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Rotating mass explained: Droppin' science on your ass

23K views 150 replies 35 participants last post by  Mac7504 
#1 · (Edited)
I'm sick of seeing people not understanding this concept so I'm going to lay it out in this thread as clearly as possible.

First off, unsprung (rotating) mass is: weight that is NOT held up by the suspension, and is SPINNING as the bike is moving.

The "weight" of unsprung mass is irrelevant. The moment of inertia (or weight distribution) is the unit of measure that really counts, and those numbers are hard to extract meaning from. It's not gonna be like "Oh man, I can REALLY feel that last couple inches^4 that we got off there". Rims do make a HUGE difference though.

The reason why brake horsepower numbers are always higher than wheel horsepower numbers is because there's extra weight to rotate before the power can get to the ground. As such, rotating mass directly affects the acceleration, deceleration and horsepower on a motorcycle. You're going to see more power on the dyno with less rotating mass (like lighter rims/rotors/chain/sprockets), because you've got less parasitic weight in the drive train to eat up power.

What I'm saying is that if you weighed a stock rim and a forged magnesium (for instance) rim on a scale and they both turned out to be the same WEIGHT, that wouldn't mean that they would perform the same. It's all about weight DISTRIBUTION, hence why the units for moment of inertia are in length^4. It's determined by cross-sectional area times the distance to the center of rotation (the axle) squared. This is why cutting a couple pounds of rotating mass from the tires has a much more dramatic effect than cutting a couple pounds of rotating mass from a rotor. The tires are farther from the axle, and that distance is squared when computing moment of inertia. This is why doing the 520 conversion for a lighter chain and sprocket than stock while leaving the gearing alone won't really do much for you. It all helps, but you aren't going to feel that little of a difference in moment of inertia. Same with rotors..

3 pounds lost off the weight of the bike (like from a lightweight battery) does NOT yield the same performance gains from mounting rims that are 3 pounds lighter, necessarily. You can't even determine the effectiveness of your lighter rims with just a scale. Yes, lighter parts are almost always better, but that's not what counts. What I'm saying is that you can't say that mounting a tire that's 1 pound lighter is going to give the same performance benefits that losing 1 pound from the rotors, because they're not the same distance from the axis of rotation (the axle in this case).

Now does everyone understand why saying "these rims are 3 pounds lighter than stock" means absolutely nothing in terms of actual performance gains?


Another member explains this concept with a great analogy:

perhaps i can assist in the understanding of this argued concept. imagine you have two baseball bats of equal weight but unequal size. one bat has most it's weight near the grip (imagine holding the bat backwards) and the other bat has more weight near the end.

which bat will be harder to swing? the bat with the weight farthest from your body.

here's why: although your are technically swinging the same amount of weight, your moving a different amount of mass a different distance. the bat with the mass closest to the grip only has to move a short distance through space. however the bat with the weight farthest from the grip is traveling a much greater distance. so instead of moving 3lbs across a two foot distance, your moving 3lbs across a 6 foot distance, thus more effort is required.

did this clear it up for anyone?
 
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#2 ·
I'm sick of seeing people not understanding this concept so I'm going to lay it out in this thread as clearly as possible.

First off, unsprung (rotating) mass is: weight that is held up by the suspension, and is SPINNING as the bike is moving.

The "weight" of unsprung mass is irrelevant. The moment of inertia is the unit of measure that really counts, and those numbers are hard to extract meaning from. It's not gonna be like "Oh man, I can REALLY feel that last couple inches^4 that we got off there". Rims do make a HUGE difference though.

The reason why brake horsepower numbers are always higher than wheel horsepower numbers is because there's extra weight to rotate before the power can get to the ground. As such, rotating mass directly affects the acceleration, deceleration and horsepower on a motorcycle. You're going to see more power on the dyno with less rotating mass (like lighter rims/rotors/chain/sprockets), because you've got less parasitic weight in the drive train to eat up power.

What I'm saying is that if you weighed a stock rim and a forged magnesium (for instance) rim on a scale and they both turned out to be the same WEIGHT, that wouldn't mean that they would perform the same. It's all about weight DISTRIBUTION, hence why the units for moment of inertia are in length^4. It's determined by cross-sectional area times the distance to the center of rotation (the axle) squared. This is why cutting a couple pounds of rotating mass from the tires has a much more dramatic effect than cutting a couple pounds of rotating mass from a rotor. The tires are farther from the axle, and that distance is squared when computing moment of inertia. This is why doing the 520 conversion for a lighter chain and sprocket than stock while leaving the gearing alone won't really do anything for you. You aren't going to feel that little of a difference in moment of inertia. Same with rotors..



Now does everyone understand why saying "these rims are 3 pounds lighter than stock" means absolutely nothing in terms of actual performance gains?
I don't understand can you please explain with smaller words and slower for me, because I deserve respect. :poke
 
#4 ·
Less rotational mass equates to faster accel and decel. Suck it. It also helps the suspension work better. Factory teams use light parts, Steve. Why is that?:laugh
 
#9 ·
Did you read what I wrote? Yes, lighter parts are always better. What I'm saying is that you can't say that mounting a tire that's 1 pound lighter is going to give the same performance benefits that losing 1 pound from the rotors, because they're not the same distance from the axis of rotation (the axle in this case).
 
#12 · (Edited)
If your not getting it, think of it this way.

When you sit in a officechair and spin, when we were kids..., if you spin and put your feet out, you would always slow down, but when you brought your legs in the chair would spin faster easier and longer.. basicaly as you out more weight further away from the axis of rotation the more the forces worked against you. Same concept maybe easier to understand...
If you were fat you could spin just as fast, but when you put your fat leg out it slowed you down even more. It was only weight furthest from axis of rotation affecting it, although total mass of course plays a role

Great post more people should know even basic physics.
 
#13 ·
If your not getting it, think of it this way.

When you sit in a officechair and spin, when we were kids..., if you soin and put your feet out, you would always slow down, but when you brought your legs in the chair would spin faster easier and longer.. basicaly as you out more weight further away from the axis of rotation the more the forces worked against you. Same concept maybe easier to understand...

Great post more people should know even basic physics.
/thread, thanks Paul :laugh:laugh
 
#20 ·
I feel some what smarter meow.
 
#22 ·
But......... Just to clarify :laugh

The benefit of lighter rims is less mass so the weight distribution is more towards the edge of the wheel, which is the tires? So ultimately, you want to save more weight near the tires instead of the center like rotors and sprockets?
 
#26 ·
You confused me..


The benefit of forged aluminum, forged mag and carbon fiber rims is that while they're all lighter than stock (so the suspension works better), they also all have a lower rotating mass than stock rims. That is, the weight distribution of the aftermarket rims is such that it is easier to get them spinning and to stop them from spinning. Moment of inertia (rotating mass) is measured as area times distance from the axis of rotation SQUARED, so the farther the weight (or lack thereof) is from the axle, the more significant of an impact it makes. This is why the Michelin Power Pures were such a big deal-- they were a couple pounds lighter than most other tires, and because the tire is the farthest thing from an axle on a wheel, that weight loss makes a huge difference. The same couple pounds of weight loss taken from some lighter rotors or sprockets, for instance, wouldn't have NEARLY the effect that it does on the tires.


Did I confuse you yet? :laugh
 
#23 ·
Thats true that a rim that is is 3 lb lighter than stock can actually perform worse. in theory then, you can have rims that are heavier and can perform better if you add weight to the center and take some off from the outside rim.

So anyone that designs a lighter rim that has a higher moment of inertia is an IDIOT. That would mean they moved weight farther from the center just because they can. So when I hear someone say "my rims are 3 lbs lighter than stock" I guess I assume whoever designed them is not an idiot and did not add random weight to the outside of the rim.
 
#28 ·
Thats true that a rim that is is 3 lb lighter than stock can actually perform worse. in theory then, you can have rims that are heavier and can perform better if you add weight to the center and take some off from the outside rim.

So anyone that designs a lighter rim that has a higher moment of inertia is an IDIOT. That would mean they moved weight farther from the center just because they can. So when I hear someone say "my rims are 3 lbs lighter than stock" I guess I assume whoever designed them is not an idiot and did not add random weight to the outside of the rim.
Right, exactly. This is why it's pretty safe to say that lighter parts are always better... but for the sake of completeness, that's not necessarily always true.
 
#25 ·
^^ Also an excellent point. And there are a bazillion other factors to consider in the design as well besides weight and rotating mass. Everything is a huge compromise. The amount of R&D that goes into sportbikes blows my mind, especially given how (relatively) cheap they are.
 
#35 ·
So I'm guessing the forged magnesium rims or even the forged aluminum wheels are ideal because the weight distribution is so even throughout the rim?
No, the even distribution on the rim is pretty standard on every rim... everywhere-- that's why you throw wheel weights on when you balance tires. You wouldn't have more weight on the left side of the rim than the right side. :dunce:
 
#45 ·
:laugh Not an argument at all, just different points of view that converge in a muddled mess....:laugh

Good points, for sure, and a good explanation, just more engineer than parts guy...:laugh
 
#49 ·
Some reason this reminded me that some people say centrifical (or centriphical however you wanna spell it). This isn't a real term. It's centrifugal or centripetal. Just my .02 that has to do with rotating mass
Did you know that neither of those things is the actual term for what's happening either? It's call angular acceleration. :fact
 
#50 ·
I wasn't neccessarily talking about your situation, just that people say centrifical trying to sound smart when it isn't a real word. Centrifugal is the force pulling you away from the center of the circle (yes I'm dumbing this down) ie in a car if you turn hard left you get pulled to the right. And centripetal is the force pulling you into the circle. In the car case it would be friction, otherwise you would just fly off the road
 
#52 ·
I wasn't neccessarily talking about your situation, just that people say centrifical trying to sound smart when it isn't a real word. Centrifugal is the force pulling you away from the center of the circle (yes I'm dumbing this down) ie in a car if you turn hard left you get pulled to the right. And centripetal is the force pulling you into the circle. In the car case it would be friction, otherwise you would just fly off the road
I get what you're saying. I'm saying that you aren't pulled by a force, you're pulled by an acceleration... An angular acceleration. :fact
 
#60 ·
Posted this on my Thread about R6 weight loss.

Felt like getting this off my chest here too...


I'm not trying to be confrontational, but for someone who is "sick of explaining" things on this forum, you sure go out of your way to do so. And this is the second time you seem to be accusing me of not knowing what I'm talking about, which is really funny for two reasons.

1. I posted this after your first insinuation back in June...

You can measure the Moment Of Inertia, but it's kinda esoteric...

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2. You wrote this on another thread just 2 days ago...
I'm not sure what you mean... That's the problem, most people don't understand this, so there are never numbers posted for improvements in moment of inertia for rims-- only weight. Even if the numbers WERE posted, it's pretty esoteric, so most people wouldn't know how to relate the units of measure (myself included).
Not only DO I understand moment of inertia, but I posted published numbers demonstrating various MoI's (which you seem to think were never published and impossible to find) and it even looks like you quoted me ("pretty esoteric").

Seriously, not trying to get in a poop slinging contest, but you seem to have a bit of smarter-than-you attitude about all this stuff. If that's your goal then maybe there's a Mensa meeting somewhere near you.
 
#62 ·
Well this explains everything as clear as day.
 
#64 · (Edited)
perhaps i can assist in the understanding of this argued concept. imagine you have two baseball bats of equal weight but unequal size. one bat has most it's weight near the grip (imagine holding the bat backwards) and the other bat has more weight near the end.

which bat will be harder to swing? the bat with the weight farthest from your body.

here's why: although your are technically swinging the same amount of weight, your moving a different amount of mass a different distance. the bat with the mass closest to the grip only has to move a short distance through space. however the bat with the weight farthest from the grip is traveling a much greater distance. so instead of moving 3lbs across a two foot distance, your moving 3lbs across a 6 foot distance, thus more effort is required.

did this clear it up for anyone?

this is why cheater bats work so awesome, the weight starts out close to your hands so it's easy to get moving. as the weight moves outward more force is required to keep the rate of acceleration the same, but you already got it going plenty fast from the beginning of the swing, so your essentially taking the normal power of a swing and adding about 20 - 30% power by shifting the weight.
 
#65 ·
perhaps i can assist in the understanding of this argued concept. imagine you have two baseball bats of equal weight but unequal size. one bat has most it's weight near the grip (imagine holding the bat backwards) and the other bat has more weight near the end.

which bat will be harder to swing? the bat with the weight farthest from your body.

here's why: although your are technically swinging the same amount of weight, your moving a different amount of mass a different distance. the bat with the mass closest to the grip only has to move a short distance through space. however the bat with the weight farthest from the grip is traveling a much greater distance. so instead of moving 3lbs across a two foot distance, your moving 3lbs across a 6 foot distance, thus more effort is required.

did this clear it up for anyone?
Perfect analogy. I'm gonna add this to the OP.
 
#68 ·
:dunce: Wat if i stop waring hevy geer an lose 10 pounds....Will i get bedder performance an gain HP?
 
#71 ·
Read this entire thread, and unless I missed it I saw zero mention of how "rotational mass" affects switchback cornering (left to right - right to left) or aka "stability".

Wheels with tires are simply flywheels, they are the largest single flywheels on the entire machine. The larger the diameter of the flywheel (or the heavier the flywheel) the greater the tendency to act as a stabilization gyroscope. That gyro tendency wants to fight the rider when tipping in, or changing lean directions (such as "corkscrew" type cornering situations). The more that the wheels acts as flywheels, the less the bike will want to lean away from whatever axis it is presently in. The bike resists tipping in, and it resists standing up. It wants to stay in the position it is in. Y'know, that whole "bodies at rest tend to stay at rest, bodies in motion tend to stay in motion" stuff.

Flywheels are stabilization devices, and it is their job to stabilize in the position they are in and remain in that same position. The FASTER the flywheel spins, or the heavier the flywheel is, (or the "greater moment of inertia" that the flywheel has, which is actually a symptom of ANY of those dimensional changes .... diameter, weight, or location of weight) the more the flywheel wants to stay right where it is.

That said, the heavier your wheelsets are, not only will the bike resist forward propulsion and braking, but it will also make the bike far more stable. Hmmm .... stability .... well hell's bells, that's a Good thing, right? Well, yes and no. If you're going for land speed record stuff or drag racing stability is good. If you're on a sportbike on a technical track stability is best left to the rider's control rather than Universal Physics.

To prove this whole "stable gyroscope" thing to yourself, take a bicycle wheel with an axle in it, and spin that wheel as fast as you can while holding it by it's axle. Now, while the wheel is spinning, attempt to move that bike wheel while it is spinning ... more it any way you want to .. or at least TRY to move it. Pretty darned difficult to do, isn't it! That gyroscope simply wants to remain in the position it's in, doesn't it! If you stand on a spinning surface of some type (like a small rotating table) while you are holding that spinning wheel, you will actually spin around on that table as you change the bike wheel's angle in your hands while the wheel is spinning. That demonstrates the utter power of rotational mass, or perhaps "rotational mass" may be better thought of as the simple gyroscope!

So then, after having absorbed all of that, imagine the following: Take two 26 inch diameter wheelsets (the typical OD of a 17" wheel with tire) weighing in at roughly 25 pounds (average weight of a mounted 17" wheel), spin it to whatever the operational RPM of said wheelset at (let's say) 75mph to simulate a L-to-R transition .. now then, just think of how much gyroscopic stability that you and your bike are attempting to overcome! Not to mention those high speed gyroscopes that the engine is packed with all spinning at a bazillion revs (crankshafts, counterbalancers, transmission shafts, et al).

So.....

*** not only are heavier wheels/tires more difficult for the engine and brakes to overcome (that rotational mass resistance thing).

*** your suspension isn't able to keep the tires an the ground as well (the unsprung weight issues thing ... jsyk most racing engineers use a rule of thumb about unsprung weight that states each ONE POUND of unsprung weight has the same handling effects as adding TEN POUNDS of sprung weight. So yea, 1:10. Wow.)

*** AND you as a rider are not able to flick the bike about nearly as well either (that gyroscopic stability issue).

That said, if I had a few grand to throw at the bike, it would NOT be headers. It would be lighter wheels, a lighter rear subframe, and if within budget perhaps a better front master cylinder. Those items would remove more time per lap (and in the end a few seconds of total race time) far better than adding power (headers).

A very graphic view of how much our wheels affect our bikes can be seen watching any Supercross race. Just watch the riders use those "gyros" to their advantage numerous times per lap. If the front end climbs too high when airborn, the rider locks up the rear brake in mid air, the lack of gyro effect in the rear makes the rear end "climb" or rise, lowering the front end and preventing a mid-air loop. Conversely, watch when the bike is nosing over too far, the rider will nail the throttle so hard the rev limiter taps in while in mid air. This action causes the rear end to sink, (hopefully) counteracting an endo. Those guys use those massive "gyros" to their every advantage at every opportunity.

BMX racers are also quite keen on rotational weight (at least we were when I raced in the 1970s). We knew the difference between the heavy duty "thorn proof" inner tubes and the ultra thin, ultra cheapo (but ultra light weight as well) inner tubes. We ran the thick tubes during the week while training, and the thin tubes on Saturday (race day). The thinner tubes were about two pounds lighter, and you could really REALLY feel the difference when attempting to holeshot everyone, or pumping your brains out on the final stretch to the checkers. This was such a common practice that I think everyone I knew did this. It wasn't "track mythology" at all, it was something you could actually feel, and not just a little bit either!

All done, shutting up in 3 ... 2 .. 1.
 
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