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Old 03-09-2018, 04:19 AM   #21 (permalink)
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Quote:
Originally Posted by ksa8907 View Post
Now take those red boxes and imagine them at half their height. Is the distance from the axis greater or lesser?
That distance is not relevant, is it? The boxes would be just as close together.

If there are 20 boxes around the wheel, each rotation would involve 20 boxes and 20 gaps in between them, and the gaps would be just as wide even if the boxes are worn to half their height.

The distance traveled each rotation is not the same as the circumference as measured over the tread surface, as that gets squeezed together in the contact patch.
It IS the same as the circumference measured at the heart-line of the belts.

Look at the snow in the grooves after a drive. You'll see it has been flattened in the longitudinal direction. That happens in the contact patch.

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Old 03-09-2018, 05:44 AM   #22 (permalink)
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Quote:
Originally Posted by RedDevil View Post



That distance is not relevant, is it? The boxes would be just as close together.
I believe it is. To relate speed of a vehicle to wheel speed, the velocity of the tire where it meets the road surface is going to dictate how many revolutions the tire must turn.

I'm trying to remember physics and trigonometry from college, bear with me. If we take the angular velocity of the tire and derive the linear velocity, the formula uses the radius. If the radius changes by some small amount due to a thicker tread, then it will result in a higher linear velocity for a constant angular velocity.

I'm glad you brought up this phenomenon because the radius that will give real world numbers is measured when the tire is under load, with a flat contact patch.
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Old 03-09-2018, 06:09 AM   #23 (permalink)
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It is remarkable that we have different views on this.
You cut off the rest of the quote where I explained why it works this way.
So, we're done with theory for now.

I'm going to measure the rolling distance of one rotation this weekend., if the weather allows.
That should either prove or disprove my point; will report one way or the other.
I expect it to be about 2 cm shorter than the circumference as measured over the tire tread, and consider it falsified if the difference is less than one cm.

The difference should be more than one inch with new tires, but mine are fairly worn.
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Old 03-09-2018, 08:16 AM   #24 (permalink)
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A couple of thoughts about rolling diameter (circumference):

If you measure the freestanding circumference (not loaded, hanging in the air), you will find that it is larger than the distance traveled in one revolution by a tire at its rated load and pressure by about 3%.

If you do the math, the rolling diameter is NOT the static loaded radius times 2 (The static loaded radius is the distance from the ground to the hub when the tire is loaded.)

- BUT –

The rolling diameter is about the same diameter as the steel belt (on a radial tire) – which sort of means that if you think the steel belt as a tank track (like RedDevil’s image above) you’ll be pretty close.

- AND –

The distance traveled in one revolution by a loaded tire varies with load/inflation (those 2 things are linked), so varying the pressure without changing the load, will result in a different answer (which is what I did some time back to prove that point)

Why? Because the steel belt pantographs – changes angle - as the belt rolls through the footprint. It gets shorter (and wider).

Further, the tire will have a different freestanding circumference between inflated and not inflated.

Is there a formula? Probably, but I don’t know what it is.

But what does a difference in tread depth do? According to the tank track theory, nothing, but I don’t think that is 100% accurate – meaning the theory is wrong, but close.

And I am nearly 100% sure that the difference in tread depth doesn't account for the difference in rolling resistance from new to worn. I am sure of that because I've tested a lot of tires for rolling resistance and the amount of tread (volume) is one of the determining values. We used to design tires to get good RR values by reducing the width of the tread and/or by having wider grooves.
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Old 03-09-2018, 08:32 AM   #25 (permalink)
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Quote:
Originally Posted by redpoint5 View Post
....... Capri- Does a 13% drop in MPG due to tire selection sound reasonable? ......
The OP seems to be reporting instantaneous fuel economy (based on a gauge on the dash?). If that is the case, then 13% is fairly reasonable. But I don't think you can achieve that as an overall value.

Just for reference, there is a chart that shows how much fuel is used during the 2 EPA tests - and rolling resistance (of the entire car) varies between 4% and 7%. There's a copy of it here: Barry's Tire Tech - Rolling Resistance and Fuel Economy
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Old 03-09-2018, 08:40 AM   #26 (permalink)
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Quote:
Originally Posted by CapriRacer View Post

But what does a difference in tread depth do? According to the tank track theory, nothing, but I donít think that is 100% accurate Ė meaning the theory is wrong, but close.
Tread depth between a new and a used up tire defiantly makes some difference in both the outside diameter of the tire (unloaded or loaded) as well as the radius from axis to patch on the ground. Take the drawing and then add one half inch more rubber to the tire tread. This instantly adds one half inch to the radius and pi x radius squared to the circumference. With pressure and load constant, this new tire will roll further with each revolution than the identical tire with less rubber (smaller circumference). Tire patch is not affected.
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Old 03-09-2018, 09:22 AM   #27 (permalink)
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Quote:
Originally Posted by davelobi View Post
Tread depth between a new and a used up tire definitely makes some difference in both the outside diameter of the tire (unloaded or loaded) as well as the radius from axis to patch on the ground. Take the drawing and then add one half inch more rubber to the tire tread. This instantly adds one half inch to the radius and pi x radius squared to the circumference. With pressure and load constant, this new tire will roll further with each revolution than the identical tire with less rubber (smaller circumference). Tire patch is not affected.
But as we have already established - at least I have in my mind - the static loaded radius doesn't relate to the rolling diameter AND the idea that there is some connection between some physical dimension (and pi) and the rolling diameter isn't valid either.

While I think there is some effect tread depth has on rolling diameter, I don't think it is as large as is being portrayed - and it is not the dominant factor when it comes to RR as a tire wears.

What I think we need here is someone actually measuring how a tire behaves as the tread wears out. Anyone have access to a tire shaving machine?
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Old 03-09-2018, 03:17 PM   #28 (permalink)
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I hadn't thought about the steel belts possibility of
stretching and allowing for increased circumference. Maybe as the tire tread wears down (decreasing circumference) the belts stretch (allowing for increased circumference) resulting in approximately the same size tire over it's life.
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Old 03-10-2018, 07:53 AM   #29 (permalink)
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Quote:
Originally Posted by RedDevil View Post
I'm going to measure the rolling distance of one rotation this weekend., if the weather allows.
That should either prove or disprove my point; will report one way or the other.
I expect it to be about 2 cm shorter than the circumference as measured over the tire tread, and consider it falsified if the difference is less than one cm.

The difference should be more than one inch with new tires, but mine are fairly worn.
So I just did.
First of all, this is the tire we're talking about: ContiWinterContact TS810 S - 175/65R15 84T Tire | Continental

Purchased in 2012, fairly worn but still 2 mm off the indicator marks.

I put a marker on the sidewall and the Devil's driveway and 2 on the tread, well away from the shoulder, but in a groove so I could position the tape measure against a lug edge.



Measured one half of the circumference of the tire (100.0 cm), rolled it backwards , measured the other half (92.5 cm)
Total circumference should be 192,5 cm.

Rolled it another half rotation so the side mark was down again:


187.5 cm.
Wait a minute, this can't be right. I expected the rolling distance to be about 2 centimeter less than the circumference, not 5!
What's wrong, did the markers move?

As they say: Measure twice, cut once.
Verification measurement of the one half over the tread measurement, mark to mark, tape measure pulled to the tread at great force: 92.5 cm
Half roll forward, measure again: 100.0 cm.

So, the circumference measurements are stable. 192.5 total it is. (but see the edit down this post!)

Roll forward to complete the rotation, measure the distance traveled again:

187.5. Can't make it any more than that...

Assuming these measurements are all correct (etc) the distance traveled is a whopping 5 cm less than the circumference of the tire!
Even while the tread has already worn down quite some.

[EDIT]
I wasn't sure whether the tape measure is the correct tool to measure circumference as it is hard to align it properly over the tread; the curved metal fights back in every way imaginable. And it would not fold flat, so the edges remained raised away from the tread.
There is the possibility that it overreports the circumference.

So I tried to measure the diameter with an angle hook:

It measures 257 mm tread to center ring of wheel cover, same in all directions.

The core is 93 mm wide:


The diameter is about (257 mm * 2 + 93 mm =) 607 mm as measured.
After consuming a pie we get the diameter: 1906.9467407290044957468245336507 mm according to Windows calculator. 190.7 cm would do fine.

If we discard the tread tape measure measurement and go with the diameter calculation the difference between rolling distance and circumference is less:
190.7 cm - 187.5 cm = 3.2 cm.

The circumference of the tire, whether measured over the tread or calculated from the diameter, does not represent the rolling distance per revolution.

It corresponds with a circle that would be 1 cm deep under the tread (of this fairly worn tire). I would not be surprised if that is the depth at which the steel belts are located; if so, the belt diameter is indeed most likely the defining factor in the rolling distance of a tire.

In that case, tread wear should indeed make no difference.
Maybe someone with brand new tires can repeat my measurements and report the results.
I expect there would be a larger discrepancy, up to 5 cm, as the new tread should be that farther away from the belt.
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Last edited by RedDevil; 03-10-2018 at 09:18 AM.. Reason: There's always a reason. There just is.
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Old 03-10-2018, 08:42 AM   #30 (permalink)
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RedDevil,

A new tire's rolling circumference at the rated load and rated inflation pressure is about 97% of the freestanding circumference - and you kind of just proved it. 5/192.5 = 2.6%

The question on the table is: Does the tire's rolling circumference change as the tire wears down? What we need is someone to measure a tire's rolling circumference, then shave off the tread, then measure it again.

It has to be the same tire without dismounting or changing the inflation pressure, because tires grow. You want the tire tested both ways in basically the same condition.

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