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Old 01-05-2009, 10:40 PM   #61
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Emphasis added:
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Originally Posted by samclem View Post
I think SteveL and HaHa are right--we care about circumference, not radius/diameter. To test this, you could put a chalk mark on the tire, inflate to 45 PSI and measure the ground distance covered by exactly one revolution (chalk mark at 6:00 to chalk mark at 6:00). Then deflate to 20 PSI and do it again. I'm fairly sure the length of the lines on the ground will be virtually the same regardless of tire pressure (those steel belts aren't getting any longer and shorter with tire pressure, I'll bet). If this is correct, the speedometer/odometer accuarcy won't vary due to tire pressure.
I guess it is possible that the tire circumference grows some tiny amount with increased pressure. But, this phenomenon would produce exactly the opposite impact on observed gas mileage: Higher tire pressures would produce lower observed gas mileage.
Why: Say we have two cars: one with larger circumference tires and one with smaller circumference tires. The car with the higher pressure (larger) tire would be traveling the same distance with fewer wheel revolutions as the other car. The speedometers/odometers in our cars measure wheel revs, so this car would (incorrectly) show less distance traveled than the car with the smaller tires. But the engines in both cars drove the cars the same distance, and did the same amount of work (discounting any other phenomenon caused by tire pressure). Even if we make the argument that the bigger tire gives the car a "taller" gearing and resulting better "true" mileage (as measured on the road surface), this will not have any impact provided we measure distance traveled by counting wheel revs.

So, even if tire circumference is effectively reduced by having reduced tire pressure (a point I'm not ready to cede as I think of those circumferential steel belts), the effect on observed mileage would be opposite to the one we see in real life, indicating other factors are far more important.
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Old 01-05-2009, 10:47 PM   #62
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Heh Heh Heh...

On this subject, my thought has always been along the line of thinking as T-Al, that an underinflated tire would have a lower effective radius, hence would require more revolutions for the same distance travelled.

Then I saw the "circumference" argument from SteveL and Haha.

To borrow from a Web link included below, let's call TAl+ERD50+myself+others the "radialist" camp, and SteveL+Haha+Samclem+others the "circumferencialist" camp.

I discovered that what I always take for granted has been challenged. And yet, it is not so easy to dismiss the "circumferencialist". To do so, their argument about "traction" must be addressed, but I got to admit I do not have an answer offhand. I can see the point that if the steel belt circumference is not very compressible, and if the tire does not slip, how do you satisfy the circumferencialist camp?

The best way to settle this is to go out and do some measurements yourself. However, I am afraid measuring the distance travelled with a few tire revolutions may not be decisive enough, due to the small errors required.

Heh Heh Heh...

The inquisitive mind just got to know who is right. Sooo, I searched the Web. This is fun. What an ER'ed guy do all day?

Heh Heh Heh...

Found several patents and engineering conference papers on applications of the "radialist" principle for mandated TPMS (Tire Pressure Monitoring System). Basically they compare the rpms from each wheel to determine if a tire has been underinflated, or use GPS for accurate determination of distance travelled.

http://ddl.stanford.edu/files/NLSlipAVEC2002.pdf

http://ieeexplore.ieee.org/Xplore/lo...number=1644529

http://www.embedded-control-europe.c...cemay04p20.pdf

Method for monitoring tire pressure variation of automobile tire and system for realizing the same - Patent 7395177


Soo, the academia and industry are in the radialist camp.

Heh Heh Heh..



Also found several forums whose members were also inquisitive on this very same question.

The most relevant info came from this guy, who has done what all of us would have done. He tapped into the wheel revolution sensors of his Prius ABS (Antilock Brake), and looked at the signals on an oscilloscope carried in his car (Obviously an EE like myself). He looked at the signals with both wheels at 43 psi, then with one deflated to 35psi.

His conclusion: Radialists win!

Quote: "But that difference between 43 and 35 psi, still quite tolerable from a safety standpoint, created a significantly measureable delta in wheel speed."

I don't profess to know how the tire gets deformed so as to deceive the circumferencialists, but the real world is what it is.

Heh Heh Heh...

Details are at

http://www.techno-fandom.org/~hobbit/cars/tpms/pts.txt

The next question is: How to explain the "circumference" enigma
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Old 01-05-2009, 11:23 PM   #63
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Originally Posted by NW-Bound View Post
Heh Heh Heh...

Then I saw the "circumference" argument from SteveL and Haha.

To borrow from a Web link included below, let's call TAl+ERD50+myself+others the "radialist" camp, and SteveL+Haha+Samclem+others the "circumferencialist" camp.

Found several patents and engineering conference papers on applications of the "radialist" principle for mandated TPMS (Tire Pressure Monitoring System).

Soo, the academia and industry are in the radialist camp.

Heh Heh Heh..

His conclusion: Radialists win!
Good catch - I can't believe I didn't think about the TPMS - I know that is one way they do it, sense air pressure through deltas in rotation. What more 'real world' proof do we need?

Quote:

The next question is: How to explain the "circumference" enigma
Ummm, I dunno. Although you correctly placed me in the "radialist" camp, I started making some sketches of tires and hubs (which ended up looking like bad cartoon drawings of bad boob jobs ), and I started having my doubts. Even though the 'apparent' radius is less - the tire still has to go around that whole circumference - you don't get rid of any tire. But I think it in fact does 'grow', and the steel belts must move around and/or displace space within/between the rubber (? got a better explanation?).

-ERD50
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Old 01-05-2009, 11:32 PM   #64
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An underinflated tire must undergo some bad deformation at the ground contact point, though I don't know if your bad boob jobs drawing would offer a plausible explanation . Whatever that deformation, it was severe enough to cause eventual catastrophic damages, a la Ford Explorer roll-overs.

Hopefully, some tire experts among us will offer an explanation.
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Old 01-05-2009, 11:48 PM   #65
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NW-Bound, that was a truly monumental and worthy effort on your part--good sources and everything.
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Originally Posted by NW-Bound View Post

The most relevant info came from this guy, who has done what all of us would have done. He tapped into the wheel revolution sensors of his Prius ABS (Antilock Brake), and looked at the signals on an oscilloscope carried in his car (Obviously an EE like myself). He looked at the signals with both wheels at 43 psi, then with one deflated to 35psi.

His conclusion: Radialists win!

Quote: "But that difference between 43 and 35 psi, still quite tolerable from a safety standpoint, created a significantly measureable delta in wheel speed."
At this juncture I must make an observation about the (impressive) research provided by the Prius (nut!) at your link:

-- His measurements (indirectly) indicated that the difference in effective tire "radius" (his term, not mine) was tiny:
Quote:
855 revs/mile --> effective radius = 11.794 in. @ 43 psi

856.875 revs/mile --> effective radius = 11.768 in. @ 35 psi
Because his device counted revolutions, he was effectively determining relative wheel circumferences (not radi), and they differed by only 0.2%. So, even if this circumferential "stretching" is occuring with higher tire pressure, the effect is miniscule.

But, back to my previous point and the association with MPG: If it is true that lower PSI tires truly do have a shorter circumference, then a car with such tires should produce a higher indicated MPG (if MPG is derived from tire roations, which is the case in virtually all cars). That's not what we see in the real world.
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Old 01-06-2009, 12:27 AM   #66
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I will admit to being excited at finding a report from this tinkerer that I missed the minuscule 0.026" difference in effective radii. However, he was going from an overinflated 43psi to a "Toyota recommended" 35psi. Had he gone from the recommended 35psi down to, say 25 or 20 psi, the effect might be quite a bit more. Still, the effective radius change would not be a large percentage.

I read further down this time, and saw that the next post in the thread said

"Sorry, but the Department of Transportation has already considered
(and rejected) indirect tire pressure monitoring based upon ABS
sensors. They found that while it could detect severe loss of
pressure, it wasn't able to detect slight losses as effectively as
they wanted.

So the mandated TPMS systems are direct pressure measurement types."


Anyway, I was not thinking about the relationship of the tire radius to the MPG at all. We all know MPG is rather severely degraded with tire underinflation, as we all can extend from our experience of physical exertion with an underinflated bicycle tire. With an underinflated tire, the higher rolling resistance swamps out the small effective radius change, and the car computer still properly displays a lower MPG. I have no problem with that at all.
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Old 01-06-2009, 10:56 AM   #67
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This would make a good final exam question for an Engineering Dynamics/Mechanics class. Maybe throw in some Mechanics of Materials and different deformation characteristics of steel vs rubber, drop a couple of moduli of elasticities, a question on elastic vs plastic deformation, and you might even stump the smart students.

If the professor were sadistic.
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Old 01-06-2009, 02:36 PM   #68
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If the professor were sadistic.
Aren't most of them?

Nah... Mundane appearing as this problem is, a definitive answer must come from an expert in this field, someone who works at Michelin, GoodYear, or at least in Detroit.
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Old 01-06-2009, 06:08 PM   #69
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While we are close to counting the angels on the head of this pin, I once more enter the discussion....

If you take the two Prius tire radii and calculate the difference, it means that the circumfrence has changed by .224 inches. These tire/wheel combos make about 854 revolutions per mile, meaning in each mile, the better inflated tire goes about 4 feet further. I submit that this difference is not material in whatever difference is mileage produced by differences in inflation.

Lower inflation means more tire on the ground, more friction, more heat, and more energy spent just moving the weight of the car on the tire.
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Old 01-07-2009, 11:02 AM   #70
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UPDATE: These tires give me significantly worse gas mileage than the Toyo Ultras did! The average of the last five tankfuls with the old tires: 40.84 MPG. The average of the first five tankfuls with the new tires: 36.98 MPG.

We drive 15,000 miles per year, and use 38 more gallons per year, or, at $3/gallon, $114.

I guess it's not worth taking the tires back.
I'm no mechanical engineer (or engineer of any stripe) but I do know that cold weather has a detrimental effect on mileage; higher density air, and other factors in a cold engine. Made even worse if you're making shorter trips and thus more miles on a cold engine. I know that my mileage is significantly and consistently lower in the winter.

I didn't see it in the discussion so I'll ask. Have you made any attempt at factoring in (correcting for) cold weather operation to the decrease in mileage? I'm not sure how to do that other than to take historical data for your car and correcting for the difference you normally see in cold weather.

Just a thought to keep you guys busy.
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Old 01-07-2009, 07:32 PM   #71
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Had missed most of this thread due to a trip of 3 days (560 miles round trip) for a family friend's funeral.

Having had a suitable nap, got a chance to read the thread. Most impressed with the amount of energy and analysis expand by both the radial and circumference camps. Most of all of the Prius owner's multi decimal results.

My interest was in the mileage question of tires. The one item not mentioned in the discussions is thread squirm. All new tires will give lower mileage due to thread squirm. SO even if replacing old worn out tires with exact same brand, the new ones will give poorer mileage than the nearly worn ones.

My other interest is in inflation pressures. Some years ago reading about pursuit driving, the instructor's opinion was to run tires at maximum sidewall pressure. It will give (very) firm ride and handling. From an old graph I copied It shows rolling resistance decreasing by about 8% if pressure is incrased from 32 psi to 40 psi. Allows for faster tuns on corners (tire will have less tendency to roll off of rim) and higher hydroplaning speeds. Hydroplaning speeds change from about 45 mph at 32 psi to about 68 mph at 45 psi.

So in that spirit on highway trips my tires are at max allowed sidewall pressure. Have no compulsion at measuring precise mileage, but do enjoy the very positive handling of the car. This 560 mile run was in DW's 2006 Buick LeSabre, average outdoor temperature of 32 F for the run, Rain by the buckets both directions, indicated mileage by the car's cpu of 27 mpg, at majority of the time speeds well north of 65.
Having recently put new tires on this car was comforting. They a very high speed rated, directional tread, max pressure on sidewall 51 psi. So, traveling at high speed in the rain at full rated pressure i had no hydroplaning issues. Dry pavement handling is phenomenal. Though for around town, to please wife's ride comfort I drop pressures to 35 psi. The high performance tires are in part to compensate for DW's (and my) lead foot. ( I nick named her Mario Andretti).

So hope i did not open another can of worms on the hydroplaning issue.

Cheers!

By the way stunt drivers driving pickup trucks, tipped on two wheels, don't roll standard LT rated tires off the rim because the inflate them to 100+ psi.
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Old 01-07-2009, 11:17 PM   #72
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You guys are hopeless and dangerous and should stick to ER discussions....or just keep havin fun, but stay away from my tires.

FWIW, the tire with the larger circumference does not tell the speedometer it's dimensions have changed. The speedo uses transmission output rpms for it's calculation. The larger circumference tire would have travelled more miles than the odo would register.

I lost the logic behind why anyone believes larger circumference hurts fuel economy...it does not. Increasing tire pressure will improve fuel economy by lowering N/V ratio (more circumfernce means more miles travelled per engine revolution) AND reducing Rolling Resistance (less tread deflection & squirm). The instrumentation discussed here is likely not precise enough to measure these differences.

Best, easiest way to measure rolling resistance is making timed coastdown runs from say 60 mph to 50 mph with a stopwatch. Repeat test in opposite direction of travel and average the results. Adjust tire pressure and repeat test in each direction. Running the test in neutral would improve accuracy, but it's generally illegal on public roads.
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Old 01-07-2009, 11:36 PM   #73
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FWIW, the tire with the larger circumference does not tell the speedometer it's dimensions have changed. The speedo uses transmission output rpms for it's calculation. The larger circumference tire would have travelled more miles than the odo would register.

I lost the logic behind why anyone believes larger circumference hurts fuel economy...it does not. Increasing tire pressure will improve fuel economy by lowering N/V ratio (more circumfernce means more miles travelled per engine revolution) AND reducing Rolling Resistance (less tread deflection & squirm). The instrumentation discussed here is likely not precise enough to measure these differences.
Everyone agrees that higher pressure = better mileage.

A car with a larger circumference tire will produce lower indicated MPG, if all other factors are equal. That's because (for the reasons you describe) the odometer will (incorrectly) show that the car with the bigger tires didn't travel as far.

And here's another hair-splitting point: We shouldn't be using the term "circumference" for the distance around the (two-dimensional representation of a) tire. As ERD50's "bad boob job" drawings would clearly show (if only they were published!), the actual shape of a rolling tire is not a true circle, and the term we should be using is "perimeter."
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Old 01-08-2009, 10:17 AM   #74
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The angels keep flapping.......
Imagine a front-wheel drive car with a solid rear axle, with one of the two rear tires under inflated enough to show a bigger footprint on the ground. As the car goes forward, the rear axle will revolve and the tires will make the same number of revolutions, since the wheels are bolted to the axle. If the lower pressure tire was making less distance, it would be dragging, and get very hot, perhaps catching fire.

Under-inflation, unless to the point where the tire comes off the rim, doesn't materially change the circumference (perimeter) of a tire.
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Old 01-08-2009, 02:15 PM   #75
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Under-inflation, unless to the point where the tire comes off the rim, doesn't materially change the circumference (perimeter) of a tire.
How do you come to that conclusion? If by 'materially', we mean enough for an indirect TPMS to measure it, then yes, it changes 'materially'.

Tire-pressure monitoring system - Wikipedia, the free encyclopedia


edit/add - come to think of it - this description is probably flawed as well (and I'm not sure what it attempts to prove anyway):

Quote:
The angels keep flapping.......
Imagine a front-wheel drive car with a solid rear axle, with one of the two rear tires under inflated enough to show a bigger footprint on the ground. As the car goes forward, the rear axle will revolve and the tires will make the same number of revolutions, since the wheels are bolted to the axle. If the lower pressure tire was making less distance, it would be dragging, and get very hot, perhaps catching fire.
You are correct that given a solid axle and two tires of different effective perimeters, that something has to give (or go in a circle). But, the most likely scenario would be that the under-inflated tire would have a larger footprint and therefore *more* traction, causing the other tire to drag (or skip-ahead I guess). No idea how much hotter it would get though.

-ERD50
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Old 01-08-2009, 04:11 PM   #76
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Here's a well-done video that illustrates some of the counterintuitive aspects of wheel motion that we've touched on tangentially here. (ugh!)
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Old 01-08-2009, 05:10 PM   #77
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Here's a well-done video that illustrates some of the counterintuitive aspects of wheel motion that we've touched on tangentially here. (ugh!)
Tried your link, didn't see a video. Do you have the youtube link?
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Old 01-08-2009, 06:06 PM   #78
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Tried your link, didn't see a video. Do you have the youtube link?
Here you go--this time as a link to You Tube:
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Old 01-08-2009, 07:29 PM   #79
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Samclem, very good.

Heh heh heh...

See where our enquiring mind leads us? Who says ER guys can't have fun without money?
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Old 01-08-2009, 08:35 PM   #80
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Another addition,for those still clinging to this thread:
- video is the one that started it all. It demonstrates that a cart (or a boat, theoretically) can be designed that will move downwind faster than the wind. It is counterintutitive, but true. This cart, powered only by the wind, can outrace a balloon floating along in that same wind. The driver of the cart would feel the wind in his face, not at his back.

- another video by the "Physicsmobile" guy that carefully explains how this is possible. IMO, if this guy dreamed up the naration and the demonstration style himself, he really should be making these videos for a livng. It is first-rate.

Yes, I know I'm a freak and that no normal person finds this interesting.

T-Al--Sorry about this. I hope you were done with this thread.
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