Page 5
UNDERSTANDING THE OUTPUT
FROM THE TRACTION DYNO SPREADSHEET

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WARNING!!!


Use of a traction dyno involves a very high tensile loading of a horizontal chain attached to the car. If the chain were to break or if its attachment at either end...to the car or to the anchor point...were to fail, the chain could inflict serious injury or death.

It is strongly recommended that a safety chain be attached near each end of the chain under load.

See Page 37 for another... possibly more convenient...form of the traction dyno. (The procedure on Page 36 can also be used, but I would recommend the Page 37 procedure, as it allows all work to be done in the shop.)



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 interpreted.

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:

                                          LF          RF
                                          LR          RR


                                          850        800
                                          750        800

Then, the chain of the traction dyno is tensioned, with the following results:

                                          500        650
                                         1100        950

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:


                                          1025      625
                                           575      975

Under maximum acceleration, the loadings would be:

                                          325        325
                                         1275       1275

(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.)