OK, it's obvious I need to explain this.
First, I am prompted to do this by this thread:
Ecomodder: Thread on LRR Tires
As you can see, the question is: Does the change in tread depth (wear) account for the change in fuel economy from new to worn out?
I already know the answer: As the mass of tread rubber is worn away, the amount of energy consumed goes down - due to less internal friction (hysteresis). In fact, when I was designing tires, if we needed to reduce the rolling resistance, we used to reduce the amount of tread rubber by making the tread narrower or having wider grooves.
But I know that the group I am talking to at Ecomodder respond strongly to data - and I was hoping to be able to prove that the change in tread depth doesn't have the effect most people are expecting. I am hoping the result would be that it changes a fraction of what would be predicted by the change in physical circumference of the freestanding tire.
OK, a couple of things folks need to understand.
First, the rolling diameter of a new tire at the rated load/rated inflation pressure is about 97% of the freestanding circumference (unloaded and hanging). That is why Tire Rack publishes Revs per Mile data with each tire. Go ahead, do the math, and you will see this is so.
Second, I've already done the rolling circumference vs inflation pressure thing and lower inflation pressure reduces the rolling circumference.
The thing to remember, is that a tire is a flexible object and not a rigid structure. The whole *Diameter times PI* thing doesn't work.
Further, there is nothing you can physically measure that will calculate out to the rolling circumference - at least not with any accuracy.
- EXCEPT -
If you do the math, the calculated diameter of the rolling tire is about the diameter of the belt - and I wonder if the belt is acting just like a tank tread, where the length of a revolution of the tire is the circumference of the belt. I called this the Tank Track Theory.
And that should mean that the difference rolling diameter between a new and a worn out tire would be almost the same.
Some other things of interest:
Tires grow. They do most of their growing in the first 24 hours. In fact we call the dimensions "24 hour growth dimensions" - and because of that, our tire testing shop used to inflate tires to 2 or 3 psi more if the test was to be conducted the next day.
There is also variability between tires. If you measure a large group of tires (for whatever parameter you wish), you will get a range of results.
That means that testing a new tire vs a worn tire won't properly answer the question - but testing the same tire would!
There is also a thing called Pantographing, where the steel belt changes angle as it rolls through the contact patch and the belt changes length and width as it does so. This angle change would be affected by the load and inflation and complicates things.
So I thought if we could find a tire, measure it's freestanding and rolling circumferences, then shave the tread off, then measure again, we could answer the question and perhaps validate the Tank Track Theory. I've measured both those circumferences before and validated the 97% value
- BUT -
I don't have access to a tire shaving machine. Without that, I can't conduct the test. While I was thinking about it, I listed some of the other things I was going to need.
While I am willing to perform the physical labor, I know that shaving equipment might be too far away for me to travel, so I listed all those other things needed to conduct the test - hoping someone might volunteer.
So there you have it.