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Suspension Basics

chevelleThis Chevelle shows what a high-HP GM A-Body car can do at the starting line without the necessary modification. An anti-roll bar would greatly improve this car’s launch.

 

Drag racing has always been popular with enthusiasts who build fast street cars. The entire “street/strip” concept has been popularized by the automotive aftermarket for decades, and is still touted by the biggest enthusiast magazines and Web sites alike. The makers of premium aftermarket performance parts have straddled the line between street and strip for years, but never as well as they can today. One look at the latest drag radial tires or double-adjustable shock absorbers will drive that point home!

To that end, making a traditional front engine/rear drive American car more capable of planting the rear tires and launching hard at the track is an essential part of the street/strip formula. Working with the basic factory foundation (rear leaf or coil springs teamed with front coils, torsion bars, or struts) and making it work as well as possible is still challenging. 

Let’s discuss the car’s physical movements while going from standing still (at an idle) to accelerating at wide-open throttle (WOT). Whatever make of car you have, the same principles apply. To further explain this discussion, the photo represents a typical car making lots of torque without proper suspension modifications.

Notice the way the car lifts the diver-side front tire, and leaves the passenger side front tire on the ground. This blatant body twist affects the rear tires as well. The body roll will transfer more weight to the passenger-side rear tire, planting it harder than the driver’s-side rear tire. This gives the car less-than-maximum traction while not launching straight.

The typical car drives itself to the right. Many drivers are so accustomed to this happening, they don’t even realize how it affects the car. If your car leaves the starting line like the car in the photo and you feel the car is leaving straight, you’re wrong. Have a friend or crew member with a telephoto camera zoom in on your hand on the steering wheel while you launch. You then see how much steering correction you must add to launch the car straight. Because it’s a natural reaction for a driver to correct the wheel, and because the car probably evolved to this point over time with gradual gains in power, correcting the wheel has become part of the launch ritual. You probably don’t even realize it’s happening.

Why does this twisting happen to a car that is perfectly level at the starting line? The answer is physics. For every action, there is an equal and opposite reaction. Imagine you are standing at the front of the car, looking back. I will start at the front of the car and work toward the rear, explaining how each movement is an action, followed by a reaction.

First of all, torque (not horsepower) makes the car accelerate from a dead stop at a fast rate of speed. Horsepower comes into effect after the car is well on its way and higher in the RPM range. Second, the car is not rolling counter-clockwise because the engine is pulling it in that direction, but rather because of a reaction.

sparkplugignitionWhen the spark plug ignites the fuel/air mixture in the combustion chamber there is an equal force on the piston to go down as well as an equal amount of force trying to lift the engine block. But it can’t because it is bolted into the car.

The spark plug ignites the compressed fuel and air in the combustion chamber, driving the piston down. This rotates the crankshaft clockwise. That is the action. The illustration on this page shows the action of the exploding charge in the combustion chamber, which is driving the piston down and forcing the crankshaft clockwise while at the same time creating an equal force on the cylinder head and block trying to rotate the engine in an opposite (counter-clockwise) direction. That is the reaction. Being attached to the frame rails, the counter-clockwise rotation of the block is trying to rotate the car along with it. The more torque being generated, the greater both the action and reaction are. The more reaction there is in the chassis, the less action there is on the piston. Therefore, a chassis with enough preload to prevent body roll has more down force on the piston, allowing for more force on the crank, and therefore more usable power.

Since the crankshaft is rotating in a clockwise direction, the torque converter or clutch (and transmission) is also rotating in a clockwise direction. The transmission, with its different gear ratios, multiplies that torque even further.

For example, let’s use a typical GM Turbo 400 automatic transmission with a first-gear reduction ratio of 2.48:1. That means the transmission’s output should be 2.48 times its input from the converter or clutch. If your engine produces 700 ft-lbs of torque at the flywheel, then the transmission output should be 1,736 (700 x 2.48 = 1,736) pounds of torque in first gear, minus the parasitic loss in the transmission.

The clockwise-rotating transmission and driveshaft also rotate the rear axle’s pinion gear in a clockwise direction. Once the pinion gear rotates in a clockwise direction, another action-versus-reaction is set up. Because the pinion gear is rotating clockwise, the entire rear-end housing tries to rotate counter-clockwise (when viewed from the front of the car, looking rearward). This causes the housing to push down on the passenger-side rear tire. If the rear passenger-side tire is being forced down (loaded), then the driver-side rear tire is being lifted (unloaded), and the housing is trying to lift that tire off the ground. Since the passenger-side rear tire is being forced to the ground by the rear-end housing rotation, less resistance is being offered against the weight transfer from the driver-side front corner. This seems good, as more weight transfer would appear to be the goal, but less resistance is not the best way to make use of this weight transfer.

piniongearAs the pinion gear turns, the pinion teeth mesh with the ring gear teeth and turn the ring gear, which then turns the axles.

As the clockwise action of the pinion gear is attempting to rotate the ring gear, a second reaction is occurring as it pushes down upon the ring gear. Since the ring gear is bolted solidly into the housing (at the bearing mounts), this action now causes the rear-end housing to try to rotate clockwise. The action of the pinion turning clockwise creates two opposing reactions, and this causes wheel hop (the rear tires jumping up and down). Without enough resistance to weight transfer, the housing goes in one direction until it pushes that tire down as hard as it can. Then it rebounds and the other reaction takes over and forces the housing into the other direction. This continues as one reaction wins, and then the other, over and over again.

upanddownpressureAs with a piston, creating equal up and down pressure, the action of the pinion gear turning the axles, wheels, and tires in a forward direction create a reaction with the rear-end housing trying to roll the pinion up and the rear end out of the car.

The pinion gear rotates the ring gear, axles, and tires in a direction that either spins the tires in a forward motion, or if the tires hook, moves the car in a forward direction. The reaction—the rear-end housing is trying to rotate in the opposite direction, attempting to roll the pinion upward and the rear-end housing out of the car. This can’t happen since the rear-end housing is bolted into the car’s chassis. But, what does happen is that the rear-end housing and its mounts (including the control arms, ladder bars, or leaf springs) rotate on an arch (pivoting on the front leaf-spring mounts, the front ladder-bar mounts, or the imaginary intersection of the extended upper and lower control arms) based upon the type of suspension system. The pivot point of the arch can be modified to achieve maximum traction.

rearviewShown from the back of the car the action of turning the pinion gear clockwise (as viewed from the front of the car) creates a reaction with the housing trying to turn counterclockwise (as viewed from the front of the car) and loading the two tires differently. Thus the need for preload in the car’s suspension. 

With the tires moving forward (without any wheel hop) and the passenger-side rear tire being loaded along with the diver-side rear tire  being unloaded, why doesn’t the passenger tire push the car to the left instead of to the right as it does? This is due to tire circumference, as shown in the bottom illustration on this page. The passenger-side rear tire being loaded (forced down) becomes shorter. Since both tires are being forced to roll in a forward direction at the same rate of speed, this allows the taller side tire (with more circumference) to push the car to the right. This justifies the need for chassis preload (tricking the car to plant both rear tires equally) to move the car forward in a straight line without intervention by the driver. The right amount of chassis preload should cause the car to plant both rear tires identically, creating equal tire circumference on both sides and causing the car to launch straight. For a good wheels-up launch you need the rear tires to be driving the car in as straight a line as possible.

actionreactionThis drawing summarizes the discussion that for every action there is a reaction. With the engine turning the transmission and pinion gear clockwise the rear axle housing is trying to turn counterclockwise. Also, with the engine firing and igniting the gas/air mixture, the piston is being forced down and the block is being pushed up. With the block connected to the frame the entire body twists, lifting the driver-side tire 6 to 12 inches higher than the passenger-side front tire.

When tuned to its maximum, a drag car’s suspension effectively transfers weight and power without wasting energy. You can learn to do this with any domestic front engine/rear drive car regardless of make or model.

 

Author Dick Miller has decades of experience making drag cars of all types work, and he shares his knowledge in the pages of How to Hook and Launch: Traction Mods for Street and Strip. In it, he explains the basic dynamics and physics of what is occurring when you launch your car from a standing start, and how to use the various energies to your advantage. What follows is an excerpt from this book.


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