The real way to make that suspension work is to apply this science to your street car.
It's never been easier or cheaper to make power. But power isn't the endgame--it's acceleration that makes heroes. Have you ever wondered how those cars with 8-inch tires are able to consistently blast low 9s and make it look effortless? Sticky tires are a big part of the plan, but you can bet that the suspensions on these cars have been heavily massaged. Low e.t.'s don't happen by accident, which means the more power your engine makes, the tougher it will be to manage that power all the way to the ground. Applying this knowledge also requires that you crawl under the car and learn a little about rear suspensions--what they are, how they work, along with a few tuning secrets the good racers use to push their cars quicker and faster. So break out those suspension wrenches, find the pizza delivery guy's number, and follow us under the car.
This Olds launch is a classic example of body twist that illustrates (to the extreme) how
Twists And Turns
It's important to understand how the rear axle moves in relation to the car before we can get into how each different rear suspension operates. Let's look at what happens when you drop the hammer on your 500hp street car at the dragstrip. Not every twist is as it appears. As torque is applied from the driveshaft to the rear axle, multiple forces begin to leverage the car. Engine torque multiplied by the transmission's First gear ratio and the rear axle ratio is equal to several thousand lb-ft of twisting motion. The first thing the pinion gear tries to do is climb the ring gear. This forces the nose of the rear axle upward. As the car begins to accelerate, the torque leverages the front of the car upward, causing weight transfer to the rear. As viewed from the rear of the car, engine torque twists the body clockwise, lifting the left front and compressing the right rear (passenger-side) spring. As the pinion continues to apply this massive torque through the ring gear, the rear axlehousing is also being leveraged in a counterclockwise direction as viewed from the rear--lifting the right (passenger) side of the axlehousing while planting the left. As the car accelerates, it appears to be planting the right rear tire when in fact axle torque motion is unloading the tire, reducing traction. That is why a car equipped with an open differential will spin the right rear tire even under light acceleration. Limited slips are used to improve traction, but as you can see, they are merely a Band-Aid on the real problem. By using proven chassis modifications and tuning techniques, it is possible to equalize the load onto both rear tires.
One simple, inexpensive way to reduce axle tramp or wheelhop on any GM or Ford leaf spring
The classic leaf spring suspension has been around since the early 1800s with horse-drawn carriages. The advantage of leaf springs is that they are simple to design, and the springs also serve as the locating points for the rear axle. Disadvantages begin to appear when massive torque is applied to leaf springs. It's difficult to control spring wrapup, which creates the dreaded wheelhop that most factory leaf spring-equipped cars experience. Let's get into what happens when we plant gobs of power through a pair of leaf springs.
Applying big power through a pair of multileaf springs generally creates what is called spring wrapup. First of all, leaf springs are designed to bend, but lots of torque tends to deflect the forward portion of the spring into an S shape. When this bend becomes severe enough, the spring binds and then bounces the tire off the road, which relieves the tension in the spring. The tire then returns to the pavement, and the process repeats itself with a nasty shudder. This violent wheelhop can quickly damage axles, housing mounts, and shock absorbers and even yank the driveshaft out of the transmission. The earliest solution for this problem was a traction bar that placed a rubber, cone-shaped snubber just below the leading end of the leaf spring. When the spring begins to wrap up, the snubber contacts the spring and prevents wrapup. While this works, there are other, more elegant solutions.
Mopars are noted for not needing traction bars, and if you study how a Chrysler leaf spring is designed, you understand why. All GM and Ford leaf springs are symmetrical, centering the rear axle between the front and rear spring eyes. Chrysler engineers cheated this deal by moving the axle mount toward the front of the spring. This shortens the length of the front segment of the spring, which increases stiffness and minimizes the effect of spring wrapup. Chrysler also placed a small rubber bumper (called a pinion snubber) just above the flat portion of the rear axle pinion area, which limits the amount of vertical pinion travel.
Here is a classic illustration of spring wrapup where torque has created an S bend in the
While the leaf spring is still around because of its simplicity, there are drawbacks. The springs themselves are heavy, which contributes to the car's unsprung weight. This is defined as the weight not supported by the car's suspension. From a dynamic standpoint, less unsprung weight is an advantage. Because of their weight and size, leaf springs are also more expensive compared with coil springs. There are composite material leaf springs available that do a great job of reducing weight, but they're also more expensive.
Another important step to help control unwanted rear axle movement is to invest in high-quality suspension bushings for the front and rear. Polyurethane is a popular and inexpensive upgrade, but you should consider the virtually bulletproof aluminum insert Del-a-lum bushings first created by Global West almost 30 years ago. The aluminum bushings use a Delrin insert that prevents metal-to-metal contact, enhancing wear while also offering near-zero deflection. Global offers these bushings for all popular performance body styles. If you're going to go fast, these bushings are an excellent investment.
Factory Coil Spring
The most popular factory rear suspension design for solid rear axle cars is the coil spring system. Under the coil spring umbrella are a number of subtle design variations that make coil spring suspensions more attractive to suspension tuners, compared with leaf springs. Because the coil spring's only job is to support the weight of the vehicle, designers still needed a way to locate the rear axle under the car. This necessitated control arms (also called trailing arms). The simplest OE coil spring rear suspension is the four-link. This design uses two parallel lower control arms located near the outboard ends of the rear axle. The two upper control arms are angled outward instead of parallel to the chassis. This creates a triangle that locates the rear axle laterally (side to side) under the car, eliminating the need for a Panhard bar or Watt's link. Popular examples of this rear suspension can be found in cars such as the '64 to '72 GM A-bodies and the '79 to '93 Ford Fox Mustangs. While the coil spring four-link system is more complex than a leaf spring design, it enjoys numerous inherent advantages. First off, the system is usually lighter than leaf springs. More importantly, leaf spring wrapup is eliminated, although wheelhop can still occur if the rear ride height is raised excessively.
This is the result of ultimate spring wrapup where the tire literally bounces off the pave
With a true parallel four-link rear suspension, the links form a right-angle box that allows the rear axle to move from side to side underneath the car. This system is most often used in drag cars and requires the addition of a Panhard bar or Watt's link (these will be described later in this story), which limits rear axle lateral movement. The main advantage of factory four-link rear suspensions is that the rear axlehousing is securely located. If you view the car directly from the side, the housing pulls on the upper links under acceleration, placing them in tension while pushing the driving force through the lower links. This pushes on the links, applying a compressive force. The point where all this force comes together is something called the instant center (IC--see Suspension Simulation sidebar). This point can be moved by adjusting the position of either the upper or lower links. Factory four-links do not offer adjustability, but aftermarket add-on components such as the original Lakewood No-Hop bars change the IC by raising the rear upper link position, which shortens the IC. Generally, this creates a situation where the car's body actually rises on acceleration.
Adding a bolt-on traction bar helps to prevent spring wrapup, which in turn keeps the tire
In early '70s NHRA Pro Stock racing, the rules required stock-type rear suspensions. Chrys
This is a CalTracs bar created by John Calvert and used here on his original '68 428 Cobra
A recent addition to the CalTracs lineup is the split-mono spring. This is not only lighte
As production cars became wider and lower in the '80s, the classic four-link suspension evolved into the torque arm rear suspension most widely used in the third-generation, '82 to '92 Camaros. It is still a coil spring rear suspension, but the upper control arms were replaced with a single long arm that bolts between the nose of the rear axle and the transmission tailshaft. Aftermarket torque arms such as those from BMR, Edelbrock, and Global West strengthen the arm and relocate it to a much stronger transmission crossmember, which then can be tuned (moved) for IC modifications. Because the triangular four-link upper arms are eliminated, a Panhard bar is required to locate the rear axle laterally under the car. Torque arms can be used successfully in drag race applications, but on cars running quicker than 10s, it's rare to find a third-gen Camaro still sporting its factory torque arm.
A fourth variation on the coil spring suspension hit parade is the three-link. As you have probably surmised, this design relies on a single upper control arm mounted on the top of the rear axlehousing. Obviously, a Panhard bar or Watt's link is also necessary to laterally locate the housing. Chevy used this configuration in its '58 to '64 fullsize cars and more recently in '05-and-later Mustangs. The advantage is the rear suspension is allowed to roll laterally with minimal bind, although a potential downside is that it places the entire upper bar tension into one single mount, which may have to be reinforced when applying serious power to the ground.
The most popular factory coil spring rear suspension is the four-link, using splayed upper
Lakewood originated the No-Hop bar that relocates the rear upper control arm off the rear
Dick Miller Racing (DMR) and others sell an adjustable antiroll bar (arrow) that bolts to
BMR makes this interesting torque arm conversion for early Camaros and Firebirds that conv
The three-link has returned under late-model Mustangs as seen here under an FR500CJ Ford d
Late-model Ford Crown Victorias use a parallel four-link rear suspension, relying on a fac
Drag Race Rear Suspensions
The demands of drag racing created a few interesting twists on the factory rear suspension designs. The most popular drag race rear suspension is the ladder bar combined with coilover shocks. This system entails a triangulated bar with two attachment points at the rear axle and a single connecting point at the chassis. The advantage of the ladder bar is that it usually does not require floorpan modifications to fit into the car, needing only a custom crossmember to attach the leading mounts. This also makes suspension tuning easier, since there are generally only three or four adjustment holes that reposition the bar's vertical placement. Ladder bars are most commonly used with coilover shocks but can also be employed in conjunction with leaf springs if you use a set of rollers or sliders that allow the leaf springs to roll through a different arc than the ladder bars. Sliders and ladder bars are most often found in NHRA Super Stock racing where rules prohibit the use of coilovers.
The classic parallel four-link system is virtually a standard in the faster drag racing classes because of its much wider adjustability. The limitation of the ladder bar is that it can only adjust the IC within a limited vertical range. The four-link radically expands the number of possible IC positions, since the upper and lower bars can be repositioned independently, offering dozens of possible locations. This allows the chassis tuner the freedom to move the IC vertically and fore/aft in the chassis. For more information on IC tuning, check out the Suspension Simulation sidebar.
The ladder bar is an axle-mounted triangle with a single forward pivot that allows easy ve
This is a Competition Engineering ladder bar system mounted in a rear subframe. Note the u
You can see how many more adjustments there are with a four-link by counting the number of
This screen capture from Performance Trends' simulation program illustrates all the differ
We've mentioned instant centers (IC) and percentages of antisquat several times, so they deserve to be defined. The rear suspension IC is the calculated point at which the forces of the upper and lower control arms meet. Its position is determined by extending the horizontal lines projected by the upper and lower control arms of a four-link rear suspension until these two lines intersect. If you study the Performance Trends simulation illustration below, the IC point is plotted at its static location (B)--but keep in mind that as the suspension moves under acceleration, the IC also moves. The IC is not always an imaginary point, however. The most obvious example of this is a ladder bar where the front pivot is the IC or a leaf spring car where the IC is the front spring eye. When adding CalTracs bars, the IC becomes the intersection of the two lines projected by the leaf spring and the lower bar. On a torque arm car, the arm's front pivot point is the IC.
Knowing the relative position of the IC is important because it is the true leverage point for the application of power from the rear suspension to the car. Performance Trends offers a computer program that simulates the IC position for your car after you input a few rear suspension measurements. Then, merely by changing the pickup points on the simulation, you can visualize where the IC falls and its relationship to the 100 percent antisquat line. If you look at the program's screen capture, the 100 percent antisquat line is the diagonal dotted line drawn from the rear tire contact point on the ground extending upward toward the point where the vertical line formed by the front axle centerline intersects with a horizontal line created by the car's center of gravity (CG). The lower the car's CG, the lower the angle of this 100 percent antisquat line.
When the IC is positioned above the 100 percent antisquat line, the rear suspension lifts as power is applied. Conversely, the rear suspension will squat if the IC is located below that 100 percent line. Another important variable includes the distance of the IC ahead of the axle centerline. Increasing the distance of the IC from the rear axle creates a longer effective lever arm. With a longer lever, power is applied to the rear tires over a longer duration. Drag racers define this as a softer initial hit. A harder hit can be generated with a shorter IC length. There are many more details about all this than we have space to discuss, but this should give you a taste as to what's involved with rear suspension design.
Insufficient rear shock travel is a common cause of tire spin. Generally, this occurs afte
Just bolting on the best suspension isn't the end of the story. There's still the necessity of tuning the rear suspension. Adjustable shock absorbers are almost a necessity if you are chasing that optimal 60-foot time.
Let's start with some basics. The term shock absorber is really a misnomer. It should be more accurately called a damper, because the device is designed to dampen or regulate spring motion. Shocks are rated by their resistance to motion in compression (bump) and extension (rebound). Most car crafters know that a typical 90/10 drag race front shock is easy to pull apart and very stiff to compress. This design allows the front end to extend easily and then stay up to assist weight transfer. But what you really want is for the front end to rise at the proper rate on the starting line and then quickly settle to keep the nose low at the top end to reduce aerodynamic drag. Equally important is the ability to adjust front and rear shocks to create the effect you desire. Most single-adjustable shocks create changes only in the rebound direction. The more expensive but better approach is to choose double-adjustable shocks that can tune compression and rebound separately.
Let's use a leaf spring Mopar as our tuning example. When the driver hits the throttle, the rear suspension separates, planting the rear tires. But let's say this hit is too harsh, crushing the sidewalls of the tires and causing them to spin. By slowing the rate at which the rear shocks allow the body to rise with a stiffer rebound, the chassis tuner can tweak the rate of torque application to the rear tires. This slows down the application of load, making it easier on the tires, which improves the 60-foot times. On the front end, let's imagine that it actually tops out too quickly, slamming up against the upper bumpstops almost instantly. When this happens, the car will sometimes porpoise, which unloads the rear tires and creates a loss of traction. Stiffening the front shock rebound slows the rate of front end rise, eliminating the porpoise action and generating a quicker run. These are just two simple examples of why it's necessary to tune both ends of the car to optimize traction.
|Art Morrison Enterprises||Global West Suspension|
|253/922-7188||San Bernardino, CA|
|BMR Fabrication|| |
|Thonotosassa, FL||Heidts Hot Rod Shop And|
|Lancaster, CA|| |
|Riviera Beach, FL|| |
| ||Guilford, CT|
|Chris Alston's Chassisworks||800/544-8894|
|cachassisworks.com||Mr. Gasket (Lakewood)|
| ||Cleveland, OH|
|Dick Miller Racing||216/688-8300|
|Eaton Detroit Spring||248/473-9230|
| ||Lakeville, MN|
| ||Morton Grove, IL|