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Old 07-27-2006, 02:43 PM   #1
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Rolling and sliding friction.

Does anyone have any info on tire/asphalt Crr compared to temperature/speed/pressure? As well as the relationship between sliding friction and speed? Fluid friction seems pretty straightforward, but the other two vary wildly in all the approximations I've seen. Aside from measuring the force required to move a tire at various speeds/pressures/Crrs, getting a dataset, and finding a nice polynomial approximation, is there any other way to accurately model rolling/sliding friction?
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Old 07-27-2006, 02:59 PM   #2
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I certainly don't. What do you need it for?

From what I am aware, the equation for rolling resistance is an approximation anyway. You are not going to get the data from tyre manufacturers, you will have to generate it yourself.
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Old 07-27-2006, 03:04 PM   #3
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I'm trying to come up with a reasonably accurate expression for the forces on a car at a given speed so that an overlay of gasoline efficiency at certain speeds/loads can give a good idea of engine efficiency compared to rpm. Diesels and electrics don't suffer from a huge change in efficiency, but pumping losses in gasoline cars yield some big changes and mask the huge increase in energy compared to speed when looking at fluid drag. From what I've gathered, rolling friction alone is pretty accurate and only Crr dependent for a given vehicle. But sliding friction seems like a PITA.
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Old 07-27-2006, 03:26 PM   #4
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Shouldn't be hard... why do you need it to be that accurate anyway? You should be able to get away with highway power load (on flat) = Crr * m * v^2 + Cd * A * v^3.

That's from memory, I think the powers are correct. Where is sliding friction going to come into things? It's a car, not a sleigh. At highway speed with no acceleration the wheels should not be losing any grip.

The alternative is to stick the car on a dyno and take an instantaneous FE meter, then you can build a proper dataset without making approximations for drag (Cd is not fixed in stone, but varies according to airspeed and wind direction), but I don't see why there still wouldn't be rolling resistance you would have to account for.

Although I'd imagine that you could still account for it by using different gears at the same rpm range and fitting a polynomial, as you suggest.
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Old 07-27-2006, 03:34 PM   #5
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I've seen sliding friction on flats approximated as (10^-2)*Crr*V*W in terms of force, so it doesn't even approach the rolling friction Crr*W until ~60mph (those powers look correct for power). Personally, I've only "felt" sliding friction above 100mph (and boy was the speedo off ), but it's probably there at lower speeds.

Something else you said that's interesting, assuming steady state/no wind, how does the Cd usually change wrt speed?
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Old 07-27-2006, 07:43 PM   #6
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How does the Cd change wrt speed? It depends on the shape of the object, and Reynolds number. I made a post about it somewhere here, too lazy to dig it up. Suffice to say, it does. I'm pretty sure that the longer, more tapered shapes do better than the fatter shapes at high speed. (Look at airplanes for examples.)

Drag coefficient is something that should be thought of as being applicable within a given range of speed.
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Old 08-13-2006, 09:48 AM   #7
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I just realized it'd be pretty easy to figure mechanical losses in gasoline engines provided we know the maximum efficiency, and sum of the forces of the vehicle, aka the power needed to move the vehicle on flat ground and transmission losses. If we have the speed the engine is operating at, we can figure out how much power each cylinder firing makes, then compare this to the delta pressure between the cylinder, depending on the A/F ratio, and crankcase to figure out the approximate pumping losses at low speed, since this is where they are >> than friction losses. This allows us to observe the approximate increase in friction losses wrt rpm as well.
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