2 experts debate tire revolutions/mile (vs. measured circumference)

[cosmick]

Noone learns much from discussions they can't read, so I'm trying to make this open. This disagreement began in Smudge's 2.7L Tacoma thread, and I guess I did feel slighted, when my laborious testing was refuted. CapriRacer kept his cool better than I did, and I should have done better.
Even so, The issue is how many times any given tire actually turns, per mile. Some thing revolutions, some think rotations, I'm side-stepping that mess with turns, to keep the focus on the real issue.
So I'm going to start by detailing my testing, that will be a long read. But it took a lot of physical labor, and I am clear on the scientific method; establish a baseline, repeat to confirm, change only one variable, test again, then repeat to confirm. Then go back to the baseline, by undoing the change, and see of original test results repeat, and re-test that. I did all that. Took me a long summer saturday and plenty of Gatorade, just for the first test.
That was a C2500 pickup truck on LT245/70R17E tires, then after that, I realized I needed a subject with short-sidewall, non-LT tires. I had an Isuzu mini-truck available, and I had a set of P245/50R16 tires. So I tested that as well.
I know a place with just over 2/10 of a mile of straight, flat, level asphalt and no traffic. Farm country. I took gloves and 2 shovels as well as my Gatorade and sandwiches, and I took my then girlfriend and her brother. He borrowed one of those measuring wheels on a long handle. You walk, it rolls and shows distance.
It was about 70 and overcast when we started. At the local salvage yard, their scales showed my C2500 had right at 2000# on the rear axle, with nothing in the 8' bed. So while she went marking distances with that device and some chalk, I equalized the rear tires at 78 psi.
Chalking a mark on each rear tire wasn't hard, Once I had one beside the first mark on the pavement. I had her drive at walking pace. Us guys counted. Then he drove back, us lovebirds counted. Multiply that by 5 to equal a mile. I lost the notes a few weeks after I had my answer, I had no idea I'd ever need them again. All that mattered to me was I got proof that the advertised numbers are always wrong.
In fact, get on www.tirerack.com, find a size with 100 or more choices, and you'll find no consistency there in r/mile even when they're all the same diameter.
Anyway, Baseline in hand, we shoveled dirt until the bumpstops touched the axle. Never got a weight, but the tires were bulging. And the count was the exact same. Why? Because the circumference didn't change, only the vertical measurement from the center of the hub down to the road. Whatever the circumference is, that's how far you will travel for one turn of the tire, unloaded or overloaded. This is basis physics. It can't be refuted, and that's why my gal thought the day wasted. All we did was confirm what the math told us.
Same deal again wit the Isuzu. She didn't help that day, nor did her brother. I had to bribe another friend with a steak dinner.
Maybe I got over invested in something the average Ecmdder doesn't need to think about. But I'll not be told I'm wrong when an independently verifiable test proves I'm not.
Fir all your future calculations, use measured circumference, if need be divide by Pi for diameter, divide diameter by 2 for real radius. Then if there are discrepancies, you'll know where it can't be.

[Fat Charlie]

Makes sense to me. I especially love the determined pursuit of data.

Originally Posted by cosmick

Some thing revolutions, some think rotations, I'm side-stepping that mess with turns, to keep the focus on the real issue.

But FWIW, TPMs is already taken.

[cosmick]

WRONG! This is the exact nonsense myth that my testing busted! Stop spouting this blatant lie!

[Xist]

Originally Posted by cosmick

WRONG! This is the exact nonsense myth that my testing busted! Stop spouting this blatant lie!

Me thinketh the lady doth protest too much.

[redpoint5]

Marry that girlfriend and stop making her do things she probably hates. Any girl willing to spend the day doing what most would see as pointless has a heart toward pleasing you. Now go take her shopping, or whatever it is you find pointless that she delights in.

Oh, and your method seems sound. I'd be interested to see the data and examine the variations, even if they are tiny.

[Hersbird]

I have to go with cosmick on this one, no matter what weird shape the weight makes the tire, the belts in the tire don't let the circumference change. It's just not a circle anymore but it's an egg with the same circumference as the circle you started with. If you were using an innertube that could expand maybe, but not a modern tire.

[bentring]

Originally Posted by Old Tele man

FWIW, in reality, the mathematical tyre diameter and rolling tyre diameter are NOT the same. The mathematical diameter does NOT account for vehicle weight-loading, which slightly flattens the tyre bottom, and thus slightly reduces the actual rolling diameter by about 3%. A good approximation is:

Dia(rolling) ≈ 0.97*Dia(math)

I never would have thought about that mattering, and I got curious so I did a 2 minute research session. According to Michelin :


The SAE procedure recognizes that within the test method itself there will be some variation. In fact, there are other factors that cause variation on Tire Revs./Miles among similar tires. Be aware that just because similar tires have the same overall diameter this does not necessarily mean that they will have the same Tire Revs./Mile. The SAE procedure determines the Tire Revs./Mile to within ± 1.5%. Some factors, which cause variation among tires, are: • Load and Pressure – A difference in Load/Pressure could alter the Tire Revs./Mile measurement by as much as 1.5%. If pressure is constant, going from an empty vehicle to a fully loaded vehicle can change the Tire Revs./Mile by 1 to 1.5%. • Treadwear – The Tire Revs./Mile varies from a new tire to a fully worn tire. This can affect Tire Revs./Mile by as much as 3% from the rated Tire Revs./Mile. • Tread Geometry – The height and stiffness of the blocks and the shape of the tread pattern can affect Tire Revs./Mile. • Torque – The presence of driving and braking torque can affect the Tire Revs./Mile. • Type and Condition of Pavement – Asphalt vs. concrete, wet vs. dry can create difference in Tire Revs./Mile.

[CapriRacer]

What prompted this thread is a PM I sent Cosmick. I suggested we do an off-line discussion to resolve the issue. I don't think embarrassing someone in public is the right way to get them on the right page, but it's obvious that isn't the way this is going to go down.

I'll quote from some of the PM I sent:

"......Where am I coming from? I'm a retired tire engineer - 40 some years. From the start of my career, I have been aware that this whole issue is problematic for the layperson - that the rolling diameter can NOT be measured directly.

For a while, I was a development engineer and would be the guy who published the dimensions of tires, including revs per mile. We had a formula we used and the result was about a 3% difference compared to the free standing (that is unloaded) diameter. But at the same time, we verified these dimensions where we could to assure their accuracy - and that included revs per mile. If I remember correctly we had a counter on a test wheel that was read while some other test was going on.

Plus, I was, for a time, a liaison to Ford, and we would pass dimensional information to Ford, so they could check for clearances and speedometer accuracy (which needed to be within 3% by regulation). They never mentioned any errors in the data we reported.

So I am extremely confident in the 3% value - keeping in mind it's an approximation and varies according to load/inflation pressure/speed.

What I would propose is that we both measure tires. Unfortunately the only vehicle I have on a hard surface is a 2006 Mercury Milan, so I will only have 2 data points - front and rear.

I suggest you do the same. One word of caution: Be careful measuring the back tires on an unloaded pickup - especially a 250 or 350 using Ford's terms (2500 and 3500 using Chevy's terms). Those rear tires can be so slightly loaded that the rolling diameter might be less than a 3% difference...... "

Well, there you have it. I've proposed a verification and will report on it when I have completed it. I'm going to suggest that others do this as well.

I appreciate the effort Cosmick put into obtaining his answer, but I'll submit that the tire manufacturers thoroughly disagree and that fact alone should be enough to cause hesitation on contradicting them.

I'll be back later with a description of my methodology and details about the results.

[CapriRacer]

OK, I've done the measurements and here are the results. But first, allow me to document the process.

Here's the car involved:

It's a 2006 Mercury Milan. It has 215/60R16's on it (as opposed to the stock 205/60R16) inflated to 34 psi (33 placard pressure).

Here are the tools:



A jack, masking tape, tape measure (flat tape), safety pin, marker (sometimes called a Sharpie!)

I jacked up each tire on the driver's side, wrapped the tape around each tire using the safety pin to hold the tape onto the tire as the tire was rotated, and recorded the measurements.



Here are the measurements;

LF: 6' - 9" = 81"

LR: 6' - 10 1/8" = 82 1/8"

I then lowered the car onto the tires, marked each tire with a radial line on masking tape, positioned the tire so the tape down down and added a line on the ground at the point of contact.



OK, OK, my line isn't the best, but it doesn't matter as you will see in a moment.

Then I pushed the car back to get one rotation of each tire, then marked the ground again - followed by measuring between the 2 marked.



Results:

LF: 6' - 7 1/8" = 79 1/8" or 97.7%
LR: 6' - 8 1/8" = 80 1/8" or 97.6%

OK, so it isn't a 3% difference, but it's more than 2%!!

[Hersbird]

I don't get the point of the above, 2 different circumference tires have 2 different rolling distances. I think the alternative point was that changes in loading and air pressure (within normal ranges) won't change the rolling distance.
I do see what you are showing here, a tire with no load at all and full pressure will crown out in the center.