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Old 08-29-2012, 09:46 AM   #18 (permalink)
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Originally Posted by some_other_dave View Post
Barry/Capri, can you please help me understand the coefficent of RR versus the RR force?

To me it seems that we have a fixed weight (to a close approximation) of vehicle, so we would be putting a fixed load on a tire. I would also assume (and it may be a bad assumption) the same inflation pressures in each case. And for comparison we also need to assume the same construction, rubber compounds, etc. In that case, the lower RR force for narrower tires would seem to mean that those tires would coast farther.

Why is it the coefficient, which you have to multiply by the load, which actually shows you which tires would coast further?

Rolling Resistance Force (RRF) = Rolling Resistance Coefficient (RRC) X load on tire.

When tires are tested for RR, they are tested at a particular load and inflation pressure. The result is a Force. If you divide by the test load, you get a dimensionless number (a coefficient) that applies to that particular test fixture and that particular test procedure at that particular inflation pressure.

If you want to compare tires, you need to run them all at the same time, at the same place, and under the same conditions.


1) Do the values vary according to the test fixture? Yes, but a correlation between test fixtures (and therefore test facilities) could be done so they could be compared. I'm not going to go into what it would take for that to happen but it is considerable!

2) Do RR values vary according to the test method? Yes, but a correlation between tests can also be done. Again a considerable coordinated effort would need to take place.

3) Is RRC constant for a given tire? No. Not only does it vary according to inflation pressure, but it varies according to load. The best information I have is the variation due to load is small enough to ignore (but it is something to keep in mind)

Coming back to the same tire in different sizes, I think the reason the data shows that "Bigger is Better" is that an increase in tire width of (say) 10mm increases the width of the tread a fraction of that (say 7 mm), but the increase in load carrying capacity is pretty close to the increase in width. The net effect is that there is slightly less material in comparison to the load.

And did I mention that while the sidewalls only play a small role in RR, every tire only has 2. Changing the tire size doesn't change how many of them there are.

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Last edited by CapriRacer; 08-03-2014 at 08:12 AM..
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