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Does rotating weight really matter?

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Does rotating weight really matter?

Old 05-05-21, 10:18 PM
  #126  
asgelle
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Originally Posted by tyrion View Post
I know the basic physics of aero being more important than mass, and rotational mass, but rotational mass is still a factor. We can't call it insignificant because most people can feel it, so if someone wants to claim it's so small it's meaningless they need to back up that claim with data.
Which is exactly what Kraig Willett has done.

We also know from virtual elevation and other field testing that people’s perceptions can be very faulty especially when confirmation bias is a factor.
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Old 05-06-21, 07:08 PM
  #127  
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I have 2 sets of wheels for my Lynskey Backroad. One set of wheels is fairly lightweight. Those are wrapped by GP4000S tires.The other set of wheel is heavy. Those are shod with Schwalbe Marathon 32s. I would guess the Schwalbe set is 2 pounds heavier. The difference is dramatic. The bike changes from feeling lively and quick to dead and slow.
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Old 05-07-21, 01:39 PM
  #128  
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Originally Posted by tyrion View Post
I call B.S. on this. (Jobst Brandt knows way more about bikes and engineering than I do, but I think this old usenet post should be challenged.)

I think minute differences in acceleration can make or break competitors at the end of the race. If someone wants to say "it doesn't matter very much" it should be quantified.
The mass at the very edge of a rotating body (e.g. at the edge of the tire on a bicycle wheel) takes equal amounts of energy to accelerate in a straight line and rotationaly. That is to say that there is a 2x contribution to any additional weight at the edge of a wheel while acceleration, as compared to weight on the frame. This effect decreases linearly as the weight gets closer to the center of rotation, meaning that changes in hub weight are nearly the same as changes in weight on the frame. However, you get that energy back unless you use the brakes, and with the case of aero wheels, you likely get much more than that back in reduced drag when pedaling at a steady rate.
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Old 05-07-21, 03:17 PM
  #129  
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Originally Posted by cuevélo View Post
The mass at the very edge of a rotating body (e.g. at the edge of the tire on a bicycle wheel) takes equal amounts of energy to accelerate in a straight line and rotationaly. That is to say that there is a 2x contribution to any additional weight at the edge of a wheel while acceleration, as compared to weight on the frame. This effect decreases linearly as the weight gets closer to the center of rotation, meaning that changes in hub weight are nearly the same as changes in weight on the frame. However, you get that energy back unless you use the brakes, and with the case of aero wheels, you likely get much more than that back in reduced drag when pedaling at a steady rate.
Correct. Basic physics.
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Old 05-07-21, 03:24 PM
  #130  
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Nothing matters.
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Old 05-07-21, 03:40 PM
  #131  
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Originally Posted by Paul Barnard View Post
I have 2 sets of wheels for my Lynskey Backroad. One set of wheels is fairly lightweight. Those are wrapped by GP4000S tires.The other set of wheel is heavy. Those are shod with Schwalbe Marathon 32s. I would guess the Schwalbe set is 2 pounds heavier. The difference is dramatic. The bike changes from feeling lively and quick to dead and slow.
Right. Put the same Conti tires on the heavy wheel set and get back to us.
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Old 05-07-21, 04:24 PM
  #132  
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Originally Posted by genejockey View Post
Nothing matters.
Nothing matters ... until it does.
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Old 05-07-21, 04:41 PM
  #133  
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Anecdotal...Back when I was a crit monkey (early 2000s), I borrowed my buddy's Mavic Cosmic Carbone wheels for a few races. They were a good bit heavier than the Rolf Primas that were my usual choice. The Mavics definitely felt more sluggish to accelerate, but once they got spinning around 30+mph, they just wanted to keep going (flywheel effect). For courses where you could carry your speed well through corners, I liked them a lot, especially if there was a long drag to the finish line where I could start my sprint early and really wind them up. For courses that demanded more punchy accelerations, I preferred the Rolfs.
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Old 05-11-21, 10:46 AM
  #134  
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On flat ground- a lighter bike helps me maintain a higher average speed. On climbs is where weight really matter to me....especially rotating weight. By that, I mean, it is easier/quicker for me to pedal a 16 lb bike up 7 miles and 2,000 ft than a 19 lbs bike with heavier wheels. Same with mountain biking- I'm always quicker or expend less effort on my 5 lb lighter bike. I'm not an especially strong rider though. For those cranking out 400+ average watts on climbs and 250 watts on average rides, the difference will prob be felt far less.
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Old 05-11-21, 12:36 PM
  #135  
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Originally Posted by Chandne View Post
On flat ground- a lighter bike helps me maintain a higher average speed.
I'm curious how you determined this. It goes against established principles of Newtonian physics.
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Old 05-11-21, 12:46 PM
  #136  
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I have several bikes. The heavier ones take more effort to keep up to speed. Heavier bikes simply require more effort to keep at 28+ for me. Not sure what Newton's Law of Motion you are referring to where it takes less effort to pedal a heavier bike. Th heavier bike will pick up speed faster (with equal tires etc.) organically on steeper downhills but maintaining speed of flats is different.

If that (your Newton's Law- prob not #2 or #3 though) were the case, I'd be flying at 30 MPG on my 30 lb mountain bike with slicks...something that I could simply never ever do on a flat but I can with my 16 lb BMC, for example. That even goes for downhill portions when you have to pedal for 30-40 flatter yards. I have almost all my PRs on the two bikes and on those 50+ MPH ones, my second lightest bike with deeper wheels. If you have to pedal hard to maintain speed, it is simply less effort to keep a light bike up to speed. I have a 28 lbs heavy gravel bike with 30s as well and while I can get that up to speed on flats eventually, it takes far more effort to keep it above 24-25.
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Old 05-11-21, 01:28 PM
  #137  
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Originally Posted by Chandne View Post
I have several bikes. The heavier ones take more effort to keep up to speed.
Lighter bike probably has less wind resistance, and/or put you in a more aero position.
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Old 05-11-21, 02:04 PM
  #138  
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Originally Posted by Chandne View Post
Not sure what Newton's Law of Motion you are referring to where it takes less effort to pedal a heavier bike.
It's not that heavier bikes take less effort, it's that it doesn't make much difference. When you're on flat ground, gravity is perpendicular to your direction of motion, so you're not experiencing any direct losses to it. Increasing weight will increase hysteretic rolling resistance, but for changing bike weight this is a very small effect and it's mostly tunable with tire pressure.

If you're seeing notable speed differences in steady flat efforts between bikes of different weights, there are probably factors other than weight having an impact. Fit and posture, tire selection and setup, aerodynamic differences in components like wheels and cockpit, aerodynamic drag from extra thingies like bags or racks, riding with slower clothing, etc.

The low effect that weight has on flat-ground cruising is why, in the HPV community where they don't have any rules regarding rider posture or bans on fairings, everyone encloses the bike+rider system in a full fairing, despite the enormous amount of weight that this adds. Fully-faired recumbents usually weight 50 pounds or more, but the HPV hour record is currently 57.4 miles; the current UCI hour record, set on a track pursuit bike, is 34.2 miles.
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Old 05-15-21, 06:27 PM
  #139  
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Interesting thread.
I'm also an ex-F1 tech guy and this question often comes up with regard to rotating components used in racing cars e.g. wheels, engine crankshafts etc. Despite having a Masters Degree in Mech Eng, I never really did get my head around the physics of a spinning wheel - especially the gyro effect. The rotational inertia (flywheel effect) is easy enough to understand and no doubt a lighter wheel set will be easier to accelerate. That could be important in racing, but only one of many other factors to consider as a whole. Less mass (regardless of it rotating or not) is also easy to understand and certainly important if there is a lot of climbing involved. For my own stats (weight & power) I calculated that 1 kg reduction in bike mass = roughly 1 min saved on a climb up Alpe d'Huez. But unfortunately I would be unlikely to save more than a couple of hundred grams on my current stock carbon wheels. So that might give me 10 seconds gain for a solid hour's slog up the Alpe! A pro racer would take that gain for sure, but it's of no practical use to me. So what about any additional benefit in that 200g being rotational mass rather than static mass? Well good luck trying to put a number on that! I honestly don't think anyone could, which is why threads like this never really come to any conclusion. They never did in F1 either. Mass is always important as a first order parameter, but rotational vs static mass is a few orders of magnitude down. If I was buying a wheel set for flat racing then I would look at the aero first and not worry too much about weight. For climbing I would look first at weight and not worry too much about aero. Most pro teams appear to do that too. Obviously the more you spend on a wheel set, the less compromise you need to make on either parameter.
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Old 05-15-21, 07:16 PM
  #140  
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Originally Posted by PeteHski View Post
So what about any additional benefit in that 200g being rotational mass rather than static mass? Well good luck trying to put a number on that! I honestly don't think anyone could, which is why threads like this never really come to any conclusion.
Just because you can’t think of how to do it, doesn’t mean others can’t. You might want to take a look at the Willett article I cited above and the Martin model of cycling kinematics, https://collections.lib.utah.edu/dl_...0d77868437.pdf The analysis is straightforward and fairly easy.
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Old 05-15-21, 07:53 PM
  #141  
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Originally Posted by asgelle View Post
Just because you can’t think of how to do it, doesn’t mean others can’t. You might want to take a look at the Willett article I cited above and the Martin model of cycling kinematics, https://collections.lib.utah.edu/dl_...0d77868437.pdf The analysis is straightforward and fairly easy.
Not planning on choking that paper down, to get back to the original question. If the total system weight is the same and one riders wheels are say 500 grams lighter how much faster would he/she/they be up the Alpe?
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Old 05-15-21, 08:16 PM
  #142  
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Originally Posted by Atlas Shrugged View Post
Not planning on choking that paper down, to get back to the original question. If the total system weight is the same and one riders wheels are say 500 grams lighter how much faster would he/she/they be up the Alpe?
If it’s not worth it to you to figure out why would it be worth it to me? Nevertheless, the short answer is your question is ill posed. However, for any reasonable choice of rider and equipment and consumer grade timing equipment, the difference between adding the weight to the rims or the frame would be below the limit of detection.
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Old 05-16-21, 02:58 AM
  #143  
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Originally Posted by Paul Barnard View Post
I have 2 sets of wheels for my Lynskey Backroad. One set of wheels is fairly lightweight. Those are wrapped by GP4000S tires.The other set of wheel is heavy. Those are shod with Schwalbe Marathon 32s. I would guess the Schwalbe set is 2 pounds heavier. The difference is dramatic. The bike changes from feeling lively and quick to dead and slow.
Yes but the question remains. Is the difference in feel due to less weight or less rotating weight? And, is there a difference?
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Old 05-16-21, 04:59 AM
  #144  
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Originally Posted by asgelle View Post
Just because you can’t think of how to do it, doesn’t mean others can’t. You might want to take a look at the Willett article I cited above and the Martin model of cycling kinematics, https://collections.lib.utah.edu/dl_...0d77868437.pdf The analysis is straightforward and fairly easy.
I never said others couldn't work it out, I merely doubted it. I will have a look at that article to see if it sheds any light on whether a few hundred grams of rotating weight is really more significant than a few hundred grams of static weight in the real world.
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Old 05-16-21, 05:43 AM
  #145  
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Okay I had a brief look through this paper, which is a pretty comprehensive power cycling power model and does indeed include the kinetic energy stored in the wheels. They approximated the moment of inertia of the 2 wheels at a value of 0.14 kg.m^2 and added this term to the overall kinetic energy of the total moving body (rider+bike). In the discussion section they stated that changes in kinetic energy (of the entire bike+rider, of which the wheels are merely a fraction) accounted for 1% of total power (1-2W). So you could safely say that a subtle change in wheel mass and inertia would be pretty insignificant in this model and totally unmeasurable out on the road.

If you wanted to work out in theory how much power/time you would gain or lose between different wheel sets, you would have to very accurately measure the mass and moment of inertia of the 2 sets of wheels you were comparing and then plug those values into the model for whatever course you were riding. They didn't go anywhere near that kind of micro comparison in this paper. Wheel inertia was merely approximated as above. But you can see from the model that the affect would be so small as to make no difference. Even if they had totally ignored the wheel inertia it wouldn't have had any significant affect on their test results.
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Old 05-16-21, 05:57 AM
  #146  
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Originally Posted by Atlas Shrugged View Post
Not planning on choking that paper down, to get back to the original question. If the total system weight is the same and one riders wheels are say 500 grams lighter how much faster would he/she/they be up the Alpe?
The answer there is approx 30 seconds due to the 500g of mass saving (there are several online calculators available for this). The fact that it's rotating is really insignificant and I haven't seen a cycling power model yet that allows you to plug in wheel inertia values to make any comparison between wheel sets (assuming you even knew exactly what moment of inertia each wheel set actually had).

Typical models allow you to change rider/bike mass, frontal area, drag coefficient, rolling resistance coefficient, gradient, wind speed, air density and drivetrain loss. Haven't seen one with wheel inertia as a variable parameter. Any links would be cool.

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Old 05-16-21, 06:32 AM
  #147  
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Originally Posted by PeteHski View Post
But you can see from the model that the affect would be so small as to make no difference. Even if they had totally ignored the wheel inertia it wouldn't have had any significant affect on their test results.
It's also worth noting that while the power to overcome the major ******ing forces scale with speed (either to the first or third power), wheel and other inertial effects scale with acceleration. Outside of starts on the track, bicycle accelerations are orders of magnitude smaller than speed. Alex Simmons has some nice examples. https://wattmatters.blog/home/2013/0...-of-parts.html though not exactly on the question here.
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Old 05-16-21, 10:15 AM
  #148  
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Originally Posted by PeteHski View Post
I haven't seen a cycling power model yet that allows you to plug in wheel inertia values to make any comparison between wheel sets (assuming you even knew exactly what moment of inertia each wheel set actually had).
Oh, those models exist--I worked on one about a decade ago, and reviewed another one this last year. In both of those cases we were focused on track pursuit where times are recorded to the millisecond. An oddity of track is that wheel speed is different from CG speed in the turns and although the entire mass accelerates and decelerates twice per lap, the wheels accelerate and decelerate more.

Edited to add: That said, I usually ignore it in my field tests not because it's zero but because most consumer-level devices record only at 1 Hz so the wheel inertia contribution gets swamped. On the track we can get higher sampling rates so it's not really a matter of the physics, it's a matter of measurement.

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Old 05-16-21, 11:00 AM
  #149  
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Originally Posted by RChung View Post
Oh, those models exist--I worked on one about a decade ago, and reviewed another one this last year. In both of those cases we were focused on track pursuit where times are recorded to the millisecond. An oddity of track is that wheel speed is different from CG speed in the turns and although the entire mass accelerates and decelerates twice per lap, the wheels accelerate and decelerate more.

Edited to add: That said, I usually ignore it in my field tests not because it's zero but because most consumer-level devices record only at 1 Hz so the wheel inertia contribution gets swamped. On the track we can get higher sampling rates so it's not really a matter of the physics, it's a matter of measurement.
Sure, just to be clear, the physics of the wheel inertia is pretty simple and any half decent model is likely to include it, but not usually as a variable parameter in the user interface. It only gets more complicated when you start talking about subjective differences in "feel" between wheel sets. Can you really quantify that in any meaningful way?

In track pursuit I can see how these lower order variables might be worth studying to some degree, but in general road cycling it just seems totally insignificant. I think it's more than enough to compare wheels by their mass and aero efficiency alone. The rule of thumb stating that rotating mass is worth double static mass is pretty misleading.
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