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Unread 07-28-2013, 08:11 AM   #1
mschi772
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Rotating Mass: You've Probably Been Misinformed

On this forum especially, the concept of rotational inertia gets invoked a lot. It may not be called that every time--rotating mass, rotating weight, spinning mass, whatever--they're all names people use for the same concept, and most times someone will give you some numbers to put it into perspective.

Most times, they're wrong.

The most common ratio I see used is 10:1. "Every pound of rotating mass is worth 10 pounds in the trunk." Wrong. Wrong and simplified as well.

THE BOTTOM LINE: The ratio is about 2:1 for most automobile tires and 1.5:1 for most automobile wheels. There, if you don't care about anything else, at least you know that. It's a bit more complicated than that because that ratio grows or shrinks depending on where the mass is distributed relative to the hub. The farther from the hub, the larger the mass's effect will be. In the end, though, the rule of thumb is 2:1, and value that however you wish. I personally don't give it much consideration because there are many things more important about tires to be concerned with.

I could go on to explain for those who are interested, but awhile back I found someone with a website that had done all the work of explaining it for me and did a better job of it, too.

For more information as well as various calculators for you to use, go to: http://hpwizard.com/rotational-inertia.html OR http://stephenmason.com/cars/rotationalinertia.html

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Unread 07-28-2013, 08:56 AM   #2
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No one cares about rotating mass as an off road topic. They care about unsprung weight. Why would anyone here care about rotating mass? Most of us are going like 2mph off road at the max. And, what really matters about rotating mass is the rotational velocity, not that other stuff you mentioned. A 65 pound wheel and tire at 2mph has like no rotating mass compared to a 60 pound wheel and tire at highway speed.

Is the point that if one puts on bigger wheels and tires one needs better brakes? Duh!

As for on highway use, don't we all know that energy is energy? Once moving the energy of an increased rotating mass is delivered back when the rig is coasting.

I just wonder who would care about 5 pounds on the tires? I mean, no one is going to put small tires on their rig to reduce rotating mass, are they?
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Unread 07-28-2013, 09:00 AM   #3
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Originally Posted by wilson1010 View Post
No one cares about rotating mass as an off road topic. They care about unsprung weight. Why would anyone here care about rotating mass? Most of us are going like 2mph off road at the max. And, what really matters about rotating mass is the rotational velocity, not that other stuff you mentioned. A 65 pound wheel and tire at 2mph has like no rotating mass compared to a 60 pound wheel and tire at highway speed.

Is the point that if one puts on bigger wheels and tires one needs better brakes? Duh!
So I guess you assume no one here daily drives their Jeep?
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Unread 07-28-2013, 09:06 AM   #4
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Originally Posted by wilson1010 View Post
As for on highway use, don't we all know that energy is energy? Once moving the energy of an increased rotating mass is delivered back when the rig is coasting.

I just wonder who would care about 5 pounds on the tires? I mean, no one is going to put small tires on their rig to reduce rotating mass, are they?
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So I guess you assume no one here daily drives their Jeep?
OK, I guess there is someone who will put little tires on his rig to reduce "rotating mass." Who knew!
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Unread 07-28-2013, 09:11 AM   #5
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OK, I guess there is someone who will put little tires on his rig to reduce "rotating mass." Who knew!
No, I did not put "little tires" on my Jeep. As soon as you have to do heavy braking, all that energy has to get transferred again, and at a much faster rate. A bigger tire = more energy to get stopped.
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Unread 07-28-2013, 09:20 AM   #6
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No, I did not put "little tires" on my Jeep. As soon as you have to do heavy braking, all that energy has to get transferred again, and at a much faster rate. A bigger tire = more energy to get stopped.
Do you think that the increased rotating mass of a tire that is a little heavier than a tire of the same size is more significant than just the increased total weight of the rig from having a heavier tire?
By how much? What, 20 pounds? Tire height compared to brake rotor diameter is only about a hundred times more important.

BTW, I like your quote about speed not killing anyone.
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Unread 07-28-2013, 09:21 AM   #7
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Not everyone here uses their vehicle the same. I certainly do a hell of a lot more than just move at 2 mph on trails with my vehicle. I drive on highways. I drive on country roads. I drive in the city. I drive on trails. I drive off trails. I drive in snow. I drive at 2 mph. I drive at 70 mph. There are more than just hardcore off-roaders around here, and I see rotational inertia get brought-up quite often around here, so take your crusade to them because they're the ones who feel it's worth considering. I personally DON'T feel that rotational inertia is worthy of much consideration, but for the people who think the ratio is 10:1 (especially the ones on the fence between a 30 lb AT tire and a 45 lb MT tire), I can understand why they might think it's more important than it really is which is why I started this thread--to show them that it's really not as big a difference as they think so that they may adjust their thoughts accordingly.

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Originally Posted by wilson1010 View Post
And, what really matters about rotating mass is the rotational velocity, not that other stuff you mentioned. A 65 pound wheel and tire at 2mph has like no rotating mass compared to a 60 pound wheel and tire at highway speed.
If you're going to burst in here and start criticizing someone who's simply trying to share info with anyone who might be interested in it, try to know what you're talking about. They have the same mass at 2 mph or 60 mph--mass is constant; velocity does not add or remove mass from an object (unless we start talking about relativistic mass instead of invariant mass, but that's all relativity, Einstein, light speed stuff; that's not Jeep stuff). "What really matters" is rotational inertia which is governed by mass and velocity, so you were partially right and as such, partially agreeing with me even when you were trying to disagree.

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As for on highway use, don't we all know that energy is energy? Once moving the energy of an increased rotating mass is delivered back when the rig is coasting.
Coasting doesn't deliver any energy back; don't be foolish. Yes, rotational inertia helps coasting, but you'll only really benefit if you can coast to a stop without brakes every time. Reality: you can't. Because you have to use your brakes in the real world, that rotational inertia is a double-whammy because it will be harder to stop as well as harder to get it going.

If you don't care about the science/details, fine. I'm not telling you that you have to (but if you don't care, then don't act like you do because you'll embarrass yourself). Some people DO, and I'm just trying to make the info available to them. You seem to be under the impression that anyone who doesn't share your opinions/priorities is worthy of ridicule. That attitude isn't welcome here, so please be more respectful of people who are sharing/seeking information.
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Unread 07-28-2013, 09:23 AM   #8
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Originally Posted by wilson1010 View Post
Do you think that the increased rotating mass of a tire that is a little heavier than a tire of the same size is more significant than just the increased total weight of the rig from having a heavier tire?
By how much? What, 20 pounds? Tire height compared to brake rotor diameter is only about a hundred times more important.
No, not necessarily. I agree, distance of the mass from the center of rotation is the biggest factor.
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Unread 07-28-2013, 01:35 PM   #9
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Even at 2:1 when multiplied by four tires the difference in wheel/tire combos becomes significant. Given the overall focus on lightening off road vehicles this is stuff that can't be ignored.
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Unread 07-28-2013, 05:26 PM   #10
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I am mostly pleased to see that not only I thought the 10:1 ratio looked preposterous.
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Unread 07-28-2013, 07:42 PM   #11
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Well as a scientist who is also a 4x4 fanatic, I will say this. You can argue numbers and math until the cows come home... but the bottom line is this: In the real world a tire that weighs 10 lbs. more than your stock tires WILL significantly hurt your fuel economy, braking ability, and performance.

There are guys who have deep pockets, and they won't care. But there are also guys who are trying to enjoy this hobby on a tight budget with a family and responsibilities...and for those folks fuel economy and braking matters. It matters a lot. And when you consider that for the average U.S. driver, at current fuel prices, every 1MPG = $15 per month it doesn't take long to see why people today are becoming more conscious about the weight of their tires.

I got better gas mileage with 33x12.50 Duratracs than I got with 285/75-16 Falken ATs. Why? The Falkens were smaller and MUCH more mild in the tread design. Why would they suck fuel? Because of one thing.. weight. Those tires were 63 lbs. each on average, compared to 49 lbs. each for the Duratracs. Made a HUGE difference at the gas pump. Also made the poor little V6 scream for a while just to hit the speed limit.

Also, I used to haul a 600 lb. ATV around with my Ranger 4x4 sometimes, and the fuel economy hit I'd take when the ATV was loaded was very very similar to the MPG hit I took when I increased my tire weight 23 lbs. each on my Jeep. So personally, just from my years of real world experience with dozens of 4x4s and dozens of tires, I think a 2:1 ratio is horse s*it. There is no way in hell that adding 10 lbs. to your tires is only like adding 20 pounds to the cargo area to haul.
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Unread 07-29-2013, 12:48 AM   #12
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I stand corrected. Just when I thought no one would care a bit about a few pounds on the tire and wheel assembly, they come out of the woodwork and claim massive fuel economy from lighter tires. Who knew. I hope the tire companies don't find out. They don't need an excuse to take more rubber out of tires. Think what carbon fiber wheels would do to help meet CAFE standards.

Look, if you want to believe your own anecdotes, fine. For me, I'll stick with science and math. And 63 pounds is still two bags of groceries or a 10 year old kid in the car having no perceptible effect on gas mileage.
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Unread 07-29-2013, 05:21 AM   #13
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Well as a scientist who is also a 4x4 fanatic, I will say this. You can argue numbers and math until the cows come home... I think a 2:1 ratio is horse s*it. There is no way in hell that adding 10 lbs. to your tires is only like adding 20 pounds to the cargo area to haul.
How can you claim to be a scientist but then refuse to believe the math and physics presented to you? Because you got great MPG gains going from Falkens to Duratracs? There's way more than weight at play there, and any self-respecting scientist must know what extraneous variables are. Just because a tread patter looks more mild doesn't mean that it must have lower rolling resistance.

Respectfully, if you want to act like a real scientist, and you really think all this math is wrong, disprove it. Don't cite confounded anecdotes--be a scientist and present data gathered from controlled experiments. Until you do so, I'll stand by my math and the math of others who've verified it such as the link in my OP.
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Unread 07-29-2013, 05:52 AM   #14
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I stand corrected. Just when I thought no one would care a bit about a few pounds on the tire and wheel assembly, they come out of the woodwork and claim massive fuel economy from lighter tires. Who knew. I hope the tire companies don't find out. They don't need an excuse to take more rubber out of tires. Think what carbon fiber wheels would do to help meet CAFE standards.

Look, if you want to believe your own anecdotes, fine. For me, I'll stick with science and math. And 63 pounds is still two bags of groceries or a 10 year old kid in the car having no perceptible effect on gas mileage.
Then you, sir, are a moron.

Why do you think Goodyear's new batch of offroad tires are the lightest in the market? Why do you think the new Toyo tires are specifically made with lighter weight materials? Why are these engineers specifically trying to reduce weight? Why are structural engineers at all the pickup truck manufacturers working day and night to shed weight from their trucks, even though those vehicles are used to do heavy duty work? Why do you think Jeep has a contract with Goodyear to produce a SPECIFIC tire solely for their vehicles that is ultra lightweight (a 32" tire that only weighs 39 lbs!)? Because weight reduction is how you get MPGs up! CAFE standards!

Like I said, you can argue the math all day. The 2:1 ratio is bulls*it. Go throw some tires on your rig that are 10 lbs. heavier at each corner and watch your fuel guage drop off on the first tank.
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Unread 07-29-2013, 05:56 AM   #15
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How can you claim to be a scientist but then refuse to believe the math and physics presented to you? Because you got great MPG gains going from Falkens to Duratracs? There's way more than weight at play there, and any self-respecting scientist must know what extraneous variables are. Just because a tread patter looks more mild doesn't mean that it must have lower rolling resistance.

Respectfully, if you want to act like a real scientist, and you really think all this math is wrong, disprove it. Don't cite confounded anecdotes--be a scientist and present data gathered from controlled experiments. Until you do so, I'll stand by my math and the math of others who've verified it such as the link in my OP.
Because everybody who's been around this hobby more than 6 months knows for a fact that weight KILLS. It hurts on the trail because it destroys drivetrain parts. It hurts off the trail because it destroys your ability to panic stop. It hurts the wallet because you're getting as much as 50 miles less per tank of gas. Weight kills.

And, no one has proven that the 2:1 ratio is scientifically fact as it applies to 4x4s, because as somebody already mentioned in this thread there are other variables. Just because you see it on the internet, that doesn't make it true.
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