Tire Roll out

[Dunebound69]

Consumers also need to buy tires appropriate for the vehicle and what they use it for. You do not need 10 ply (E range) tires on a 1/2 ton. The tires could carry more than the truck ever should. Also I have been told by an engineer with Goodyear that your tires should always be with in 10% of max pressure if running a D range or heavier tire. Said tires are designed for load. To low of pressure can cause the belts to separate or at very least shorten the life of the tire. We must remember that auto manufactures are making a compromise with suggested tire pressure. Balancing ride quality, economy, contact patch etc.

[fabr]

YUP

[LiveWire]

For the moment, I am only talking about steel or kevlar belted radial tires.

Mileage will go down with a tire inflated much more than a couple PSI below the door sticker. This is not because the circumference of the tire tread changes. It is because as some of the sidewall makes contact with the road, it has a circumference different than the tread so it therefore is being dragged. The friction causes a loss in mileage. I am only talking about inflating over the door sticker in an attempt to improve mileage, not the difference between a low tire and proper inflation.

For those that believe the tire changes in radius and therefore circumference, please explain how steel or kevlar belts stretch.

Vehicle manufactures determine based on a particular tire size what air pressure is needed to keep the tread flat to the road for traction and wear. When you put a large tire on a light vehicle such as Explorer, it needs to run a lower pressure to not wear out the center of the tire. The requirements are then sent to tire manufacturers to quote a tire that meets the specs. Ford requested quotes for tires that were capable of running at 26 psi on the Explorer even though side wall pressure is 35. Firestone tried to blame Ford for setting the pressure to 26 even though Firestone had said their tire was capable of running at 26 psi. Once the tires were replaced, the replacements were still run at 26 psi without issue. The same Firestone tires failed on other vehicles, but due to the high sales numbers of the Explorer, those incidents were much more highly publicized. Ford also requested quotes for Excursion tires. The Excursion, due to not being able to pull a fifth wheel, cannot carry the load of an F250. While it can carry a load greater than load range D tires are rated to carry, Firestone said their load range D tires could carry more than the rating and enough for the Excursion. There were actually a higher percentage of Firestone load range D tire failures than the Explorer tires. The lower total vehicles and incidents prevented news coverage. I now have load range E tires and run them at the original tire pressure which is lower than that listed for the F250. I have no degradation issues including the splits in the sidewall like the Firestones had. The only issue I see is the center of the tires are wearing faster than the outside. I believe this is due to the load range E tires flexing less. So I may run the pressure lower to compensate for the stiffer carcass.

[LiveWire]

As Masterfabr pointed out, if you run wider tires, you need to lower your pressure. A way to find out how much is to wet a strip across the tread then roll over a piece of paper. The mark left should be the entire tread. Too low and the center will cup upward and . Too High and only the center will leave a mark.

[fabr]

For the moment, I am only talking about steel or kevlar belted radial tires.

Mileage will go down with a tire inflated much more than a couple PSI below the door sticker. This is not because the circumference of the tire tread changes. It is because as some of the sidewall makes contact with the road, it has a circumference different than the tread so it therefore is being dragged. The friction causes a loss in mileage. I am only talking about inflating over the door sticker in an attempt to improve mileage, not the difference between a low tire and proper inflation.

For those that believe the tire changes in radius and therefore circumference, please explain how steel or kevlar belts stretch.

Vehicle manufactures determine based on a particular tire size what air pressure is needed to keep the tread flat to the road for traction and wear. When you put a large tire on a light vehicle such as Explorer, it needs to run a lower pressure to not wear out the center of the tire. The requirements are then sent to tire manufacturers to quote a tire that meets the specs. Ford requested quotes for tires that were capable of running at 26 psi on the Explorer even though side wall pressure is 35. Firestone tried to blame Ford for setting the pressure to 26 even though Firestone had said their tire was capable of running at 26 psi. Once the tires were replaced, the replacements were still run at 26 psi without issue. The same Firestone tires failed on other vehicles, but due to the high sales numbers of the Explorer, those incidents were much more highly publicized. Ford also requested quotes for Excursion tires. The Excursion, due to not being able to pull a fifth wheel, cannot carry the load of an F250. While it can carry a load greater than load range D tires are rated to carry, Firestone said their load range D tires could carry more than the rating and enough for the Excursion. There were actually a higher percentage of Firestone load range D tire failures than the Explorer tires. The lower total vehicles and incidents prevented news coverage. I now have load range E tires and run them at the original tire pressure which is lower than that listed for the F250. I have no degradation issues including the splits in the sidewall like the Firestones had. The only issue I see is the center of the tires are wearing faster than the outside. I believe this is due to the load range E tires flexing less. So I may run the pressure lower to compensate for the stiffer carcass.

And that is where the safety issue comes in.

[fabr]

As Masterfabr pointed out, if you run wider tires, you need to lower your pressure. A way to find out how much is to wet a strip across the tread then roll over a piece of paper. The mark left should be the entire tread. Too low and the center will cup upward and . Too High and only the center will leave a mark.

When I was drag racing I used a carbonless transfer paper that I got from Dana corp/Victor Reinz at the time . It was used to demonstrate the clamping consistency at varying torque values on a cylinder head. You dry assembled the head to the block,torqued it to spec,disassembled and inspected the paper for the results. I used it by putting a piece of it under the slick and reading the results. It gave a very good place to start from when changing to a different type of slick. I also out of curiousity did the same with my tow vehicle loaded and unloaded. Made it very easy to see what it took to get a consistent footprint on the ground.

[SPEC]

Kinda like white shoe polish on on the slicks of a roundy-round car to get the true roll out of a tire...It's more accurate than the tire tape

[trojan]

Livewire: what is this "roll out" you speak of? any definition I can find online referrs to it being equal to circumference (diameter x pi), but you imply it's not circumference without stating explicitly what it is you are referring to.

technically the ONLY way to test whether you have optimum pressure is with a pyrometer. When the tyre is hot, the temperature should be the same for the outside edge, middle and inner edge of the tread face. Optimum is only achievable on a set piece of road for a particular driver on a particular day. Everything else by comparison is WAG.

[chrishallett83]

[trojan]

little point: pi times R squared gives you the area of the circle not circumference (you're thinking of 2 x radius x pi)

As to sidewall bulge, I expect it to be negligible if the tyre is inflated properly?

[chrishallett83]

First point - Oh yeah... Whoops.

Second point - nope. Even low profile tyres have a degree of 'squash'. Obviously, the higher the tyre profile, the softer the sidewalls are, and the lower the pressure, the more 'squash' there will be.


It only takes a couple of millimetres of differece to a circular objects diameter to make a noticeable difference to its circumference.

[trojan]

I would suggest the tyre illustrated is underinflated?

[LiveWire]

Trojan,

The roll out is less than the circumference as calculated from the diameter of the tire. I want to wait a bit before posting why that really is so more people can try and explain the faults I point out in the bulge theory. It has nothing to do with the bulge as Chris states. That is the Wiki answer. It is not right, just what most people think.

If you lower the air pressure, the flat spot on the tire gets bigger. The wiki term 'rolling radius' gets smaller. That is why many say inflating the tire more and decreasing the bulge and contact patch (forgetting the safety issue), that the tire will now roll out more. That would have to mean the steel belts in the tire shrink when the pressure is lowered and the bulge increased. Also by that definition, you could run 2 different size tires and just put more air in one. Quite a few people actually believe that last one too since it would be true if the definition of 'rolling radius' were true. Other's who believe in 'rolling radius' will acknowledge that is absurd, but can't explain why it is while fitting their definition.

Another thing to ponder is that the the 'rolling radius' is measured in the center of the contact patch. The very center of the contact patch is the only point that has that radius. As you more fore or aft of it, the radius gets bigger. The tire not only bulges at the side walls, it bulges fore and aft as well. If you measured the radius at the points were the tread just meets the road surface, it is actually greater than the tire with no load. That is where the length of the belts displaced in the center end up. Averaging the radius measured over the entire contact patch may be very close to that of the unloaded tire.

[fabr]

This whole overinflate thing is based on decreased rolling resistance and nothing to do with rolling radius. Street tires have little to no change in rolling radius at different pressures. 

[LiveWire]

Street tires have little to no change in rolling radius at different pressures.

While I do, most people do not think that.

So here it is: the reason that the roll out on a tire varies from the calculated circumference is that the tire does not flex at the surface of the tire. It flexes at the belts below the surface under the tread. The tread blocks are squeezed together as they go through the contact patch. If it were even possible and you inflated the the tire to the point that the contact patch was a single point, the roll out would increase because then the tread and rubber outside the belts would be affecting roll out. Between vehicle sticker pressure and sidewall max pressure, the belts remain the same length so the roll out remains unchanged. Roll out is also changed little to none as the tire wears.

It just so happens that the amount a typical tire is compressed at the bottom is similar to the distance the belts are from the ground. Therefore using the center of the wheel to the ground as the radius in calculating circumference may often give a result similar to the roll out. It is just a fudged number though and often will not give that good of an approximation. Roll out is the only true way to measure a tire.