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Originally Posted by SuperTrooper
It's easy to say someone is wrong because they don't reach the same conclusion as you. The text book he references is 539 pages. He did get the ISBN number wrong.
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I realize it is easy to say someone is wrong, but when he is wrong, he's wrong. Referencing a textbook doesn't make him right. But more importantly, he never used that bit of information again to see if the conclusion is right. In fact, the rest of the paper deals with building a machine to test RR and he used it to test pavement. I think our Finnish friend would not qualify as an expert in tires, but based on his research he might be considered an expert on pavement.
Nevertheless, I am in the process of going through the reference to see where he found this conclusion and why he is mistaken about aspect ratio.
Quote:
Originally Posted by SuperTrooper
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At the beginning of Section 2 he references a couple of papers from the SAE defining the factors in RR, of which 90% is due to material hysteresis. Sidewall stiffness contributes to hysteresis inversely. Shorter sidewalls are stiffer. Less sidewall = lower hysteresis. Any disagreement here?
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Yes, but I'll come back to that point.
Quote:
Originally Posted by SuperTrooper
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I was speaking more of a theoretical nature. As I noted in my posts it would be tricky to achieve this. I referenced the tires/wheels on supermileage racers as the ideal, not as a practical solution for daily drivers.
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First, I'm going to reference a document that referenced in a previously posted link. Here's the whole document:
http://www.energy.ca.gov/transportat...%20Testing.pdf
The problem with this paper is that they are looking at a HUGE!!! volume of tires of all different types - large sizes / small sizes, all season/high performance, different speed ratings, OE / replacement, etc. - all of which cloud the issue and make it difficult to draw conclusions. We need to sort this out a bit.
On page 34, there is a graph that shows the relationship between RR and Max Load (I'm going to refer to this as "Rated Load") Notice there is a pretty strong correlation between RR and Rated Load - R squared = 86%. Considering all the different types of tires they looked at, this is a remarkable correlation.
If we want to compared the affect aspect ratio has on RR, then we need to hold the Rated Load constant.
-- As an aside - Look at pages 11 and 12, where they compare the RR values for the same size (and therefore, the same Rated Load). There is an incredible range of values - more than plus or minus 25%. This makes the data being published highly variable and we have to be careful with it.
Now look at page 30, where they publish the RR vs tire size (and tire size controls Rated Load). If we choose carefully, we can pick out tires that have about the same rated load, but different aspect ratios. Here's an example:
P195/75R14 (LI92)/ P205/70R14 (LI93)/ P205/65R15 (LI92):
From the graph on page 30, here's the order for RR: P195/75R14 (LI92) / P205/65R15 (LI92) / P205/70R14 (LI93)
Notice that this is more or less in the order of Load Index (LI) - LI being a quick way of referencing rated load - and NOT in the order of aspect ratio.
If you try to keep the width the same - P185/75R14 (LI89) / P185/70R14 (LI87) / P185/65R14 (LI85) - not only is the rated load going down, but the graph on page 30 shows the rack order to be P185/65R15 / and the other 2 are nearly identical. This is probably where the confusing is coming from.
The problem is that there are confounding issues. So if you want to define the role aspect ratio plays in RR, you are always going to get confounded by something. And since we know that rated load plays a HUGE role in RR, we have to hold it as constant as we can. The net effect is that every time you go to a lower aspect ratio, you have to go wider in section width (and therefore tread width) to maintain the load carrying capacity - and that means more volume of material - and that means more RR.
Please note: RR testing takes place at the rated load and rated pressure - which is not the way a vehicle using tires. A vehicle holds the load constant - another confounding issue.
Try a couple of other sizes and see what you get. Remember: The elephant in the room is "Rated Load".