Why does my car do that?
How can I make it do what I want it to?
This is just a problem Porsches have, right?
Before pilot candidates begin to fly, they spend time learning a lot of the basic principles
and properties of flight and airplanes in a classroom. In this column, I hope to cover topics
that affect how vehicles respond in the same type of approach, since an understanding of some
basic principles may translate to improving mastery of your car on the road and track.
The three questions above were frequent queries I heard during a recent car control clinic.
In this first column, I would like to address some basics of vehicle dynamics, weight shifts
and balance, and hopefully answer these questions. In future columns, I intend to explore
oversteer/understeer/drifting, cornering techniques and how to adjust your line on the track.
If you have questions or suggestions you'd like addressed, feel free to email me and I will
try to answer them as well. For today, we need to cover some basic science to begin.
Joel demostrates extreme weight trasnfer for his instructor. By the
end of the Car Control Clinic, the car was much more balanced, Joel was a lot
smoother and quicker as well. Photo Credit: Rink Reinking
As with everything in the physical world, our cars are governed by the laws of physics.
Newton's First Law (Inertia) states that objects in motion remain in motion and objects
at rest remain at rest unless acted upon by an outside force (Miss E ? that explains why I
can never get around to cleaning out the garage ? its against the Law!). Whether you drive
a 997 GT3 RS, 356 or a Tucson City Bus, you directly control the outside forces of the
throttle, the brakes, and the steering. Gravity, wind resistance and the friction between
your tires and the road surface are also acting upon your vehicle. Obviously spoilers,
body shape and damage, etc can affect wind resistance, but we'll assume it's largely beyond
our control once we're behind the wheel. Gravity is also beyond our control (except for
Walt Harrington ? I'm sure he had a gravity control device in his old 911!). It's the last
item, the traction of our tires, which we can modulate to our advantage while driving.
Traction is a function of the road surface, the tire rubber compound and surface area,
and the pressure between the tires and road. While we can't control the grip of the
road surface, there are conditions while negatively effect it. Ice, snow, water, sand,
gravel, debris, and cracks all deteriorate the quality of the road. For high performance
driving, this means avoid the debris, sand, gravel, and "marbles" (chips of rubber from
tires) to maximize the grip of the surface. We can address water and "the rain line" at
a later time. As for our tires, a "slick" or similar tire with minimal voids between
tread blocks has more rubber in contact with the track, which translates to more traction,
all other things being equal. If you were driving in snow, sand or mud, increasing the
tread void depth increases the surface area contact as these soft surfaces push into these
voids, hence mud & snow tires for your SUV. The actual rubber compound itself is a crucial
part of the tire's grip ? imagine an old dried out tire which is hard and smooth versus the
soft gummy surface of a track once it has heated up. This is why racecars use slicks or "R
compound" tires on the track, as they are a much softer, stickier rubber. In general, the
lower the treadwear rating, the faster a tire will wear down, but the stickier it will be
on the track, and it is not unusual for race tires to wear out and be replace several times
during a race! Tire width is somewhat dictated by your car's design, but clearly as wide a
tire as you can fit without rubbing will increase the amount of rubber you have on the road.
Obviously, tire selection and its influence is dictated by car design & determined before
you get behind the wheel, but after the road surface and the tire factors are managed,
we're left with the last factor, the pressure between the tire and the road. This is
where a good driver uses vehicle dynamics to his or her advantage.
Remember Newton and his First Law? The car's inertia will resist the forces you apply
with brakes, throttle and steering. You experience this as a perceptible leaning of your
car opposite the direction you are encouraging it to go. Push the throttle and the vehicle
weight shifts to the rear, increasing the weight on the rear wheels. For a rear wheel drive
vehicle, this is useful and reduces wheel spin as you accelerate. This is also why many
of our cars have wider tires on the rear wheels to increase grip for acceleration. For a
front wheel drive, however, this weight shift moves the weight off the drive wheels and may
induce wheel spin or hop if done abruptly. In a similar way, when you apply the brakes,
the weight of the car shifts forward (and you do too, pushing you against your seat belt)
applying more weight to the front wheels. This is why most vehicles have larger brakes in
the front than the rear, as the front wheels apply most of the stopping force. When steering,
the vehicle's weight shifts towards the outside wheels of the turn, improving their grip, as
it shifts off the inside wheels, reducing their traction.
In a perfect world, the reduced grip experienced by some wheels during these maneuvers
would be exactly offset by the improvement experienced by the other. Alas, this is not
the case, with the most traction obtained when the car is at constant speed in a straight
line. Any change will upset this balance and degrade the total traction present. Your
car's suspension is designed absorb bumps in the road while keeping all 4 tires in contact
with the road surface, and yet keep the car as level as possible. The driver's job is to
control the car in such a way as to optimize the functioning of the suspension. The more
lean experienced, the bigger the loss of traction provided by the sum of all 4 tires, which
is why high profile trucks won't corner as well as a go-cart. As you operate the controls,
realize that the balance of the car will change with control inputs, and allow the weight
shifts to occur, and the suspension to reach steady state. The smoother you apply
steering/throttle/brakes, the less it will upset the suspension overall, and the better
traction you will have to control the car. It also means that if the weight is shifted to
the rear tires while you accelerate, there is less weight on the front tires for steering.
This is why your car will drift toward the outside of the turn as you accelerate out of a
corner, a phenomenon called "throttle steering". Let off gently on the throttle, more
weight will transfer to the front tires, and the steering angle will increase. We'll explore
this more when we examine oversteer and understeer.
One last facet of traction to consider before we move on is that there is a certain
maximum traction available, and you can use that for accelerating, steering, or slowing,
or a combination of steering with either of the other two, up to that maximum. The more
you use for steering, the less is available for speed changes and vice versa. If you
exceed that maximum, your tires will break loose and slide. This is why you'll hear
Wendy Walker warning you against "braking and turning". While advanced drivers do this
as "trail braking" you risk loss of traction and control if not done properly. It is
sometimes suggested to imagine a sting from the bottom of the steering wheel to your
right foot, so as you turn the wheel more it will lift your foot off the brake or throttle.
Applying more "pedal" will similarly pull on the string and tend to return the wheel to
a neutral position. I like this image because it reinforces that what your hands and feet
do is interconnected and each affects the other.
Now that your head is spinning, go clear it with a spirited drive and start preparing
for your next track session!
Keep the shiny side up and the greasy side down.
Greg
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