From a stoplight, the guy next to you gives you "the look," and we all know what that means. As the light turns green, you put the pedal to the metal. In the ensuring launch, the truck shakes violently, bucks, hops, and heaven forbid, you stay on it, and eventually hear a loud "BANG" from a grenaded U-joint, followed by a "clink clink" as the driveshaft hits the pavement Perhaps if you were smarter, you would have known that the axle wrap was giving you traction problems, and you should have backed out of the throttle. But poor judgment (and perhaps a little machismo) kept you from doing otherwise.
So what causes this axle wrap phenomenon? Well, it’s a combination of tremendous amounts of torque from the engine, combined with a leaf spring type rear suspension, and perhaps taller tires. Axle wrap is occurs during heavy acceleration, when the axles apply torque to the rear wheels. As the tires try to turn forward, the axle is fighting to keep itself from turning backward (causing the pinion to point upwards). The only thing preventing the axle from turning upwards are the leaf springs. As the leaf springs load, or "wrap" around the axle, the springs will reach a maximum deflection, and invariably unload itself. The unloading process is less gradual, as it snaps back into place. Now, as the tires fight for traction, the axle is loading and unloading, giving you the sensation that the rear end if hopping up and down, which it really is. As the pinion angle changes, it is not longer in alignment with the driveshaft, stressing the universal joint (u-joint). With all this happening in a cyclical fashion, and the driveshaft under heavy load, the fuse in this situation will be the u-joint, as it grenades and no longer transmits power from the transmission to the axle.
There are two solutions to this problem: never put your foot to the floor, or install a set of traction bars, also known as ladder bars. Most people choose the latter option, and Four Real Steel, a division of Daystar, has one of the best products in the market. Daystar, working with Off Road Unlimited, has designed a full floating traction bar system for the Ford Superduties (F-250 and F-350). They work with both single- and dual-rear wheel configurations.
The purpose of the traction bar is to prevent the pinion angle from changing during heavy acceleration. The pinion rolling up may be felt as a vibration in the cab as the u-joints are temporarily out of alignment. As the acceleration decreases, the axle rolls back into place, and the vibration is gone. That being said, proper pinion angle is critical to the life of the u-joint. So the key to keeping the proper pinion angle is to find a fixed point on the axle and locate it with a bar(s) to prevent it from rotating about itself.
The full-floating design from Daystar works a great deal better than a single bar setup. A floating design allows the suspension to cycle through its full range of travel without binding. Binding inevitably causes a harsh ride, as the suspension no longer functions properly to dampen out the road irregularities. The part that makes this traction bar full-floating is the use of a yoke near the frame attachment point. As the suspension cycles up and down, the traction bar is able to flex up and down, in addition to changing lengths simultaneously to prevent binding. However, to keep the axle from wrapping, it has two attachment points on the axle, and one on the frame (per side). The two fixed points on the axle is what prevents the axle from wrapping. This provides the same functionality as having a shackle on the upper frame mount, but without the added complexity.
The design works out great. The bracket on the axle is a weld on piece, many times superior to a bolt on style. The bolt on style, when used in conjunction with high torque motors (read: diesels), will slip, and cock the axle in a loaded, pinion-up position – which is a BAD thing.
After installing the Four Real Steel traction bars, a long drive down the highway proves that it did not alter the ride at all. Foot-to-the-floor excursions resulted in a smooth, albeit with a lot of wheel spin, launches, without any tell tale signs of axle wrap such as wheel hop (or worse, broken u-joints). A few days later, at the NHRDA season opening sled pull event, the bars were really put to the test. With the Superduty hooked to a 45,000 pound sled, a diesel motor putting out in excess of 800 foot-pounds of torque, and a lead footed driver, the bars worked flawlessly. They kept the rear axle in perfect alignment with the driveshaft, never changing the pinion angle, even at the end of the track, where the weight box of the sled is fully loaded on the sled pan, and the 40 inch tires on the truck feverishly clawing for traction.
The final verdict? Four Real Steel’s Traction Bars are an awesome product. The powder coated parts (available in both black and silver) are a great addition to any rig, and the work perfectly to prevent any axle wrap. They are plenty strong to hold up to the excessive torque put out by a diesel truck, lifted to make matters worse! Even when they are pushed to their design limits during sled pulling.
Now, the next time someone gives you the "look," you sure as hell will be ready.