Rotating Mass...

Discuss light weight issues concerning road bikes & parts.

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by yourdaguy

More resistance=more heat. If you push the same then you go faster with lighter rims, means less heat. It is because everything bends to some extent and more resistance means some of the extra energy is turned into heat in the pedals, cranks, etc. because they bend more with more resistance and the same push. Remember, I said if you want to count every watt. The emount of energy is trivial, but it is measurable.
For certain parts stiffer is more important than lighter.

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by drjones96

HOLY CRAP! Mark MCM has done the math! I wasn't really interested enough to pull out the physics book but apparently he is!

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by Mark McM

While we are counting every watt of energy, an equally small wattage due to the loss caused by greater speed fluctuations are the losses caused by the fact that during the act of pushing the heavier rotating mass wheel you effectivly experience more resistance and therefore put a minute amount of extra energy in the form of heat into the pedals, crank, chainrings, chain, sprockets, hub, spokes, etc.

This argument shoots itself in the foot. It is true that if you push a higher inertia with the same power, there will be less acceleration, and more flexing of the load bearing components. There will be some amount of hysteresis losses due to this flexing. However, the majority of the drivetrain components are metal, which has little hysteresis loss - most of the energy is returned when the components un-flex.

Contrast this with the other drag losses on the bicycle, which are much higher, including the viscous losses from aerodynamic drag, and the high losses from the elastomeric tires.

So, in the case of higher inertia, you get more energy going into component flexing, but because of the low hysteresis losses most of that energy is returned. In the case of lower inertia, you get more acceleration, which results in higher higher peak speeds and their accompanying higher viscous losses, which are not returned.

So the higher losses due to the velocity fluctuations result in more total energy loss, and thus lower average velocity.

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by yourdaguy

I admit the loss is trivially small, but you said it yourself most of the energy is returned and less is stored in the first place with lighter wheels and any time there is storage and retreval there is always loss due to local heating. The loss due to greater variation in speed causing more losses to the air are also very small. That was my whole point. I loved your post and agree with it totally, but I am saying we might as well account for every watt. Your drag effect is very small and probably 5 times as many watts as the effect I explained, but both are small and I thought it would be interesting to include this effect also since your post was otherwise exhaustive of the subject. I thank you again for doing all the hard cogitating required for your post. When I said equally small, I didn't mean equal as in the same I meant equal as in also trivially small. Basically the steeper the hill the less the air drag effect and the more the flex heating effect will be relatively speaking, but in real world hills, the drag effect will be more.
For certain parts stiffer is more important than lighter.

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by cadence90

"Gimondi è un eroe umano, che viene sconfitto ma che continua la sua corsa fino a tornare a vincere." - Enrico Ruggeri

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by fdegrove


So, in the case of higher inertia, you get more energy going into component flexing, but because of the low hysteresis losses most of that energy is returned.

Just wondering how much "most" actually is....

I'm having a little difficulty imagining a bicycle being an efficient system.

Knowing that anything that can move inevitably loses energy, and I don't mean just move forward but rather move as in flex, vibrate, resonate and considering the number of mechanical links a bicycle is made of which are by nature lossy in transfering energy, I wonder.....

If we take for instance a chain with well over a hundred of links we have an average efficiency of 0.98. Not too bad but it's not just the chain losing energy either....

So I wonder how much of the power put on the pedal/crank assy is effectively converted into forward motion.

I read drag increases with speed yet measurements clearly show that aero wheels yield more power gain with increasing speed.
Seems pretty logical to me but from what I've read from postings above one could easily be misled into thinking otherwise....
Naturally you're all going to be all over me telling me the reducing drag only becomes effective when sufficient drag is there to reduce it in the first place which occurs when a certain amount of spedd is reached etc. etc.....I know, no need to tell me.... :wink:

Then there's friction, stiction and no doubt a number of other factors that are far from ideal and it should become clear that once the initial power needed to overcome these factors it's going to take a lot less power to just keep the bike rolling. Aren't wheels just great? 8)

The more efficient we can make that system, the less energy that's going to take.
In order to optimize that efficiency we try to make bikes as stiff as we can.
Stiff cranks, pedals and their cleats, shoe soles from stiff carbon, everything is optimized for stiffness and if it weighs less then it can't be a bad thing either.
What most of us probably don't realise is that the lighter/stiffer the bike becomes the faster it will release energy. IOW it won't store it very long as it just doesn't have enough mass for that.
So the sooner it can be converted into forward motion the better. By making the bike less lossy it automatically will become a more efficient bike or so that should be the goal.

Conversely, the more mass available the longer the energy is going to be stored but....
Nothing guarantees us that that energy is not going to go wasted as a big chunk of it is going to be converted to heat or just going to end up not being used to give the bike forward thrust....
Mass then becomes a storage tank of energy, energy that is not going to be used in any useful way for sure.

That aside and, after an admittedly quick read through this thread and having read a number of contradicting statements from various contributors, I'm still left wondering if high rotational mass could indeed create a flywheel effect (an energy storage tank if you like) that actually would give the rider an advantage and if so under which circumstances.

If we'd have sufficient gyroscopic energy generated we'd still have to transfer that energy to the ground surface effectively as the ground surface would act as a brake on the flywheel.

In short, ahem....Is there any practical condition in any commonly held type of race a pro cyclist would have any measureable benefit from an increase in rotational mass?

If that's not the case, would it be safe to assume that the lighter bike has a clear advantage over the heavier one (rotational mass kept more or less out of the picture) and if so, have practical testresults been published on this?

To clarify a little, I don't need any answers on what I suggest.
I'm just throwing some things at the audience as I often am under the impression that quite a number of us don't seem to see the trees for the forrest and that includes me.
Concentrating on one aspect of engineering without actually knowing why if you like.
Not that I'd know anything about that but I s'pose you get the picture....

TIA, :wink:

P.S. Sorry for the long, rant. :oops:
Being a snob is an expensive hobby.

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by phatdad

just go to the bathroom before you ride! you'll drop lots of weight, and it's free and good for you!

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by elmer

53-11 wrote:Spinning a crankset in a circle is definitely not the same as rowing a boat.

Wow, brilliant.

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by Mr_Potatohead

Mark McM wrote:...

The complete equation of motion is thus:

dV/dt = {(P/V)(1+sin(2RT)) - [ (1/2)CdRhoAV^2 + (Ms+Mr)gCrrCosine(S) + (Ms+Mr)gSine(S) ] } / (Ms + 2Mr)

This equation is non-linear, so I solved it numerically with a 4th order Runge-Kutta numerical differentiation...

I'd be happy to send the Excel spreadsheet to anyone that is interested.

This has to be the best post ever to a bicycle related thread.

Compare this to: " And if you watch a rowboat it accelerates and slows down, so therefore..."

I think the thread can be locked now, debate over.