Rotating Mass: You've Probably Been Misinformed - Page 3 - JeepForum.com
Search  
Sign Up   Today's Posts
User: Pass: Remember?
Advertise Here
Jeep Home Jeep Forum Jeep Classifieds Jeep Registry JeepSpace Jeep Reviews Jeep Gallery Jeep Clubs Jeep Groups Jeep Videos Jeep Events Jeep Articles
Go Back JeepForum.com > General Technical Discussions > Tires & Wheels > Rotating Mass: You've Probably Been Misinformed

FS: Wrangler RGB Multicolor Fog Light LEDs: Awesome EffectFS: 2007-2013 Jeep Wrangler "HALO" Angel Eye KitRockridge 4WD IS Taking Zone Offroad Suspension Lift Kits

Reply
Unread 07-31-2013, 12:16 PM   #31
mschi772
Web Wheeler
 
mschi772's Avatar
1997 XJ Cherokee 
 
Join Date: Apr 2012
Location: Racine, WI
Posts: 2,061
Quote:
Originally Posted by WXman

They didn't swap tires. They swapped wheels/tires and used that to point out how the weight changes the game.
In changing the tires, they changed a major variable. There is a lot more about tires that affect performance than just their weight. Like I said, the one Pirelli configuration in that example was lighter than stock but still got worse mileage.

Quote:
Originally Posted by KKiowaTJ View Post
They are jeeps, Not Yugo's and if you want that mpg go buy one. The only stupid idiot, Is the one who bought a jeep, A block into the wind, For mpg and then *****es about it.
I wish people like you would just keep to themselves. That's a little rude of me to say, but it's true. I'm sick of seeing the "if you care about MPG, why'd you buy a Jeep" argument. Why'd I buy a Jeep? For Jeep reasons obviously; we're not idiots. I've NEVER met anyone who bought a Jeep for its fuel economy. Never. Buying/owning a Jeep doesn't mean that one is no longer allowed to care about improving their MPG. Better MPG doesn't become irrelevant just because it's a Jeep. What's better than owning a Jeep XJ and all the awesomeness and utility that comes with it? Owning a Jeep XJ and all the awesomeness and utility that comes with it AND saving an MPG or two (and the satisfaction simply from the achievement itself; I gain priceless satisfaction simply from improving on a design even if it's not always super cost-effective). Gaining 5 MPG [for example] is still 5 MPG gained whether you're going from 15 to 20 in an XJ, going from 25 to 30 in a Buick, or going from 30 to 35 in a Corolla. It's not like we Jeepers are trying to go from 15 MPG to 35; again, we're not idiots. Heck, even avoiding or just minimizing MPG losses can be satisfying. If you think gasoline is meaningless to you because you drive a Jeep, have fun with that, but stop trying to spread your careless attitude to everyone else.


Look, this ISN'T an MPG thread. I made this thread to share relevant physics with people to use or not use for whatever they feel like. It's just information. I don't know how someone may or may not use it, and I'm really telling anyone how to use it either. I just wanted it to be there for them. If you don't care about inertial mass, then what are you doing in this thread? People need to stop trying to derail this thread, and I will have a mod take action if I have to. A little discussion of MPG among other this is totally fine as it's obviously related to this topic, but let's keep it all in perspective.


P.S.
I share sm31's feelings about that article, and Wilson's post was quite decent as well. It's a bit simplified (probably for the best since not everyone here is necessarily great with physics) and I wouldn't really explain it like that personally, but it's a fine post and totally relevant. It's a much better post than your earlier one that seems to have been trying to communicate the same idea. Thanks, guys.

__________________
My 96 XJ is for sale in the event you want something to restore or build into a wheeling rig. Sale ad is HERE.
mschi772 is offline   Reply With Quote
Unread 07-31-2013, 07:08 PM   #32
KKiowaTJ
Registered User
1995 YJ Wrangler 
 
Join Date: Oct 2005
Location: Iowa
Posts: 3,452
Well its a fact on most models of jeeps, X mpg is the best you will get. Even more so when you mod your jeep and run a larger tire. All side forums have their 'best' list and from there is wasting money.

There are a lot of scam items new jeepers fall for, Im sorry if I have been on here and read the same thing a million times and done it myself countless times. Sorry it offended you, But I don't sugar coat **** and was only replying to what I was called.

Only reason I replied was to try and help save jeepers from wasting time and money chasing something that don't exist as many have tried and failed and I too fell for some 'bolt on' items that cause more harm. Again sorry and I wont post in 'you're thread' anymore.
__________________
YJ, TJ, XJ, ZJ..... Too many Jeeps!
KKiowaTJ is offline   Reply With Quote
Unread 08-01-2013, 01:20 AM   #33
Charley3
Web Wheeler
1999 XJ Cherokee 
 
Join Date: Mar 2013
Posts: 3,597
I don't know what the ratio is, but I do know from personal experience that a heavier tire/wheel SUBSTANTIALLY affects braking, acceleration, and ride quality. Also, don't ignore increased parts wear/breakage.

Instead of getting hung up arguing what the math ratio is, just know it matters a lot.

Also, tire/wheel weight affects unsprung weight, which matters a lot to ride quality.

Larger tires (particularly wider tires) have a lot more rolling resistance.

Higher load rated tires (D & E) have much more rolling resistance and are heavier too.

Often larger tires have a higher load rating.

I've run small, medium, and large tires (3 Jeeps). I've learned to appreciate tires on the borderline between small and medium. I prefer 30s or 31s on a Jeep (or any 1/4 ton SUV) after having tried sizes from 28" to 33".

The affects of larger, heavier tires on braking, acceleration, and ride quality are plainly evident when you try larger, heavier tires. Most people drive on highway more than off road.

Also, when off road, you can rev and spin a smaller, lighter tire without much risk of breaking drive train parts. With less unsprung weight you can drive faster off road. Not everyone wants to crawl. Some like to zoom along faster.

---

I can't put a scientifically proven number to it, and I'm not convinced the Website Mschi quotes is right or wrong on the numbers. What I do know is that when I had heavy 33 x 11.5 R16D (285/75R16D) tires on a stock LJ, it was very sluggish. When I switched to a much lighter 33 x 10.5 R15C tire/wheel combo the Jeep had MUCH better acceleration, braking, and gas mileage. The original tires were 31s, which were even better yet on highway.

I currently run 30s on an XJ and am very happy with them on and off road. Eventually, I might increase size to 31 x 9.5 R16C (245/75R16C) for off road gain. I will never go larger than 31s again. 30s & 31s are a great DD size being big enough for off road, small enough for on highway performance and economy, and light enough for all things.

---

I'm all about DD Jeeps. Good performance and gas mileage on and off road increases fun for me. These medium size tires do all that.

---

Tire size and weight matters a lot. Chose what suites your needs, and realize your choice matters.
__________________
Warning: Sometimes I edit a post a few times to get it how I want it.
Charley3 is offline   Reply With Quote
Unread 08-01-2013, 01:25 AM   #34
Charley3
Web Wheeler
1999 XJ Cherokee 
 
Join Date: Mar 2013
Posts: 3,597
Quote:
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!

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?
Wheel/tire weight matters on highway, and highway performance and economy matters to many people. Yes many people choose small to medium size tires for highway handling, acceleration, braking, and gas mileage reasons.

Weight matters off road if you want to go fast, or spin tires for traction in mud.

Even if you just want to crawl around at 2 mph off road, weight still matters when you want to climb a steep challenge.
__________________
Warning: Sometimes I edit a post a few times to get it how I want it.
Charley3 is offline   Reply With Quote
Unread 08-01-2013, 01:31 AM   #35
Charley3
Web Wheeler
1999 XJ Cherokee 
 
Join Date: Mar 2013
Posts: 3,597
Quote:
Originally Posted by biffgnar View Post
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.
Right on.
__________________
Warning: Sometimes I edit a post a few times to get it how I want it.
Charley3 is offline   Reply With Quote
Unread 08-01-2013, 01:33 AM   #36
Charley3
Web Wheeler
1999 XJ Cherokee 
 
Join Date: Mar 2013
Posts: 3,597
Quote:
Originally Posted by mschi772 View Post
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.

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.

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.
Well said.
__________________
Warning: Sometimes I edit a post a few times to get it how I want it.
Charley3 is offline   Reply With Quote
Unread 08-01-2013, 02:24 AM   #37
schitzangiggles
King of Macastan
 
schitzangiggles's Avatar
1993 YJ Wrangler 
 
Join Date: May 2005
Location: Rigby, Idaho, Macastan (Idaho)
Posts: 6,169
Quote:
Originally Posted by wilson1010 View Post
It costs money to create kinetic energy stored in a rotating mass. True. Miniscule amounts of gasoline, but a cost nonetheless. But, that rotating mass (kinetic energy) does not disappear. It is stored in the rotating mass. None is lost. As the wheel and tire slows, the kinetic energy is transferred back to the vehicle, cashing in by moving the vehicle a little tiny bit further or faster down the road. This process of investing in kinetic energy and recovering kinetic energy continues, back and forth with no losses as the rig goes down the road, changing speed in tiny amounts. Not one bit of kinetic energy is lost until one puts on the brakes. When the brakes are applied, there is a loss of kinetic energy because it is converted to friction and heat instead of moving the rig. A heavier rotating mass suffers a larger loss to heat than a lighter mass. Tiny, miniscule amounts, but more just the same. In city driving this is more of a loss than on the highway. Either way, it is insignificant.
So you have never heard of leverage?
The larger or heavier tire has more leverage than a lighter one. Don't think it matters? Ask a race car mechanic if there is a noticeable difference in engine performance with a coupla pounds heavier/lighter fly wheel.

Not to burst your bubble, but if you are going to use big werds, it would be helpful in your argument to actually know what they mean. You have basically described a perpetual motion machine which any one with a rudimentary grasp of elementary physics knows is impossible. You discount friction, and still haven't figured out that a heavier tire will have a greater moment of inertia than a lighter one and that force has to go/come from some where. If heavier tires are insignificant to the forces they impart, then why do your steering components and brakes fail/wear at an accelerated rate when larger or heavier tires are put on?

But since werds fail...
https://commons.wikimedia.org/w/inde...of_inertia.ogv
Oh and there is math involved so get some aspirin.
__________________
"Axeheads are supposed to be sharp, but you are one blunt tool."
FLynes



If you love wealth more than liberty,the tranquility of servitude more than the animating contest of freedom, go home from us in peace. We seek not your counsel nor your arms. Crouch down and lick the hand that feeds you. May your chains sit lightly on your shoulders and may posterity forget that you were our countrymen.
-Samual Adams

A government big enough to give you everything you want is big enough to take everything you have.
Thomas Jefferson

Calling Illegal Aliens "Undocumented Immigrants" is like calling drug dealers "Unlicensed Pharmacists"

Nemo Me Impune Lacessit
"No one provokes (attacks) me with impunity".


"Your right, the Tea Party and the OWS are the same, just like the America Revolution and the French Revolution were the same. The only problem is the American Revolution ended with Liberty, Freedom, Justice for all and a shining light upon a hill. The French Revolution ended with lynchings, guillotines, murder and socialism. So yeah, I could see how one could confuse the two..."

Mac Wilson
schitzangiggles is offline   Reply With Quote
Unread 08-01-2013, 03:01 AM   #38
Charley3
Web Wheeler
1999 XJ Cherokee 
 
Join Date: Mar 2013
Posts: 3,597
Just a thought, but if there were an all electric SUV (or hybrid), then heavier tires really would return energy on braking (regenerative braking).
__________________
Warning: Sometimes I edit a post a few times to get it how I want it.
Charley3 is offline   Reply With Quote
Unread 08-01-2013, 04:58 AM   #39
mschi772
Web Wheeler
 
mschi772's Avatar
1997 XJ Cherokee 
 
Join Date: Apr 2012
Location: Racine, WI
Posts: 2,061
Yeah, regenerative braking can return some energy. They're not 100% efficient though, and there will always be energy lost while driving: friction with the road and air, noise generation, drag from drivetrain parts, etc.

But yeah, regenerative braking does take a huge edge off of how wasteful lots of braking can be. Some hybrids and electrics can also use their own form of engine braking instead of brakes during light braking in order to recharge even more efficiently.

Off-topicish: Charley, the link I provided covers all rotational interia on a vehicle--not just tires/wheels. I know how you like to chow-down on the nitty-gritty and how you're always on the look-out for outside-the-box ways of enhancing your vehicles. No guarantees, but skim through that link. You might read something useful or at least inspirational. Maybe not though.



It has occurred to me that a bit of the fussing people have done in this thread so far probably wouldn't have ever happened had they clicked on a link and even just skimmed over the "easy" stuff on Stephen Mason's page. I provided a link for a reason: there's a lot to say on this topic, but I wasn't about to waste my time repeating it all when Mason has it covered so well already.
__________________
My 96 XJ is for sale in the event you want something to restore or build into a wheeling rig. Sale ad is HERE.
mschi772 is offline   Reply With Quote
Unread 08-01-2013, 05:55 AM   #40
wilson1010
Registered User
1999 XJ Cherokee 
 
Join Date: Aug 2006
Location: Cincinnati Ohio
Posts: 1,638
The heavier tires return essentially all of the energy they consume other than as lost through braking as I described above. The additional kinetic energy lost by braking an increased rotating mass is insignificant since the weight of the entire vehicle is the denominator of any equation seeking to quantify it. Same for friction.

For those who do not understand kinetic energy, read this again. For those who teach high school physics, write it on the board. Your students will thank you.

Quote:
It costs money to create kinetic energy stored in a rotating mass. True. Miniscule amounts of gasoline, but a cost nonetheless. But, that rotating mass (kinetic energy) does not disappear. It is stored in the rotating mass. None is lost. As the wheel and tire slows, the kinetic energy is transferred back to the vehicle, cashing in by moving the vehicle a little tiny bit further or faster down the road. This process of investing in kinetic energy and recovering kinetic energy continues, back and forth with no losses as the rig goes down the road, changing speed in tiny amounts. Not one bit of kinetic energy is lost until one puts on the brakes. When the brakes are applied, there is a loss of kinetic energy because it is converted to friction and heat instead of moving the rig. A heavier rotating mass suffers a larger loss to heat than a lighter mass. Tiny, miniscule amounts, but more just the same. In city driving this is more of a loss than on the highway. Either way, it is insignificant.
Finally, apologies to Mschi. I should not have flamed the original post. Your mistake was trying to introduce a complex topic to Jeepers, including myself.
__________________
03 Rubicon; 99 xj with too much stuff to list; Unimog 406
wilson1010 is offline   Reply With Quote
Unread 08-01-2013, 04:18 PM   #41
Charley3
Web Wheeler
1999 XJ Cherokee 
 
Join Date: Mar 2013
Posts: 3,597
Quote:
Originally Posted by mschi772 View Post
Off-topicish: Charley, the link I provided covers all rotational interia on a vehicle--not just tires/wheels. I know how you like to chow-down on the nitty-gritty and how you're always on the look-out for outside-the-box ways of enhancing your vehicles. No guarantees, but skim through that link. You might read something useful or at least inspirational. Maybe not though.

It has occurred to me that a bit of the fussing people have done in this thread so far probably wouldn't have ever happened had they clicked on a link and even just skimmed over the "easy" stuff on Stephen Mason's page. I provided a link for a reason: there's a lot to say on this topic, but I wasn't about to waste my time repeating it all when Mason has it covered so well already.
I did read at his Website the prior time you posted the link in an earlier thread. I might go back and read more.

Now how bout you mount up some big heavy ole tires and wheels so you can experience it first hand. Then tell me what you think. At this point you're still running stock tires. You need to try other sizes and weights of tires and wheels to gain first hand experience with the topic.

In the past on other Jeeps I went from 225 (28) to 235 (29) to 30, to 245 (31) to 33 x 11.5 to 33 x 10.5 to 30.

I might try 245 (31) again someday, or I might stay with 30s.

I've tried enough to compare, and in the case of my 33s I had, they were same diameter, but substantially different weight wheels and tires.

A little extra rotational mass is signifigant. I can't put a number to it, but I have experienced it, and it makes a big difference.

The reason you got some gruff treatment in this thread is these guys have all tried multiple tire sizes, and some have various weight tires of same size, and therefore these fellows (including me) have more real world experience with the topic than you do.
__________________
Warning: Sometimes I edit a post a few times to get it how I want it.
Charley3 is offline   Reply With Quote
Unread 08-01-2013, 09:38 PM   #42
mschi772
Web Wheeler
 
mschi772's Avatar
1997 XJ Cherokee 
 
Join Date: Apr 2012
Location: Racine, WI
Posts: 2,061
Quote:
Originally Posted by wilson1010 View Post
Finally, apologies to Mschi. I should not have flamed the original post. Your mistake was trying to introduce a complex topic to Jeepers, including myself.
Yeah, I pretty much give-up. I'm no stranger to non-scientists insisting on arguing science with scientists. It's bizarre how so many people think they can "out-science" professional scientists (not talking about just this thread anymore; I see this kind of stuff a lot). I love how people keep jumping to the conclusion that I'm just aping something I read online. I'm quite educated and have done my OWN calculations. I simply cited a source that shows all the math and has all the explanations already because it saves me a TON of time to do that instead of typing all the stuff out myself. Plus, I'd probably do a worse job of conveying it if I had to do it myself.

I'm not here to teach people how to understand this stuff. Anyone with the right background/education can check the theory and the math and correct/debate with me on my level if they want to, but I don't have the patience to play physics professor for people who just want to play the "nuh-uh" card. They can go on operating as they always have.

Seriously, though--if anyone here wants to talk real science and/or throw some physics equations around, THAT stuff I welcome. Some of you can probably go over my head quite easily on this topic because I'm not a mathematician, engineer, physician...I'm formally a biologist. I just have broad interests and am good at math.
__________________
My 96 XJ is for sale in the event you want something to restore or build into a wheeling rig. Sale ad is HERE.
mschi772 is offline   Reply With Quote
Unread 08-02-2013, 09:04 PM   #43
3X4X4
Registered User
2006 TJ Wrangler 
 
Join Date: Jan 2013
Location: Valley Center, Kansas
Posts: 601
I bought my Rubicon with 35" DC Crushers full of that pellet material. I removed them and went to BFG 33X15X12.5 (still heavy), removed the huge, massive steel bumpers on both ends and the spare (for local use) and spare tire carrier tire swing (gone forever) and my mileage went from 12ish to 15ish and even hits the occasional 16. Nothing has changed in my driving cycle or habits or speeds and my speedo was then and is now correct (Yeller-box).

How much heavier are the BFG 33s compared to the OEM Rubicon tires?

I have a MS and I am an engineer, I am going with the "science" guy.

J
__________________
2006 TJ Rubicon, Custom 4 inch lift with Currie Arms, TF springs, RS9000 shocks, TF High Steer Kit, TF ST Dual rate anti-rock, TF front track bar, Yellow Box speedo correction, Black Rock 15X10 Streetlock wheels with BFG AT 33X15X12.5 tires, Line-X tub, Bed Rug carpet, Tom Woods shaft, Spidershade, Savvy corner and rocker armor, Exogate.
3X4X4 is offline   Reply With Quote
Unread 08-03-2013, 06:37 AM   #44
SLADE
Registered User
2004 TJ Wrangler 
 
Join Date: Jul 2010
Location: 26537
Posts: 1,795
Quote:
Originally Posted by mschi772 View Post

THE BOTTOM LINE: The ratio is about 2:1 for most automobile tires and 1.5:1 for most automobile wheels.


Wouldn't the ratio change with tire height?
__________________
2014 Wrangler JKU
2013 Wrangler JK
2004 Wrangler TJ on LEAFS
2001 Dodge Ram 2500
SLADE is offline   Reply With Quote
Unread 08-03-2013, 07:51 AM   #45
mschi772
Web Wheeler
 
mschi772's Avatar
1997 XJ Cherokee 
 
Join Date: Apr 2012
Location: Racine, WI
Posts: 2,061
Quote:
Originally Posted by SLADE View Post
Wouldn't the ratio change with tire height?
Absolutely correct, but typical automobile tires don't vary in diameter by enough to really swing that ratio too much one way or another, so I just say 2:1 to keep it nice and simple. My 225/75r15's actually calculate as having ever so slightly less than a 2:1 ratio. The reason I feel comfortable using a simplified figure like 2:1 is because my math was simplified as well. Even Mason talks about how he had to simplify his approach to the problem. To strive for perfection would take a staggering amount of work. Yes, the math would be more precise and accurate, but the end result wouldn't be that much different. Tires and wheels are diverse products that get used in a chaotic world, so that level of precision just isn't worth the effort to me.

Mason's page has calculators, and his tire calc does obviously take size into account, so you can play with the numbers by just plugging-in a tire's size and weight. The only figure that Mason's java calc requires that's not as easy to determine is tread-to-sidewall mass ratio. He defaults it to saying that the tread area is twice as massive as the sidewall areas, but for tall, beefy tires like we Jeepers use, I'm not so sure about that ratio. The only way to really know without guessing would be to cut a tire apart and compare the mass of the tread to the mass of the sidewalls.

Mason's work contextualizes most things with speed freaks in mind, so keep that in mind if you read his stuff. I say this because he notes that the tread-sidewall ratio isn't very important for low profile tires. The higher the profile, the more important it is to get that ratio right for his java calc to be accurate, and we Jeepers use some pretty high profile tires.
__________________
My 96 XJ is for sale in the event you want something to restore or build into a wheeling rig. Sale ad is HERE.
mschi772 is offline   Reply With Quote
Reply
Thread Tools






Jeep, Wrangler, Cherokee, Grand Cherokee, and other models are copyrighted and trademarked to Jeep/Chrysler Corporation. JeepForum.com is not in any way associated with Jeep or the Chrysler Corp.