This chapter explains the basic physics occurring on the starting line when you launch your car, along with why it does (or doesn’t do) what it should. Your understanding of what is happening can give you an advantage over your competitor. Put this advantage to full use, and your car will maximize its starting-line bite. Maximum bite means quicker and more consistent reaction times, as well as quicker and more consistent elapsed times (ETs).
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For Every Action There is a Reaction
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 on this page 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 on this page 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.
Remember to picture yourself standing at the front of the car, looking to the rear. Most novice racers assume that the car in the photo on page 8 has so much horsepower, the engine is lifting the front driver’s side of the car, transferring weight, and forcing the rear passenger side of the car to squat. What is really happening is much different! Let’s clear up that misconception.
First of all, torque (not horse-power) 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.
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-clock-wise) 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 (see Chapters 2, 3, and 4) 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. This is discussed in-depth in Chapter 4.
As the clockwise action of the pinion gear is attempting to rotate the ring gear, a second reaction is occurring as it pushes down uponthering 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.
The illustration above shows that 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. That is the action. The illustration at right (top) is 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 (see Chapters 2, 3, and 4).
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.
The quickest way from point A to point B is the straightest line possible. The illustration (below left) gives a summary of this chapter. Be sure you understand it completely before you proceed.
One thing that is very important, yet often overlooked, are the rear shock absorbers. Even if you have the right spring on your car, it is up to the shock to control that spring. As with most race car parts, you get what you pay for. An inexpensive set of shocks (such as the ones advertised as 50/50 or a three-way adjustable) should work on cars with as much as 300 to 350 hp. However, I have seen these shocks tested on a dedicated shock dyno and very seldom are they identical to one another out of the box. Cars at this horsepower level simply don’t need a lot of suspension work to make the tires hook, so even the most rudimentary upgrade shock absorbers should work. Once your car’s horse-power level starts going beyond 350, I strongly suggest moving up in the quality of the shocks.
I’ve found that the QA1 single-adjustable (one knob) rear shocks are the next logical step up, and are good for cars with 350 to 500 hp.
QA1 single-adjustable shocks have one knob at the bottom with 18 different adjustment settings. Turning the knob counterclockwise creates a softer shock setting, while turning the knob clockwise stiffens it. There are 18 positions (or clicks) that change both the compression and extension (rebound) settings. As a car approaches 500 hp (and definitely by 600 hp), I suggest going to the QA1 double adjustable (two knob) rear shocks, which boast 18 positions on each knob. This gives you a total of 324 different valving combinations and most importantly the ability to adjust the shock’s compression independently from its extension (rebound).
With most adjustable shocks, it’s best to start near the center of however many adjustments that particular shock has. If the front of your car needs more weight transfer (pitch rotation), a softer setting may be needed. Don’t get too soft or you may lose the control your shock has on your suspension, thus allowing it to do unexpected things.
For a car that squats in the rear upon acceleration you may need to loosen the shock to get maximum weight transfer (pitch rotation). For a car that raises in the rear upon acceleration, you need to be in that middle range and careful not to get so stiff upon compression that the rear suspension does not allow the front suspension to raise.
Remember, if a car is standing on its back bumper, it may be getting maximum traction off the line but it’s not necessarily getting maximum forward motion. Since the car is pivoting on the rear tires, the front of the car is actually going rearward in order to get to that height and making the car much harder to push forward.
Due to the popularity of muscle cars, coil-over conversion kits have been developed for the rear. You can’t mount a true coil-over spring and shock in the factory shock mount locations, as they were never designed to hold up the weight of the car. You can buy kits that include additional strengthening upper and lower mounts to allow these mounts to properly support the weight of the car. Also included are the coil-over shocks and matching springs, and Torrington bearings for the springs to sit upon, which make spring adjustments easier.
I have designed three kits. One raises the rear of the car 2 inches; another lowers the car 2 inches, and a third maintains the rear factory ride height. The bottom mount is also further adjustable. I have worked on muscle cars enough over the years to be able to pick the right parts for an application although you should have the rear of your car weighed to be accurate.
Written by Dick Miller and Posted with Permission of CarTechBooks