Rotating Weight - Fact or Urban Legend
Comments
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dennisn wrote:rhext wrote:I think the answer to this perhaps depends on whether you buy into the theory that (say) a few hundred grams weight saving makes a practical difference to a bike/rider combination that weighs many many times that. From my point of view, there's a limit to how much it's worth spending on reducing the weight of the bike when there's still so much scope to reduce the weight of the rider.
If you do buy into the theory that saving a couple of hundred grams makes a difference then, yes, it is more effective to lose it from the wheel-rims than anywhere else. To accelerate a wheel you have to impart it with linear momentum and angular momentum. To accelerate anything else, just linear momentum. To get a feeling for what this means, put your bike on a stand and instead of gently rotating the wheel, try and get it spinning as fast as you can as quickly as you can. You'll feel some real resistance then: and the lighter the wheel, the less that resistance will be!
Well, just to play devils advocate, I would argue that just because you "buy into" the theory of a couple hundred grams making a difference doesn't mean that the theory is true. Isn't that kind of like saying "well, I bought these because they are supposed to be fast - I believe what was said about them - therefore they are fast". I guess I'm a pretty skeptical person but I don't see that as a bad thing. Drives my wife crazy but other than that.......
The key words in rhext's post are 'practical difference'. Physics says that lighter= better (and more so for rotating weight) and that's true whether you buy into it or not, so in that sense the benefit of lighter wheels isn't an urban myth. Whether you'd notice a difference in practice is something else, although that said I reckon I did notice when I got lighter wheels on my MTB- or maybe I was just trying to convince myself that my money was well spent :?
But as others have said for most people there are things that will make a bigger difference than bling wheels.0 -
MrChuck wrote:dennisn wrote:rhext wrote:I think the answer to this perhaps depends on whether you buy into the theory that (say) a few hundred grams weight saving makes a practical difference to a bike/rider combination that weighs many many times that. From my point of view, there's a limit to how much it's worth spending on reducing the weight of the bike when there's still so much scope to reduce the weight of the rider.
If you do buy into the theory that saving a couple of hundred grams makes a difference then, yes, it is more effective to lose it from the wheel-rims than anywhere else. To accelerate a wheel you have to impart it with linear momentum and angular momentum. To accelerate anything else, just linear momentum. To get a feeling for what this means, put your bike on a stand and instead of gently rotating the wheel, try and get it spinning as fast as you can as quickly as you can. You'll feel some real resistance then: and the lighter the wheel, the less that resistance will be!
Well, just to play devils advocate, I would argue that just because you "buy into" the theory of a couple hundred grams making a difference doesn't mean that the theory is true. Isn't that kind of like saying "well, I bought these because they are supposed to be fast - I believe what was said about them - therefore they are fast". I guess I'm a pretty skeptical person but I don't see that as a bad thing. Drives my wife crazy but other than that.......
The key words in rhext's post are 'practical difference'. Physics says that lighter= better (and more so for rotating weight) and that's true whether you buy into it or not, so in that sense the benefit of lighter wheels isn't an urban myth. Whether you'd notice a difference in practice is something else, although that said I reckon I did notice when I got lighter wheels on my MTB- or maybe I was just trying to convince myself that my money was well spent :?
But as others have said for most people there are things that will make a bigger difference than bling wheels.
I have bought into the physics part of it. Like you say though - in practice....???
I'm going to try an experiment if I can find a hanging digital scale. With the bike on the work stand pull down on the pedal with the scale and see what it takes to start the wheel moving. Remove the tire, put the wheel back on(instant lightweight wheel), and repeat with the scale. Maybe I'll find something interesting. :? :?
I need to get a life.0 -
balthazar wrote:NapoleonD wrote:There is a shortish steady hill near me where I could test this, do, say, 5 runs with each wheel at 12MPH.
I could also try and accelerate in a Fixed gear from, say, 12mph to 17mph uphill as quickly as possible in a particular gear to try and get some semblance of acceleration. This may be quite inaccurate though.
It would be interesting to see the results...
Well, in my opinion anyway. I really don't think there will be much, if any, difference.
I think this scenario introduces obfuscating complexity to the matter: the original experiment of spinning a wheel up to high speed in your hand is more revealing. Try it with a light and a heavy wheel, and note the difference: particularly, how easy it is to spin any normal wheel up to 20mph-ish in 2 seconds or so, with your thumbs on the spokes. That's an acceleration rate much higher than you could manage cycling the bike – yet you did it easily with only your thumbs. You don't need to "time" or measure anything, to reveal that the efforts we are talking about are nothing to the power generated by two legs (quad power?!)
I disagree, because you don't ride your bike by spinning a wheel with your hand with no resistance, it is not a fair comparison. Furthermore, the power figures don't lie. We will be able to quantify if there is any difference then...0 -
The thumb test on spinning up wheels isn't all that revealing as you don't spin them up to anything like the velocity you would whilst riding.
I think maybe people perceive that wheel weight makes a big difference because when most people experience a light wheelset for the first time it's a big upgrade from an entry-level set of wheels. Yes, they accelerate better, but is this also due to them being stiffer too? Note that I'm not saying that lightweight wheels are stiffer, but that a light £500 set of wheels will probably be stiffer than a heavy £100 set.Bike lover and part-time cyclist.0 -
Maybe try reading this article ( I know its Wiki but makes more sense than the thumb test)
http://en.wikipedia.org/wiki/Bicycle_performance
Basically the physics shows that a unit mass on the wheel has twice the kinetic energy of a unit mass on the frame. This will only have an affect when changing speeds and will only have a comparatively very small impact, but in a last minute sprint, but it would appear not so much in climbing. If you are a pro with a UCI weight limited bike it makes sense to save as much weight as practical on the wheels as this has more impact than shaving weight off the frame. Since most stock wheels are heavy trash I think upgrading to lighter wheels is always top of the list.0 -
why not? You accelerate a bicycle wheel up to circa 20 mph in a couple of seconds in your hands. How do you see that as different from accelerating the wheels on the bike? What do you imagine happens at higher speeds that's different?AidanR wrote:The thumb test on spinning up wheels isn't all that revealing as you don't spin them up to anything like the velocity you would whilst riding.0 -
I think Dennis should repeat the experiment with his bike in the biggest gear and try maintaining a reasonable cadence with just his little finger then report back on how effortless it is....More problems but still living....0
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Slight digression but relevant - I've just ordered a nice new road bike. One of the wheel recommendations the shop made was Campag Neutrons based on my request for good value, reasonably light wheels that would suit long rides in hilly country. Just to check on that recommendation, I searched the wheels in this forum and found lots of favourable comments but the vast majority specifically recommended the Neutron Ultras. Now, I think the only difference between the Neutron and Neutron Ultra is that the latter have carbon fibre hubs. If so, all the weight saving (all of 70 grammes I think) is in the hub. Therefore, for most peoples purposes, the only real benefit of the Ultra is the bling rather than the performance???Faster than a tent.......0
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With the risk of putting a spanner in the works, we should consider what is meant by "Acceleration". This is, in fact, a rate of change of velocity. Since velocity is a speed in a fixed direction, any change in direction is "acceleration". This will not, I believe, affect the rotational energy of the wheel itself but it needs energy to "accelerate" including changing direction. So, a twisty windy course will always be slower that a straight course, which, I presume, is why all the really fast TT courses are on virtual straight roads. In a similar vein, the gyroscopic effect of the wheel means that a lighter wheel will turn, i.e. change direction, more easily that a heavy wheel thus giving creadence to the the "reduce rotating mass" theory.
My brain hurts now.I have only two things to say to that; Bo***cks0 -
balthazar wrote:
why not? You accelerate a bicycle wheel up to circa 20 mph in a couple of seconds in your hands. How do you see that as different from accelerating the wheels on the bike? What do you imagine happens at higher speeds that's different?AidanR wrote:The thumb test on spinning up wheels isn't all that revealing as you don't spin them up to anything like the velocity you would whilst riding.
You won't get the wheel up to 260 rpm with your thumb that easily. Try spinning up the wheel by putting your thumb on the tyre, then do it by turning the crank with your hand when the bike's in a high gear, up to a cadence of around 90rpm. You'll note that the wheel is spinning a lot faster in the latter situation.
As for taking into account acceleration when you change direction, whilst technically this is true by far the larger issue is that most people will decelerate linearly when approaching a corner. Breaking by definition is all wasted energy. If you were to keep the same speed through a corner the rotational kinetic energy of the wheels wouldn't change. Planets are constantly moving in big circles but don't lose speed. Gyroscopic effects would, I imagine, be negligible.Bike lover and part-time cyclist.0 -
Whenever I encounter this sort of "put down the credit card, log off Wiggle, lose some weight and get fitter" heresy, I put my fingers in my ears and chant "Assos! Assos!" until the person goes away. Sometimes I am so upset I have to console myself with another doughnut. OF COURSE new wheels would make me MUCH faster, otherwise why would they cost £900? Next some apostate wil be saying that replica team kit, lycra shoe covers, a tiny cap under your helmet and Oakleys don't make a performance difference - which is just ludicrous.0
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Rolf F wrote:Slight digression but relevant - I've just ordered a nice new road bike. One of the wheel recommendations the shop made was Campag Neutrons based on my request for good value, reasonably light wheels that would suit long rides in hilly country. Just to check on that recommendation, I searched the wheels in this forum and found lots of favourable comments but the vast majority specifically recommended the Neutron Ultras. Now, I think the only difference between the Neutron and Neutron Ultra is that the latter have carbon fibre hubs. If so, all the weight saving (all of 70 grammes I think) is in the hub. Therefore, for most peoples purposes, the only real benefit of the Ultra is the bling rather than the performance???
I've got a pair of Neutrons and they have been fantastic. I use them for hilly sportives and general Summer training. They are light and very durable. I think they are one of the most underrated wheelsets. The Ultras have, AFAIK, slightly lighter rims as well as the different hub. My impression is that the standard Neutrons offer more bang for bucks.0 -
Double post0
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AidanR wrote:balthazar wrote:
why not? You accelerate a bicycle wheel up to circa 20 mph in a couple of seconds in your hands. How do you see that as different from accelerating the wheels on the bike? What do you imagine happens at higher speeds that's different?AidanR wrote:The thumb test on spinning up wheels isn't all that revealing as you don't spin them up to anything like the velocity you would whilst riding.
You won't get the wheel up to 260 rpm with your thumb that easily. Try spinning up the wheel by putting your thumb on the tyre, then do it by turning the crank with your hand when the bike's in a high gear, up to a cadence of around 90rpm. You'll note that the wheel is spinning a lot faster in the latter situation.
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Unless you're taking air resistance into account, which for a discussion on rotating mass is largely irrelevant, the RPM makes no difference whatsoever to the physics in any normal physical condition (i.e. nowhere near the speed of light).
The physics tells you that, all other things being equal, a lighter bicycle wheel will be faster than a heavier wheel. Fact!0 -
most people have computer sensors on their front wheels, and can spin the front wheel to 20mph ish with the bike on a stand, or holding it off the ground with one hand. That will do for the purposes of this exposItion. I think it's not hard to do much the same thing in your hands, but that point is moot. The intention is to make plain how little effort is used in accelerating a wheel. Do you agree that acceleration on bicycles occurs primarily in the sub-20mph range, that this simple test synthesises?AidanR wrote:balthazar wrote:
why not? You accelerate a bicycle wheel up to circa 20 mph in a couple of seconds in your hands. How do you see that as different from accelerating the wheels on the bike? What do you imagine happens at higher speeds that's different?AidanR wrote:The thumb test on spinning up wheels isn't all that revealing as you don't spin them up to anything like the velocity you would whilst riding.
You won't get the wheel up to 260 rpm with your thumb that easily. Try spinning up the wheel by putting your thumb on the tyre, then do it by turning the crank with your hand when the bike's in a high gear, up to a cadence of around 90rpm. You'll note that the wheel is spinning a lot faster in the latter situation.0 -
Yup, as a simple test it's a good demonstration.
I did the maths earlier in this thread - the numbers are there.Bike lover and part-time cyclist.0 -
Rick Chasey wrote:
Unless you're taking air resistance into account, which for a discussion on rotating mass is largely irrelevant, the RPM makes no difference whatsoever to the physics in any normal physical condition (i.e. nowhere near the speed of light).
The physics tells you that, all other things being equal, a lighter bicycle wheel will be faster than a heavier wheel. Fact!
Eh? I don't think anyone is saying we're about to stray outside the realms of Newtonian physics here... I was merely saying that if you're taking perceptions of exertion into account you need to get roughly the right RPM.Bike lover and part-time cyclist.0 -
Nickwill wrote:I've got a pair of Neutrons and they have been fantastic. I use them for hilly sportives and general Summer training. They are light and very durable. I think they are one of the most underrated wheelsets. The Ultras have, AFAIK, slightly lighter rims as well as the different hub. My impression is that the standard Neutrons offer more bang for bucks.
Thanks for that - couldn't see much reason to go for the Ultras tbh - besides, the bike is black and white and there is too much red writing on the Ultras
As a matter of interest, what wheels are you using when not on the Neutrons?Faster than a tent.......0 -
AidanR wrote:Rick Chasey wrote:
Unless you're taking air resistance into account, which for a discussion on rotating mass is largely irrelevant, the RPM makes no difference whatsoever to the physics in any normal physical condition (i.e. nowhere near the speed of light).
The physics tells you that, all other things being equal, a lighter bicycle wheel will be faster than a heavier wheel. Fact!
Eh? I don't think anyone is saying we're about to stray outside the realms of Newtonian physics here... I was merely saying that if you're taking perceptions of exertion into account you need to get roughly the right RPM.
*sigh*
I fear I am misunderstood, or misunderstanding.
I though the OP was whether the advantages of lighter wheels were fact or myth?
The physics says it's a fact! The examples used with spinning wheels with hands or whatever were only there to conceptualise the physics!0 -
I think what the OP was getting at was are these effects, whilst obviously real, significant enough to provide much of an advantage? Enough to be perceived by a rider who isn't fighting for tenths of seconds in a race?
The physics isn't in doubt, just the numbers.Bike lover and part-time cyclist.0 -
AidanR wrote:I think what the OP was getting at was are these effects, whilst obviously real, significant enough to provide much of an advantage? Enough to be perceived by a rider who isn't fighting for tenths of seconds in a race?
The physics isn't in doubt, just the numbers.
I buy the physics and math of lighter is better. It's the reality of any rider being able to notice the difference that I meant to discuss.0 -
Rick Chasey wrote:
*sigh*
I fear I am misunderstood, or misunderstanding.
I though the OP was whether the advantages of lighter wheels were fact or myth?
The physics says it's a fact! ....!
*SIGH*
*SIGH*
I do wish people wouldn't do this.
Perhaps you would like to explain which bits of physics actually apply, including equations with sample workings to show what benefit is accrued by lighter wheels, and under which circumstances.
That way we can judge if you actually know what you are on about, rather than just stating that 'it is a fact'.0 -
Chris James wrote:Rick Chasey wrote:
*sigh*
I fear I am misunderstood, or misunderstanding.
I though the OP was whether the advantages of lighter wheels were fact or myth?
The physics says it's a fact! ....!
*SIGH*
*SIGH*
I do wish people wouldn't do this.
Perhaps you would like to explain which bits of physics actually apply, including equations with sample workings to show what benefit is accrued by lighter wheels, and under which circumstances.
That way we can judge if you actually know what you are on about, rather than just stating that 'it is a fact'.
Great post. Really.
OK, who's up for that challenge 'cause it sure ain't me?0 -
In my experience, there is no doubt that a lighter wheelset (obviously including the tyres and tubes) makes a bike feel quicker. I am sure that this is not due to the minor reduction in effort required to spin the wheels up whilst accelerating, but is more to do with the small reduction in the gyroscopic effect which reduces the "straight line" stability of the bike a little. This makes the bike feel more responsive, which makes you think that you are accelerating more. It's all psychological really. That helps of course!0
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OK so if you have 2 sets of wheels the same mass, one with a rim 100g more mass and one with 100g more mass at the hub centre then:-
in an acceleration to 30km/h in 10 seconds you will need about 0.7Watts more for the wheel set with the heavier rim (that is for both wheels)
roughly for a 70kg rider/bike combo you will need 245W to accelerate to this speed so it's a pretty small percentage, almost negligible but not entirely if you are competing at a high level and can afford the kit.
If the wheels are 100g lighter at the rim with the same hub (therefore 100g lighter per wheel) well you twice the benefit and save yourself a huge 1.4W.
Basically it's choice. Light wheels are nice, feel good because they are light. Most of us will not appreciate a lower moment of inertia during the periods of acceleration but some of us will feel a small benefit from lighter wheels on long climbs, but the weight saving doesn't have to be at the rim for this. Personally, if I had the cash I'd go for more areo wheels at the expense of a few grams, more overall benefit for the type of riding I do even though I climb some pretty big cols. 8)0 -
Right, here we go.
The moment of inertia is like the mass of a rotating object. It takes energy to accelerate a book through the air if you are throwing it, and the heavier it is the more energy it takes to accelerate it. Similarly, if you spin up a wheel by hand, it takes energy to do this even though the wheel itself isn't moving anywhere. The heavier the wheel, the more energy it takes to spin it up. Also, the further that mass is from the centre of rotation, the more energy it takes to spin it up, i.e. a heavy hub but light rim will take less energy than a heavy rim but a light hub, even though the total mass is the same.
This is how I came up with my figures that are on the first page:
The rough calculation of the moment of inertia (I) of 1 wheel, treating the wheels as a ring, i.e. ignoring all mass other than the rim and the tyre (spokes are very light, and hubs have all their mass at the centre, therefore a very low moment of inertia):
I=m*r^2 (mass times the radius squared)
I then looked at the linear kinetic energy of a body (the cyclist and bike together) travelling at 20mph, ignoring rotating parts:
KE=0.5*m*v^2 (half mass times velocity squared)
I then looked at the rotational kinetic energy of the wheels when spinning at 260rpm (which is how fast they'd be spinning if the body was travelling at 20mph):
RKE = 0.5*I*w^2 (half moment of inertia times rotational velocity squared)
I then added the linear kinetic energy and the rotational kinetic energy together to give the total kinetic energy of the body, and compared this to just the rotational kinetic energy of the wheels to get an idea of how much of the total energy went into spinning the wheels up to speed.
The total kinetic energy the body has at 20mph is the amount of energy the rider has had to impart to get him and the bicycle up to that speed. However, it ignores friction (rolling resistance of the tyres and bearing losses) and air resistance. At low speed rolling resistance is significant, but by 20mph air resistance accounts for around 70% of the retarding force on the body.
The actual amount of energy the cyclist has had to put into the system to accelerate it to 20mph is far more than just the kinetic energy that the body has at 20mph due to these losses. So if losses count for 2/3rds of the energy put into the system, and the rotational kinetic energy is, say, 15% of the total kinetic energy at 20mph, it probably accounts for only 5% of the total energy the rider has put into the system.
Unfortunately if you model the system taking into account friction and wind resistance the calculations become very complex indeed.Bike lover and part-time cyclist.0 -
AidanR wrote:Right, here we go.
The moment of inertia is like the mass of a rotating object. It takes energy to accelerate a book through the air if you are throwing it, and the heavier it is the more energy it takes to accelerate it. Similarly, if you spin up a wheel by hand, it takes energy to do this even though the wheel itself isn't moving anywhere. The heavier the wheel, the more energy it takes to spin it up. Also, the further that mass is from the centre of rotation, the more energy it takes to spin it up, i.e. a heavy hub but light rim will take less energy than a heavy rim but a light hub, even though the total mass is the same.
This is how I came up with my figures that are on the first page:
The rough calculation of the moment of inertia (I) of 1 wheel, treating the wheels as a ring, i.e. ignoring all mass other than the rim and the tyre (spokes are very light, and hubs have all their mass at the centre, therefore a very low moment of inertia):
I=m*r^2 (mass times the radius squared)
I then looked at the linear kinetic energy of a body (the cyclist and bike together) travelling at 20mph, ignoring rotating parts:
KE=0.5*m*v^2 (half mass times velocity squared)
I then looked at the rotational kinetic energy of the wheels when spinning at 260rpm (which is how fast they'd be spinning if the body was travelling at 20mph):
RKE = 0.5*I*w^2 (half moment of inertia times rotational velocity squared)
I then added the linear kinetic energy and the rotational kinetic energy together to give the total kinetic energy of the body, and compared this to just the rotational kinetic energy of the wheels to get an idea of how much of the total energy went into spinning the wheels up to speed.
The total kinetic energy the body has at 20mph is the amount of energy the rider has had to impart to get him and the bicycle up to that speed. However, it ignores friction (rolling resistance of the tyres and bearing losses) and air resistance. At low speed rolling resistance is significant, but by 20mph air resistance accounts for around 70% of the retarding force on the body.
The actual amount of energy the cyclist has had to put into the system to accelerate it to 20mph is far more than just the kinetic energy that the body has at 20mph due to these losses. So if losses count for 2/3rds of the energy put into the system, and the rotational kinetic energy is, say, 15% of the total kinetic energy at 20mph, it probably accounts for only 5% of the total energy the rider has put into the system.
Unfortunately if you model the system taking into account friction and wind resistance the calculations become very complex indeed.
When I saw this post I said to myself "Why did I even ask this question?" But I'm actually going to put a bit of time into it and see if I can't get a bit better picture of bike wheels than my funky experiment of spinning one with my little finger. This could take well into the next decade for someone like myself, what with my limited brain powers.
Out of curiosity AidanR do you think that my proposed experiment of hooking a hanging bike scale on the pedal and pulling to see how much force is required to put the wheel in motion(with tire), then removing the tire(for a so called lighter wheel) then pulling again with the scale, will yield any result worth mentioning? Of course my first problem is locating an accurate scale that will work. Whatever you do don't tell me to "just do the math". That's too much to expect, although I'm going to dig into your formulas to see if my brain still works. :oops: :oops:0 -
Ariege cyclist wrote:Personally, if I had the cash I'd go for more areo wheels at the expense of a few grams, more overall benefit for the type of riding I do even though I climb some pretty big cols. 8)
But if the calcs posted here imply that a lightweight wheel has little benefit to most of us, what are the numbers that justify spending the money on the aero? Unless there is no wind or it his head or tailwind, the benefits of the more aerowheel are going to be compromised by crosswind.Faster than a tent.......0 -
dennisn wrote:Out of curiosity AidanR do you think that my proposed experiment of hooking a hanging bike scale on the pedal and pulling to see how much force is required to put the wheel in motion(with tire), then removing the tire(for a so called lighter wheel) then pulling again with the scale, will yield any result worth mentioning? Of course my first problem is locating an accurate scale that will work. Whatever you do don't tell me to "just do the math". That's too much to expect, although I'm going to dig into your formulas to see if my brain still works. :oops: :oops:
You need to be able to measure the force times distance, and the chances of you being able maintain and exact force over the entire distance is pretty slim. You'll also need to measure the maximum rpm of the wheel. Whilst in theory the experiment would work fine, you might have difficulty working it in practice.Rolf F wrote:But if the calcs posted here imply that a lightweight wheel has little benefit to most of us, what are the numbers that justify spending the money on the aero? Unless there is no wind or it his head or tailwind, the benefits of the more aerowheel are going to be compromised by crosswind.
No point in even trying to calculate that - it's a fantastically complicated model that you'd need a lot of computing power to solve. These things are best measured in a wind tunnel, as the likes of Zipp do. Not that I'd believe everything they say as they're a tad biased
If you want to measure that kind of thing you'll need a still day and then do multiple roll-down tests with different wheel combinations. Anything other than a very still day and you're likely to get variations in wind velocity which will render the results useless.Bike lover and part-time cyclist.0 -
This has been great, heretics well and truly told, and I'm printing it off to show the wife. "Yes, they may be 700% more expensive than my RS10's but look, they're almost certainly 0.05, 0.5 or 1.5% faster!!"
Could we have one next on the physics of why tyres with a colour band that matches your bar tape, roll faster than black ones?0
