rec.autos.simulators

Understeer in open wheeled sims are gonna get better

Jim Sokolof

Understeer in open wheeled sims are gonna get better

by Jim Sokolof » Tue, 08 Dec 1998 04:00:00



> >  Im mixed up here.As the front tires experience sidewall flex,causing the
> > steering wheel to get tighter in relation to the amount of slip angle(self
> > aligning torque),then how can the steering go light when the fronts lose
> > grip?If the tire is sliding heavily,the slip angle is even greater,then the
> > self aligning torque should be even greater.But instead the self aligning
> > torque gets lighter.Sidewall flex is lessened???

> I think that aligning torque is an effect of
> the spinning of the tire, and doesn't relate
> directly to tire deflection.

It's not related to the spinning per se, but the effect of that
spinning on deflections in the tread and suspension geometry.

This depends on the castor of the wheel. If the effective center of
the (sliding) contact patch is to one side of the axis about which the
wheel can pivot, there will be an aligning torque.

The following picture illustrates the case. Assuming the forces are
balanced in magnitude (they will be in a sliding car tire example),
the fact that they are offset will result in an into-the-page
(counterclockwise) torque.

       ^
       |
----------------
         |
         V

That's a little different. If the wheel is already aligned, then the
aligning torque from castor will be zero, and the wheel will not tend
to center (it already is). If the wheel is not aligned, it's the same
case as above. (assuming a non-zero castor)

---Jim

Doug Millike

Understeer in open wheeled sims are gonna get better

by Doug Millike » Wed, 09 Dec 1998 04:00:00


Since this has gotten pretty technical, it's time for some standard
terms...even though I swore off this thread earlier...!

Aligning Torque (AT) is the torque about a vertical axis (SAE "Z" axis)
through the axle and the wheel centerplane.  It is one of the tire forces
and moments and is independent of steering and suspension geometry.  It's
primarily a function of slip angle and load.  It's not quite zero when
the slip angle is zero (tires don't really run quite straight).

Trail results from Castor angle and/or Offset in the steering geometry.
Tire lateral force at the ground produces a torque about the kingpin.

Steering torque about the kingpin is a combination of the AT and the
lateral force x trail torque.  Then to get it up to the steering wheel
there is a gear ratio, and (usually) quite a bit of friction.

-- Doug

                Milliken Research Associates Inc.


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