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Page 10
DISCUSSION OF LAUNCH CHARACTERISTICS

The following pertains, specifically, to rear wheel drive cars with beam axles.
 
As you're probably aware, as the car starts from rest at the dragstrip, the forward thrust at the rear tire patches, acting with the rearward inertial force at the center of gravity, results in a couple that unloads the front tires and loads the rear tires. That inertial force is, of course, proportional to the product of the weight of the car and its forward acceleration. What you might not have considered are inertial reactions resulting from other accelerations. Due to the rear suspension geometry, the rear of the car might be accelerating either upward or downward. And, since the front of the car will always be rising, there is most likely a rotational acceleration to be considered.
 
First, the rotational acceleration: If the rear of the car rises as much as the front, there will, of course, be no rotation, but this would certainly be a rare situation. So, with the rotation normally expected, the first effect will be to decrease weight transfer. As the rotation reaches its maximum value, however, the rotational inertia of the car tends to increase the weight transfer. The result is a "hit" on the rear tires that can, during a wheelstand, exceed the total static weight of the car. But, before we get too excited about this extra weight transfer, we must remember, first, that it was preceded by a decrease in weight transfer, and, second, that this benefit lasts for only a fraction of a second. The car quickly settles into its final position and all losses and gains, from the rotational acceleration, are gone.
 
The vertical acceleration of the rear of the car, like the rotational acceleration just described, lasts for only a very short time interval. And, within that small time interval, there are included both gains and losses.
 
If the rear of the car squats (is accelerated downwards), the first effect is to unload the rear tires. As the rear of the car reaches its lowest point, rear tire loading is increased.
 
If the rear of the car rises (is accelerated upwards), the first effect is to increase the load on the rear tires. As the rear of the car reaches its highest point, the rear tires are unloaded.
 
So, there are both gains and losses to be considered with either situation and they all take place within a very short time period.
 
If all of the weight transfer is taken through the suspension links, the rear of the car will neither squat nor rise. This requires a particular geometry. Specifically, the instant center, as viewed from the side, must fall on a line which passes through the rear tire patch and the intersection of two other lines, one a horizontal line through the center of gravity and the other a vertical line through the front tire patch. (Actually, it passes about an inch and a half below the rear tire patch when the weight of the rear axle assembly is taken into consideration. Most reference books ignore the rear axle assembly weight and place the line through the rear tire patch, which, for all practical purposes, is perfectly valid.)
 
I realize there are those of you who prefer a certain amount of squat and others a certain amount of rise. Personally, I believe maximum traction is achieved when transient loading changes are minimized. So, I would endeavor to eliminate squat and rise and minimize rotation.
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