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When the car launches, the front wheels come up, sometimes off the ground, and that puts more weight on the rear tires, which helps them get traction. If both from wheels are off the ground, 100% of the vehicles weight is on the rear tires, so theoretically they should have maximum traction. That's a gross oversimplification of what is really happening, but it helps to explain how tires are 'planted' with the body movement. Weight transfer doesn't just occur front-to-back. The rotation of the engines comes into play. Imagine looking at the rear of the car. Remember that the crankshaft turns counterclockwise (as viewed from the flywheel), which makes the engine block rotate in the opposite direction (action-reaction) and lift the driver side of the frame. The energy from the crankshaft is ultimately relayed to the third-member, where another set of reactive forces applies. AS power is applied to the rearend, the front of the rearend housing rotates upward. Because the tires are stuck to the ground, the pinion gear tries to climb the ring gear instead of turning the tires, which causes the axle housing to rotate. The housing also rotates side to side in reaction to torque. The right-side tire at the back of the car lifts toward the body and the left-side rear tire gets pushed away from the body. When you combine these motions with front to read transfer and watch the car leave, it looks like the body is putting a lot of weight on the right rear tire. In fact, the torque is trying to pull that right rear tire off the ground, and plant the left rear. That is why the right rear is usually the tire that spins. On The Straight and Narrow: Watch fast cars leave the line and you will notice that every one of them reacts differently. Sometimes the car turns to one side, forcing the driver to correct with the steering wheel. A car that pulls is not planting both tires effectively, and therefore not getting proper bite. It is also dangerous. Before adding a traction device to the car, the chassis must be inspected for things that keep it from going straight. Is the housing mounted straight in the chassis? Bent or twisted housings, worn out bushings, bent leaf springs, or unevenly adjusted ladder bars will cock the rearend in the car. Where the rearend housing goes, the car goes, and this must be perfectly straight ahead. Even if the rearend is plumbed straight, a flexing chassis, suspension parts and axle housing, or a combination of ineffectual shock-mount location, worn-out springs, incorrect or uneven preload, and uneven tire pressure are all liable to steer the car off course. Further, an undercarriage that hasn't been strengthened to resist torsional forces with sub-frame connectors or a rollcage will be impossible to tune successfully. All this twisting and rotating is abusive, especially to an unmodified axle housing. Spring perches and axle tubes twist a bit under high-torque loading, and that plays hell with the rear suspension, too. In a drag race car or serious street machine, the spring perches must be gusseted and the factory welds inspected for full coverage. Things such as worn or broken springs and uneven tire pressure seem simple enough to check, but you would surprised at the number of racers who don't bother. Finally, the obvious: An uneven water burnout heats up one tire more than the other and makes it stickier. If the left rear tire hooks better then the right rear, the car will turn right. All the above variables should be duly noted or fixed outright before you install a traction device. The Nature of the Leaf Spring: Leaf springs are bulky and heavy, but they represent a simple and effective way to locate and suspend the rearend. Unfortunately, they tend to bend under the big power. As power is applied to the rearend, the front of the housing is rotates upward. The rearend is bolted to the leaf springs, and when the housing rotates it bends the leaf front into an S shape and pulls the tire off the ground. The spring snaps back violently, but power is still being applied so it happens over and over again, causing the tires to bounce up and down with a vengeance. This is called 'wheelhop.' The primary purpose of a rudimentary traction device is to keep the spring from wrapping up and to keep the tires on the ground, although some devices serve other purposes as well. Think of a leaf spring as two separate entities. The springs front half locates the housing in the chassis; the rear half is responsible for most of the 'spring' function. The front half wraps up under power, pulling the tire off the ground. The stiffer the springs front half is, the more resistant it is to wrapping up. That is one of the keys to the success of the legendary Chrysler Super Stock leaf springs. The front half is shorter and stiffer than 'normal,' and designed to preclude the use of any type of traction device whatsoever. A leaf spring custom-made from thicker steel and with increased spring rate has more natural resistance to bending. There are two types of leaf springs: multileaf (usually have 3-5 leaves, each one progressively shorter then the next), and the monoleaf type (with single main leaf). The monoleaf is light and locates the rearend, but it wraps up much easier and usually requires a traction device to work. Traction Action: The leaf spring benefits from a plethora of traction aids to make the car launch like a Super Stocker, and most are easy bolt-ons that don't require a ton of tuning. · Spring Clamps: Spring clamps are so simple to make, you can do it in the privacy of your home. Clamping the leaves together on the front half of the spring is one way to solidify it and make it more resistant to wrap-up. The clamps force the rear half of the spring to work harder, so it will wear out faster, and longer spring shackles are usually required to maintain the ride height and allow that part of the spring sufficient movement. · Slapper Bars: Also known as traction bars, but 'slapper' is more accurate because they actually slap up against the bottom of the leaf spring. The slapper bar bolts solidly to the leaf spring and to the rearend housing at the mounting pad. In fact, most slapper bar designs replace the stock lower mounting pad. As the housing rotates,, so does the bar, which contacts the front portion of the leaf spring and keeps the rearend from rotating further and the spring from becoming S shaped. A rubber snubber is mounted between the slapper bar and the spring to soften the initial contact. Slapper bars are easy to install, allow some preload to be dialed into the spring, and they are inexpensive - all of which makes them the worlds most popular traction aid. · Southside Bars: The southside bar bolts solidly to the bottom of the rearend housing spring mount, and clamps securely to the front of the leaf (depending on application), just behind the mounting eye. It changes the lifting point of the suspension and, when the housing rotates, the bar pushes up on the body (hence, 'lift bar') and plants the tire. · Cal-Trac Bars: The Cal-Trac system allows slight changes to the instant center (or 'pushing point') of the rear suspension, and it also prevents leaf spring wrap-up through methods different from anything else. Calvert also claims it can handle corners, so it can be used on dual-purpose vehicles. · Pinion Snubbers: A pinion snubber is similar to a slapper bar, but it applies force to the top of the rearend instead of the spring. The pinion snubber is a rubber bumper mounted to the floorpan (sometimes the pinion snout itself), just above the pinion snout of the rearend housing. When the housing rotates, it contacts the snubber, which stops the rotation and helps prevent spring wrap-up. As with the slapper bar, many racers adjust the snubber so it touches the rearend (or floorpan) in a static mode; some use it to preload the pinion. · Ladder Bars: Ladder Bars are normally used with coilover shocks, but they have been used with leaf springs that have a housing floater. The ideal length of a ladder bar is longer then the front half of the leaf spring, and one of its functions is to locate the rearend. If the ladder bar and leaf are both locating the rearend, and their lengths are different, the arcs they follow will also differ and eventually bind the suspension. Floaters allow the rearend to slide back and forth on the springs a small amount. The springs still support the vehicle, but the ladder bars locate the rearend and transfer the energy of the movement from the housing to the chassis. The advantage to ladder bars is that they provide a longer lever to act on the car ( as compared to leaf springs) and the ability to change the instant center and pinion angle, and therefore the 'hit' on the tire. Ladder bars are more violent than bolt-on traction devices and four-links, and they are not much fun on the street, but they work very well on the dragstrip and allow more compensation for varied track conditions than slapper bars. Preload: Sometimes, no matter how well the chassis is sorted out, the car still will not go straight. In this case, the suspension can be preloaded on one side. This means that the traction device exerts force on the spring while the car is sitting still. This is equivalent to adding rate to just one spring. Whichever way the car turns, the tire on that side isn't doing its job, so adding more rate to that side in the rear may straighten the car out. Many racers set up their cars with just a touch of preload on both sides. If the car only goes straight when the preload is different side to side, there is something wrong with either the car (weight distribution is off, the rearend isn't straight, the frame is bent, or the tires aren't of equal pressure or pliability) or the surface of the race track. Most traction devices can be adjusted for preload, some easier then others. Pinion Angle: Pinion angle, defined as the angle of the pinion gear in relation to the driveshaft, is critical to the whole, but many racers don't know what affects it, or they simple ignore it. On a street car with street tires, pinion angle can be useful in getting the car to hook up. Ideally, as the car moves down the track under power, the pinion angle should be zero degrees. Because the rearend rotates (and the pinion rises) under power, when at rest, the pinion must be pointed downward so that is at zero when under power. The less the static pinion angle (the closer to a straight line it is with the driveshaft), the harder the suspension hits the tires. On a slippery track or on the street, more pinion angle will shock the tires less and help them bite better. Shock Absorbers: Leaf springs and traction devices, with their various mounting options, rates, and so forth, are adjusted to get the suspension close to optimum, but shock absorbers are the key to fine-tuning the whole enchilada. You would be amazed at what can be done with a quality adjustable valving. Instead of altering spring rates side to side and asking for a bunch of other problems, adjust the shocks as a pair to compensate. A shock absorber works in two different directions: compression (pushing it together), and rebound (pulling it apart). If the car heaves itself on the right rear tire, adjusting the shocks so that the right rear pushes in harder and the left front pulls out harder will lessen or at least slow down that motion, just as if you had put a stiffer spring on the right side. This requires quality adjustable shocks, such as a pricey Koni set. But if you are serious about maximum bite, consider them mandatory. Don't Forget the Front Suspension: The reaction of the car on launch is a function of much more then just the rear suspension. The front suspension is a vital element in the traction equation and it must not be ignored. Each car and each combination is unique. A suspension setup that works on one car, may not compliment another even though the two seem identical. There are no guarantees when it comes to the 'right' suspension parts, spring rates, shock valving, and so on. Gearing and clutch/converter are paramount when tuning the suspension, so the only way to find out what works best is by testing, testing and more testing. It can be expensive and time consuming, but mandatory for complete exploitation if the leaf-spring suspension. |
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