[Closed] Rotating weight calculation.

I don't know. But I do know it wont make any difference, that rotating weight 'a pound off the wheels is worth three off the frame' business only really affects acceleration, and the acceleration on a bike is not really significant unless you're racing crits and aren't that good at cornering.

Plus, rotating mass is much more important the further away from the axle. This means that you will notice weight reductions in tyres, tubes and rims much more than hubs, for example. What you are actually trying to reduce is the moment of intertia about the rotating axis (I think), not simply the mass of the wheel.

that rotating weight 'a pound off the wheels is worth three off the frame' business only really affects acceleration

wrong. It affects the difficulty of changing the direction of the wheels, and influences the suspension by increasing unsprung weight, but the wheels travel at the same speed as the rest of the bike (you hope) - on average. If your wheels are lighter they are probably going to be more prone to damage 🙁

I think that when climbing becomes difficult less rotating weight helps as the bike's speed isn't constant.

Ever held a gyroscope? Thats why light weight wheels rule 🙂

General ruel of thumb is 1g lost off the wheels is worth 2g off the rest of the bike. The maths behind it is because the wheel has to both accelerate in the direction of the bike and its rotation.

wrong. It affects the difficulty of changing the direction of the wheels

I will think you'll find that acceleration is a vector not a scalar quantity and therefore changing direction is an acceleration regardless of whether the speed (the scalar quantity has changed)
Losing the weight depends on where its from, if its from the hub as said, this wouldn't make a massive effect on te gyroscopic forces but obviously would still reduce the weight of the bike, if its from the rims then this would be more useful. Obviously as well as possibly reducing strength if material has been removed, there is also the fact that gyroscopic forces are useful in maintaining the rubber side down position given they make you more stable...
Tbh given i weigh about half a car i can't say i would notice such a thing but if it makes you happy!

wheels travel at the same speed as the rest of the bike

Your wheels may "travel" at the same speed as your bike but the two are different, one is normal velocity and the other is angular velocity...

So, it's more significant on bigger dia. wheels - say 29"

Your wheels may "travel" at the same speed as your bike but the two are different, one is normal velocity and the other is angular velocity...

which are locked together. The part of the tyre in contact with the ground is stationary (unless you are skidding), and the top is travelling at twice the speed of the bike, so, like I said, the average speed of the wheel is the same

how does unsprung weight affect the suspension? i think the bottom line is, that lighter wheels are faster and what poppa said about tyres and rims as they're on the outside.

Personally I find wheel weight makes the difference between a "dead" feeling bike and a more "live" feeling. Obviously wheel weight affects suspension (recently putting tyres on that weighed twice as much as my old ones totally screwed up my handling, the damning on my forks was all to pot), but since the rotational inertia is particularly susceptible to weight at the rim and affects acceleration mainly, I find it very hard to accelerate after slowing for an obstacle. While the 4pots scrub the extra energy off easily, that extra energy has to go back into the flywheel to get back to the same speed. Even at low speeds, especially when tired, you tend to be a bit pulse-like in your riding style (or at least I do!) and this just gets worse and worse - the more you pulse the more energy you waste accelerating the mass and although you roll a little further with each pedal (which should in theory give you back that energy) my body doesnt like powering it at that speed and the power loses in the tyres tends to sap it away. The bike just feels sluggish really.

I think that when climbing becomes difficult less rotating weight helps as the bike's speed isn't constant.

Almost exactly the opposite (ignoring the fact that less weight in general is better).

Bomberman - ultimately unsprung mass is harder for the suspension to control, needing more damping, and so makes the fork feel more rigid and horrible. Lighter unsprung mass allows the fork to "work less" on controlling the mass of the wheel and more on controlling the motion correctly without affecting the rider as much. Obviously thats a gross simplification, but the maths is too much for this time of night lol.

aracer - I disagree, I think the flywheel effect is not dominant at such low speeds, the problem becomes the fact that we arent very good at producing smooth power at low pedal revs which makes the climb quite disrupted and energy sapping. If we could use the flywheel effect at speed to maintain momentum and spin over the surface you'd be fine. A bit like moving a rhino on ice - once you get it moving it'll move over any rut in the ice fairly easily but if you're trying to get it moving starting in a rut you wont stand a chance.

ah, this thread has been quite useful to me!! 🙂

coffeking - ultimately unsprung mass is harder for the suspension to control, needing more damping, and so makes the fork feel more rigid and horrible. Lighter unsprung mass allows the fork to "work less" on controlling the mass of the wheel and more on controlling the motion correctly without affecting the rider as much

I think you are talking regurgetated pish and what you go on to describe says far more about your own bad technique than fork damping.
lighter wheels/tyres do acelerate a lot faster/easier but heavier ones hold their momentum better through and over rough stuff - finding a sweet spot for the terrain you ride and how you ride it is more appropriate than simply choosing the lightest.

to put simply in my experience - light wheels are nicer to ride ( at the expense of strength so keep at it and learn to ride light and pick good lines - i wrecked a lot of rims when learning to ride 17 years ago as a kid and i just dont now at all! )

Lighter wheels also seem to have more lively ride my 717 hoops are more sprightly than my old 521s !- i hate putting heavier tyres on as i notice a difference!

paul

*regrets ever starting to read this thread*

If i ever feel it necessary to ask such a question or even wonder why, thay it'll be comforting to know some sad b'stids taken the time to find out for me.

I think the flywheel effect is not dominant at such low speeds

Inertia is inertia - it's independent of speed. The only difference being that 1mph change in speed is more significant the slower you're going. The point is that any momentum at all is a good thing, and so less inertia certainly isn't advantageous.

Inertia is inertia - it's independent of speed

interestingly, the definition of inertia is mass x velocity :o)
The independent part is called [b]mass[/b], though that begins to increase as velocity approaches that of light...

The point is that any momentum at all is a good thing

not when you want to stop!

SFB your wrong! And here's why...............

spin one of your wheels, its average speed is zero, but its still taken energy to get it spinning.

so take an idealised wheel, where all the weight is on the outer edge, weighing 1kg

F=ma and E=Fd

therefore

E=mv^2

so a wheel flying through the air at 10ms-2 has 1000J of energy (10x10x10)

a wheel spinnig on a stationary bike also has a velocity (at its rim) of 10ms-2, and so an energy of 1000J.

now put it on the ground, and roll it at the same speed, the rim is spinning at 10ms-2 and the wheel is moving at 10ms-2 so the total energy is 2000J.

Obviously a real life wheel isnt entirely weighted right on its outer surface. so the real life multiplier is only rim and tire weight should be doubled.

As for un sprung weight, lighter wheels accelerate easier, this also applies in the up an down direction so are easily moved up and down over bumps. Hence why I hate hub geared bikes, they put too much unspung mass right at the back of the bike, making it crash into things.

Also................... (no ones mentined this yet)
the wheels are the part of th bike furthest form its COG, and furthest from you, so have the greatest leverage, so are the harest bit to move arround, couple that with a gyroscopes natural inclination to stay put and you can see why lighter is better.

Dont think the size of the wheels has an effect, as they would be spinning at the same rim speed, so the energy required to spin them up should be the same? On the other hand they weigh more, and anecdotaly BMX's have a much lower gyroscopic effect.

interestingly, the definition of inertia is mass x velocity

The most interesting thing is that that's completely wrong!

The most interesting thing is that that's completely wrong!

indeed, I got it mixed up with momentum 🙂

SFB your wrong! And here's why...

yes, I'd worked that out for myself, seems like it's my evening for mechanical inaccuracy 🙂

Don't argue engineering principles with an engineer 😉

bleh.

If an object has a large moment of inertia about an axis, which is characteristic of increased mass at a distance from the axis, it will be more difficult to change the rotational velocity of the object about the axis in question.

In other words, it is harder to increase the rotational velocity of heavy wheels than light wheels. This will be felt most when pedalling to accelerate the bike, and on steep climbs when the wheels are repeatedly being accelerated from near zero rotational velocity.

It should be noted that this also means light wheels will slow down quicker than heavy wheels too, e.g. when rolling over rough terrain.

but bear in mind, even if the effective inertia is double the mass of the wheels, this is still very small compared to the mass of the rider ie less than 10%, so [b]differences[/b] in wheel inertia will be much less significant

This will be felt most ... and on steep climbs when the wheels are repeatedly being accelerated from near zero rotational velocity.

Rubbish.

Agreed sfb - to use the original numbers, 1lb more on the wheels, even if it's all at the outer edge, still makes only ~1% difference to the total inertia of an average rider and bike (and only ~0.5% difference compared to 1lb extra on the bike - or on your gut). I'd challenge anybody to be able to notice 1% difference in their rate of acceleration. Realistically, the effects you mentioned in your first post (direction changing and unsprung weight) are far more significant.

Fair enough...