Re: Acceleration and curves

[brian whatcott <inet@INTELLISYS.NET>]

At 19:46 8/19/99 -0500, you wrote:
> As I drive through a curve there is the need to
> brake just before and during the initial segment of the curve...
> After the vehicle is well into the
> curve however, I can take my foot off of the brake and press on the
> throttle ... Why can I accelerate through the end of such a turn but not
> through the beginning? ...
> Cliff Parker

We can all understand why you prefer to brake while driving in a
straight line. The weight transfer is to both front wheels which
are set up to provide most of the braking effort.
 But why does your car roll outwards while turning?

   It must be a geometric relation between the center of gravity,
and the effective rolling axis, the roll center.   Although it is
possible to design a suspension to roll inwards while turning,
 the car body is always arranged to roll outwards.

 In that case the outer wheels are taking more than half the
 reacted forces.
 Racing cars can easily produce an inwards force providing 0.75g
and I have read of peak accelerations of 1.1 g.

   In this circumstance, the forces are unevenly shared between
the front and back tires, and both tires have an effective path
at an angle to the plane normal to their spin axis.

   It is usually preferred to arrange for understeering,
particularly on road cars, whereby the front wheels drift out more
than the back tires. Near the adhesion limit, brake application will
favor the front tires breaking away into a skid.

Understeering provides a certain 'negative-feedback' action, in that
 a too vivid turn is countered by increased front wheel drift.
 In contrast, an oversteering car needs a control correction in the
reverse direction. (This then is the mode for positive feedback
 favoring oscillation.)

But all this does not directly answer the question, 'why can I
accelerate when established in a roll during a turn?'
 The answer, I believe, is the converse of braking during a turn,
in that weight shifts a little further back, providing the front
tire with a little relief from the maximum shear force needed,
increasing the shear on the rear tire, and providing a helpful
inwards orientation to the car's long axis compared to its
instantaneous tangential direction.


brian whatcott <inet@intellisys.net>
Altus OK