The following is for rear wheel drive cars with beam rear axle:
To obtain maximum launch performance, the dynamic loading of the driving tires must be
known. This knowledge can be obtained with onboard instrumentation, but many racers find
this method too costly.
This is the third procedure I have presented whereby this information can be obtained
in your shop and in a comparatively inexpensive manner. Pages 4, 5, and 6 describe a
procedure similar to that which was employed in the automotive industry before the use of
full chassis fixtures. Its advantages include the gathering of many data points and the
ability to place the car at any point within the full launch range. In other words, a
tether load could be imposed which would cause one front tire to have zero load. (This
would be equivalent to the "lifting" of that tire during launch.) The car would, during
this procedure, also squat or rise exactly as it would at the strip.
Its disadvantages, however, proved sufficiently large to discourage its use by many
racers. First, a structurally sound location must be available for the attachment of the
tether. If such a tether location is available, it is then necessary to find an
equally sturdy attachment point for the other end of the tether.
These disadvantages forced me to seek a simpler method. I considered using gravity
instead of a tether load. This resulted in the procedure of Page 36. I know engineering
students visit this site, so I'm not going to remove Page 36. They might find it
interesting. The small wheel scale changes and the necessity of finding a suitable
road grade, however, cause me to discourage its use.
This third procedure brings the testing back into the shop, is quick and inexpensive, and
requires a minimum of fabrication. Its disadvantages are that there is only one set of
wheelscale readings and the car does not squat, rise, etc. during the test. The answers,
however, provide everything needed to evaluate your present setup and any changes you
might make.
First, it is necessary to scale the car. It is necessary to rescale the car
before testing after any suspension change or adjustment.
As the page title indicates, this procedure involves the use of two chocks. I realize
you don't normally work with wood, but I found that these chocks can be quickly made
using 2x4's, half inch plywood, and lag screws. Cutting diagonally across the width of a
2x4, between lines spaced 6 and a sixteenths apart, yields a 30 degree angle. I recommend
30 degrees, but, if you end up with something else, you can input the value into the
spreadsheet.
After scaling the car, the rear wheel scales are removed and replaced with the chocks.
The tires are to bear entirely on the sloped surfaces of the chocks and are NOT to contact
the shop floor. Obviously, the engine must not be allowed to rotate under the load. With
an automatic, it is only necessary that the transmission shifter be placed in "Park." With
a manual transmission, it will probably be necessary to engage 2 gears or use some other
means to prevent engine rotation. Even with a fresh engine, air trapped in the cylinders
will leak by the rings and the car will slowly "walk" down the chocks. With the chocks in
place, note and record the front wheel scale readings for insertion into the spreadsheet.
As you've probably figured out, the purpose of the chocks is to provide a known torque
for the driveshaft. From the scale readings and the other parameters supplied in the
spreadsheet, it is then possible to calculate the equivalent weight transfer and the manner
in which the driveshaft reaction torque is distributed by the chassis. An extrapolation then
provides the tire loads when one of the front tires loses contact with the strip surface
during launch.
The goal, of course, is to equalize the launch loads for the left rear and right rear
tires. It is assumed that the user will follow the procedure, make an appropriate suspension
adjustment, and then repeat the procedure. The number of adjustments and procedure repeats
is determined by the user's adjustment skills and accuracy requirements.
Finally, don't concern yourself excessively with the angle accuracy of the chocks or the
center of gravity height. What you're concerned with is the loading difference and
that difference is going to be evident with less than "perfect" values for these parameters.
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