Rear Suspension Guide - Drag Racing And Simulation

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 vertical adjustment to the fixed instant center location. The turnbuckle adjuster on the lower bar allows for easy changes to pinion angle.
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 use of a track bar, which triangulates the two ladder bars to prevent lateral rear axle movement. Track bars can also be successfully used on parallel four-link systems.
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 front adjustment holes for the upper and lower bars. There are four holes for the upper and lower bars, but most racers discover their car will generally operate best with one or two particular combinations.
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 different four-link IC points (crosses) that can be generated by this four-link suspension. The center of gravity (A), the instant center (B), and the 100 percent antisquat line (C) are all included to make the visualization easier. You can actually download a demo version of this program directly from performancetrends.com to see how it works.

This screen capture from Performance Trends' simulation program illustrates all the differ

Suspension Simulation
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.