Two questions: Why would you gain 5mm? I guess I should say, if you change nothing other than the crank arm length (meaning your seat height and arm pads and overall position doesn't change, then I'd think you'd only gain 2.5mm. Now the diameter of your turning circle for your cranks shrink by 5mm, but at TDC (so to speak) your leg would be 2.5mm lower than before and since your torso didn't change position I don't understand where the other 2.5mm is coming from.
Second, I don’t think it’s fair to say that you need additional beats in your cadence. You could keep the same cadence as before but would just be required to push harder (more lbs to make up for less ft) to produce the same amount of torque (same amount of overall power, just applied differently). You can use less force spread over a bigger distance (longer crank) or more force over a shorter one (smaller crank) and both would have the same cadence (but different foot speeds). At the extremes I would understand saying you’d just have to spin more times since you couldn’t apply enough force per revolution with super short cranks. Am I missing another piece (very likely)? Normally only a chain ring tooth count difference would cause you to have to put in extra revolutions to make the same amount of torque and therefore speed.
“The way I understand it, your muscles have a preferred range of contraction speed for sustained efforts”
jim martin likes to talk in terms of negative power, that is, power that is fed back into the system, working against forward propulsion, because the muscles are contracting arrythmically or in any case inefficiently. the signal gets from your brain to the muscle to contract, or to release its contraction, but it’s getting there late, or maybe early, but not with precision. martin, as i recall, seems to think this is significant at about 115bpm. but what you’re saying is that this might change depending on crank length, that the issue is not cadence but foot speed. i don’t know, maybe so.
“shortening the cranks by 2.5 mm effectively raises your position by 5mm? Am I interpreting that correctly?”
tom has it. the issue is your hip angle at the top of your pedal stroke. this is an important metric, because it determines what sort of leverage you’ll have as you start pushing on the pedals. if you shorten the crank by 2.5mm, you increase the “clearance” between your torso and your thigh by 5mm. you can do whatever you want with this extra clearance. you can use it to increase your armrest drop by 5mm, flattening your back, if it ought to be flatter. or you can give yourself that clearance if you need it, which may or may not be the case with your position.
but you lose torque when you shorten the crank, so you have to make up for this with an extra beat or two (or three) a minute in cadence.
So, would “optimal” be a flat upper leg at the top of the stroke? Or is that going to be one of those things that’s different for everyone?
“The way I understand it, your muscles have a preferred range of contraction speed for sustained efforts”
jim martin likes to talk in terms of negative power, that is, power that is fed back into the system, working against forward propulsion
The issue is more the speed of muscle contraction itself: a muscle fiber is most efficient at ~1/3 of its maximal shortening velocity, so if your feet are moving in smaller circles, you need to pedal faster (if you can) to avoid having them contract too slowly, and also to avoid having to recruit more muscle fibers (due to the increased force requirement).
i think you’re right. let me think about it some more, but i think you’re right.
yes, you could simply apply more torque, and if your problem is that your cadence is borderline too fast, say, 105bpm for a shorter course race then, yes, fine. but that’s typically not the problem. seat angle, effort, and cadence all seem to want to track or flow together, so, for 19 people out of 20, or 49 out of 50, the concern or goal as their seat angles get steeper and their efforts increase is getting their cadences higher. running a shorter crank more easily allows for that, while also increasing the gap between knee and torso (how much that gap increases will require just one more cup of coffee before i just up and give in entirely on that one).
“The way I understand it, your muscles have a preferred range of contraction speed for sustained efforts”
jim martin likes to talk in terms of negative power, that is, power that is fed back into the system, working against forward propulsion
The issue is more the speed of muscle contraction itself: a muscle fiber is most efficient at ~1/3 of its maximal shortening velocity, so if your feet are moving in smaller circles, you need to pedal faster (if you can) to avoid having them contract too slowly, and also to avoid having to recruit more muscle fibers (due to the increased force requirement).
How does that affect someone using compact cranks? Or is that not really a factor since the crank arm length stays the same?
“would “optimal” be a flat upper leg at the top of the stroke?”
i would guess not. but i’ve never looked at the angle of the femur relative to the horizon. it might be an important metric, but my sense says that it’s not and, in any case, i don’t know the answer. i’m much more concerned with your hip angle, so, that angle you’re talking about would presuppose a specific, immutable seat angle – no rotating your body around the face of the clock, as it were.
That makes sense. I don’t know as much about the human involvement side of things and had to think in dumb physics terms. Obviously this isn’t a machine we’re talking about so I figured it had to be something human related. Thanks.
If you don’t raise the saddle, you only gain 2.5 mm at the top of the pedal stroke, while losing 2.5 mm at the bottom of the pedal stroke (i.e., you won’t be able to extend your leg as much, and/or will need to drop your heel while pedaling to do so).
If you do raise the saddle to keep hip, knee, and ankle angles at the bottom of the pedal stroke constant, then you gain 5 mm.
“The way I understand it, your muscles have a preferred range of contraction speed for sustained efforts”
jim martin likes to talk in terms of negative power, that is, power that is fed back into the system, working against forward propulsion
The issue is more the speed of muscle contraction itself: a muscle fiber is most efficient at ~1/3 of its maximal shortening velocity, so if your feet are moving in smaller circles, you need to pedal faster (if you can) to avoid having them contract too slowly, and also to avoid having to recruit more muscle fibers (due to the increased force requirement).
How does that affect someone using compact cranks? Or is that not really a factor since the crank arm length stays the same?
Not a factor.
“The way I understand it, your muscles have a preferred range of contraction speed for sustained efforts”
jim martin likes to talk in terms of negative power, that is, power that is fed back into the system, working against forward propulsion, because the muscles are contracting arrythmically or in any case inefficiently. the signal gets from your brain to the muscle to contract, or to release its contraction, but it’s getting there late, or maybe early, but not with precision. martin, as i recall, seems to think this is significant at about 115bpm. but what you’re saying is that this might change depending on crank length, that the issue is not cadence but foot speed. i don’t know, maybe so.
Well…I know that Doc Coggan likes to show his “Quadrant Analysis” plots as “Average Effective Pedal Force” vs. “Average Effective Pedal Speed”…and not Torque vs. Cadence, since that more closely aligns with the well studied “Force vs. Velocity” relationships of muscular contractions:
Another dumb question (why stop when I’ve done so well with them so far in life), why is it more important to maintain hip/knee/angle angles at the bottom of the stroke as opposed to the top? If you shorten the crank arms 2.5mm and then also raise the saddle 2.5 mm you’re heavily changing the angles at the upper part of your stroke to maintain them at the bottom. Since you’re applying mor power (at least in my awkward way of pedaling) at the top than at the bottom (coming into the back of your stroke) wouldn’t it make more sense to maintain the top or heck even the middle where ostensibly you’re applying the most power?
jesus, i know i’m going to regret this, and it’s probably going to end up in your goddam sig, but thank you andy.
of course, if you don’t raise the saddle, then the distance at the top of the pedal stroke increases by 2.5mm. if you raise the saddle 2.5mm to normalize for seat height, then you’re at 5mm, just as andy says.
Another dumb question (why stop when I’ve done so well with them so far in life), why is it more important to maintain hip/knee/angle angles at the bottom of the stroke as opposed to the top? If you shorten the crank arms 2.5mm and then also raise the saddle 2.5 mm you’re heavily changing the angles at the upper part of your stroke to maintain them at the bottom. Since you’re applying mor power (at least in my awkward way of pedaling) at the top than at the bottom (coming into the back of your stroke) wouldn’t it make more sense to maintain the top or heck even the middle where ostensibly you’re applying the most power?
Thanks for the insights guys.
I think a little “quality time” experimenting with a leg press machine might answer this question for you
“why is it more important to maintain hip/knee/angle angles at the bottom of the stroke as opposed to the top”
the answer changes depending on what angle you’re talking about. it is not important to maintain a particular hip angle at the bottom of the pedal stroke, but it is at the top, because of the leverage (or lack thereof) during femur extension. the knee angle at bdc must also be appropriate, for best leverage during tibia extension. but the appropriate hip angle at TDC ought to be maintained, not enlarged, so shortening the cranks has more to do with improving aerodynamics and, secondarily, enhancing a higher cadence.
what i don’t know is whether that shorter crank also improves aerodynamics down around the BB. mostly i’m talking about flatter backs, but there’s also the question of whether feet spinning a smaller circle might be aerodynamically beneficial. i just don’t know.
and speaking of things i don’t know, andy (since he’s on a roll) might give you a better explanation than i did two paragraphs above on muscle firing patterns and mechanical advantage.
It makes sense in terms of power delivery, that part I got, I just wasn’t sure if the angles were important in terms of the initial application of major power on the downstroke. I guess since you’re also moving backwards when you move your seat up that it helps maintain some of the angles as well.
Well from an **aerodynamics **perspective I can tell you that you HAVE to use less power to move your foot slower in smaller circles than faster in larger ones. If it’s a difference of power that is in anyway measurable I don’t know, but it WILL use less power to move your foot through the air of the pedal circle a shorter distance in the same amount of time (assuming again we keep cadence the same to keep power application the same) because it’s slower.
From an aero perspective it certainly makes sense. It also make sense to me in terms of giving you more room (as a guy who gets close to my chest with longer cranks when in an aggressive position), the part I don’t understand, well ONE of the parts I don’t understand, is how well the body deals with trying to apply that same amount of power over a shorter distance with inherently different angles. That’s where you smart guys come in
If one was needing to raise ones cadence to compensate for the shorter cranks then the smaller chainring could allow for that assuming the same rear cassette. Of course you could just go up one gear on the back
I’m too lazy today to figure it out, but assuming you needed 3-4rpm to maintain the same contraction speed would simply going from a 53 to say a 52 chainring make the difference?
Just came back today from my 2x20 at threshold, and looks like my power is now back to where it was with the longer crankarms, and my hip/back feel a lot more comfortable.
If one was needing to raise ones cadence to compensate for the shorter cranks then the smaller chainring could allow for that assuming the same rear cassette.
You’re assuming that cadence is a dependent variable. It isn’t (wasn’t), at least under the circumstances in which I did my testing (i.e., on an ergometer).
