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On page 18, I describe the setup for an asymmetric rear suspension which would dynamically cancel the driveshaft torques
that upset the rear tire loadings of a beam axle RWD car as it accelerates. On this page, a change of the front suspension,
which accomplishes the same thing, will be described.
As weight is transferred from the front of the car during launch, the front suspension springs extend and the front of
the car rises. This front end rise, during forward acceleration, is unavoidable. If the right front spring has a higher rate
than the left front, any rise of the front of the car will remove more weight from the right front than from the left front.
Since the sum of the right front and right rear wheel loads must remain a constant (or the car would be in the process of
rolling over), this means that most of the weight transfer will go to the right rear. Since the front end rise is proportional
to the driveshaft torque and since the driveshaft torque is tending to unload the right rear, it follows that there is a particular
value of the ratio of right front spring rate to left front spring rate...for a particular car...which will provide equal
rear tire loading for any value of driveshaft torque.
For the following procedure to be totally effective, NO front swaybar should be used.
With the car on the scales, record the readings. These values will go into the "original" boxes in the spreadsheet. The "jacked"
side can be either the right or left.
Place a jack anywhere under the side of the car, but preferably near the middle of the wheelbase and as far from the centerline
of the car as is conveniently possible. Jack the car until the wheel scale values are appreciably changed. (See the example
in the spreadsheet.) Record the scale readings. These values will go into the "after jacking" boxes in the spreadsheet.
Measure and record the front and rear tracks for entry into the spreadsheet. When you click the "Compute" button, you'll
see a "ratio of spring rate difference to spring rate sum" appear at the bottom of the page. If, for instance, the RF spring
has a rate of 800 pounds per inch and the LF spring has a rate of 600 pounds per inch, a value of (800-600)/(800+600) or 200/1400
or 0.143 would be the equivalent ratio. With the values for your car in the spreadsheet, spring selection based on the calculated
ratio will result in complete dynamic cancellation of the driveshaft torque and equal rear tire loading will be assured.
Unfortunately, a change of only one spring will result in a change of roll stiffness distribution. In other words, BOTH
springs should be changed with the goal of having the sum of the two new rates as close as possible to the sum of the two
original identical springs.
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