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Rear Suspension Tech Guide

All That Monster Power Is Of Little Benefit If You Can't Put It To The Pavement. Traction Is The Key That We Unlock In This Rear Suspension Tech Guide

Photography by Brett Kepner, Courtesy Of Competition Engineering, ,

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.

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.

Leaf Springs
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.

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