rec.autos.simulators

I am trying to clarify the standard definition of "longtitudinal slip
velocity" and hence "slip ratio".

Let Vx be longtitudinal velocity of wheel. Let w be angular velocity
of wheel, R be wheel radius, and Vsx be "longtitudinal slip velocity".

Now, is it:

Vsx = Vx - Rw
and, slip ratio = -Vsx/Vx

or is it,

Vsx = Rw - Vx
and, slip ratio = Vsx/Vx

Erik Lowndes thesis gives the first definition as the standard
definition, but I felt terribly confused by it, since I had previously
thought the second definition was it. (Note the only difference is the
sign of the expression).

Can someone please set me straight about the correct sign of
"longtitudinal slip velocity"?

Thanks,

Ruben

As I understand it its positive when accelerating, and negative when
breaking, if Rw>Vx your accelerating, and therefore if Rw<Vx your
deccelerating.
But really its a relative measuerment, and only depends on what you use it
for to what it needs to be. That said I've not yet come across any official
convention at all, and this standard form of calcualting it is quite useless
at low speed, and with divide by zeros its not even a continuous function
and
all rather numerically unstable. There is a differential equation that can
be used at low speed, in some SAE paper somewhere it was suggested, but this
doesnt work to well at high speed. Ah well.
Hope this helps and I havent confused even more =]