There’s more than meets the eye when it comes to setting up and measuring for a U-joint equipped drive shaft. While the driveshaft allows for movement of the rear end there are also important operating angles and shaft length criteria that must be adhered to. Exceeding these limits can lead to driveline vibration and/or failure, transmission damage, and can also reduce the amount of power being transmitted to the rear wheels.
In a perfect world the centerline of the transmission output shaft would be exactly parallel to the pinion centerline on both vertical and horizontal planes (but not “pointed” at each other). But for most applications, there will be some offset. Centerlines should be parallel, within 1/2 of a degree (see diagram below), the operating angles of the front and rear U-joint would be the same. The combined angles should not exceed 5 degrees.
A simple method that can be employed to ensure that the trans output shaft and pinion are parallel is to place a digital angle finder on a machined surface of the engine block (i.e. oil pan rail etc.). Using a floor jack, raise the front or rear of the car as needed to bring the angle finder to zero (note: if the rear needs to come up jack under the housing). Then place the angle finder on the face of the pinion yoke. If this angle is anything but zero, the pinion angle must be adjusted to bring it to zero.
Driveshaft length is also very important. The function of the slip yoke is to compensate for suspension movement. A shaft that is too long can bottom-out the slip yoke in the trans, causing damage. A shaft that is too short doesn’t provide the yokes needed support and has potential to bind. The shorter the shaft (4-link dragsters in particular) the more likely this can occur.
To properly determine optimum driveshaft length it’s important that the car is on the ground with the suspension set to racing conditions and all weight should be in the car, including the driver, when measuring. The technicians at Mark Williams Enterprises like to get as much information as possible, so that measurements can be cross-referenced to assure accuracy.
A. End of trans yoke to end of pinion yoke B. End of pinion yoke to U-joint center C. U-joint center to U-joint center (make sure that your measurement includes 3/4" to 1" of slip) D. End of trans yoke to U-joint center E. Transmission seal to U-joint center (most common & easiest to measure) F. Trans seal to pinion seal G. Trans seal to end of output shaft
M-W only builds driveshafts utilizing 1350 or 1480 series U joints. 1350 series joints will handle the majority of drag racing applications while the 1480 series is recommended for ultra high horsepower applications.
If you are in the process of building a new race car it’s best to wait until the final phases of construction before getting a driveshaft, as experience has shown that transmission changes, etc. can occur and disrupt the best-laid plans. Should you have any questions, feel free to call M-W’s tech reps toll-free at 866-508-6394.
