Driveshaft Doin'sTwo problems prompted me to install a new driveshaft. The tighter yoke requires a larger U-joint than used by my stock shaft. Researching information provided by Bob Parquet, manager of education for Dana Corporation's Spicer Driveshaft Group, I learned that since modifications increased my car's top speed, my existing, OE-type, 3-inch diameter, 57 13/16-inch-long, steel driveshaft had a reliability problem unrelated to vibration.
Anything made of elastic material-and metals are elastic-has a resonant bending frequency. When a rotating shaft's speed approaches its resonant frequency, the shaft begins to vibrate as if it were unbalanced. If allowed to spin for very long in that speed range, known as "critical speed," the shaft will fail catastrophically. Don't be in the car when that happens.
Another noteworthy industry standard is "safe operating speed," defined as 75 percent of critical speed. Engineers (along with corporate risk management staffs, we figure) believe a driveshaft can run continuously at safe operating speed and be unaffected by resonant frequencies. Critical and safe operating speeds are a function of a shaft's length, diameter, stiffness, and tubing wall thickness. Using a calculator on the Spicer Driveshaft Web site (www.dana.com) I found that the safe operating speed of my Malibu's shaft was 4,370 rpm. Dividing that by 0.75, I got a critical speed of 5,827 rpm. A slide rule version of this calculator is available at many Spicer dealers.
For a given driveshaft speed, vehicle speed is a function of axle ratio (in this case, 3.73:1) and tire diameter (255/50ZR16 F1 GS at 26.06 inches). Using the formula: speed = (tire diameter x driveshaft rpm x .002975) (axle ratio), I determined at the shaft's critical speed, vehicle speed would be 120 mph. As I'd seen 120 a few times, it was a miracle my Malibu's driveshaft hadn't blown right through the floor.
To eliminate this danger, Inland Driveline built us a 57.25x3.5-inch driveshaft out of .120-wall aluminum tubing and fitted it with larger Spicer U-joints (PN 5-447X). Our new shaft's critical speed is 6,976 rpm, or 145 mph, and its safe operating speed is 5,232 rpm, or 109 mph. Though it was larger, as it was made of aluminum, there'd be no weight penalty. Inland machined the TH400 yoke to the correct length for the Malibu's transmission, a transplanted ZF S6-40 six-speed out of a '92 Corvette, and installed it on the new shaft.
Bent HousingWe pulled the rearend a third time to install an Inland Driveline pinion yoke which fits the larger U-joints, so new suspicions I had about the axle's effect on our vibration could also be addressed. After the second rebuild, pinion rotation felt rough, as if there was a bad bearing. Tom's Differentials told me to drive the car 500 miles, and if the trouble persisted, call again. I believed there was a problem right then-time to try a different service.
Jim Cook at Performance Differentials in Ontario, California, tore down the axle. As I suspected, the rear pinion bearing was damaged due to either incorrect preload or improper installation of the bearing race. Worse yet, Cook found my axle housing was bent by .125 inch, which could cause a vibration.
Cook straightened the housing and installed new pinion bearings and the new pinion yoke. We put the rear back in the car, filled it with Red Line Heavy Shockproof Gear Lubricant, installed the new driveshaft, and went for a road test. I didn't get very far when, crossing a railroad track, I heard a knock on the floor.