Hmm, has my thread of determining what axle ratios are available been
hijacked? As tot eh formula, I always go back to first principles and
figure it out for myself, rather than use friendly formulas. Just derive it
yourself and it is forever stuck in your head. Interestingly, the point of
the tire that is in contact with the surface on which it is rolling has zero
velocity. And the top of the tire is going twice speed. And that the
tires actually grow as a function of speed making the vehicle faster, the
faster it goes. After chatting with Goodyear some time ago, I found that my
tires grew about an inch in diameter at the speeds planned. And that of
course ads to the torque or thrust because of that lever arm of tire radius
needed to move down the surface. And it needs to be accounted for in the
simple formula. There is always more to the iceberg than meets the eye,
eh?
I asked the question because I have a Sunbeam Tiger in my shop (owned it
since 1967) and I was considering a higher gear ratio for it for motoring
around in if I ever get it put back together. It came with a 2.88:1 Dana,
but has a 3.54 Posi in it now. I have a Mexican 302 block that I am
considering a stroker kit for and that would give me a lot of low end to
push those gears at highway speeds for better mileage.
See, not every question is related to LSR, lol..
mayf
-----Original Message-----
From: Land-speed [mailto:land-speed-bounces@autox.team.net] On Behalf Of
Kirkwood
Sent: Wednesday, November 5, 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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