The tire's "rolling radius" takes the load deflection into account so it
isn't exactly equal to one half the tire diameter.
Regards, Neil Tucson, AZ
-----Original Message-----
From: Kirkwood
Sent: Wednesday, November 05, 2014 10:18 PM
To: land-speed
Subject: [Land-speed] Rear Axle Question
There are essentially two speed formulas. The one Ed gave uses the tire
DIAMETER and you divide by 336. The other formula uses the tire RADIUS and
you divide by 168. Since a radius is half of a diameter it stands to reason
the 336 constant would be divided by 2. So what is the difference you say?
The lever arm from the center of the axle to the ground is what propels the
car forward. On a tire the radiuses are NOT equal! No, this is not geometric
heresy. The radius at the top is longer than the one at the bottom due to
the tire footprint or the squashed part of the tire. IOW, the bottom radius
does all the work and is more accurate. The top part of the tire (i.e., the
diameter) has nothing to do with the cars speed. It is the distance from the
axle to the ground that determines speed. Now you say our tires are inflated
to 70 psi and there is no footprint. Well maybe you believe the tire-to-salt
interface is also perfectly tangent.
(RPM x W) / 168 x R = MPH
W = axle to ground radius
R = Overall gear ratio.
The list has been too quiet . . . just stirring the pot! LOL :-)
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