[Land-speed] Rear Axle Question

[Jim Dincau]

     Think about this, for every revolution of the tire the full 
circumferance must also  rotate 360 degrees in contact with the ground. This 
is why the relevant measurement is the actual circumferance not radius or 
diameter (I know they are related by geometry). Goodyear used to give away 
roll out tapes to measure tires with. The fact that the radius is shorter on 
the bottom side only affects the torque required to turn the wheel.

-----Original Message----- 
From: Kirkwood
Date: Wednesday, November 05, 2014 9:18 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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