Quote:
Originally Posted by aptizzle
Hey Ken,
If you haven't read 'The Leading Edge' by Goro Tomai, you should. The less rolling resistance claim is a sum of all of the forces, tire forces, bearing windage losses, brake drag, etc.
SteveF
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Hi Steve,
Tamai's concerns are only vaguely applicable to the solar racer world and not applicable at all the the world of real cars. Bearing windage and even bearing seal drag (a separate effect with a different equation) taken together are not measurable, in any realistic sense. You can prove this to yourself by jacking up a car wheel and adding a one ounce weight to the rim. The heavy spot ends up at the bottom. If you have brake drag, this is not the case, but we should not have brake drag, in the pad-to-steel friction sense. (We can have aerodynamic brake drag, from the effect of the pads trying to shear off the boundary layer -- but that is better put into the category of aero drag, and is rarely modeled -- it is not an effect that you can separate from all the turbulence around the brake calipers.)
These types of drag are more than an order of magnitude smaller than the traditional rolling resistance, which is heavily dependent on tire construction and rubber compound. A 2000 lb car has circa 20 lb of rolling resistance: a couple ounces is not enough to worry about, let alone reliably calculate.
If you look at the
Wikipedia article on solar racers, you will see that there is a term that could show an effect due to number of wheels, but that coefficient is 0, and the term drops out entirely in the "usefully simplified" performance equation:
So even in the esoteric solar racer world the term drops out... but especially in the real car world, things like bearing windage have no effect in the difference in rolling resistance between 3-wheelers and 4-wheelers. (The main component of bearing drag, from ball and race deformation and skidding, scales with weight.)
You can pull a 1000 pound three wheeler and 1000 lb four wheeler across a shop floor and measure drag with a load cell or spring scale. Given tires with the same rolling resistance spec, the rolling resistance will be the same.
Regards,
Ken