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Page Nomenclature

c.g. = center of gravity

H =  c.g. height above datum, in.


c.g. height, H  and dynamic c.g. shift

We will now investigate the significance of c.g. height (H) above the surface datum. We  will continue along this path for a while until the formulas are introduced. Let it be sufficient to say at this stage that increasing height of the c.g. is very damaging to good vehicle stability on a curve. To visualize this phenomenon, look at Fig. 3.

Fig. 3

Vector analysis, a special branch of math, has revealed that this imaginary car has a very dangerous property. Under certain conditions, this car will upset on a turn. In this case above, the vehicle has entered the 100 foot radius curve at 33.5 mph, the same as in the previous example. However, this car has a c.g. too high or a track too narrow, and the c.g. has moved from the center more than is tolerable according to the laws of physics. You will notice that the dynamic c.g. has moved across the tip-over line. The left side wheels are weightless, the total vehicle weight is on the overloaded right hand side tires, and the whole car has begun to roll over to its right. The driver cannot now correct the situation and the rolling moment will be self-increasing. This car has cashed in. 

Adequate safety factor 

The interesting thing to note now is that this car may have been driven for years with no problems. If the speed limits on curves were observed and the driver never had a real emergency, he would not even have known this situation above was going to happen. The c.g. would of course move back and forth laterally as on any car, but unless he really pushed his car to the limit, the c.g. would have never moved enough to contact or cross over a tip-over line. Adequate SF ensures the dynamic c.g. shift will remain within safe limits under all driving conditions encountered on the road. A table will be provided further along in the web to quantify what is adequate SF.

Just to show that this fatal situation above is perfectly predictable mathematically without actually risking a test driver, I will tell you that this car above has a track, T, of 57.6 inches, same as the previous safe car example, but differs because it has a very high c.g., where H = 42.4 inches. It is not likely you would find a car offered to the public with such a configuration, but if I can conceive of a modified pickup truck that has been jacked up (because it looks "cool"), then loaded, to be this tipsy. The distance the c.g. has traveled outside the tip-over line is 3 inches. It has a negative SF, thus, this vehicle deserves to labeled UNSAFE.


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