Tire circumference and distance traveled...

[Quattro Pete]

Let's say we have a tire with an overall diameter of 25". From this we can calculate that the circumference is about 78.5". But once the tire is on the car, will one tire rotation result in a distance traveled of 78.5" or something less than that because the sidewall is a bit flattened? Another words, a radius of a tire off the car and on the car is not the same, correct?

I noticed some manufacturers report a value called "static load radius":

http://www.bridgestonetire.com/tireselector/dpp/sizespecs.asp?passproductid=1113

How do I interpret this value?

Does changing PSI in the tire by 5 or 10 affect the rolling radius?


I got into an argument with someone that great changes in PSI will affect the distance traveled during one full rotation. He claims that the amount of tread in a tire is fixed, say a slab of 78.5" in length, and since all of this tread has to make one full rotation, the distance traveled will not fluctuate with PSI or load carried. I understand what he's saying, but then again, we're talking about rubber here - a flexible material that can stretch and contract, different tread blocks coming together and moving apart as the tire flexes. I thought that causes the distance traveled to vary.

Capri, can you shed some light on this?

Thanks.

 

[CapriRacer]

Quote:

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.......Capri, can you shed some light on this?....

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I'll see what I can do.

Let's start with static loaded radius. This is the distance between the ground and the center of the axle at rated conditions (Rated load and rated pressure)

Needless to say other conditions will result in different values.

But the important point out of this is that not only does the sidewall bulge out (and the tread flattens) in the footprint area, but the hole in the center of the tire (or the rim) is pulled off center. Put another way, if you measure a tire and rim in a loaded state, you will find the distance from the tread at the "top" of the tire to the center of the axle is larger than it was in the unloded state. In some respects, it's like the rim is "hanging" from the tread "ring".

I'll bet you'll need to read that a couple of times before it sinks in. I know I had to think long and hard before I came to grips with the concept.

The only useful data you get from static loaded radius is a way to calculate the ride height of a vehicle.

OK, let's try another difficult concept.

The rolling circumference: I'm going to use circumference in this discussion because it is easier to visualize than diameter.

The rolling circumference is about 97% of the free standing circumference. It turns out that this is the value using the center of the belts. As a result, I tend to think in terms of a tank track - you know the Abrams M1A1 type - where the length of the track controls the distance traveled in one revolution. It doesn't matter what the shape of the tire is or how much tread depth is remaining, the circumference is always the same. One way to look at this is that for practical purposes you can consider the belts as inextensible. After all steel is many, many times stiffer than rubber, so it dominates the situation.

BUT, the quirk is that rubber IS flexible and extensible, so there is a very slight (and measureable) difference when different pressures are used - which is why the ABS system can be used as a pressure monitoring device.

Further, the tread depth left on a tire also affects the distance travled in one revolution. I suspect this has to do with the ever so slightly difference in stiffness caused by the additional tread rubber. To understand this, you have to consider how flexible the sidewall is compared to the tread - the amount of deflection doesn't have a lot to do with the rolling circumference.

I hope this helps.

 

[John K]

In the old days when I used to participate in sports car rallys we used to start out and warm up the tires before checking our odometers over a known distance so we could calibrate our readings. Since we were required to cover a specified route in a specified time, it could throw off your results. Most people running steel belted tires did not notice a difference, those with fabric radials a bit more, and those running bias belted tires varied the most. I know this isn't exactly what was asked, but it is sorta related. If you ran low tire pressures you got a bigger variance, because they would heat up more and thus expand. If you are trying to be exact you want the tires to be at operating temperature when you start the route. So the circumference can change....