The driveshaft loop must be placed in a location that allows it to effectively restrain the shaft and be replaced on a regular basis. A two-piece bolt-together design can make driveshaft removal much easier. The loop is typically placed about two thirds of the length of the shaft from the front U-joint and must be large enough to allow a smooth insertion of the shaft into the transmission while the rear axle is at full drop. A little thought expended here during the mock-up phase can save a lot of frustration on Saturday afternoon.

The mock-up phase of car assembly is a good time to begin checking driveshaft alignment. Position the engine to meet all of its location constraints for best weight distribution and ensure the rear axle location meets wheelbase requirements.

Using a tool like the True Laser Track System, you can make sure that the rear axle and the transmission output shaft are aligned in two planes and that the resulting U-joint angles will be as close to zero as possible at running ride height. Any significant offsets in alignment should be corrected at this time. The new True Laser Track Driveline Alignment tool is very helpful in determining how to reposition the major components to bring them into alignment.

Your test day is now scheduled and your new car is finally coming together. Your new engine is sitting on the floor, that new transmission you first saw at PRI is now here, and your gear shop has brought over the correct rear ratio. If your mock-up process went well, it should all bolt together on that shiny new frame. But many variables and adjustments can affect the final choices you need to make before ordering that critical last link in the drivetrain-the driveshaft.

First, you must research the appropriate rules for your racing series as they apply to the driveshaft. For example, in most cases, NASCAR will specify a required material ("magnetic steel"), a minimum diameter ("no less than 4.0 inches"), a minimum thickness ("no less than 0.065 inch"), and a required paint color ("painted white"). These specifications have been implemented for safety reasons and are based on industry recommendations for critical speed capability and durability for your series. But there still are several choices to be made before you can order the right shaft.

If your rules are not specific about material, you can choose from a bewildering menu of steel, chrome-moly steel, aluminum, MMC aluminum, or carbon-fiber composites. Low-cost short-track shafts are typically steel or aluminum. High shock load applications such as those found in road racing and drag racing will demand chrome-moly steel or MMC aluminum shafts for durability. The lightest and most expensive choice for professional road racing and dirt track racing is the carbon-fiber composite shaft.

A racing driveshaft can be made in a variety of tubing diameters, from typically 2.5 to 4.5 inches. The correct choice for your application is best determined by understanding the concept of critical speed. Critical speed is an engineering calculation based on the tube's thickness, material, and length. This calculation determines the rpm at which a given shaft enters its critical speed zone.