# Rotating weight versus static weight?



## gordy748 (Feb 11, 2007)

Hi all. As an ex motorcycle racer, I learned a lot about rotating weight and centrifugal force created by heavy wheels, not to mention the effects on braking and acceleration. I accept that bicycles are very mildly affected by these things, as few people can do 40 - 80 mph inside 3 seconds or vice versa. But they're still affected by them, nonetheless. 

I've seen lots of great posts about removing a pound of weight to achieve X seconds faster time over Y distance on Z gradients, but what about the impact of taking a pound off your rims versus your frame (or a$$?). Granted it'll be less than a motorbike, but what is the difference?


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## Mike T. (Feb 3, 2004)

gordy748 said:


> what about the impact of taking a pound off your rims versus your frame (or a$$?).


I'll guess that the wheels will spin up with less effort, slow down faster and when up to speed make no more difference than a pound off anywhere else on the bike or the body. Scientists, how did I do?


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## krisdrum (Oct 29, 2007)

Mike - I believe that seems to be the general concensus.

I'll add that it seems to be the case with actual rotating weight, so the rims, tires/tubes, nipples, and to some extent the spokes. The hubs have minimal impact (if any) on Mike's proposed effects. Or at least that is my understanding.


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## Pirx (Aug 9, 2009)

gordy748 said:


> I've seen lots of great posts about removing a pound of weight to achieve X seconds faster time over Y distance on Z gradients, but what about the impact of taking a pound off your rims versus your frame (or a$$?). Granted it'll be less than a motorbike, but what is the difference?


Like you said, given the usually moderate to slow accelerations of bicycles, rotating weight is mostly irrelevant. It does make a bit of a different under extreme acceleration, so if you race, in particular if you race professionally, and for crits in particular, that fraction of a second when you "jump" for that final sprint could make a difference. For a hobby rider it is completely and utterly irrelevant, meaning it doesn't matter if it's rotating weight in the rim of your wheel, or your frame, or your @ss...


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## ericm979 (Jun 26, 2005)

The change in rotating weight does not have much effect on accelleration. See Bicycle performance - Wikipedia, the free encyclopedia

Mass on the tire has about twice the kinetic energy of mass on the frame (or rider). Losing say 220g off the tires would equate to 1 lb off the frame. Now pour out 16oz of water from your bottle, then accellerate. How much faster are you? Not much. The effect is small and in most situations, heavier but more aerodynamic wheels would be more of an advantage.

Lighter rims *feel* faster, but a lot of that is in the rider's head. Maybe it's the lighter feel when rocking the bike side to side. Or placebo effect. Or both.


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## Mike T. (Feb 3, 2004)

ericm979 said:


> Lighter rims *feel* faster, but a lot of that is in the rider's head. Maybe it's the lighter feel when rocking the bike side to side. Or placebo effect. Or both.


My lighter wheels (1410 grams) undoubtedly "feel" faster on acceleration than my heaviest wheels (1852 grams) but that pound of weight loss doesn't pan out in faster overall ride speed. Between the above two sets plus a set of carbon 50mm deep wheels (1579g) over three years of recorded average speed data there is just 0.3mph of difference. The day-to-day differences in my rides due to other factors (time of year, temp, humidity, wind, feeling, etc etc) is a whopping 3mph. So, home-testing of wheelsets for the benefits of lightness or aerodynamics (unless it was limited to roll-down testing), for me anyway, is totally useless.


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## rruff (Feb 28, 2006)

ericm979 said:


> Mass on the tire has about twice the kinetic energy of mass on the frame (or rider). Losing say 220g off the tires would equate to 1 lb off the frame.
> 
> Lighter rims *feel* faster, but a lot of that is in the rider's head. Maybe it's the lighter feel when rocking the bike side to side. Or placebo effect. Or both.


For accelerations, rotating weight that travels at you bike speed (like rims, tubes, and tires) has 2x the effect of weight elsewhere. 

Light wheels *do* make a bike feel and handle quicker. The spinning wheel has a fair amount of momentum and resists accelerations that take it out of plane. I also suspect that the sprinter's preference for light wheels might be based on a real effect... *if* their style involves a lot of side to side motion. The heavier wheels will resist this to a greater degree and result in a power loss. This is in addition to the extra power needed to just spin up a wheel.


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## Kerry Irons (Feb 25, 2002)

*How am I doing?*



Mike T. said:


> I'll guess that the wheels will spin up with less effort, slow down faster and when up to speed make no more difference than a pound off anywhere else on the bike or the body. Scientists, how did I do?


You did great!


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## Mike T. (Feb 3, 2004)

Kerry Irons said:


> You did great!


I should have as I got it all from you I think


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## chase196126 (Jan 4, 2008)

Check out this excellent post on weigh weenies: 
Weight Weenies • View topic - Rotating Mass...

Basically when racing a lighter wheelset will not accelerate noticeable faster than a lighter wheelset. Even going from a super heavy 1800 gram set of wheels to 900 gram ultra lights will not make a noticeable difference in your ability to accelerate. 

Sure, if the choice is between two wheels with identical aerodynamic profiles you might want to choose the lighter set, but in the real world this is not a likely scenario. Most of the rims that are cutting edge aerodynamically will weigh more than those that are simple V shapes and light weight. Lets say your choice is between a set of Firecrest Zipp 404s and a set of Reynolds DV46 tubulars. The Reynolds will weigh around 300 grams less than the Zipps, but their less aero rim shape will have a MUCH larger detrimental influence on your accelerations and overall speed than the modest reduction in weigh could ever offer.


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## TomH (Oct 6, 2008)

Mike T. said:


> My lighter wheels (1410 grams) undoubtedly "feel" faster on acceleration than my heaviest wheels (1852 grams) but that pound of weight loss doesn't pan out in faster overall ride speed.


Thats the winning comment. 

You wont "feel" a pound of frame weight. Try riding without a water bottle, you wont notice. You'll feel something with a pound of wheel missing though. 

Tangibly, you're seconds faster over an hour or so of riding. If you're not racing, its irrelevant. 

You can take it further and talk about stiffness, now were into the realm of completely intangible gains in speed.. but stiff wheels feel better, no one likes a sloppy, floppy wheel.

Its a hobby, it should be fun. If you can afford it, you should ride gear that makes you happy. The real difference is understanding what that gear is actually doing for you in terms of performance.. little to nothing. In terms of feel? Big difference. Ive found that Well built high spoke wheels that are mid 1500g are completely reliable for me, and I can build up a stiff set. They're just fun to ride. I sold off my 2100g set a long time ago and dont miss them, but Im certainly not faster! 

If you do the hard math on a 2500g wheelset vs a 1300g wheelset, the raw numbers are pretty small. They're not much faster, on paper it looks pretty bad. If you ride the two, you'd have to be dead not to notice a drastic difference... but smart consumers should understand the difference between "feel", and real gains


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## tarwheel2 (Jul 7, 2005)

All theories and formulas aside, most cyclists seem to notice heavier wheels. Undoubtedly some of it is psychological. However, I have several years worth of commuting data to back up my contention that heavier wheels are slower. Most of my bikes have the old standby 32H Ultegra hubs with Open Pro rims. Two years ago, I had some new wheels built with 36H Ultegra hubs and Velocity Dyad rims. The wheels felt heavier and slower, and my commuting data bear that out. Here are the stats from the 3 main bikes I commute on, carrying similar loads:

Bob Jackson World Tour, Dyad wheels - 2800 miles, 14.56 mph avg
Salsa Casseroll, OP wheels - 3080 miles, 15.60 mph avg 
Waterford RST, OP wheels - 989 miles, 15.29 mph avg

The mileage on these bikes is mostly commuting over the same route, with occasional longer weekend rides and tours. The Jackson and Waterford both had Continental GP 4 Season 28 mm tires, and the Casseroll had Michelin Pro2 Race 25 mm tires (which actually measure 27 mm wide), but the tires all weigh the same. The frames are all steel and weigh about the same, and are set up with similar components.

The Bob Jackson's average speed is a full 1 mph slower than the Salsa and 0.7 mph slower than the Waterford. I have only owned the Waterford since late August and suspect that it's average speed would be very close to the Salsa if I had ridden over the course of a year since most of my riding on it has been during cold weather.

BTW, the Dyad wheels are probably more aero than the OPs because they are higher profile, but they are heavier. Weight is probably a bigger factor for than aerodynamics because most of my riding is on hilly roads.


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## Pirx (Aug 9, 2009)

rruff said:


> For accelerations, rotating weight that travels at you bike speed (like rims, tubes, and tires) has 2x the effect of weight elsewhere.


Yep, that's correct. The factor of two is for wheels that have all of the weight concentrated at the outer perimeter, however. For real wheels, rim weight is more important than spokes, or hubs, as far as rotational effects are concerned.



rruff said:


> The spinning wheel has a fair amount of momentum and resists accelerations that take it out of plane.


Hmm, yes, angular momentum conservation has an effect, but keep in mind that the reaction moment is exactly orthogonal to the imposed non-axial rotation, so it does _not_ appear in the energy balance. In the case of the sprinter rocking the bike side-to-side, the reaction moment is in the vertical axis. This will affect steering, of course (that's part of the story of how bicycles can stabilize themselves), but it does not affect required power.



rruff said:


> The heavier wheels will resist this to a greater degree and result in a power loss.


No, there is no power loss associated with angular momentum conservation, see above. Or did I misunderstand what you were saying? Of course, the inertial moment of the wheel around an axis parallel to the bike and the ground has an effect as well, but if we assume the motion is periodic, then there's no obvious energy loss associated with that one, either.


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## krisdrum (Oct 29, 2007)

tarwheel2 said:


> All theories and formulas aside, most cyclists seem to notice heavier wheels. Undoubtedly some of it is psychological. However, I have several years worth of commuting data to back up my contention that heavier wheels are slower. Most of my bikes have the old standby 32H Ultegra hubs with Open Pro rims. Two years ago, I had some new wheels built with 36H Ultegra hubs and Velocity Dyad rims. The wheels felt heavier and slower, and my commuting data bear that out. Here are the stats from the 3 main bikes I commute on, carrying similar loads:
> 
> Bob Jackson World Tour, Dyad wheels - 2800 miles, 14.56 mph avg
> Salsa Casseroll, OP wheels - 3080 miles, 15.60 mph avg
> ...


Not to throw a wrench in your data and assumptions, but are the 3 bikes set-up identically and fitted to you in the same manner? In other words, I question whether you are generating the same power/being as efficient and/or catching more/less wind on one or more of the bikes to see the 1 mph difference. There are alot of confounding variables in there that may not be accounted for.


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## rruff (Feb 28, 2006)

Pirx said:


> No, there is no power loss associated with angular momentum conservation, see above. Or did I misunderstand what you were saying?


The wheels resist the motion that the rider is imparting on them, so some of his power must go to overcoming this resistance. If the wheels and bike were weightless, there would be no resistance to overcome. 

I'll admit my understanding of this is rusty... dates back to college which was 30 years ago... But f you are ambitious and have the time you could calculate about how much power is absorbed.


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## jnbrown (Dec 9, 2009)

Finally some sanity here.
There is nothing wrong with lighter wheels as long as durability is not sacrificed and it makes sense financially. The speed gain as stated already is very small and the most noticeable effect is "feel".
I think where weight matters is the bike as whole. If you can reduce by 1/4 lb in 10 places then the overall weight savings is 2.5 lbs which would be much more significant on a long and/or steep climb but still not all that much. Wheel "spin up" is a complete myth. If you don't believe it, take the wheel off your bike and spin it up to speed with one finger. Now how many watts did your one finger put out?


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## JimP (Dec 18, 2001)

One area I haven't seen mentioned is climbing. The wheels are accelerated for each pedal stroke more than acceleration on the level. Now I have heard that it is my problem with poor pedal stroke but if you watch even the top pros on a steep hill you can see that they too don't have a perfectly smooth pedal stroke while climbing. So, a heavy wheel with more mass to accelerate or a lighter one, which one is better for climbing? It would depend on where the mass is located. If the mass is in the rim and tire, as noted, the wheel has more rotational mass which will affect the acceleration of the wheel. I have a Sun Mistral M14 rim at 295g with a Conti Podium tire. That wheel will accelerate with any "climbing wheel" even though it has a heavy Phil Wood hub.


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## krisdrum (Oct 29, 2007)

jnbrown said:


> Wheel "spin up" is a complete myth. If you don't believe it, take the wheel off your bike and spin it up to speed with one finger. Now how many watts did your one finger put out?


Huh? Last I checked, the tire is contacting the ground when we are seated on a bicycle and has to over come other forces besides air resistance when "spinning up". So I'm not following how that is an accurate depiction of the effort needed to spin a wheel up to speed?


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## tarwheel2 (Jul 7, 2005)

krisdrum said:


> Not to throw a wrench in your data and assumptions, but are the 3 bikes set-up identically and fitted to you in the same manner? In other words, I question whether you are generating the same power/being as efficient and/or catching more/less wind on one or more of the bikes to see the 1 mph difference. There are alot of confounding variables in there that may not be accounted for.


The bikes are all set up to fit essentially the same -- crank arms, saddle height, handlebar height, reach. All of my bikes have the handlebar tops even with the saddle height. Saddle is positioned the same with regard to the crank. The frame/fork weights are very similar and all have mostly Ultegra components with a few DA bits.

I also compared average speeds on the Bob Jackson before and after I installed the Dyad wheels. The average speed with Open Pros was 0.3 mph faster than with Dyads. All of these data are averaged over many, many commutes totaling several thousand miles.


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## jnbrown (Dec 9, 2009)

krisdrum said:


> Huh? Last I checked, the tire is contacting the ground when we are seated on a bicycle and has to over come other forces besides air resistance when "spinning up". So I'm not following how that is an accurate depiction of the effort needed to spin a wheel up to speed?


The only forces due to the wheel contacting the ground are frictional which would have almost nothing to do with wheel weight. Get it now?


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## Pirx (Aug 9, 2009)

rruff said:


> I'll admit my understanding of this is rusty... dates back to college which was 30 years ago... But f you are ambitious and have the time you could calculate about how much power is absorbed.


This is easier if you pick the right reference system. Ask yourself this question: Imagine holding a spinning wheel, and then rocking it back and forth around an axis that is orthogonal to the axis of rotation. What happens with the power you claim is absorbed? Do you think the wheel will speed up/slow down, or just keep spinning?


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## Pirx (Aug 9, 2009)

JimP said:


> The wheels are accelerated for each pedal stroke more than acceleration on the level.


This is true, in principle, but the accelerations during the pedal stroke you are referring to are minute.



JimP said:


> So, a heavy wheel with more mass to accelerate or a lighter one, which one is better for climbing?


In addition, the heavy wheel with its higher inertia tends to slow down less during the low-torque portions of your pedal stroke. So the answer is, wheel rotational inertia makes no noticeable _objective_ difference for climbing performance.




jnbrown said:


> Wheel "spin up" is a complete myth.


Umm, no, not if you understand the phsyics. There is energy in the rotational motion of the wheel, which is in addition to the translational energy. It requires power to provide that energy.



jnbrown said:


> If you don't believe it, take the wheel off your bike and spin it up to speed with one finger. Now how many watts did your one finger put out?


The amount of _Joules_ (=Watts*seconds) your finger put out will be exactly the same amount as the energy that is now in the spinning wheel, one half times the inertial moment times the rate of rotation squared, if you must know.



jnbrown said:


> The only forces due to the wheel contacting the ground are frictional which would have almost nothing to do with wheel weight. Get it now?


No idea what you are trying to say here.


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## jnbrown (Dec 9, 2009)

Pirx said:


> Umm, no, not if you understand the phsyics. There is energy in the rotational motion of the wheel, which is in addition to the translational energy. It requires power to provide that energy.
> 
> 
> 
> The amount of _Joules_ (=Watts*seconds) your finger put out will be exactly the same amount as the energy that is now in the spinning wheel, one half times the inertial moment times the rate of rotation squared, if you must know.


The point being it is very small.


[/QUOTE]No idea what you are trying to say here.[/QUOTE]

Just stating what forces come into play when the wheel is touching the ground compared to not touching the ground. Also known as rolling resistance. What is not to understand?


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## Pirx (Aug 9, 2009)

jnbrown said:


> The point being it is very small.


Yes, and you are quite right about that. Like I said, unless fractions of a second under the most extreme conditions count, then there's no reason to worry about rotational mass.



jnbrown said:


> What is not to understand?


I just did not understand the point in the context of the argument that's going on here, but nevermind, it's not important.


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## rruff (Feb 28, 2006)

jnbrown said:


> If you don't believe it, take the wheel off your bike and spin it up to speed with one finger. Now how many watts did your one finger put out?


The test I prefer is to put your bike on a trainer, but leave the rear wheel "free". Now hop on and see how hard it is to spin the wheel up to 40 mph. How much quicker would you get there if the inertia was cut in half?


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## Pirx (Aug 9, 2009)

rruff said:


> The test I prefer is to put your bike on a trainer, but leave the rear wheel "free". Now hop on and see how hard it is to spin the wheel up to 40 mph. How much quicker would you get there if the inertia was cut in half?


Excellent example. You could actually do this exact experiment once with your full wheel, and once after you have taken the tire and tube off. This way you will get a good feeling for the difference between a light and heavy rim. You may be surprised.


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## rruff (Feb 28, 2006)

Pirx said:


> This is easier if you pick the right reference system. Ask yourself this question: Imagine holding a spinning wheel, and then rocking it back and forth around an axis that is orthogonal to the axis of rotation. What happens with the power you claim is absorbed? Do you think the wheel will speed up/slow down, or just keep spinning?


It takes energy to move it back and forth... more energy for a spinning wheel than a non spinning wheel. The power is supplied by the rider, which is power that isn't being used to turn the cranks around.


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## nightfend (Mar 15, 2009)

...and in the trainer you still are overcoming the drivetrain resistance as well. So it would be even less than what you'd feel spinning it up on the trainer.


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## rruff (Feb 28, 2006)

Pirx said:


> You may be surprised.


Yes, it only takes a small fraction of a second to get my heavy trainer wheel up to 40 mph... one hard stoke of the pedal.


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## Pirx (Aug 9, 2009)

rruff said:


> It takes energy to move it back and forth... more energy for a spinning wheel than a non spinning wheel.


Where does the energy go? How about the non-spinning wheel? Does it not "take energy" to move that one back and forth? If yes, where does that energy go? Think about it.


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## Mike T. (Feb 3, 2004)

Pirx said:


> Where does the energy go? How about the non-spinning wheel? Does it not "take energy" to move that one back and forth? If yes, where does that energy go? Think about it.


It is dissipated as heat energy by the person.


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## Pirx (Aug 9, 2009)

Mike T. said:


> It is dissipated as heat energy by the person.


There is no person in my experiment. People tend to just cloud the issues... 

Seriously now, imagine the rocking motion being caused by a spring: Let's say we mount the spinning wheel with its axle held horizontally (so the plane of the spinning wheel is vertical) in a fork that's held horizontal by a torsion bar, and everything supported such that the fork can only rotate around a single axis normal to the axis of revolution of the wheel. The fork is perfectly straight (not a real bicycle fork), so the axis around which the fork can rotate intersects the axle of the wheel. We then start the experiment by rotating the fork so that the plane of rotation of the wheel is off the vertical, and let go. Now idealize to assume friction-free bearings, no aerodynamic drag, and a perfectly elastic torsion bar. What happens? Now where does the energy, if any, go?


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## Mike T. (Feb 3, 2004)

Pirx said:


> There is no person in my experiment. People tend to just cloud the issues...


Oh I didn't read this then - "Imagine holding a spinning wheel, and then rocking it back and forth around an axis........ What happens with the power you claim is absorbed?"

So if it isn't a person or your imaginary friend imparting the energy, who or what is it? If you do have an imaginary friend then you will fit in quite well around here.


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## Pirx (Aug 9, 2009)

Mike T. said:


> Oh I didn't read this then - "Imagine holding a spinning wheel, and then rocking it back and forth around an axis........ What happens with the power you claim is absorbed?"


That's entirely beside the point. In principle, there is no difference if a person does this, other than it being easy to claim that somehow, mysteriously, "energy is dissipated".



Mike T. said:


> So if it isn't a person or your imaginary friend imparting the energy, who or what is it? If you do have an imaginary friend then you will fit in quite well around here.


Careful now. You're really not in any position to comment on theoretical mechanics, let alone argue same with me. The experiment I have outlined in my previous post describes what is known as a Hamiltonian system, with fully holonomic constraints. Everything follows from there. Feel free to look it up. No energy is lost or gained. Fundamental mechanics, my friend.


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## Mike T. (Feb 3, 2004)

Pirx said:


> That's entirely beside the point. In principle, there is no difference if a person does this, other than it being easy to claim that somehow, mysteriously, "energy is dissipated". The experiment I have outlined in my previous post describes what is known as a Hamiltonian system, with fully holonomic constraints. Everything follows from there. Feel free to look it up. No energy is lost or gained. Fundamental mechanics, my friend.


Do you people who think, talk and write about this stuff ever *ride*?


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## rruff (Feb 28, 2006)

Pirx said:


> Seriously now, imagine the rocking motion being caused by a spring... What happens? Now where does the energy, if any, go?


Your example does not resemble the real system. The rocking motion of the bike side to side is caused by physical exertion... it is nothing like a spring at all. 

When the cyclist is pushing down with the right leg, he pulls the bike to the left to facilitate the motion and to balance forces. The energy required to accelerate, and then decelerate and reverse the direction (repeat) of the bike requires energy. It is related to the inertia of the bike in this axis and rate of acceleration. Heavy spinning wheels have more resistance than light spinning wheels... ie more of his energy will be wasted just to make the bike tilt back and forth.


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## Pirx (Aug 9, 2009)

Mike T. said:


> Do you people who think, talk and write about this stuff ever *ride*?


At peak performance, I'm just a bit shy of 4W/kg FTP, max power on a good day 900W, maybe 1,000W if it's a REALLY good day. Although I don't race anymore I am doing o.k. among Cat3s. Anything else you need to know?


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## Pirx (Aug 9, 2009)

rruff said:


> When the cyclist is pushing down with the right leg, he pulls the bike to the left to facilitate the motion and to balance forces.


Oh please, not this sh!t again. This is exactly like those clueless discussions about the performance impact of frame stiffness. I once swore that I will not comment on stuff like that anymore, and I will stick to my promise. Today, anyway... 

A bit disappointing, though, coming from you.


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## rruff (Feb 28, 2006)

What is controversial about it taking energy to tilt a bicycle side to side at 100+ rpm?


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## Bill Bikie (Jul 24, 2010)

I live in the Coolee region, where Wisconsin, Minnesota, Illnois, and Iowa come together. Your either on a flood plane or your climbing through, up, around or down the bluffs. 

I plan on doing some competitive hill climbing this summer, therefore I'm opting for HED Ardennes SL 1450gm wheels to get up the hill more quickly. My Pinarello came with enrty level Fulcrum 5 wheels which are 300gms heavier than the HEDS. 

I feel the lighter wheels will be an advantage for hill climbing. And of course no advantage during a steady-state effort on the flat. Pros have the advantage of many types of wheels, some more subtlely advantages than others.


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## Mike T. (Feb 3, 2004)

Pirx said:


> At peak performance, I'm just a bit shy of 4W/kg FTP, max power on a good day 900W, maybe 1,000W if it's a REALLY good day. Although I don't race anymore I am doing o.k. among Cat3s. Anything else you need to know?


OMG I'm in the presence of greatness!  (Mike T. bowing deeply)


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## Pirx (Aug 9, 2009)

rruff said:


> What is controversial about it taking energy to tilt a bicycle side to side at 100+ rpm?


Alright, here are some of my thoughts on this topic: 

First of all, implicit in your question is the assumption that the human musculo-skeletal system always consumes energy whenever forces are exerted, no matter whether the forces are parallel or anti-parallel to the direction of motion. If that was so, then any motion associated with a force would always consume power, and never recover any. As a consequence, indeed any periodic motion that from a purely mechanical point of view would neither consume nor generate any power, would always consume power for the human. 

Unfortunately, that assumption is false: Our musculo-skeletal system is indeed often capable of recovering part of the mechanical energy exchanged during the cycle of a periodic motion. It's not perfect, but the associated efficiency is certainly non-zero.

As an aside, a corollary assumption that is often made is that we consume power while just holding a weight, without any motion. Again, while the mechanical power is zero, people assume, "because it takes an effort" to hold that weight, that metabolic power is consumed. Again, that assumption is not correct, or at least inaccurate. The fact that exerting a force takes a muscular effort, and will tire out your muscles eventually, does not necessarily mean that your muscles in fact need to consume a significant amount of power. My colleagues in the bio-medical field have investigated this quite extensively, and it turns out that the ATP consumption of muscle cells during pure holding efforts even at high forces, although not entirely zero (due to tiny muscle movements that cannot be avoided), is quite small.

I should say that I'm not really an expert in this area, which is one of the reasons I avoid going there. But I know that, for example, running on a level surface does take a lot less power than one might think. Yes, we all know it's not quite as efficient as riding a bike on such a surface, but it's not completely terrible, either. As an even better example, the mode of locomotion of kangaroos, for example, is a lot more efficient than the above naive assumptions might suggest.

Finally, now, when we are talking about that sprinter rocking back and forth on his bike, we need to appreciate that we are considering a system that is quite complex. It's not clear at all what effects the increased resistance of the spinning wheels to the rocking motion might have. What is fairly obvious is that the dynamics of the sprinter-bike system will change, but what exactly those changes are, let alone what the effect of those changes on the actual power consumption of the sprinter versus the power output at the wheel are, is completely unclear. On top of that, let's not forget that power consumption alone might not even be sufficient to judge which wheel is "better": If two wheels result in the same power consumption for the same power at the wheel, but somehow due to the particular way forces are distributed over one cycle of the motion, one tires out the sprinter's muscles faster than the other, we might prefer the latter. 

In any case, what is clear is that, _if_ there are such effects, then they are entirely due to changes in the bio-mechanical efficiency of the sprinter as an _indirect_ effect of the different dynamics of those wheels. I for one am not knowledgeable enough to predict with any confidence what the result might be. I am not sure there is anybody who could tell us for sure, without having performed controlled experiments to find out.


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## Pirx (Aug 9, 2009)

Mike T. said:


> OMG I'm in the presence of greatness!  (Mike T. bowing deeply)


Heh, I don't feel so great when I get dropped by the guys in my group, which happens quite regularly...  Builds character, though.


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## Mike T. (Feb 3, 2004)

Pirx said:


> Heh, I don't feel so great when I get dropped by the guys in my group, which happens quite regularly...  Builds character, though.


You sound like a tiger to me Pir.


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## rruff (Feb 28, 2006)

Pirx said:


> If two wheels result in the same power consumption for the same power at the wheel, but somehow due to the particular way forces are distributed over one cycle of the motion, one tires out the sprinter's muscles faster than the other, we might prefer the latter.


Don't get too carried away here. If we were talking about whether or not it is "good" to have a lot of side to side motion in sprint, then your points would make sense. That isn't what we are talking about though... I've already assumed that this rider prefers a lot of side to side motion. So will a high inertia system (ie heavy wheels) suck more energy than a low one? The cyclists motions are exactly the same in both cases, only with higher inertia, more energy is needed. You can argue that the arms and upper body are supplying this energy and it has little negative effect on the power produced at the cranks, because the effort is short and anaerobic... but it is still something.


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## Pirx (Aug 9, 2009)

rruff said:


> The cyclists motions are exactly the same in both cases,


No, they're not. But I have spent more than enough time on this by now, severely violating that pledge I had made, so I'll just leave it at that.


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## vettracer (Jan 12, 2011)

Pirx said:


> There is no person in my experiment. People tend to just cloud the issues...
> 
> Seriously now, imagine the rocking motion being caused by a spring: Let's say we mount the spinning wheel with its axle held horizontally (so the plane of the spinning wheel is vertical) in a fork that's held horizontal by a torsion bar, and everything supported such that the fork can only rotate around a single axis normal to the axis of revolution of the wheel. The fork is perfectly straight (not a real bicycle fork), so the axis around which the fork can rotate intersects the axle of the wheel. We then start the experiment by rotating the fork so that the plane of rotation of the wheel is off the vertical, and let go. Now idealize to assume friction-free bearings, no aerodynamic drag, and a perfectly elastic torsion bar. What happens? Now where does the energy, if any, go?


Well, you did have to work to move the wheel off center and energize the torsion bar. And you are locking out the precessional gyroscopic forces with this imaginary setup. 

You do not get back the initial energy you put it in the system even though it oscillates forever.


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## gordy748 (Feb 11, 2007)

Whether any of you get dropped on rides or not, you've all proved yourselves rather knowledgeable and frightening. I'll take my lighty-light carbon wheels and will run with them. Or pedal. Ta muchly!


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## Pirx (Aug 9, 2009)

vettracer said:


> Well, you did have to work to move the wheel off center and energize the torsion bar.


Of course. But the point is, no energy is lost. 



vettracer said:


> And you are locking out the precessional gyroscopic forces with this imaginary setup.


If that bothers you, then just put the whole contraption on a turntable. Similarly to what you saw before, the whole shebang will keep wobbling forever, just in a more complicated fashion.


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## woodys737 (Dec 31, 2005)

tarwheel2 said:


> The bikes are all set up to fit essentially the same -- crank arms, saddle height, handlebar height, reach. All of my bikes have the handlebar tops even with the saddle height. Saddle is positioned the same with regard to the crank. The frame/fork weights are very similar and all have mostly Ultegra components with a few DA bits.
> 
> I also compared average speeds on the Bob Jackson before and after I installed the Dyad wheels. The average speed with Open Pros was 0.3 mph faster than with Dyads. All of these data are averaged over many, many commutes totaling several thousand miles.


Can you attribute any of the difference to the difference in tires? From my own experience tires make a noticeable difference in speed while wheels do not.


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## aclinjury (Sep 12, 2011)

two wheelsets with a total 200g - 400g difference won't make you notice much on the flat.

but a 400g difference on a 7 mile climb averaging 8% - 9% with some segments of 15%,17%, 20%... your legs and lungs WILL feel the difference by the end of the climb!

And on such climb, taking off 1 water bottle from your frame will also make you feel the difference by the end of the climb.


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## Pitts Pilot (Dec 5, 2011)

Hold a wheel out in front of you with both arms extended. Give it a good spin forward. Now try to quickly lean the tire to the left, as it would if you were quickly leaning the bike to the left. You will find that the leading edge of the tire TURNS quickly to the left. Forces placed on a spinning wheel have an effect 90 degrees further along the rotation than where the force is applied. (This is all really important when flying aerobatics, which I used to do, because you are tossing the plane around with a heavy spinning object on the front and weird stuff happens.) This phenomenon is called "gyroscopic precession." I think this is what people are feeling when they get a lighter wheel set.

But I could be totally wrong.


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## Pirx (Aug 9, 2009)

Pitts Pilot said:


> This is all really important when flying aerobatics, which I used to do, because you are tossing the plane around with a heavy spinning object on the front and weird stuff happens.


Heh, try one of those World-War-I Fokker DR-I triplanes: For better cooling these things had rotary engines where the crankshaft was bolted to the fuselage, and the propeller was fixed to the engine housing. So you had the whole engine (which was probably more than 50% of the total weight of the aircraft) spinning in front. Some pilots, (like the Red Baron) loved them, because the angular momentum conservation effects allowed you to make unexpected maneuvers, and flip the aircraft around suddenly, in order to evade your enemy. On the other hand, keeping them under control while rotating for take-off required some serious skill.

Sorry, completely off-topic here...


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