Carbon wheels, why would you bother?

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jowwy

Can't spell, Can't Punctuate....Sue Me
^^^ ok, your turn but make sure it is controversial.
aint i always
 

jowwy

Can't spell, Can't Punctuate....Sue Me
carbon wheels are lighter and faster than wheels made of concrete, but not as fast as the wheels on my car which are heavier but powered by an engine.
 

ayceejay

Guru
Location
Rural Quebec
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.
Which doesn't contradict what I wrote at all - an inexpensive wheel will meet your criterion if it lasts as long as you expect it to considering the price you paid - a wheel costing five times as much might meet that criterion better.
 
U

User6179

Guest
Which doesn't contradict what I wrote at all - an inexpensive wheel will meet your criterion if it lasts as long as you expect it to considering the price you paid - a wheel costing five times as much might meet that criterion better.

Never said it did contradict what you said , just saying better means different things to different people :smile:
 

ayceejay

Guru
Location
Rural Quebec
Never said it did contradict what you said , just saying better means different things to different people :smile:
Go back to my first post Eddy where I say "better (however you measure that)" i.e. better means different things to different people - if you are not contradicting that, which apparently you are not, what are you saying?
 

jowwy

Can't spell, Can't Punctuate....Sue Me
5 pages in and we've realised that wheels are still round and that Jowwy doesn't want to explain why I was tagged in his post.
None required
 
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 are missing the point in a monumental fashion. You seem unable to explain why two bikes which both weigh 8kg will consume different amounts of energy to move a specified distance if one has lighter rims. You are unable to explain it, because it cannot be explained. They both use the same energy to cover the same distance, with the same weight regardless. If you don't agree with this, you need to prove me wrong. If you could also point out where Newton was wrong, that would also be useful.
 
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.

Not really sure why you are making this point. As I said before, the point about heavier rims offering more momentum - and requiring more energy to gain that momentum - has never been in question.
 
2731926 said:
In your model, have you factored in braking?

Not sure what you mean by 'my model'? But anyway, all of this presumes that the brakes are not being used - why would they be if you are accelerating or simply riding along a road? Having said that, of course braking will recover the energy in the rims. This has already been covered on page 3, or 4, or something. If it isn't recovered through the brakes, it will be released in the form of slower decelleration (for a heavier rim) or faster decelleration (for a lighter rim). But the difference is tiny, either way.
 
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2731956 said:
My commute is often a series of sprints between traffic lights, accellerate brake, accellerate brake, repeatedly. I don't get back that stored momentum in a useful fashion. Lower rotational momentum is definitely an advantage under that circumstance.

I think you are probably over-interpreting the 'rotational momentum' thing by a significant amount. In the majority of cases, the difference between lighter rims and heavier rims will not be noticable. If you are doing constant 'stop/go' on your commute, then the overall system weight is far more important than any particular weight carried at the rim.
 
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