4 ms·

When you read the latter, you'll find: Yes, when accelerating, wheel mass is worse than frame weight, but only a tiny amount. and Wired's physics ar

by rawland 10y ago

When you read the latter, you'll find: Yes, when accelerating, wheel mass is worse than frame weight, but only a tiny amount. and Wired's physics are correct in the case of accelerating a wheel to the steady state of a spinning wheel. Edit: To expand on this a bit: This "tiny amount" is a lot when you are among people close to your performance. And I'm talking about stiffness vs. weight. Stiffness is just so much more important, because the sum of these infinitesimal losses is just huge: Modern riders rpm: 100 which equals to 200 energy pulses per minute, because each leg has a rest phase and a full force phase per revolution. In a 4h race: 240 * 200 which are 48000 pulses. Now imagine you have a super light but not very stiff wheel (because it's light, you know)... that's a lot of lost energy.

Natanael_L 10y ago
What's the estimated lost energy? Where does it end up as the wheel flexes?
- rawland 10y ago
  Two very good questions! I'm thinking about these since a few years. > What's the estimated lost energy? From my point of view, that's almost impossible to calculate. But one can measure it. See: Stiffness and Damping in Mechanical Design by Eugene Rivin Click: https://amzn.com/0824717228 https://amzn.com/0824717228 Or Gavin's paper I cited in the other comment. > Where does it end up as the wheel flexes? Stiffness & Damping are closely related, as the book title above already implies. A part of the energy is lost in heat and structural changes. And another part is basically unused as it's unused by design: Imagine the following experiment. You need to push a box using many impulses over a specific length. At the side of the box is a plate, which you are punching, which is attached to a spring which itself is attached to the box. Now, let's go the extreme. Your impulses are so short and weak, that the spring absorbs most of it and the box hardly moves. The energy quantum you transferred via impulse, was used up by the spring and the spring released it by its own = Lost by design. Now, let's get your impulses bigger and longer. You'll move the box better. And now let's gradually fixate or, say, stiffen the spring. Each hit moves the box directly. That's stiffness. You hit the pedal and you experience how the bike just instantly starts to accelerate your body forward. Which a very stiff wheelset and bike would do. Now get on a fully. Thick tires, springs everywhere. Hit the pedal. You'll feel like cycling under water. PS: I'm not Eugene Rivin.
  - wiredfool 10y ago
    It should be possible to put limits on the lost energy of individual components, and in doing so, quantify the range of the problem. e.g.: For now, let's assume that the wheel is a torsional spring, we're putting a force of 1000N (~100kg*g) at the rim, (roughly 300NM torque) and it deflects 1 degree. The energy that goes into the elastic deformation of a torsional spring is k(theta)^/2, t=k(theta), so energy = t(theta)/2. Plugging the numbers, you get 2.5 joules. Now, 1000N is a lot of force, and you're only likely to have that level of force with a heavy rider in a very low gear. (i.e., your tires have a coefficient of friction around 1, so that's implying 100kg on the back tire just to prevent wheelspin) But 2.5 joules starts to sound like it _might_ be measurable, and maybe look at the numbers a little more closely to see if they're reasonable. I'm only a fairly decent rider, but even then that's only about 1% of my 5 minute output, (if you assume 1 power stroke per second).
    - rawland 10y ago
      World Class Levels (Duration vs. Watts/kg of Rider): 5s: 23 1min: 11 5min: 7.5 20min: 6.5 UCI Div. 3 pro (my [former] level): 5s: 20.28 (that's touching UCI Div 1/2 - sprinter type) 1min: 10.14 5min: 6.43 20min: 5.14 More details: https://www.cyclinganalytics.com/blog/2012/06/watts-kg-on-the-power-curve https://www.cyclinganalytics.com/blog/2012/06/watts-kg-on-th... Side note: Rumors were that the big LA had 20min+ values above 7.5, which is pure alien. The 5min test is hell, because one stresses _all_ energy systems in the human body to their maximum effort. So you ride in pain after something like 30 secs till the finish, which feels eternal. The 20min test is more easy compared to that, as it is very close to your general endurance. Thus, a top rider these days would rip his legs & mind out, when climbing besides the former LA.
      - wiredfool 10y ago
        Well, my 5 minute number is how long it takes me to climb a steepish local hill, when I'm doing a few reps. So it's imprecise, and there's like one significant figure. 250-300 watts is my guess. And half as fast as cat3? Yeah, I could see that. My point still is, it's possible to calculate the upper limit on losses from wheel flex under power load, and I'd assert that they are small enough to not matter. And what's more, the effect of torsional spring type flex goes down (at a constant power output) as speed goes up.

by rawland 10y ago

Natanael_L 10y ago

What's the estimated lost energy? Where does it end up as the wheel flexes?

rawland 10y ago
Two very good questions! I'm thinking about these since a few years. > What's the estimated lost energy? From my point of view, that's almost impossible to calculate. But one can measure it. See: Stiffness and Damping in Mechanical Design by Eugene Rivin Click: https://amzn.com/0824717228 https://amzn.com/0824717228 Or Gavin's paper I cited in the other comment. > Where does it end up as the wheel flexes? Stiffness & Damping are closely related, as the book title above already implies. A part of the energy is lost in heat and structural changes. And another part is basically unused as it's unused by design: Imagine the following experiment. You need to push a box using many impulses over a specific length. At the side of the box is a plate, which you are punching, which is attached to a spring which itself is attached to the box. Now, let's go the extreme. Your impulses are so short and weak, that the spring absorbs most of it and the box hardly moves. The energy quantum you transferred via impulse, was used up by the spring and the spring released it by its own = Lost by design. Now, let's get your impulses bigger and longer. You'll move the box better. And now let's gradually fixate or, say, stiffen the spring. Each hit moves the box directly. That's stiffness. You hit the pedal and you experience how the bike just instantly starts to accelerate your body forward. Which a very stiff wheelset and bike would do. Now get on a fully. Thick tires, springs everywhere. Hit the pedal. You'll feel like cycling under water. PS: I'm not Eugene Rivin.
- wiredfool 10y ago
  It should be possible to put limits on the lost energy of individual components, and in doing so, quantify the range of the problem. e.g.: For now, let's assume that the wheel is a torsional spring, we're putting a force of 1000N (~100kg*g) at the rim, (roughly 300NM torque) and it deflects 1 degree. The energy that goes into the elastic deformation of a torsional spring is k(theta)^/2, t=k(theta), so energy = t(theta)/2. Plugging the numbers, you get 2.5 joules. Now, 1000N is a lot of force, and you're only likely to have that level of force with a heavy rider in a very low gear. (i.e., your tires have a coefficient of friction around 1, so that's implying 100kg on the back tire just to prevent wheelspin) But 2.5 joules starts to sound like it _might_ be measurable, and maybe look at the numbers a little more closely to see if they're reasonable. I'm only a fairly decent rider, but even then that's only about 1% of my 5 minute output, (if you assume 1 power stroke per second).
  - rawland 10y ago
    World Class Levels (Duration vs. Watts/kg of Rider): 5s: 23 1min: 11 5min: 7.5 20min: 6.5 UCI Div. 3 pro (my [former] level): 5s: 20.28 (that's touching UCI Div 1/2 - sprinter type) 1min: 10.14 5min: 6.43 20min: 5.14 More details: https://www.cyclinganalytics.com/blog/2012/06/watts-kg-on-the-power-curve https://www.cyclinganalytics.com/blog/2012/06/watts-kg-on-th... Side note: Rumors were that the big LA had 20min+ values above 7.5, which is pure alien. The 5min test is hell, because one stresses _all_ energy systems in the human body to their maximum effort. So you ride in pain after something like 30 secs till the finish, which feels eternal. The 20min test is more easy compared to that, as it is very close to your general endurance. Thus, a top rider these days would rip his legs & mind out, when climbing besides the former LA.
    - wiredfool 10y ago
      Well, my 5 minute number is how long it takes me to climb a steepish local hill, when I'm doing a few reps. So it's imprecise, and there's like one significant figure. 250-300 watts is my guess. And half as fast as cat3? Yeah, I could see that. My point still is, it's possible to calculate the upper limit on losses from wheel flex under power load, and I'd assert that they are small enough to not matter. And what's more, the effect of torsional spring type flex goes down (at a constant power output) as speed goes up.