Big Chevys from '58s to '64s utilize a two-piece driveshaft. There is a universal joint in the middle as well as a center bearing to support the shaft. This means there are more opportunities for parts to wear out or fail, and thus more things to repair. Stick with us while we give you some pointers on what you can do with these drivelines, what you can't do, and how to rid yourself of this frequent and often frustrating repair job.
Why did the big Chevys from '58 to '64 have two-piece shafts, and can you replace the two-piece with a one-piece? To answer this question we need to look no further than the overall design of the '58 Chevy. The '58s were longer and lower than the '57s. To accomplish this an X-frame was used, prompting the use of a small-diameter two-piece shaft in order to clear the frame. Replacing the two-piece shaft with a large-diameter, one-piece version would necessitate cutting the X-crossmember to gain clearance, weakening the already flexible frame. The doors may begin to hang up, and the roof may even begin to buckle. Unless you plan to build a completely new tube frame similar to a race car chassis, leave the two-piece in place.
The two-piece shaft with a center support was necessary because of a thing called "critical speed" which, if ignored, can cause catastrophic driveline failure (see figure 1). Critical speed is the rpm at which a driveshaft will destroy itself and is determined by tube diameter, weight, and length. Smaller diameter, greater weight, and longer spans lower the critical speed; larger diameter, lighter weight, and shorter spans raise it. Shaft speed alone produces this characteristic failure; horsepower and torque are not important considerations.
The driveshaft center support originally contained a natural rubber cushion which was very durable, although it was sensitive to oil. Believe it or not, we see some of these cushions still in service 40-plus years later, although many of them have been replaced with a synthetic material. The synthetic isn't bothered by oil, but it weather-checks and disintegrates rather quickly causing the driveline to "jump rope" within the metal housing, resulting in severe vibration. Incorrect reassembly, however, is the leading cause of failure in this part, regardless of the material (rubber or synthetic).
In some of these stock-configuration '58-64 big cars the yoke holding the front portion of the two-piece driveshaft was intended to be at 90 degrees to the front U-joint (at the transmission end). This controlled secondary couple loads. The rear half of the driveshaft then went on the required angle to properly connect with the differential. If your '58-64 big car has the stock rearend setup, and you are experiencing a severe vibration between 12 and 20 mph followed by a lesser one between 30 and 40 mph, it means that the yokes are in line rather than at 90 degrees. This will drastically shorten the life of the support.
A safe and long-lasting repair technique for these center supports has been developed by Inland Empire Drive Line. Beginning with the correct alignment of the yokes and U-joints, a reinforced center support using a polyurethane cushion is installed (figure 2). A slipping member has been added between the urethane cushion and the driveshaft. This absorbs fore and aft travel caused by the rearend because of acceleration, braking, or changes in level on hydraulic-equipped cars. This slip member will not work with the synthetic cushion since that material does not hold the bearing tight enough.
When the '58 -64 big cars were new, the 409s were the biggest engines available, and they were brutes. The original natural rubber support and small-diameter shafts worked well (figure 3), but as we now see installations of blown big-blocks and injected small-blocks putting out major amounts of power, along with overdrive transmissions using locking torque converters, it may be tempting to consider re-engineering the driveshaft. Such a move might even be brought on by hearing that critical parts are out of production. Rest assured, these parts are and will continue to be available allowing owners and builders of these classic cars to update the driveline safely and without unexpected, dismal, and irreversible consequences.