All the Angles

Imagine a world without universal or constant-velocity joints. Engines and transaxles mounted rigidly to the drive wheels with nothing but gears between crankshaft and tire tread. Older techs have seen vehicles like this — older Volkswagen Beetles and pre-1965 Corvairs, while everyone has seen the most typical four-wheeled example, go-carts.

In the real world, however, even swing axles can’t generate a car that handles well without the ride of a forklift truck. Until the long-predicted hub motors and electric/fuel cell vehicles arrive, drive shafts and axles will be flexibly jointed, which means wear and breakage.

There’s a law in physics that basically says “there’s no free lunch,” and that applies to drivelines too. Universal and CV joints aren’t perfectly efficient, so part of the energy used to drive them is lost as heat. Generally speaking, the greater the angle, the greater the heat and wear, so anything that disturbs the factory angles will have an impact on driveline wear. Chronic overloading, sagging springs, badly repaired collision damage and even worn and broken engine or transmission mounts can play a part in premature failure, especially in driveshaft U-joints in light trucks and SUVs.

While CV joints in front wheel drive and IRS RWD models are dominated by failures due to loss of grease via a torn boot, “open” universals are a different story. With needle bearings taking the primary torsional load plus shock loads from the transmission output shaft and tire grip, there’s a lot of torque acting through a very small surface area. To spread wear and load over a greater surface area (more needles) the joint naturally “rocks” the spider back and forth as the shaft rotates, with the amount of needle bearing movement increasing with increasing driveline angle. It’s not all bad, since a zero angle would mean no needle bearing movement and “brinelling” of the bearings.

What does this have to do with joint service? The actual rotational speed of a conventional driveshaft isn’t constant, but varies with increasing driveshaft angle. In fact, it cycles up and down contributing to driveline vibration, one of the reasons why many high-end vehicles use constant velocity joints in RWD drive shafts. Reducing or eliminating noise and vibration in typical light truck and SUV applications (as well as the few RWD sedans on the road today) means dealing with a few key factors:

1. Angularity. Because the rotational speed varies with increasing joint angle, less is better here. Lift kits, heavy duty springs, aftermarket air suspension or sagging and lowered springing, changes the factory angles. The important point to remember is that angularity is measured relative to the fixed component in the system, which is the transmission/transaxle. Oversize wheels and tires, for example, only increase angularity if the axle centres are lifted relative to the output shaft. Similarly, drag-race style rake won’t affect angularity as long as the axle/output shaft relationship remains constant. The driveshaft doesn’t care if the whole chassis is tilted, just the front and rear joints.

2. Offset. The driveshaft’s front and rear U-joints are phased to cancel out the rotational speed variation generated by each joint. Since the amount of speed variation created by each joint depends on its angle, the angle of front and rear joints must be the same, regardless of the total offset between tail shaft and pinion flange. The less offset the better, but not if it’s achieved by altering the joint angles (usually by changing the transmission mount or rotating the axle housing) so they’re not the same relative to parallel. Like angularity, this is measured with the vehicle’s weight on its tires.

3. Balance. Drive shafts are factory balanced, but there are conditions where that balance can be compromised. The simplest is undercoating clinging to the shaft, but a common problem is also dented or bent shafts, especially modern thin-wall aluminum units. Damaged drive shafts should be replaced, while modified (usually shortened) shafts should be rebalanced.

Naturally, U-joint service is still the first step. If you must replace worn joints, remember to mark the yoke’s position relative to the pinion flange and to properly support the yoke when pressing out caps. They’re not as tough as they used to be, so leave the sledgehammer under your bench.