Joe:
Thanks for the thoughtful response. There is no question the
circumference of the tire and its revolutions are tied to a finite
distance. However, I don't think that is the issue.
The engine applies torque through a lever arm. (The distance from the
axle to the ground), i.e. what I call the drive radius because it is
always different from the radius on the top side. That length (and of
course, the gear train) determines just how far the crank will have to
revolve for any given amount of wheel rotation. Obviously for small
wheel diameters the crank will turn more times than for larger wheel
diameters for the circumference to go the exact SAME distance. In
effect, you might say it is a gear ratio even though you said "if you
use the "drive radius" instead of the "true radius", you are, in effect,
putting in a "gear ratio change" that does not belong there."
However, you later said . . "At high speeds the circumference stretches
and increases the tire radius--a really big "gear ratio" change for top
fuel dragsters as they go down the course!" So you do agree that a
radius change does have a gear ratio "effect". :-)
In that comment you indicate both the circumference and the radius
become larger. I agree because it means that the lever arm (drive
radius) also becomes larger meaning(less turns of the crank required to
rotate the circumference a given distance). Who hasn't seen the lever
arm grow, i.e., the axel of a Top Fueler lift 2"-4" due to tire growth
in the lights! What we are doing here is coupling the crankshaft to
the lever arm, that is touching the ground, which is reacting the
torque, to propel the vehicle forward. The lever arm is the only thing
doing useful work. Conversely, if you shorten the lever arm via a tire
"footprint" you are requiring the crank to turn more for the
circumference to travel a given distance. More in Part 2. -Elon
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