At this speed, the shaft is likely to experience lateral bending that will make it try to leave the vehicle. Sustained operation at or above this critical speed means certain and rapid failure of the shaft will occur. It is important that a racing driveshaft be specified to ensure that its critical speed is above the normal operating rpm range of the vehicle.
For example, a Late Model two-barrel car operating below 7,000 rpm can safely use a 3.0-inch shaft, but a Hooters Pro Cup Late Model operating near 8,500 rpm cannot. If in doubt, your driveshaft distributor and/or manufacturer can recommend the best combination to avoid any critical speed issues.
12. DIAMETER SELECTION BY SELECTION
In most cases, a series' rules will dictate the desirable diameter. For the NASCAR Sprint Cup COT, Busch car, Craftsman truck, and ARCA car, the driveshaft is required to be 4.0 inches. The USAR Hooters Pro Cup Series cars and the NASCAR Busch East and West Series cars typically use a 3.5-inch shaft with 1350 U-joints for critical speed reasons. Low-horsepower short-track Late Models should use such robust shafts as well, but most still use the weaker 3.0-inch driveshaft and 1310 U-joint combination.
13. MINIMUM THICKNESS
For NASCAR and short-track steel driveshafts, there are also several choices for tubing thickness. The lightest and most expensive thickness is 0.065 inch. This helps reduce rotating mass to help the race car accelerate faster out of the corner. For durability and cost reasons, some teams specify a thickness of 0.083 inch. These shafts are somewhat heavier but give crew chiefs added peace of mind when tackling the rigors of concrete short tracks.
For heavy-duty applications, such as drag racing and off-road trucks, shafts with even thicker walls may be preferable. Once again, consult your driveshaft distributor and/or manufacturer to get the best recommendations for your needs.
NASCAR and other race sanctions require a driveshaft to be painted white. The reason for this goes back to the days of true stock car racing, when driveshaft failures were more common. If a car lost a driveshaft on the track, the drivers following often could not see the dark and heavy shaft lying on the black asphalt and would run it over, turning it into a deadly missile or crashing themselves.
A white driveshaft is very visible and can be easily avoided. Of course, today, driveshafts leaving a car are very rare and an errant lightweight driveshaft would probably crumple like an empty soda can. But this requirement still exists as a safety feature.
The U-joints shown are the 1350 (left) and the 1310. The 1350 has a 45 percent larger bear
15. U-JOINT SERIES IDENTIFICATION
Two of the most critical choices to be made are size and construction of the U-joints. It is necessary to specify a racing-quality U-joint of sufficient strength to survive a full season of hard use. Street cars and low-horsepower Late Models typically use a 1310 series U-joint.
Heavier trucks and stock cars with high horsepower use the larger 1350 series part. The 1350 U-joint has 45 percent larger bearing cups and a thicker cross to better handle the load. A racing-quality U-joint does not have a grease fitting and does not need periodic greasing. The hole for the grease fitting in a U-joint weakens the strength of the cross and becomes the immediate failure point for a highly stressed joint.
16. OUTPUT SHAFT BEARING IDENTIFICATION
Most stock car racing aftermarket transmissions use the tried-and-true GM 32-spline output shaft design to interface with the driveshaft. Even though this allows the slip-yoke specification to be standardized, there are still choices to be made.