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Old 03-10-2018, 09:17 AM   #31 (permalink)
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Hi CapriRacer, thanks for that.
In the meantime I edited my post and added another measurement of the circumference, for reasons mentioned in the post, and that gave a smaller circumference and hence a much smaller difference; less than 1.7%

It is safe to assume that the tread diameter went down as the lugs wear. The reduction of the difference between rolling and actual diameter for my worn tires suggests the rolling diameter has not changed much.

But, as I wrote in my first post on this topic, the rolling distance has actually changed over time; my commute became about 0.6% shorter by the odometer, give or take 0.2% but definitely shorter even though the route has not changed one bit.

So the rolling distance did increase over time, probably due to gentle stretching and settling of the belts.

Googling and reading into rolling circumference, everyone seems to agree that it varies with load and inflation pressure. Tread wear does not get mentioned, at least not by the experts.

This puzzles me.
I can't see how load influences rolling distance.
The contact patch would always be flat unless the road surface gives way (mud, sand, snow). Even if it does deform, as long as it approximates flatness it will still be very near constant as the difference reduces quadratically.
Pressure may stretch the belts a bit, but every tire has some pressure so they are already tight.

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Old 03-10-2018, 11:59 AM   #32 (permalink)
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Thanks for doing this experiment!

Quote:
Originally Posted by RedDevil View Post

This puzzles me.
I can't see how load influences rolling distance.
The contact patch would always be flat unless the road surface gives way (mud, sand, snow). Even if it does deform, as long as it approximates flatness it will still be very near constant as the difference reduces quadratically.
Pressure may stretch the belts a bit, but every tire has some pressure so they are already tight.
My understanding of this is that the distance from the axis of rotation to the road surface is what determines the rolling distance. Which means that load and inflation pressure are the primary influences.

Circumference is irrelevant.

While calculations comparing tires using the typical tire measurements xxx/xx/xx are relevant, they are not absolute since they don't consider deformation at the contact patch.

I am very skeptical that the steel belts stretch over time
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Old 03-10-2018, 12:02 PM   #33 (permalink)
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If you ignore the tire for a moment and imagine a leg and foot on the hub, the distance of the leg is going to dictate how far the vehicle can travel for a constant rotation speed.
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Old 03-10-2018, 01:33 PM   #34 (permalink)
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Contact patch vs load vs PSI

Quote:
Originally Posted by RedDevil View Post
Hi CapriRacer, thanks for that.
In the meantime I edited my post and added another measurement of the circumference, for reasons mentioned in the post, and that gave a smaller circumference and hence a much smaller difference; less than 1.7%

It is safe to assume that the tread diameter went down as the lugs wear. The reduction of the difference between rolling and actual diameter for my worn tires suggests the rolling diameter has not changed much.

But, as I wrote in my first post on this topic, the rolling distance has actually changed over time; my commute became about 0.6% shorter by the odometer, give or take 0.2% but definitely shorter even though the route has not changed one bit.

So the rolling distance did increase over time, probably due to gentle stretching and settling of the belts.

Googling and reading into rolling circumference, everyone seems to agree that it varies with load and inflation pressure. Tread wear does not get mentioned, at least not by the experts.

This puzzles me.
I can't see how load influences rolling distance.
The contact patch would always be flat unless the road surface gives way (mud, sand, snow). Even if it does deform, as long as it approximates flatness it will still be very near constant as the difference reduces quadratically.
Pressure may stretch the belts a bit, but every tire has some pressure so they are already tight.
the answer to the puzzle is :
The change of load does not change the flatness of the contact patch only the size of the contact patch if there's 4000 pounds on a tire and the tire has 40 pounds of air in it each square inch of the contact patch has 40 lb of pressure applied. So if load÷psi= ~ size of contact patch. 4000÷40= ~100sq in contact patch. This equation only works for approximation as it does not account for sidewall stiffness tread block stiffness steel belt stiffness or any of the other load bearing traits of a tire. My understanding is the difference between a low-rolling-resistance tire and a conventional Tire is how much of an effect these other traits( tread/ sidewall stiffness) have on the footprint size.



Capri.
Is a run-flat the same as a low rolling-resistance if not which one would actually have the better rolling resistance?
Y Rated Tire (186mph)vs Q rated Tire(100mph) which one, all other conditions being the same, would have better rolling resistance , and would any gains be consumed by the extra mass of the reinforcement requiring more horsepower to make it rotate?
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Old 03-10-2018, 02:05 PM   #35 (permalink)
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The size of the contact patch has no influence whatsoever.
The tread pattern is the same once it is flat, just as it would be if you completely cut off the sidewall and lay the tread flat out on the tarmac.
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Old 03-11-2018, 10:08 AM   #36 (permalink)
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Wow, this thread generated a lot of responses – some of them needing clarification. Sorry for the long post!

Here’s the first one:
Quote:
Originally Posted by RedDevil View Post
Hi …….. In the meantime I edited my post and added another measurement of the circumference, for reasons mentioned in the post, and that gave a smaller circumference and hence a much smaller difference; less than 1.7% ……..
Did you use that same tape measure? Didn’t it buckle when you tried to measure the circumference? I would think that would hurt the measurement in the direction of a larger distance, thus lowering the percentage.
Quote:
Originally Posted by RedDevil View Post
Hi …….. I can't see how load influences rolling distance.
The contact patch would always be flat unless the road surface gives way (mud, sand, snow). Even if it does deform, as long as it approximates flatness it will still be very near constant as the difference reduces quadratically.
Pressure may stretch the belts a bit, but every tire has some pressure so they are already tight.
A couple of thoughts:
If the belts don’t stretch, then they must bulge out in front of and behind that contact patch in order to make up for the shorter distance through the contact patch. Either that or they shrink a bit (or perhaps both!)
We already know that a steel belted tire expands when it is inflated. You can see that when the tire is first mounted and adding air causes it to expand. So why wouldn’t deflecting the tire under load shrink it back a bit?

Quote:
Originally Posted by ksa8907 View Post
…….. My understanding of this is that the distance from the axis of rotation to the road surface is what determines the rolling distance. ……..
Sorry, that is NOT true. That distance is called the Static Loaded Radius and it determines the ride height of the vehicle.
Quote:
Originally Posted by ksa8907 View Post
…….. Which means that load and inflation pressure are the primary influences. …….
That is correct, even though how it was derived was wrong.
Quote:
Originally Posted by ksa8907 View Post
…….. Circumference is irrelevant. …….
Not exactly. We can use the freestanding circumference to estimate the rolling diameter of a new tire at the rated load/ rated inflation pressure.
Quote:
Originally Posted by ksa8907 View Post
…….. I am very skeptical that the steel belts stretch over time
Remember, the steel belt isn’t solid steel. It is a series of parallel wires set at an angle in 2 layers in opposite directions. The wires themselves don’t stretch, but they do change angle as the belt goes through the footprint, thus changing the length and the width of the belt. The process is called pantographing.

The other part of the process of growth is called *Material Creep*, where are a material permanently deforms over time. Sagging springs is an example. Yes, rubber under goes material creep, but tires generally encounter it in the form of compression set in the tread compound.
Quote:
Originally Posted by ksa8907 View Post
If you ignore the tire for a moment and imagine a leg and foot on the hub, the distance of the leg is going to dictate how far the vehicle can travel for a constant rotation speed.
But if the leg is bent, does it travel the same distance as when it is fully straightened out? I think not.

Quote:
Originally Posted by gumby79 View Post
…… My understanding is the difference between a low-rolling-resistance tire and a conventional tire is how much of an effect these other traits( tread/ sidewall stiffness) have on the footprint size. ………
Ah ….. Mmmmmm ……. Sorry, No!

The primary difference between a low RR tire and one with a high RR is the tread compound – and that tread compound’s hysteresis (internal friction).

Another way to reduce the RR of a tire is using less tread rubber – either by making the tread width narrower, or by using wider grooves, or even starting off with less depth.
But tread/sidewall stiffness hardly matters because it is the inflation pressure that is stiffening the carcass, and by comparison, the rubber, and even the steel wire belts, are more flexible than the amount the inflation pressure stiffens things.

Quote:
Originally Posted by gumby79 View Post
…… Capri. Is a run-flat the same as a low rolling-resistance if not which one would actually have the better rolling resistance? ……
No, and that is a good example of a tire with a fairly stiff carcass. Again, by comparison, the inflation pressure adds so much more stiffness than the tire itself has.
Quote:
Originally Posted by gumby79 View Post
…… Y Rated Tire (186mph)vs Q rated Tire(100mph) which one, all other conditions being the same, would have better rolling resistance , and would any gains be consumed by the extra mass of the reinforcement requiring more horsepower to make it rotate?
The primary difference in the speed rating of a tire is the number of cap plies – typically nylon – layered over the steel belts. Nowadays these are spiraled on in a thin strip – and that means the number of layers can vary across the face of the tread.

A Y rated tire will generally have 3 layers at the belt edge (and perhaps 3 at the center), but 2 layers everywhere else (and the sidewall will be marked as having 2 nylon plies in the tread). A V rated tire will have 2 layers; an H will have one, a T will have strips at the belt edges (and the sidewall will not say anything about a cap ply), and an S rated tire will have none. Q rated tires are generally that low because they are winter tires and a winter tread compound isn’t up to the heat being generated.

(OK, time for the standard disclaimer for the above paragraph: That is about passenger car tires and there are lots of exceptions, some for reasons not related to speed rating. Other tires will have different constructions, so what was said there would not apply to – say - LT tires)

But all other things being equal, the more material in a tire, the worse its RR – ergo, the Y speed rated tire will have the worst RR.

- BUT –

Tread compound plays such a large role in RR that a Q rated tire might actually be the worst – and due to the fact the Q rated Passenger Car tires are generally winter tires and they have a large amount of tread rubber – which also negatively affects RR.

Further, Y speed rated tires generally have tread compounds with good grip – and those tread compounds are worse for RR, while many S and T rated tires have tread compounds that are good for tread wear – and those compounds are also worse for RR.
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Old 03-11-2018, 12:42 PM   #37 (permalink)
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Quote:
Originally Posted by CapriRacer View Post
...
Did you use that same tape measure? Didn’t it buckle when you tried to measure the circumference? I would think that would hurt the measurement in the direction of a larger distance, thus lowering the percentage.
I did use the same tape measure for the 'two halves' circumference measurement and the roll test.
I used the alu hook to measure the diameter, but checked its scale to the tape measure and they were, surprise surprise, identical.

As I wrote I wasn't satisfied with my circumference measurement.
I planned to measure it when I swap to summer tires, as then it would be off the ground.
Then it hit me - I don't have to wait for that!

As the wheel drops there's more space above it. I could simply drag the lead around the wheel, then align it perfectly and pull it tight.

Came out like 192.0 cm. 5 mm less than my first measurement, still more than I expected from the diameter check (which read 190.7 cm)
Looking around the wheel I see gaps between the lead and the tread. The bent metal tape fights being bent around so it forms a polygon rather than a circle.
So I pull it with all my might, and it reduces to 191.5 cm. Even then it polygons and there are faint slits between the tread and the nods of the polygon letting light through. But the real circumference must be close to this anyway.

Conclusion: the circumference of my tire can be anything in the range of 190.7 cm to 191.5 cm. I'd take 191.1 with a 5 mm variance.
Thus the rolling distance would be 36 mm less than the circumference, 1.9% difference.
Assuming it was 3% when new the difference got 1.1% (of the circumference) less through wear and use.
Also, metal tape measures really suck at measuring the circumference of objects with a 1 foot radius.

Quote:
Originally Posted by CapriRacer View Post
A couple of thoughts:
If the belts don’t stretch, then they must bulge out in front of and behind that contact patch in order to make up for the shorter distance through the contact patch. Either that or they shrink a bit (or perhaps both!)
We already know that a steel belted tire expands when it is inflated. You can see that when the tire is first mounted and adding air causes it to expand. So why wouldn’t deflecting the tire under load shrink it back a bit?
I'd think the whole cross-section of the tire pivots at belt level when it bends sharp ahead and behind the contact patch. The thread would be stretched there, the inside of the tire squeezes together. Then at the contact patch it is the other way round.

The force on the belt would be radius times width times pressure.
At a 30 cm radius at 3 Bar and a 175 mm width that would be:
(0.3 meter) * (0.175 meter) * (3 * 100,000 Newton / meter / meter) = 15,750 Newton.
1.55 times a metric tonne. Enough to lift my car and then some.
And it will very likely stretch out some if the pressure gets higher.

Each wheel supports just about 300 kg (2943 Newton), less than a fifth of the force on the belt.
So the force on the belt marginalizes all other forces in the contact patch.
I'd say the belt is all determining.
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Old 03-11-2018, 02:06 PM   #38 (permalink)
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I wanna endorse almost everything redpoint5 says above. As usual, lots of good points from redpoint5 and others here. But if I understand his post correctl, he suggests LRR tires wear more quickly. It's not true that LRR tires are *necessarily* gonna wearout faster. Always check the wear ratings (and stay skeptical of them, too). My Michelin Defenders LRRs have an amazing rating, longer than most other tires, period. And actual driving experience over tens of thousands of miles confirms they will last and last... Just two cents from a driver with a wrench and some time behind the wheel.
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Old 03-11-2018, 02:19 PM   #39 (permalink)
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Quote:
Originally Posted by CapriRacer View Post
.... Remember, the steel belt isn’t solid steel. It is a series of parallel wires set at an angle in 2 layers in opposite directions. The wires themselves don’t stretch, but they do change angle as the belt goes through the footprint, thus changing the length and the width of the belt. The process is called pantographing. ...
I have been on this site for seven year this month and have seen steel belt stretch debated repeatedly. Never yet until now do I recall the term "pantographing" being used. This is the first time I have ever heard this phenomena explained here or anywhere. (A search of EM shows it was mentioned once in June 2012 by another user.)

Thanks for that. Great little explanation of an elegant concept. I effin love that.

Https://www.merriam-webster.com/dictionary/pantograph
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Old 03-22-2018, 08:32 PM   #40 (permalink)
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Quote:
Tire slip is quantified by a formula known as the "Pacejka magic formula" developed primarily by the Dutch university professor Hans Pacejka. A curious aspect of this extremely complex subject is that some tire "slip" ( not sliding or wheelspin) is necessary for a tire to generate traction, and that there is an optimum amount of tire slip. This does not imply that wheelspin or skidding is desirable in land speed racing or in drag racing. .paterstreamliner.com/
As the tire wore out the volume of slip went down ?

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