[andy669]

NHRA rules General Regulations section 2:4.

.....all cars in competition using open driveshafts must have a retainer loop 360 degrees of enclosure, 1/4 inch minimum thickness and 2 inches wide, or 7/8-inch x .065-inch welded steel tubing, securely mounted and located within 6 inches of the front universal joint for support of the driveshaft in event of U-joint failure.

Andy