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SUV STABILITY

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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 An example of an oversteering car is the outdated "Beetle" which has a rearward weight bias.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.

 

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