Rock, But Don't Roll

National Dragster, Feb 27, 2004 by Smith, Evan J

One of the hottest trends in drag racing is to install a rear anti-roll bar. Virtually any new vehicle has one, and now they're on everything from stock-type doorslammers to tube-chassis Pro Stock machines and even Super Comp dragsters. An anti-roll bar, or anti-sway bar as it's also called, is a torsion bar. When installed in a drag race car, it does a pretty important job.

As the name implies, an anti-roll bar limits chassis roll, which enhances launch and vehicle stability. Anti-roll bars are used in a road-going vehicle to keep the car flat during cornering. By using an anti-roll bar, auto manufacturers can install softer springs to allow for a smoother ride yet maintain good handling in the turns.

Front-mounted anti-roll bars are attached to the car's lower A-arms by links at each end and to the chassis in the center. During a turn, the body will lean toward the outside, and the lower control arms will try to move in opposition to each other. The anti-roll bar will resist this motion, thus keeping the vehicle's body from rolling too much. In road racing, crew chiefs can change tire pressure, spring rates, shock valving, and the stiffness of the anti-roll bars to change the car's handling characteristics.

Aside from resisting the motion of the body during cornering, the front anti-roll bar limits chassis lift during acceleration and nose dive during - braking. That's why drag racers remove the front anti-roll bar in order to "free up" the front end and enhance lift for purposes of quicker weight transfer during launch.

The rear anti-roll bar at the other end of the car is often smaller than those on the front of a stock car. The rear bar will either be connected to the lower control arms or be attached to the rear housing and to the chassis or car's underbody.

In drag racing, the most efficient path from the starting line to the finish line is a straight line, and, for the most part, any motion other than forward motion is wasted and will slow the car's elapsed time - that is unless the motion or energy is used to help traction. Our goal is to limit unwanted motion because unwanted motion consumes energy that could be used for forward acceleration.

Once the burnout is complete and the car is staged, the car will be sitting in a neutral position with the weight divided over the four wheels. When the driver determines that it's time to go, he or she will do one of the following: release the transbrake, dump the clutch, or release the footbrake and floor the gas pedal (assuming the car doesn't have a two-step). In the next instant, during the period we term "launch," some amazing things take place under the car.

First and foremost, the driveshaft spins and applies force to the pinion and ring gears. If at this moment you could watch the-rear housing and suspension closely, and I mean closely (from a low angle behind the car), you'll see that the initial hit of power causes the left side of the housing (along with the left rear tire) to plant downward. This occurs as a result of the twisting or rotating force from the driveshaft as it applies torque to the ring gear. You will also see the housing be driven downward, also due to the torque applied to the ring gear.

In the very next moment, another reaction to the torque causes the chassis of the vehicle to "roll rotate," or twist to the right if viewing from the rear. This twisting force can be so strong that it can lift the wheel on the driver's side off the ground while the passenger-side tire stays planted to the track (see top photo). Despite all of this other movement, the torque makes its way to the tires and the car launches. The tires will dig in and turn forward, but because every action has an opposite and equal reaction, the forward rotation of the tires causes the rear housing to rotate counterclockwise to the rotation of the tires.

But we, being smart racers, have attached our suspension links (or leaf springs) to the housing, and the rotation causes the torque to be applied to the chassis. In turn, this causes the vehicle to transfer weight from front to back. In a perfect world, the torque will be applied equally to both drive tires, the weight will be transferred evenly over both rear tires, and the car will leave dead straight. Sadly, we don't live in a perfect world, and at times the rear tires or the chassis may be loaded unevenly.

Thankfully, the fix may be as simple as installing an aftermarket rear anti-roll bar. Quite a few companies sell them; some are universal kits, and others are specially designed to fit a particular chassis. Most kits will include the actual torsion bar, a system for mounting it to the chassis, and a series of links that connect the bar to the rear housing.

"For an instant during launch, you will have more or less torque on one tire or the other," said noted chassis builder Jerry Bickel. "When the car rolls to the right, the weight of the body will tend to plant the right rear tire, and that alone can equalize the traction at the rear tires. But even if the car doesn't roll-rotate, an anti-roll bar is still a good idea. By installing an anti-roll bar, you can help to equalize the torque because when one side [of the housing] tries to drop, the torsion bar will fight to keep the rear square."