Quote:
Originally Posted by some_other_dave
Perhaps it's because I'm unreasonably dense, but isn't the Force the thing that directly affects coasting?
Are you saying that we can never assume that the load is the same from tire to tire, even on the same car and if we set the same pressures?
I think, that if I am looking for tires to put on one specific car (e.g., my own car) that if I am allowed to set whatever tire pressure I want, the load would be the same no matter what the specific tire was. So in that case, the tire with the lowest measured force resisting movement would be the best one for efficiency.
Can you please show me where I am getting this wrong?
-soD
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Let me put it like this:
- and for the purposes of discussion and simplicity, I am going to confine this to passenger car tires - standard load only.
When tires are tested for RR, the test method will describe the testing conditions. Typically the pressure is specified (remember, we are talking about SL PC tires!) - say 30 psi.
What is also specified is the load. Since the rated load on a tire varies by tire size, this is usually expressed as a percent of some value in the load table. Larger tires will have more load than smaller tires, but the way it is calculated would be the same.
So when you try to compare tires of difference size, since they were tested at different loads, the RRF values are NOT comparable. The only way to compare tires of different size is by RRC - which takes the load out of the picture.
You should also be aware that RRF varies by inflation pressure - and to my knowledge, no one has developed a formula that covers every tire to account for changes in inflation pressure. Put a different way, there isn't a standard way of comparing tires tested with different inflation pressures.