Does rotating weight really matter?
#126
For The Fun of It
Join Date: May 2007
Location: Louisissippi Coast
Posts: 5,851
Bikes: Lynskey GR300, Lynskey Backroad, Litespeed T6, Lynskey MT29, Burley Duet
Mentioned: 12 Post(s)
Tagged: 0 Thread(s)
Quoted: 2135 Post(s)
Liked 1,646 Times
in
828 Posts
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.
Likes For Paul Barnard:
#127
Full Member
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.
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.
#128
Senior Member
Join Date: Oct 2015
Location: San Diego, California
Posts: 4,077
Bikes: Velo Orange Piolet
Mentioned: 28 Post(s)
Tagged: 0 Thread(s)
Quoted: 2228 Post(s)
Liked 2,011 Times
in
972 Posts
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.
#129
Klaatu..Verata..Necktie?
Join Date: May 2007
Location: SF Bay Area
Posts: 17,958
Bikes: Litespeed Ultimate, Ultegra; Canyon Endurace, 105; Battaglin MAX, Chorus; Bianchi 928 Veloce; Ritchey Road Logic, Dura Ace; Cannondale R500 RX100; Schwinn Circuit, Sante; Lotus Supreme, Dura Ace
Mentioned: 41 Post(s)
Tagged: 0 Thread(s)
Quoted: 10424 Post(s)
Liked 11,886 Times
in
6,090 Posts
Nothing matters.
__________________
"Don't take life so serious-it ain't nohow permanent."
"Everybody's gotta be somewhere." - Eccles
"Don't take life so serious-it ain't nohow permanent."
"Everybody's gotta be somewhere." - Eccles
#130
just another gosling
Join Date: Feb 2007
Location: Everett, WA
Posts: 19,528
Bikes: CoMo Speedster 2003, Trek 5200, CAAD 9, Fred 2004
Mentioned: 115 Post(s)
Tagged: 0 Thread(s)
Quoted: 3886 Post(s)
Liked 1,938 Times
in
1,383 Posts
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.
__________________
Results matter
Results matter
#131
Senior Member
Join Date: Jul 2009
Posts: 5,948
Bikes: Colnago, Van Dessel, Factor, Cervelo, Ritchey
Mentioned: 5 Post(s)
Tagged: 0 Thread(s)
Quoted: 3949 Post(s)
Liked 7,295 Times
in
2,946 Posts
#132
Habitual User
Join Date: Jan 2020
Location: Altadena, CA
Posts: 7,981
Bikes: 2023 Niner RLT 9 RDO, 2018 Trek Procaliber 9.9 RSL, 2018 Storck Fascenario.3 Platinum, 2003 Time VX Special Pro, 2001 Colnago VIP, 1999 Trek 9900 singlespeed, 1977 Nishiki ONP
Mentioned: 5 Post(s)
Tagged: 0 Thread(s)
Quoted: 4943 Post(s)
Liked 8,083 Times
in
3,824 Posts
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.
__________________
"Swedish fish. They're protein shaped." - livedarklions
"Swedish fish. They're protein shaped." - livedarklions
#133
Senior Member
Join Date: Dec 2009
Location: Denver area (Ken Caryl Valley)
Posts: 1,803
Bikes: 2022 Moots RCS, 2014 BMC SLR01 DA Mech, 2020 Santa Cruz Stigmata, Ibis Ripmo, Trek Top Fuel, Specialized Levo SL, Norco Bigfoot VLT
Mentioned: 8 Post(s)
Tagged: 0 Thread(s)
Quoted: 464 Post(s)
Liked 182 Times
in
118 Posts
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.
#134
Senior Member
Likes For asgelle:
#135
Senior Member
Join Date: Dec 2009
Location: Denver area (Ken Caryl Valley)
Posts: 1,803
Bikes: 2022 Moots RCS, 2014 BMC SLR01 DA Mech, 2020 Santa Cruz Stigmata, Ibis Ripmo, Trek Top Fuel, Specialized Levo SL, Norco Bigfoot VLT
Mentioned: 8 Post(s)
Tagged: 0 Thread(s)
Quoted: 464 Post(s)
Liked 182 Times
in
118 Posts
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.
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.
#136
climber has-been
Join Date: Dec 2004
Location: Palo Alto, CA
Posts: 7,102
Bikes: Scott Addict R1, Felt Z1
Mentioned: 10 Post(s)
Tagged: 0 Thread(s)
Quoted: 3427 Post(s)
Liked 3,563 Times
in
1,790 Posts
#137
Senior Member
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.
#138
Senior Member
Join Date: May 2021
Posts: 8,415
Mentioned: 12 Post(s)
Tagged: 0 Thread(s)
Quoted: 4397 Post(s)
Liked 4,843 Times
in
2,997 Posts
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.
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.
Likes For PeteHski:
#139
Senior Member
#140
Senior Member
Join Date: Mar 2010
Posts: 1,659
Mentioned: 7 Post(s)
Tagged: 0 Thread(s)
Quoted: 1248 Post(s)
Liked 1,323 Times
in
674 Posts
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.
#141
Senior Member
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.
Likes For asgelle:
#142
Senior Member
Thread Starter
Join Date: Jul 2006
Location: Lebanon (Liberty Hill), CT
Posts: 8,473
Bikes: CAAD 12, MASI Gran Criterium S, Colnago World Cup CX & Guru steel
Mentioned: 6 Post(s)
Tagged: 0 Thread(s)
Quoted: 1743 Post(s)
Liked 1,281 Times
in
740 Posts
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.
#143
Senior Member
Join Date: May 2021
Posts: 8,415
Mentioned: 12 Post(s)
Tagged: 0 Thread(s)
Quoted: 4397 Post(s)
Liked 4,843 Times
in
2,997 Posts
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.
#144
Senior Member
Join Date: May 2021
Posts: 8,415
Mentioned: 12 Post(s)
Tagged: 0 Thread(s)
Quoted: 4397 Post(s)
Liked 4,843 Times
in
2,997 Posts
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.
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.
Likes For PeteHski:
#145
Senior Member
Join Date: May 2021
Posts: 8,415
Mentioned: 12 Post(s)
Tagged: 0 Thread(s)
Quoted: 4397 Post(s)
Liked 4,843 Times
in
2,997 Posts
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.
Last edited by PeteHski; 05-16-21 at 06:16 AM.
#146
Senior Member
#147
Perceptual Dullard
Join Date: Sep 2009
Posts: 2,417
Mentioned: 36 Post(s)
Tagged: 0 Thread(s)
Quoted: 918 Post(s)
Liked 1,148 Times
in
490 Posts
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.
Last edited by RChung; 05-16-21 at 10:42 AM.
#148
Senior Member
Join Date: May 2021
Posts: 8,415
Mentioned: 12 Post(s)
Tagged: 0 Thread(s)
Quoted: 4397 Post(s)
Liked 4,843 Times
in
2,997 Posts
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.
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.
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.