When you hit the throttle on your Mopar's engine, the carburetor(s) dumps fuel into the engine, and the spark plugs ignite this fuel, in-turn, spinning the flywheel. The flywheel then transfers energy through the transmission, which then transfers that inertia through the driveshaft to the differential, spinning the tires and putting the power to the ground—or putting your tires up in smoke. Theoretically, the driveshaft is simply a link between the transmission and the car's rear end. While this is basically true, a driveshaft does a lot more than simply act as a connector.
To many people, a driveshaft is just a piece of tubing with a yoke at each end that is designed to accept a U-joint. For the best performance and highest level of shaft strength, the shaft itself needs to be as straight as possible, with the yokes properly aligned. This alignment of the yokes means a driveshaft is considered in phase. When the shaft is out of phase, one of the yokes is twisted in relation to the other. To make a good driveshaft, you need to build a shaft that is straight, has the yokes in proper phase, and uses appropriate material. But what material is appropriate for your Mopar? We thought you guys might want to get a little lesson in driveshafts, so we took a trip to Dynotech Engineering in Troy, Michigan, to get the low-down on spinning tubes.
A driveshaft is a driveshaft, right? Maybe back in 1970 that was the case, but now you have a choice of several different types of shaft material, depending on your application. Combine these different types of material with the endless combinations of slip yokes, u-joints, and pinion yokes, and there are literally hundreds of combinations for a custom driveshaft. When talking about driveshafts, there are a few terms that you should know about.
This is a measurement of how straight your driveshaft is as it rotates about its diametric centerline (axis of rotation). Tube run-out is a direct contributor to driveshaft balance. If you have too high run-out (bent tube), your driveshaft will not balance without welding excessive weight on one side. Dynotech recommends tube run-outs at 0.020 inch or less over the entire length of the tube. During both the weld and balance phase of the driveshaft development, run-out should be checked and minimized (Drawing A).
U-Joint Phasing and Clearance
This can play another significant role in the build and balance of your driveshaft. During the assembly of your driveshaft, you must make sure that the U-joint is centered in the weld and slip yoke. If the U-joint is off center, the entire weight of the driveshaft is rotating out of the driveshaft's centerline. In addition, if you do not have your U-joint phasing correct, you will create a severe vibration as a result of the non-constant velocity of the standard U-Joint. U-joint phasing is the alignment of the weld yokes during the build process. The centerline of the U-joint hole on the weld yoke at one end will be in line with the centerline of the U-joint hole on the other end. Dynotech uses specialty fixtures to ensure that the driveshaft is built with less than 0.0025 inch joint clearance after the U-joint installation and weld yokes are phased to within 0.1 degrees from one end to the other.
As you can see in the image,...
As you can see in the image, the universal receiver yokes that are getting welded to the driveshaft tube are aligned so that they are oriented in the same direction.
This is the final ingredient in making a good driveshaft. Dynotech balances all of their driveshafts to 0.20 oz-inches or less, and they tell us that they expect their measurement to repeat within 0.05 oz-inches. An ounce-inch is the standard balance measurement unit that relates to imbalance. Balance speed is another key element of driveshaft manufacturing. Your driveshaft should be balanced as close to operating speed as possible. Whatever imbalance is generated at 2,000 rpm is increased four times at 4,000 rpm, and nine times at 6,000 rpm. Even the smallest imbalance force at slow speed can make your car shake at high speed.