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I'm not sure if rotational mass is the correct term. Someone correct me if it's wrong.

It's pretty easy to find the actual mass/weight of a wheel. But not the rotational mass. I'd like to find 17" wheels with less of a rotational mass than the stock 15" ones I have.
For example, it takes more energy for a full rotation of a 17" wheel than a 15" wheel of the same actual mass. You see this effect when figure skaters pull their mass inwards for a faster spin.

And is there any place that would have this info listed?

 
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but the overall tire diameter should stay the same anyways, right. bigger rims, less rubber, smaller rims more rubber. So I dont think rotational mass changes any significant amount..imo

 
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Sounds like a bunch of confusing physics nonsense

The angle of the dangle is directly proportional to the heat of the meat times the mass of the ***

Does that help you any?

 
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Think about this. Take a 10lb sledgehammer, grab it at the very end of the handle and try to spin it around with one arm. Now grab the same sledgehammer and grab it half way down the handle and try to swing it the same way. Which was easier? They weigh the same but it seemed easier to rotate when you grabbed half way down....

 
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i dont think finding a 17 with the mass of a 15 is possible.
im not sure if thats exactly what your asking tho

 
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Mokushi
What you are talking about is dealing with leverage. Regardless of wheel/tire size the force is still coming from the center of the wheel.

 
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we did something like this in my physics class. it took like 3 pages of notes to do it.

 
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Hmmm, interesting question...

However, when the skater pulls in their mass, don't they actually decrease their diameter? I would think the MAIN thing to consider is the total mass. After all a 10 lb wheel weighs less than a 12 lb wheel, and would take less energy to move. The tire/wheel combo circumference should be the same for your speedo to be accurate, so total mass is really the primary factor. I would think the total mass of any wheel is pretty much evenly spread throughout the wheel. Most of it would be in the outer rim because that is where most of the metal is. Having more mass on the hub than the outter rim would probably make it easier to move, but I doubt the wheel manufacturers take this into consideration. After all they know that LIGHT weight wheels are the ones that sell for high performance and the only way to keep the wheel weight the same would be to move more mass to the hub area and take mass away from the outter rim, and that would probably weaken the wheel. Making a heavier wheel, by adding weight to the hub are might be interesting though. But then you would get into more weight, and braking becomes a factor to consider also.

 
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its not real mass its weight and momentum [IMG]i/expressions/face-icon-small-smile.gif[/IMG]

el edit: the amount of mass in something is the same regardless of gravity or speed (well, we will stay under 2000mph to stay accurate)

Its Inertia that must be overcome to start the tire, and to stop the tire. the more mass the tire has, and the more mass that is in the rotational area, and the vector that that mass desires to travel in vs. the direction it actually travels will make the difference

 
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Rotational Inertia
it is the factor of momentum, mass, and inertia...the lighter you make your wheels the faster it accelerations, brakes and reduces unsprung weight (see below)...the heavier the opposite...i like riding along in my 88 crx hf 60hp! (stock for now) and whupping the slammed civics off the line with their 18" rims...they don't understand it severely affects performance

wheel weights for rims you can purchase (a word to the wise...forged wheels are stronger than cast and therefore can be much lighter than a cast wheel of equivalent displacment [size]...but they cost more)

calculating your rotational inertia
you'll have to convert to metric but its fun

Unsprung Weight
however when adding heavier rims and tires it affects the suspension b/c of the increase in unsprung weight...in other words the stock suspension cannot handle the accleration and decleration caused by suspension travel...this also wears out your bushings and shocks making things ever worse in the long run...vtec net article

maintaining comfort with larger wheels
you have to consider the air volume in the stock tires and get the equivalent on a larger set to keep it comfortable on rough pavement (see motor trend response in the vtec article)

larger wheels also fight against acceleration b/c the outer wheel has more distance to travel to make one revolution as compared to a smaller wheel...however smaller wheels suffer from lower top speeds nad cruising at higher rpms than their longer diameter counterparts..this is what affects the speedos when you change wheels

 
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yeah whatever [IMG]i/expressions/face-icon-small-wink.gif[/IMG]

 
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One other thing to consider... The tires are lighter than the rims. Think of a skater with some hoola-hoop thing on the outside. Or attached to some really light cylinder. They would still spin faster when pulling some weight inwards. The force is still coming from the center but think about it this way... Rotating at the same speed, the outside of a larger circle is moving faster than a smaller circle. Thus it takes more energy to do it.

Rotational Inertia sounds more like what I was trying to get at. [IMG]i/expressions/face-icon-small-smile.gif[/IMG] Thanks guys!

Jaje, thanks for the links! I'll check them out!

 
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if your wheel is spinning faster than your tire, youre gonna have more to worry about than weight