paulhus@euclid.DEC (Chris Paulhus 223-6871 MLO8-3/T13) (10/23/84)
I've read a lot of discussion about snow tires for front wheel
driven cars on the net, but I have not read a good explanation of
why 4 tires are best - from a vehicle dynamics standpoint (if such
has been presented, I missed it due to our VAX's intermittent contact
with the rest of the world). Since this is an important safety and
$$ issue, I'd like to present a thorough explanation of why 4 snow
tires are needed on front-wheel and 4-wheel drive vehicles.
First, a few definitions:
Stable: The tendency of a system to return to a previous condition
after being acted on by an outside influence.
Unstable: Tendency of a system to depart from a previous condition
after being acted on by an outside influence.
Example: Stable - a ball bearing inside a beach ball. It tends to
settle at the bottom.
Unstable - a ball bearing balanced on the top (outside) of
the beach ball. It tends to fall off.
[Tire] Slip Angle: The angle between where the tire is pointing and
its actual path over the road.
Understeer: Condition where the front tires' slip angles are larger
than the rear tires' slip angle.
Oversteer: Condition where the rear tires' slip angle is larger than
the front tires' slip angles.
Example: Understeer - Buick Le Sabre
Oversteer - VW Beetle with swing axles
Limit behavior/conditions: [Behavior when a ] Vehicle is operating near
its maximum (cornering/braking/etc.) ability.
Discussion
A vehicle that understeers is stable - it tends to go straight
ahead. (This may not be where you want it to go, but it IS stable.)
A vehicle that oversteers requires unusual control inputs at near
limit conditions (steering into a slide, etc) because it is unstable.
[Oversteering cars are preferred by racers for low speed (say, under
130 mph) cornering because this attitude is usually faster; high speed
racing (Indy, sweeping turns on road races) is usually done with a
slight understeer condition (except qualifying, sometimes). You can
change a racer's under/oversteer as a function of speed with it's
aerodynamics.]
(I learned to drive fast in karts and a F Vee, both
oversteering vehicles, so oversteer is my preference on dry roads.
But when its slippery, see below.)
An untrained (at limit behavior) (or non-alert/unsuspecting/sleepy/
etc.) driver is safest in an understeering vehicle since all the typical
near limit reactions - steering more, lifting off the gas - result in
the desired changes to the car's path while maintaining a stable attitude.
A word on brake behavior at limit conditions
If, during limit conditions, the front wheels lock before the rears,
the vehicle is stable (understeers) under braking. If the rears lock
first, the vehicle becomes unstable and tends to spin. All properly
designed/set-up vehicles have front brakes that lock just before the
rears on dry roads (with less weight shift on slippery roads, this
tendency increases - a safety factor).
(My karts had only rear brakes - darn unstable under hard braking -
but I'm glad I learned to handle it: it prepared me for my 1980 Chevy
Citation X-car.)
Snow Tires
Snow tires tend to run at lower slip angles than regular tires
during slippery conditions (especially if they have a hydrophilic
tread compound). Putting snow tires on only the rear of a rear wheel
driven vehicle increases the understeering tendency (stable condition)
of the vehicle. Since the vehicle gets more stable, this has been
accepted by generations of rear wheel driven vehicle drivers.
(The exception is people who like to go fast under slippery
conditions - like stage rally drivers. They use snow tires on all 4
wheels of rear wheel driven cars.)
Putting snow tires on the front only of front wheel driven vehicles
tends to lessen the understeer (stable) condition of the vehicle. Now
the front has a better grip than the rear. The rear slip angle (due
to regular tires) tend to become larger than the front slip angle and
you've got an oversteering (unstable) vehicle! To keep the vehicle
balanced (slight understeer condition desired) it is necessary to
install snow tires on all 4 wheels of front wheel driven [or 4 wheel
drive (my little sister has 4 new hydrophilic snows already mounted
and ready to install on her new 4000 Quattro)] vehicles.
So, if you see a white Cavalier wagon with huge driving lights (I
like to see at night - they are relayed with the hi-beams) you'll see
4 hydrophilic snows under it during the winter. During the summer
you'll see fat low profile tires on the front, but - surprise! - you'll
still find snow tires on the rear! Why? Like I said, I like oversteer
under dry conditions but understeer under slippery conditions. This
set-up delivers it with a very slight decrease in limit capabilities.
Summary - 4 SNOW TIRES ARE RECOMMENDED FOR FRONT-WHEEL DRIVE AND
4-WHEEL DRIVE VEHICLES.
N. Chris Paulhus DEC - Maynard (617) 493-6871