[CapriRacer]

Wow, this thread generated a lot of responses – some of them needing clarification. Sorry for the long post!

Here’s the first one:

Originally Posted by RedDevil

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.

Originally Posted by RedDevil

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?

Originally Posted by ksa8907

…….. 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.

Originally Posted by ksa8907

…….. Which means that load and inflation pressure are the primary influences. …….

That is correct, even though how it was derived was wrong.

Originally Posted by ksa8907

…….. 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.

Originally Posted by ksa8907

…….. 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.

Originally Posted by ksa8907

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.

Originally Posted by gumby79

…… 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.

Originally Posted by gumby79

…… 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.

Originally Posted by gumby79

…… 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.