greatly. Those forces come about when a joint speeds up and then slows down, as it would going from a higher gear to a lower one and back to a higher one. Those inertial forces are significant—there are riders who suffer from joint pain when riding at high to extremely high cadences because of them.

Last, there's really no need to exert any pedal force at top dead center. Pushing down at top dead center is a waste of energy because all it does is flex the frame. Pushing forward doesn't do too much for you either, although many people still believe that. If you generate most of your pedal force from 2 to 4 o'clock on the crank circle, your concern about top dead center forces may not even be warranted. For what it's worth: most elite riders pedal from 2-4 o'clock, with hardly anyone generating significant pedal forces pushing forward or pulling back.

Fascinating stuff. and yes, discuss all this with your doc.
 

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Discussion Starter · #12 ·
You're also ignoring inertial forces that can affect a joint greatly. Those forces come about when a joint speeds up and then slows down, as it would going from a higher gear to a lower one and back to a higher one. Those inertial forces are significant—there are riders who suffer from joint pain when riding at high to extremely high cadences because of them.
I can understand that, as I personally experienced that years ago when I was a "rabid" spinner.
Last, there's really no need to exert any pedal force at top dead center. Pushing down at top dead center is a waste of energy because all it does is flex the frame. Pushing forward doesn't do too much for you either, although many people still believe that. If you generate most of your pedal force from 2 to 4 o'clock on the crank circle, your concern about top dead center forces may not even be warranted. For what it's worth: most elite riders pedal from 2-4 o'clock, with hardly anyone generating significant pedal forces pushing forward or pulling back.
Yes, I understand. That makes sense. At fist I thought, "Well, something has to carry you thru those dead spots, so wouldn't this make sense?"
I had also considered the issue of inertial force causing stress on the joint, but didn't know how to describe it. I'm glad you were able to clearly put it into words. As I stated earlier, I had experienced this myself years ago when trying to explain to someone else why I thought my fast spinning was actualy causing some knee pain (years ago). But when I "smoothed out" my spin/force with a slightly higher gear and slightly slower cadence, the pain disappeared.
Thank you so much for affirming this.
 

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Going from 175 mm to 170 mm cranks will reduce your hip flexion quite a bit at top dead center, especially so if you raise the saddle at the same time by 5 mm
It will also lessen the stress on your knees, which may be helpful in the long run as you get older. I'm 5'6" with a 30" inseam, and rode 175mm cranks for about 10 years. My left knee, from time to tie, would begin to feel as though I was pinching a nerve, especially on longer rides. I just accepted it as normal and didn't think much more about it.

Eventually, I went to a road bike with a 170mm crank, thought I'd hate it, and ended up with zero knee pain even on century rides. Now all my bikes have 170mm cranks.
 

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Discussion Starter · #14 ·
It will also lessen the stress on your knees, which may be helpful in the long run as you get older. I'm 5'6" with a 30" inseam, and rode 175mm cranks for about 10 years. My left knee, from time to tie, would begin to feel as though I was pinching a nerve, especially on longer rides. I just accepted it as normal and didn't think much more about it.

Eventually, I went to a road bike with a 170mm crank, thought I'd hate it, and ended up with zero knee pain even on century rides. Now all my bikes have 170mm cranks.
I have a 34" inch inseam and I've never had a problem with 175, but it does sound a bit long for a 30" inseam. I'm no doctor, but I wonder if that sensation of nerve pinching was actually your plica band. I think this can be more of a problem with you introduce more flexion of the knee.

I probably won't like going back to 170s, but if it saves my joints, it should be worth it. Thanks for your feedback.
 

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For years on the MTB'ing world the thought was if you wanted more torque go with a longer crank arm and thus 175 and 180 were quiet popular. Wouldn't the opposite be true then for road bikes? Wouldn't going to a 165 or maybe a 170 reduce the amount of torque and take stress off hips and knees? Isn't that the same thing as a person going up a flight of stairs taking 2 steps at a time vs another taking just one step at a time? Wouldn't the person taking one step at a time have less stress on the hips and knees? Wouldn't that same effect apply to a using a shorter crank?

Now using a shorter crank then the leg size suggests one should use may not be the best thing for performance, but for stress on the joint it may be better?

I'm not a scientist in this stuff, I'm just making observations and opening questions about it.
 
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