01-05-2009, 09:40 PM   #61
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Quote:
 Originally Posted by samclem 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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01-05-2009, 10:23 PM   #63
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Quote:
 Originally Posted by NW-Bound 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

 01-05-2009, 10:32 PM #64 Give me a museum and I'll fill it. (Picasso)Give me a forum ...   Join Date: Jul 2008 Posts: 20,715 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.
01-05-2009, 10:48 PM   #65
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NW-Bound, that was a truly monumental and worthy effort on your part--good sources and everything.
Quote:
 Originally Posted by NW-Bound 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.

 01-06-2009, 09:56 AM #67 Give me a museum and I'll fill it. (Picasso)Give me a forum ...   Join Date: Nov 2007 Posts: 7,653 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.
01-06-2009, 01:36 PM   #68
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Quote:
 Originally Posted by FUEGO 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.

 01-06-2009, 05:08 PM #69 Recycles dryer sheets   Join Date: Aug 2005 Posts: 380 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.
01-07-2009, 10:02 AM   #70
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Quote:
 Originally Posted by TromboneAl 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.

 01-07-2009, 10:17 PM #72 Thinks s/he gets paid by the post   Join Date: Aug 2004 Location: Laurel, MD Posts: 3,325 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. __________________ ...with no reasonable expectation for ER, I'm just here auditing the AP class.Retired 8/1/15.
01-07-2009, 10:36 PM   #73
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 Originally Posted by jazz4cash 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."

 01-08-2009, 09:17 AM #74 Recycles dryer sheets   Join Date: Aug 2005 Posts: 380 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.
01-08-2009, 01:15 PM   #75
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 Originally Posted by SteveL 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

 01-08-2009, 03:11 PM #76 Moderator   Join Date: May 2004 Posts: 11,921 Here's a well-done video that illustrates some of the counterintuitive aspects of wheel motion that we've touched on tangentially here. (ugh!)
01-08-2009, 04:10 PM   #77
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 Originally Posted by samclem Here's a well-done video that illustrates some of the counterintuitive aspects of wheel motion that we've touched on tangentially here. (ugh!)

01-08-2009, 05:06 PM   #78
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Here you go--this time as a link to You Tube:

 01-08-2009, 06:29 PM #79 Give me a museum and I'll fill it. (Picasso)Give me a forum ...   Join Date: Jul 2008 Posts: 20,715 Samclem, very good. Heh heh heh... See where our enquiring mind leads us? Who says ER guys can't have fun without money?
 01-08-2009, 07:35 PM #80 Moderator   Join Date: May 2004 Posts: 11,921 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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