Date: Sun, 27 Sep 2009 18:00:32 -0400
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: Science of Washboarding
In-Reply-To: <4ABF9B86.5040709@gmail.com>
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At 01:06 PM 9/27/2009, Rocket J Squirrel wrote:
>On the return trip I tried going sufficiently fast (40 mph or so) that the
>tires and suspension could float a little. The ride was nearly smooth.
>While it felt miles better in the driver's seat when I was
>going fast, I wonder if I wasn't putting a lot of stress on the tires or
>suspension.
>
>Thoughts? Or even better, knowledge?
I know a few things and the rest is conjecture:
In the semi-analogous case of rotating machinery passing through its
"critical" (resonant) operating speeds, I know that a) continued
operation at those speeds will shake the machine apart; however b)
the internal forces in the bearings are *lower* at the resonant
speeds, because the forces are being coupled into the rest of the
machine instead of being absorbed inside the bearing.
The "trick" to passing over any sort of bump smoothly is to do it
rapidly enough that the vehicle itself doesn't respond, but only one
or more components in the suspension. A rule of thumb I've heard for
speed bumps is ten times the speed that the bump is designed to
prevent. In the case of an isolated bump like a speed bump, this
devolves to the rotating-machinery case: you're not allowing the
forces to spend themselves moving the van up and down, so they have
to be dissipated entirely in the tires/suspension, and those forces
increase with the square of the velocity. That's the suspension's
job of course, but you've definitely given it a lot more work (in the
sense of energy conversion) to do.
Doing it on a washboard, though is much more complicated because it's
possible that you may indeed be experiencing the bumps faster than
the suspension can respond to them. My knowledge stops at this point
but my intuition says that the total energy conversion by the
suspension might be less in this case; and that surely the
distribution of work tends more and more toward the tires.
As a practical thing, it's still comparable to the initial case --
operation above or below the "critical speed" keeps the bulk of the
vehicle from shaking itself to bits either by reducing the frequency
and intensity of the inputs or by forcing the suspension to work
harder dissipating the energy within itself.
>imprint the corrugations into the road surface. It consisted of two large
>drums, each about 6' wide and 4' high, roughly, mounted side-by-side on an
>axle. The circumferential surface of the two drums had steel corrugations
>spaced about a foot apart all around the drum.
I strongly suspect that its purpose is rather to smooth out said
roads. and the cleats are to keep the thing rolling rather than
building up a "bow wave" and sliding under some condition of use.
Yours,
David
