Carbon wheels, why would you bother?

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VamP

Banned
Location
Cambs
Good god, I have just realised the most important reason for lighter, more aero wheels.

Firstly I would like to apologise to all poster for not having included it earlier and beg your forgiveness. :tired:

The most important reason for lighter, more aero wheels is cyclist top trumps! Lighter beats heavier and aero beats non aero.

Two wins in one wheel.

If ever you needed convincing, there it is :whistle:

You forgot matt beats shiny.
 

T.M.H.N.E.T

Rainbows aren't just for world champions
Location
Northern Ireland
The lightbulb could do with an update, anyone want to reinvent that?
 
Shouldn't we only judge whether I know what I am talking about after the dust has settled, the winner has ridden into the sunset, and loser bitten the dust, in this debate? :gun: :becool: ^_^

To answer the first question. I presume you agree that it takes energy to accelerate a mass (even in vacuum)? In the case of a rotational mass like a piece of the bicycle rim, it has to be accelerated and travel BOTH in the direction of the bike as well as other directions, and which I have already described twice above. As a result we do not have the same energy expenditure compared to a non-rotational mass.

So Newton was wrong? Someone should tell the Nobel Institute! You still haven't explained why two objects of identical weight covering the same distance would use different amounts of energy. Can't wait to see that... ^_^

As to your second question why braking is relevant, it appears you have forgotten I was answering your statement "Do you not get the bit about conservation of momentum and how the energy used to accelerate a heavier rim is returned in the form of slower decelleration?", returning energy is only possible if said energy wasn't already turned into heat in the brake pads after you have used your brakes.

But nobody is talking about 'braking' - except you. Braking is covered in the BR post I quoted earlier. Braking would recover the energy which would otherwise be returned in the form of slower decelleration in a heavier rim. Of course that's correct, but let's pretend you didn't mention that, as it's not relevant to this discussion
 

RecordAceFromNew

Swinging Member
Location
West London
So Newton was wrong? Someone should tell the Nobel Institute! You still haven't explained why two objects of identical weight covering the same distance would use different amounts of energy. Can't wait to see that... ^_^

But nobody is talking about 'braking' - except you. Braking is covered in the BR post I quoted earlier. Braking would recover the energy which would otherwise be returned in the form of slower decelleration in a heavier rim. Of course that's correct, but let's pretend you didn't mention that, as it's not relevant to this discussion

While Newtonian physics is no longer universal truth in physics nowadays, Newton was not wrong in the present context. I think your visualisation of what the systems are doing is.

As you know Newton's 2nd law of motion says force is mass times acceleration. I trust you are also aware that work done (or a change in energy) is force times distance travelled in the direction of the force. Ok? If not please see this.

Your problem in understanding shows up as an error in your question - the fact of the matter is those two accelerating objects have not covered the same distance.

To visualise what we are talking about, we could look at a spoke nipple where one normally finds it on the wheel, and another welded to the centre of the wheel's axle. Since the two are otherwise identical, they illustrate two masses with the former being a rotational mass with significant inertia because it is far from its centre of rotation (i.e. the centre of the wheel's axle), while the other nipple essentially only translates in line with the bike (if you don't want to ignore its rotation along its own axis at the axle you can consider this tit nipple is sitting on the saddle of the bike instead, ok?).

I hope you can now be convinced, by visualising the very different trajectories of the two spoke nipples, that when the bike is accelerated it takes more energy to accelerate the nipple on the rim. Not only does it travels in a straight line (since after a mile the bike has travelled the nipple has also travelled a mile), it is also spinning up and travelling in a circular trajectory with a radius of the wheel. In contrast the nipple at the axle or on the saddle only has a trajectory in a straight line only. Since the spinning up trajectory experienced by the first nipple is the only difference and is an add-on, unless you think Newtonian physics says it can be accelerated and moved about this circular trajectory by consuming no energy at all, the nipple at the rim has to consume more energy than the one on the axle/saddle to be accelerated for the same mile with the bike.

If you believe spinning that spoke nipple up consumes no energy, then you would effectively be saying it takes no more effort to spin up from still a rim made of lead than of alloy while both rims are on a bike on a stand.

What the above shows, is saving rotational mass has greater effect on performance than an equivalent non-rotational mass. In some idealised conditions (constant speed, no braking etc.), the two nipples may end up consuming the same amount of energy over distance in theory, but those are idealised conditions that are not achievable by cycling in the real world.

From what you wrote, I believe your visualisation for comparison is different to the above - you were thinking about items that essentially have parallel but otherwise identical trajectory as the bike, and which do require the same energy to accelerate if they have the same mass. The problem with that visualisation, is that a rotational mass does not have the same trajectory as a non-rotational mass.
 

400bhp

Guru
While Newtonian physics is no longer universal truth in physics nowadays, Newton was not wrong in the present context. I think your visualisation of what the systems are doing is.

As you know Newton's 2nd law of motion says force is mass times acceleration. I trust you are also aware that work done (or a change in energy) is force times distance travelled in the direction of the force. Ok? If not please see this.

Your problem in understanding shows up as an error in your question - the fact of the matter is those two accelerating objects have not covered the same distance.

To visualise what we are talking about, we could look at a spoke nipple where one normally finds it on the wheel, and another welded to the centre of the wheel's axle. Since the two are otherwise identical, they illustrate two masses with the former being a rotational mass with significant inertia because it is far from its centre of rotation (i.e. the centre of the wheel's axle), while the other nipple essentially only translates in line with the bike (if you don't want to ignore its rotation along its own axis at the axle you can consider this tit nipple is sitting on the saddle of the bike instead, ok?).

I hope you can now be convinced, by visualising the very different trajectories of the two spoke nipples, that when the bike is accelerated it takes more energy to accelerate the nipple on the rim. Not only does it travels in a straight line (since after a mile the bike has travelled the nipple has also travelled a mile), it is also spinning up and travelling in a circular trajectory with a radius of the wheel. In contrast the nipple at the axle or on the saddle only has a trajectory in a straight line only. Since the spinning up trajectory experienced by the first nipple is the only difference and is an add-on, unless you think Newtonian physics says it can be accelerated and moved about this circular trajectory by consuming no energy at all, the nipple at the rim has to consume more energy than the one on the axle/saddle to be accelerated for the same mile with the bike.

If you believe spinning that spoke nipple up consumes no energy, then you would effectively be saying it takes no more effort to spin up from still a rim made of lead than of alloy while both rims are on a bike on a stand.

What the above shows, is saving rotational mass has greater effect on performance than an equivalent non-rotational mass. In some idealised conditions (constant speed, no braking etc.), the two nipples may end up consuming the same amount of energy over distance in theory, but those are idealised conditions that are not achievable by cycling in the real world.

From what you wrote, I believe your visualisation for comparison is different to the above - you were thinking about items that essentially have parallel but otherwise identical trajectory as the bike, and which do require the same energy to accelerate if they have the same mass. The problem with that visualisation, is that a rotational mass does not have the same trajectory as a non-rotational mass.

You're wasting your time fella.

Spend it on someone who appreciates being educated.:whistle:
 

Hacienda71

Mancunian in self imposed exile in leafy Cheshire
You're wasting your time fella.

Spend it on someone who appreciates being educated.:whistle:
My wheels were rotating faster than yours this morning. That according to Newton is because my two hot chocolates weighed less than your two Americanos. :tongue:
 

02GF74

Über Member
So Newton was wrong? Someone should tell the Nobel Institute! You still haven't explained why two objects of identical weight covering the same distance would use different amounts of energy. Can't wait to see that... ^_^
Newton was not wrong, you are. You cannot seem to grasp that the wheels are rotating but keep banging on about bike of the same weight. Read this link about flywheels and then come back.
http://simple.wikipedia.org/wiki/Flywheel

If you cannot be bothered, here is the part relevant to this discussion, if you feel it is wrong, contact the Wikipedia and Nobel Institute and ask them to get the laws of physics updated.

The amount of energy stored is a function of the weight and the speed of rotation - making a heavier wheel rotate faster takes more energy. Another factor is the radius (size) because the farther from the axis a part of the wheel is, the more energy it takes to make is rotate.

You have seen it now.
 
U

User6179

Guest
What about the stiffness of a wheel, surely a stiffer wheel will be quicker with the same amount of power applied to it ?

Are carbon wheels not stiffer?











:popcorn::popcorn::popcorn::popcorn::popcorn::popcorn::popcorn::popcorn::popcorn:
 

ayceejay

Guru
Location
Rural Quebec
Leaving science aside for a minute let me say that the quality of the build and materials is a major part of what makes one wheel better (however you measure that) than another.
 
U

User6179

Guest
Leaving science aside for a minute let me say that the quality of the build and materials is a major part of what makes one wheel better (however you measure that) than another.

Depends what you mean by better, a light wheel might perform better but may not last as long .
For me better means lasting longer as I do not race so a £200 wheelset might be BETTER than a £500 wheelset even if the £500 wheelset is 200 grams lighter.
 
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