Muscle Mustangs & Fast FordsHow To Drivetrain
Quick Tech: Driveline Diagnostics
Driveline vibrations are easy to feel, but solving them can puzzle even the most methodical of knuckle-busters. Does your car have the highway vibes? Just sending your driveshaft out for a balance job may not work…or make things even worse. To minimize frustration, here are some tips.
Check your driveline angles: because of geometric hocus-pocus, the input and output velocity of universal joints (U-joints) varies with operating angle. If the input velocity of a U-joint is constant, the output will speed up and slow down twice per revolution. But, a universal joint angled in the exact opposite direction will cancel out this phenomenon, and the output velocity will match the input.
So just make sure the tail shaft of the trans is the same angle as the pinion, and we’re good, right? Well, not exactly. If your car has rubber bushings, leaf springs, or any type of non-parallel rear control arms, the pinion angle will change under load. So, you need to compensate for this by having the pinion angle nose down a couple degrees, depending upon your rear suspension setup, and on much the pinion climbs under load. The goal is to have the pinion end of the driveshaft at the same angle as the transmission end when driving.
But, there’s also another catch (we told you this was complex). The universal joint operating angles (the angle between, say, the transmission and the drive shaft or the pinion and the drive shaft) need to be kept at a minimum, usually 3-degrees or less. This minimizes vibrations of the drive shaft between the U-joints.
Summary: Keep operating angles less than 3 degrees, and get the input and output angles equal under load.
Check run-out: Have you ever picked up a vibration after changing rearend gears? Not only is the drive shaft spinning at a different speed, but also the pinion flange may be causing the vibration. On most cars, there’s wobble that stems from the transmission output shaft and pinion flange, and after changing rear gears, the pinion has a different wobble than before. This is called run-out. Ever notice the dots that are painted on pinion flanges and driveshafts from the factory? A drive shaft with perfect balance will vibrate when connected to a transmission or pinion with excessive run-out, so manufacturers make drive shafts that are imbalanced on purpose! When a slightly imbalanced driveshaft is oriented correctly on a pinion flange or transmission output shaft correctly, the drive shaft imbalance cancels out the run-out.
Check your transmission output shaft and pinion flange for run-out using a dial indicator. If you have more than a couple thousandths of run-out on either part, try indexing the driveshaft at different positions on the flange and transmission output shaft until the vibration is reduced. You can further compensate for run-out by placing weights (washers) under one of the driveshaft bolts, or have weight added to your driveshaft.
Balance the driveshaft: Notice this the last thing to do. If your driveline angles are within spec, your transmission output shaft and pinion flange run-out measurements are only a couple thousandths, the last thing to do is have the driveshaft checked and balanced.
Thinking outside the box: Today’s popular overdrive transmission swaps don’t fit really well in the narrow tunnels of classic cars, so often the only solution is to lower the tail shaft of the transmission for clearance. This may make it difficult to keep the driveshaft operating angles under 3-degrees. To tackle this problem, some creative driveshaft builders use a constant velocity (CV) joint in the shaft, which can operate at higher angles while the input and output velocities are equal (hence the name). These shafts allow the transmission end of the shaft to operate at a greater angle, while pinion angle can be set close to zero at the differential end without regard to the transmission output shaft angle.