Page Nomenclature |
c.g. = center of gravity |
Center of gravity
Before a vehicle can be calculated for
stability, one must clearly understand the term center-of-gravity, c.g. Do not be
concerned with simplifications, the equations to be used can determine the
required information to within a few percent; this presentation is not intended to be a substitute
for a formal math or physics course. Fig. 1 below illustrates what is
known as center-of-gravity.
Fig. 1
Static c.g.
The top view of the car is shown stripped of
its structures and its wheels are turned at an exaggerated angle. The c.g. is an imaginary point where all the mass of the vehicle can be
considered to be concentrated. In the illustration c.g. is shown in two of its
three dimensions: longitudinal (length) and lateral (width). Longitudinal
position of c.g. is an important factor in its steering behavior because it
helps determine whether a car "understeers" or "oversteers"
in turns. Understeering is desirable when a car is pushed to its limits in
curves and this car has a forward located c.g. at about 35% from the front
datum and definitely understeers. An example of an oversteering car is
the outdated "Beetle" which has a rearward weight bias. Notice that the lateral position is on the
longitudinal centerline; that is, centered left and right. That is to be
expected if the car is perfectly symmetrical and remains at rest or is
in constant-velocity linear motion.
Intuition tells us that if the driver alone takes a seat the c.g. will be
shifted slightly to the left. Sometimes it is hard to visualize the c.g. by
the conventional description, so I will try a different approach: If one could
take a single hydraulic jack fitted with a pointed ram and place it directly
below the c.g. symbol, on an imaginary jack pad, one could raise the entire
car off the surface. It would just teeter there. All its weight would be on
the jack at one tiny point.
Tipover lines
Now notice the two lines connecting the front
and rear wheels at each wheel-to-road contact point. Since the car is
symmetrical, there is one line for the left two wheels and one for the right
two.
These two lines are 57.6 inches apart laterally at the longitudinal position
of the c.g. These lines are important for they establish a boundary limit for
the c.g. beyond which the car will tip over. This may seem mysterious because
the c.g. as shown is about as far away from the lines as possible. But this
can change as will be soon seen. It should be noted that the distance between
the wheel centers on an axle is termed the wheel track (T), often shortened to
simply "track".
Now for a taste of math. If this car weighs
2600 pounds, I would expect the two front wheels to carry 845 pounds each and
the two rear ones 455 pounds. It is a light station wagon with a rear wheel
drive. We will find out later why it would be better to have front wheel
drive.