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Old 03-08-2018, 10:36 AM   #11 (permalink)
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Quote:
Originally Posted by RedDevil View Post
By the way, there should be no difference in distance reported between new and worn tires, even though the new tires have a slightly larger circumference.

Amazing, no?

Consider this: A tire on a car is not a perfect circle. The contact patch is flattened out, and there the tread shrinks; the grooves narrow as the lugs get squeezed together.

By and large the distance a wheel travels with each rotation is the same as the length of the steel belts in the tire. The distance between the belts and the tread surface has no influence; again, the contact patch is flat.

Actually, if anything it is the other way round.
As my tires wear I see a very gradual reduction in the reported distance of my commute on the odometer. The same route that was 35.6 km when the tires were new now takes just 35.4 km.
I bet the belts have been stretched ever so slightly.

Worn tires slightly underreport the mileage.
So you're saying a smaller diameter tire has magical properties and actually travels a greater distance with each revolution?

The tire obviously deforms when in contact with the road, inflation pressure will play a large role. In order for a smaller diameter tire to travel the same (or greater) distance would mean the tread on the larger tire is being compressed much more than the smaller tire. How?

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Old 03-08-2018, 12:27 PM   #12 (permalink)
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Quote:
Originally Posted by RedDevil View Post
By the way, there should be no difference in distance reported between new and worn tires, even though the new tires have a slightly larger circumference.

Amazing, no?

Consider this: A tire on a car is not a perfect circle. The contact patch is flattened out, and there the tread shrinks; the grooves narrow as the lugs get squeezed together.

By and large the distance a wheel travels with each rotation is the same as the length of the steel belts in the tire. The distance between the belts and the tread surface has no influence; again, the contact patch is flat.

Actually, if anything it is the other way round.
As my tires wear I see a very gradual reduction in the reported distance of my commute on the odometer. The same route that was 35.6 km when the tires were new now takes just 35.4 km.
I bet the belts have been stretched ever so slightly.

Worn tires slightly underreport the mileage.
I'm not convinced. Take your examples to a little more extreme to magnify the effects. Use a much larger tire and see how few revolutions it takes to any stated distance compared to the smaller one. I don't care how large the footprint is, bigger going further per rev.
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Old 03-08-2018, 01:53 PM   #13 (permalink)
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Quote:
Originally Posted by ksa8907 View Post
So you're saying a smaller diameter tire has magical properties and actually travels a greater distance with each revolution?

The tire obviously deforms when in contact with the road, inflation pressure will play a large role. In order for a smaller diameter tire to travel the same (or greater) distance would mean the tread on the larger tire is being compressed much more than the smaller tire. How?
Did you actually read what I wrote?

To say the tire just 'deforms' in contact with the road is missing the whole point.

The contact patch is FLAT.
Every lug is aligned parallel to the next lug, and it really does not matter how high it is. The distance between the lugs at the tarmac is identical to their distance at the belts.

Nothing magical happening, just plain logic.

Then, over time, the constant bending of the belts combined with the pressure in the tires will elongate them ever so slightly. So, indeed, the worn tire with its shallower grooves, but stretched belts, will roll ver so slightly further over one rotation.
From personal experience I can account for how much that can be: namely, 200 meters over 35.6 kilometer, about 0.6 %.

I knew my post was going to be criticized, as it seems to go against gut feeling and simple logic.
But once you understand the mechanism it should not be hard to get.
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Old 03-08-2018, 02:05 PM   #14 (permalink)
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Quote:
Originally Posted by davelobi View Post
I'm not convinced. Take your examples to a little more extreme to magnify the effects. Use a much larger tire and see how few revolutions it takes to any stated distance compared to the smaller one. I don't care how large the footprint is, bigger going further per rev.
What can I say.
There's a farm with a speed bump at exactly 5 kilometers into my commute.
Or so it was when my car and tires were new. Nowadays the digits flip to 5.0 some 30 meters beyond the bump.
So, my worn tires need less rotations to get there, right?

This puzzled me greatly at first, as obviously the circumference of the tires IS smaller. But the distance the tire travels is not determined by the length as measured over the tread in free air.
It squeezes together in the contact patch.

Here's an example with the very large footprint you wanted:
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Old 03-08-2018, 02:21 PM   #15 (permalink)
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Old 03-08-2018, 02:46 PM   #16 (permalink)
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Quote:
Originally Posted by RedDevil View Post
Did you actually read what I wrote?

To say the tire just 'deforms' in contact with the road is missing the whole point.

The contact patch is FLAT.
Every lug is aligned parallel to the next lug, and it really does not matter how high it is. The distance between the lugs at the tarmac is identical to their distance at the belts.

Nothing magical happening, just plain logic.

Then, over time, the constant bending of the belts combined with the pressure in the tires will elongate them ever so slightly. So, indeed, the worn tire with its shallower grooves, but stretched belts, will roll ver so slightly further over one rotation.
From personal experience I can account for how much that can be: namely, 200 meters over 35.6 kilometer, about 0.6 %.

I knew my post was going to be criticized, as it seems to go against gut feeling and simple logic.
But once you understand the mechanism it should not be hard to get.
Sorry for being overly critical, I believe inflation pressure will influence what you are describing. Distance traveled is dependent on the distance between the axis of rotation and the ground where the tire is making contact.

If inflation pressure changes, then so does that distance. If the contact patch becomes shorter from front to back because of inflation pressure, then distance between the axis and the ground must have increased. Does the contact patch change in length as the tire is worn?

I might design an experiment to test this.
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Old 03-08-2018, 02:51 PM   #17 (permalink)
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Does anybody have a mix of new and used tires on their car?
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Old 03-08-2018, 02:54 PM   #18 (permalink)
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Just to clarify, my recommendation to take the circumference of the old tire vs new was to highlight the difference between different models of tires, not worn vs new. Different tire models will have different circumferences despite being labeled the same "size". Sites like Tirerack.com will list the revolutions per mile for the particular tire, and that number will be different between the models in the same size tire.

As our resident tire/wheel guru CapriRacer pointed out, tire design involves compromise. My point is that if a tire causes a drop in MPG, it doesn't necessarily cost more if it lasts much longer, or is cheaper to purchase. You have to factor in more frequent tire replacements, the lost earning potential of waiting at a tire shop to change the tires, and the cost of the tires themselves.

It was unclear in the OP, but it sounds like they got 80,000 miles on the LRR tires before replacing them? That is quite good.

Capri- Does a 13% drop in MPG due to tire selection sound reasonable?

EDIT:

FYI- Tirerack shows the Sumitomo turning 848 revs per mile and the Toyo at 832. The Sumitomo over-reports speed and mileage by 2%.
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Old 03-08-2018, 03:11 PM   #19 (permalink)
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Quote:
Originally Posted by ksa8907 View Post
Sorry for being overly critical, I believe inflation pressure will influence what you are describing. Distance traveled is dependent on the distance between the axis of rotation and the ground where the tire is making contact.

If inflation pressure changes, then so does that distance. If the contact patch becomes shorter from front to back because of inflation pressure, then distance between the axis and the ground must have increased. Does the contact patch change in length as the tire is worn?

I might design an experiment to test this.
No problem, glad you take it sportingly

IIRC someone already tested whether higher tire pressure influenced the circumference and rolling distance, and it did not noticeably.
The steel belts keep the tire at the same diameter.

From my own experience: When I discovered Ecomodder shortly after buying my Insight I raised the tire pressure by almost 10 PSI, and I was curious whether it had an effect on my reported commute distance. But it didn't; the speed bump from my example remained right there at 5.0 (at that time).

To graphically demonstrate the tread shrink effect (and my lack of drawing skills) I made this conceptual picture of a treaded tire:


As you see, when the contact patch is flat the notches align and their height becomes irrelevant to the distance between them.
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Old 03-08-2018, 05:39 PM   #20 (permalink)
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Quote:
Originally Posted by RedDevil View Post
No problem, glad you take it sportingly

IIRC someone already tested whether higher tire pressure influenced the circumference and rolling distance, and it did not noticeably.
The steel belts keep the tire at the same diameter.

From my own experience: When I discovered Ecomodder shortly after buying my Insight I raised the tire pressure by almost 10 PSI, and I was curious whether it had an effect on my reported commute distance. But it didn't; the speed bump from my example remained right there at 5.0 (at that time).

To graphically demonstrate the tread shrink effect (and my lack of drawing skills) I made this conceptual picture of a treaded tire:


As you see, when the contact patch is flat the notches align and their height becomes irrelevant to the distance between them.
Now take those red boxes and imagine them at half their height. Is the distance from the axis greater or lesser?

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