Carbon wheels, why would you bother?

[RecordAceFromNew]

No - we specifically disagree on the issue of rotating mass.

There was a massive discussion on Bikeradar on this exact same topic last year - here's one of the posts which sets out the physics (and how it relates to cycling) pretty clearly...

If you don't agree with me, how do you explain the statement "In other words, a mass on the tire has twice the kinetic energy of a non-rotating mass on the bike." in the Wikipedia link you also refer to? If something has twice the kinetic energy in motion, doesn't it consumes twice the energy to get to that point?

The bikeradar link you provided is about climbing. If one climbs at a constant speed forever (presumably possible in reality if one is en route to heaven... ) then whether the mass is a rotating mass or not makes absolutely no difference in power consumption because there is no acceleration or deceleration. But because in reality that is never possible, the moment acceleration and deceleration take place, the rotating mass will always consume more energy.

 

[Dusty Bin]

I don't believe that. The inertia is greater when the weight is at the rim. If you have a bike, remoe one of the wheels, hold it by the axle. Give it a spin and see how easy or not it is to turn it (gyroscopic effect).

Now remove tyre - equivalent to a light rim and repeat.

Then come back and tell me there is no difference in the amount of effort required to turn the wheel.

Nobody is disputing that a heavier rim has more inertia, or a lighter rim has less inertia. As has been commented on already, something called the conservation of momentum cancels these forces out, which renders them largely irrelevant from a cycling perspective. Have you not read the thread?

 

[Dusty Bin]

If you don't agree with me, how do you explain the statement "In other words, a mass on the tire has twice the kinetic energy of a non-rotating mass on the bike." in the Wikipedia link you also refer to? If something has twice the kinetic energy in motion, doesn't it consumes twice the energy to get to that point?

Energy is consumed in accelerating a body (in this case, a bike, a human being and some riding kit) - precisely where the weight is distributed on that body is largely of no consequence. The post I quoted earlier sets this out quite clearly.

The bikeradar link you provided is about climbing. If one climbs at a constant speed forever (presumably possible in reality if one is en route to heaven... ) then whether the mass is a rotating mass or not makes absolutely no difference in power consumption because there is no acceleration or deceleration. But because in reality that is never possible, the moment acceleration and deceleration take place, the rotating mass will always consume more energy.

You seem to be saying that two 8kg bikes will not take the same amount of energy to accelerate if one has a set of lighter rims within it's total 8kg weight. Why do you think that?

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?

 

[02GF74]

The bit about wheel inertia (ie rotating weight) is also consistent with established physics. It doesn't matter where the weight is on a wheel.

err, unless I don't understand Endlish, the poster above is disputing the fact that a heavier rim has more inertia by saying "it doesn't matter where the weight is on a wheel".

I am not convinced by conservation cancels tese forces out. There is linear momentum and rotational momentum or gyroscopic effect.

Imagine your bike has solid concrete disc wheels each wighing 50 kg and you are racing against someone on a normal bike both travelling at the same speed. Now you both come to a corner - you are really telling me that the amount of effort to turn both bikes is the same? How is that possible?

 

[02GF74]

You seem to be saying that two 8kg bikes will not take the same amount of energy to accelerate if one has a set of lighter rims within it's total 8kg weight. Why do you think that?

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?

The lighter wheels will take less energy to get to up to speed.

Heavier rim is takes longer to slow down (or decelerate if you prefer) but why if you are racing do you want that? You want to go as fast as possible to cover the distance in as short a time as possible, and where you need to slow down such as corners, you want to slow down from a fast speed as quickly as possible, get round the corner then back to max speed. That is how races are won, not by a gradual slow down from a long way away. Why do you think brakes are so shoot hot on F1 cars?

 

[400bhp]

Black & Yellow.

 

[Dusty Bin]

err, unless I don't understand Endlish, the poster above is disputing the fact that a heavier rim has more inertia by saying "it doesn't matter where the weight is on a wheel".

Yes, that was me - I was saying that - in plain 'Endlish'...

I am not convinced by conservation cancels tese forces out. There is linear momentum and rotational momentum or gyroscopic effect.

Momentum is momentum. Not sure what your point is. We are talking about the momentum of a bicycle - with wheels attached, obviously.

Imagine your bike has solid concrete disc wheels each wighing 50 kg and you are racing against someone on a normal bike both travelling at the same speed. Now you both come to a corner - you are really telling me that the amount of effort to turn both bikes is the same? How is that possible?

Really?

 

[Dusty Bin]

The lighter wheels will take less energy to get to up to speed.

Two 8kg bikes - one with lighter wheels, one with heavier wheels - but both still weighing 8kg. Which one will use less energy to get 'up to speed' ??

Heavier rim is takes longer to slow down (or decelerate if you prefer) but why if you are racing do you want that? You want to go as fast as possible to cover the distance in as short a time as possible, and where you need to slow down such as corners, you want to slow down from a fast speed as quickly as possible, get round the corner then back to max speed. That is how races are won, not by a gradual slow down from a long way away. Why do you think brakes are so s*** hot on F1 cars?

Really?

 

[400bhp]

Two 8kg bikes - one with lighter wheels, one with heavier wheels - but both still weighing 8kg. Which one will use less energy to get 'up to speed' ??

The one with less rotational wheel weight.

 

[RecordAceFromNew]

You seem to be saying that two 8kg bikes will not take the same amount of energy to accelerate if one has a set of lighter rims within it's total 8kg weight. Why do you think that?

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?

Exactly right regarding your first question. It is because a mass on the rim has to be accelerated in directions different to the direction of the bike as well as the direction of the bike. The work required to accelerate the rim material up (rear edge of wheel), down (front edge of wheel), and backwards (bottom of wheel) e.g. consumes energy that accelerating the lump on the saddle e.g. forward does not require.

Regarding your second question, it is indeed the case but apparently possible only e.g. if one never use one's brake (which I believe is also a sure way to go to heaven... ).

 

[Dusty Bin]

Exactly right regarding your first question. It is because a mass on the rim has to be accelerated in directions different to the direction of the bike as well as the direction of the bike. The work required to accelerate the rim material up (rear edge of wheel), down (front edge of wheel), and backwards (bottom of wheel) e.g. consumes energy that accelerating the lump on the saddle e.g. forward does not require.

Sorry - you clearly think you know what you are talking about - but you don't. You're going to have to explain why two objects (bicycles, in this case) of the same weight will require different energy expenditure to move the same distance in the same time. I'd like to see that.

Regarding your second question, it is indeed the case but apparently possible only e.g. if one never use one's brake (which I believe is also a sure way to go to heaven... ).

Why would you use your brakes when accelerating? Rotational inertia only matters for a split second when pressure is applied to the pedals. After that, you are in exactly the same boat (or bike) regardless of what wheels you have.

But hey, I'm open-minded though - if you can explain to me how you think Newton has got it wrong, then I'm all ears...

 

[Hacienda71]

Why would you use your brakes when accelerating? Rotational inertia only matters for a split second when pressure is applied to the pedals. After that, you are in exactly the same boat (or bike) regardless of what wheels you have.

Not wishing to get into your debate, but don't you apply pressure (power) to the pedals all the time apart from when you are freewheeling? Or am I missing something.

 

[Dusty Bin]

Not wishing to get into your debate, but don't you apply pressure (power) to the pedals all the time apart from when you are freewheeling? Or am I missing something.

Yes, you are effectively (albeit minutely) accelerating and decellerating constantly due to the natural pedal motion. The BR post I quoted earlier covers this already.

 

[RecordAceFromNew]

Sorry - you clearly think you know what you are talking about - but you don't. You're going to have to explain why two objects (bicycles, in this case) of the same weight will require different energy expenditure to move the same distance in the same time. I'd like to see that.

Why would you use your brakes when accelerating? Rotational inertia only matters for a split second when pressure is applied to the pedals. After that, you are in exactly the same boat (or bike) regardless of what wheels you have.

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?

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.

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.

 

[mattobrien]

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.

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