On another page, I describe the traction dyno and explain the principles
behind its use. Here, I'll give an example as to how the traction dyno output would be
Before the traction dyno is "hooked up," the car would be scaled. Suppose the following
is the result of that initial scaling:
The scale readings are usually expressed in the following order:
Then, the chain of the traction dyno is tensioned, with the following results:
It can be seen that 500 pounds have been transferred from the front tires to the rear
tires. It did not, however, transfer evenly! Whereas the RR was originally 50 pounds
heavier than the LR, it is now 150 pounds lighter. This is due, of course, to the action
of the driveshaft torque on the rear axle assembly.
So, a total of 150 + 50 or 200 pounds has been switched from left to right. If we
divide that 200 by the weight transfer (500), we see that, for every pound of weight
transfer, the LR tire loading grows by 0.4 pounds over the RR.
Suppose, further, that we have calculated the total weight transfer to be 1000 pounds.
(See another page for the procedure.) This would mean a total of 400 pounds difference
between LR and RR. Since we would want the rear tires equally loaded at this maximum
launch acceleration, it would be necessary to alter the original wheel scale readings so
that, after the driveshaft torque has been applied, we would have the equal rear tire
loading. If the car has adjustable coilovers, the following could be achieved:
Under maximum acceleration, the loadings would be:
(The preceding assumes the center of gravity is on the centerline of the car in plan
view. From a practical standpoint, this is a pretty good assumption, for, unless the car
is very light, it's difficult to move the center of gravity, laterally, to a very great
extent. If, in this case, the CG was moved 10% to the right, the 975 loading for the RR
could be reduced to 875, but I can't appreciate any extensive effort to accomplish this.)
With a competition 4link, preloading of the links will improve the static wheel loading
and, more importantly, decrease the loading shift due to driveshaft torque.
It should be noted that the above procedure provides equal rear tire loading ONLY at a
single value of driveshaft torque. With an asymmetric linkage (see other pages), it is
possible to totally eliminate driveshaft torque effects and thus provide equal rear tire
loading with any value of driveshaft torque. Still, it is wise to use the traction dyno to
verify your calculations.
(As mentioned in the page describing the traction dyno, the horizontal chain should,
ideally, be at the same height as the center of gravity. Unfortunately, this might not
always be convenient, so the chain might be either higher or lower than the ideal. If it's
within a couple of inches, I wouldn't worry about it, but a correction can be made if it's
markedly off. We shall assume that, in the above example, the chain was at the CG height.
If the chain was then moved down to half the CG height, the results would have indicated
that, for every pound of weight transfer, the LR would have grown by 0.8 pounds over the
RR. In other words, this indicated LR to RR difference value of 0.8 would have to be
corrected by multiplying by the ratio of actual chain height to CG height.)