[Land-speed] Rear Axle Question

Richard Wallendal richardwallendal at yahoo.com
Thu Nov 6 11:42:29 MST 2014


Mayf, thanks for bringing the list to life

Sent from my iPhone

On Nov 6, 2014, at 8:36 AM, "Larry Mayfield" <drmayf at mayfco.com> wrote:

> 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 at 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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