Horsepower and torque numbers are great for impressing your car buddies. Even rear-wheel dynos are fun, because big numbers at the rear wheels will inspire even the most jaded enthusiast. But the final arbiter isn't flywheel torque, rear-wheel horsepower, or even the number of solenoids under your hood. The ultimate deal is how well your car can apply all this power to the ground. If you want to create shock and awe, hook all that torque through the two small patches where the rear tires meet the road, and the dragstrip groupies will beat a path to your garage door.
Different StylesWhen it comes to rear suspensions, there are many different ways to make it happen. We'll deal with solid, live-rear-axle applications going in a straight line for this story and leave the corner-turning tricks for another time. We'll specifically look at leaf springs, four-links, and torque-arms, which are the most popular systems for high-performance street machines.
Leaf SpringsLeaf springs are the simplest form of rear suspension since they both locate the rear axle and suspend vehicle weight. The idea dates back to a time just after the invention of the wheel. Leaf springs are both heavy and also prone to wrap-up under high torque loads, which wasn't a problem for our ancestors in Conestoga wagons. Spring wrap-up occurs when the leading end of the leaf spring bends sufficiently to bind the rear suspension, at which point it bounces the tire and wheel off the ground, causing wheelhop. This is an extremely violent torque reaction that can be easily cured with traction bars that stiffen the front spring section. Unfortunately, slapper traction bars also contribute to rear-suspension bind. The most popular leaf-spring traction devices are CalTracs or Competition Engineering's Slide-A-Link bars, which act as a lower control arm to prevent spring wrap-up while eliminating the bind.
Factory Four-LinkThis design exchanges heavy leaf springs for much lighter and more compact coil springs. However, this requires control or trailing arms to connect the rear axle to the frame. Factory four-links use a pair of lower control arms matched with a pair of uppers that are angled outward. This angling of the upper arms locates the rear axle laterally to eliminate the need for a Panhard bar. This is the rear suspension used in all GM A-bodies like the Chevelle and also the Fox and SN-95 Mustangs. This rear suspension operates roughly similar to a drag race four-link, where the upper and lower control arms are parallel to the framerails. We'll get into more depth with the drag race four-link shortly.
Torque ArmThe torque-arm suspension is the latest factory version of rear suspension evolution. The torque arm replaces the two upper control arms and is used to accommodate the application of power through the rear axle and to locate the rear axle. This requires the help of a Panhard bar to locate the rear axle laterally. One advantage to eliminating the two upper control arms is gaining valuable space between the rear axle and the body.
The Application Of PowerWhen power is applied to the pinion gear and into the ring gear of the rear axle, the pinion tries to climb the ring gear. When viewed from the front of the car, the clockwise twist of the pinion attempts to lift the right (passenger-side) rear tire off the ground and plant the left (driver-side) tire. This is the natural reaction of all rear axles to torque input. This also explains why drag racers place a certain amount of preload on the right rear tire to counteract this force. An example of this is the use of an airbag over the right rear axle that preloads the chassis to counteract this torque reaction.
At the same time that the axle is attempting to lift the right rear tire, the body is twisting in the opposite direction, which normally results in the body squatting over the right rear. All of this is the reaction to torque input. The more torque you apply or the more gear ratio you use to multiply the torque, the more twisting effort is applied to the chassis. Drag racers and suspension engineers have collaborated to create very specific ways to explain how all this happens and have also come up with ways to manage the power in a systematic fashion.
Instant CenterAll vehicles have a specific point around which the entire car will balance called the center of gravity (CG). For most domestic front-engine, rear-drive cars, the CG is generally located forward of the mid-point of the car at around camshaft height off the ground. While all rear suspensions pivot around a given point, this is not necessarily the point at which the rear suspension applies power or lift. Suspension engineers call this lift point the instant center (IC). Different suspensions place this IC at different positions in the car. Because suspension components tend to shift as the body lifts or squats, this position is dynamic, meaning that it moves as the car pitches or rolls. One definition of IC is the unseen center of an arc created by the moving suspension links. The simplest instant center is a drag race ladder bar. The forward mounting point for the ladder bar where it hooks to the chassis also happens to be its instant center. With other rear-suspension designs, the instant center is an imaginary point in space.
Kevin Gertgen's Performance Trends has created a drag race four-link computer simulation program called 4 Link that offers pictures that tell the story much easier. If you look at the illustration, you'll notice a pair of dotted lines that extend from the lines drawn by the two upper control arms and the two lower control arms. The intersection point of those two lines is called the instant center. The 4 Link program allows you to reposition the IC by moving the mounting points of the upper and lower control arms. Also notice the dotted line that extends from the rear-tire contact point forward at an angle. This line intersects a point created by the intersection of the horizontal CG line with a vertical line drawn through the front spindle. This angled line is called the 100 percent antisquat line, or sometimes called the neutral line.
By changing the location of the upper and lower four-link bars, you can move the IC location either above, directly on, or below that 100 percent antisquat line. When the IC is positioned below that 100 percent antisquat line, the rear of the car will squat on acceleration and "hit" the tires relatively softly. When the IC is positioned above the 100 percent antisquat line, the rear of the car will tend to rise on acceleration and "hit" the tires harder. Obviously, if the IC is placed directly on the 100 percent line, the rear will remain neutral.
This explanation holds true for all rear-drive cars, but there is plenty of confusion around the location of the instant center with different suspension systems. For example, with leaf-spring cars, the IC is the front spring eyes, but with ladder bars, the IC is the front pivot point. Factory four-link cars are determined exactly the same way as drag race four-link systems. If you extend imaginary lines forward on a factory four-link rear suspension, the IC generally falls in front of the car, well below the 100 percent antisquat line. This is why all factory four-link cars squat on acceleration. By installing the Lakewood anti-hop bars (for example), this kit raises the rear locating point of the upper control arms roughly 2 inches. This shortens the IC length and also places it above the 100 percent antisquat line, which now helps plant the rear tires.
ConclusionSo, what have we learned here? The main thing to take away from this rear suspension discussion is that there's more to improving traction than just dumping 50 pounds of ballast in the trunk. You can use specific suspension components to help you create optimal traction, but only if you understand how all these components work. This has been a primer intended to introduce you to the ideas around rear suspension science. There are dozens of other variables like weight distribution, engine torque, shock tuning, tire pressures, and of dozens more that contribute to improving traction. That's why this is as much art as it is science. But when you get it to work for you, your car will make you out to be a low-e.t. hero.
SimulationsThere are several computer simulations that do a great job of implementing the ideas around four-link rear suspensions. It might be useful to hit both of these Web site to investigate the information these simulations can deliver. The first is Performance Trends' 4 Link Calculator ($80). An interesting visual cue for this program is a side view of a drag-race doorslammer equipped with a four-link rear suspension. The illustration helps you understand how the instant center moves in dynamic situations and how the dynamics affect the car during the launch. Performance Trends even offers a limited but free downloadable version of the program you can use to experience it first hand.
The second program is by Racing Systems Analysis. FourLink ($169) produces a chart output that shows shock-absorber separation, force acting on the shocks, and rear tire load, along with average tire force and tire force variation in percentage during the first 60 feet. These outputs are great, because you're looking to produce the highest average tire force in pounds along with the least amount of variation in tire force applied to the pavement. One misconception is that once the force is applied to the rear tires, it is consistent throughout the entire run. The reality is that due to dynamic changes as the car accelerates, the IC moves, and this constantly changes the force applied to the rear tires loading and unloading. The idea is to minimize these force variations, making the car more predictable and stable as it runs down the track.
