Driveline Geometry 101 - Continued
General Application

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TOOLS, SETUP & TESTING

To measure these angles, you'll need a tool called an inclinometer. Every driveshaft or chassis shop will have one or more of these, and anyone doing chassis work should own one as well. You can spend more for more accuracy, but most hardware stores will sell you a perfectly serviceable tool for around $15. I purchased mine from Eastwood, also your local Sears store has them.

To check this geometry, you'll need to have the truck sitting on a flat surface, wheels pointed straight ahead, and with the truck complete and carrying whatever weight it carries under most conditions.

Setting up or correcting driveline geometry will involve shimming transmission mounts, putting angled shims between the rear axle mount pad and the springs, or otherwise rotating the rear axle to achieve the correct angles.

CRITICAL ANGLES

To get right to the point, the angle formed between the transmission output shaft and the driveshaft should be equal but opposite the angle formed between the driveshaft and the pinion. In Fig. 1 (ABOVE), this is shown by ANGLE 1and ANGLE 2. You'll notice that the reason these angles are the same is because the transmission output shaft and the pinion share parallel to each other. In practical terms, this also means that the angle between any flat shop floor (whether or not it is level) and the output shaft and pinion shaft will be the same.

Figure #1

Although not always obvious, proper driveline geometry is based on the fact that there must be at least one degree of static angle between the transmission tailshaft and driveshaft and the driveshaft and the pinion. This is required so that the U-joints will oscillate back and forth enough to push grease around inside the bearing cups. If no angle were maintained, the action of the roller bearings would eventually wipe the U-joint cross-shaft dry and destroy it. You may have seen the ribs on U-joint crosses. Called "brinneling", it indicates wear and lack of roller bearing rotation around the cross. This condition is terminal for the U-joint. Because of this pre-set joint angle, the bearing cup not only rotates in a circular motion, but oscillates from front to rear twice per driveshaft revolution .

You can check this measurement by placing your inclinometer atop the center of your driveshaft and observe the angle. Next, place the inclinometer on the transmission tailshaft with the flat surface of the inclinometer perpendicular to the output shaft. Use an old yoke to get a good flat surface to measure the angle. Record your reading. Subtract the driveshaft reading from the transmission reading. An angle greater than 1 degree will achieve proper bearing lubrication.

Now, repeat this procedure on the pinion. An angle of four to five degrees is usual for the transmission tailshaft and pinion. Your goal should be equal, but opposite angles. In order to adjust components to achieve the proper cancellation, shimming of the axle and transmission mounts maybe necessary. [more]

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