AutoX For December 2011
The resistance of a car to leaning, or roll, while cornering is a primary factor in how a car behaves. There are two reasons for this: Roll affects how the tires touch the pavement and relative roll resistance determines how much weight transferred each corner of the car will carry in a corner. It is clear that a car’s body roll influences grip by tilting the tires on the pavement, but tires are also sensitive to how much vertical load is placed on them. More load equates to more grip, but the tire is also less efficient. Increasing the roll resistance of one axle of a car causes more load to be transferred at that axle, making the tires on that axle work less efficiently. It is this principle you work with by stiffening or softening the chassis. Sway bars and shocks are part of a car’s roll resistance. They are commonly adjustable and work in different ways to affect a car’s behavior.
Sway Bars: Fundamentally, a sway bar acts as a spring that connects the two wheels on an axle. This spring is only activated when the axle is twisted relative to the chassis. If both wheels move up and down together, no sway bar force is generated. In other words, sway bars only affect roll. A car’s springs offer the primary resistance to roll, but sway bars can help fine-tune the balance of a car. Also, in cars with soft springs, sway bars can help augment inadequate roll resistance without swapping out the factory springs.
Physically speaking, sway bars are a mechanical link between the left and right springs on an axle. The most common sway bar design today is that of a torsion bar. As one spring is compressed and the other extends, the sway bar is forced to twist to allow the two springs to move in different directions. The harder it is to move the springs relative to each other, the greater the resistance to roll motion at that axle.
The most important characteristic of a sway bar is that the force in a sway bar is displacement based. The greater the difference in wheel position between one side of the car and the other, the greater the force in the sway bar will be. In other words, sway bars are most effective when a car is leaning heavily to one side, such as in the middle of a corner. Adjusting a sway bar will affect steady-state cornering much more than transient motions. Think sweepers.
For the average Autocrosser, adjusting a sway bar is one of the most effective methods of changing a cars behavior between runs. Most sway bars are easy to adjust and have a relative wide range of adjustment. The most common adjustment is to adjust the leverage between the suspension and the sway bar. The shorter the distance between the sway bar and the sway bar end link, the stiffer the bar will be. A stiffer arm will put more force into the sway bar than a softer arm.
Common adjustment methods:
1. On a car that cannot get enough camber, increasing sway bar stiffness reduces roll and allows the tires to work more effectively.
2. A car that understeers can often benefit from a softer front sway bar or a stiffer rear sway bar.
3. A car that oversteers can often benefit from a stiffer front sway bar or a softer rear sway bar.
Shock Absorbers: Shocks are primarily used to control the motions of the wheels and keep the tires smoothly pressed onto the pavement. Without shocks, a wheel that strikes a bump will keep moving up and down until friction stops the motion. When talking about using shocks to adjust the handling of a car, always keep the shocks primary function in mind: to keep the tires on the pavement. Going overboard with stiff shocks does not improve grip, however.
Physically, shocks use viscous friction to dissipate energy from the hub/wheel/spring assembly. A piston rests in a tube of oil. The piston is attached to the wheel. When the wheel is moved up or down, the piston is forced to move and the oil must move around the piston. The faster the piston moves, the harder it is for the oil to move through the piston. If the wheel and the piston are not going up and down, the shock is not doing anything. In other words, the shocks are only effective when in motion. Think slaloms.
Most cars do not come from the factory with adjustable shocks. That said, it is easy to find aftermarket adjustable shocks. Adjustable shocks generally come in two flavors: off the shelf direct replacements for the stock shocks (think Bilstein PS9 or 10’s) and high end custom built shocks intended for racing (think Motons). Every shock has its own niche and purpose, but they all fundamentally function in the same manner.
In order to get the most out of an adjustable shock, it is critical to understand what the external adjustments actually do. Shocks use different pieces to make their force, depending on which direction the wheel is traveling. If the wheel is traveling up it is called compression and if the wheel is traveling down, it is called rebound. External adjustment on a single-adjustable shock (a shock having only one adjustment knob) can only adjust compression, only rebound or compression and rebound together. Commonly, single-adjustable shocks adjust rebound only or both compression and rebound together.
Used properly, compression and rebound adjustments are made for different reasons. Compression damping should be used to adjust to compensate for very smooth or very bumpy road surfaces. Rebound damping can be used to adjust the balance of a car in slalom type elements. Adjusting both the compression and rebound together make a bumpy surface feel harsher, but also stiffens the car in slaloms.
Common damper adjustment methods:
1. A car that responds slowly in a slalom will benefit from more overall damping force.
2. A car that oversteers in a slalom will benefit from more front damping force.
3. A car that understeers in a slalom will benefit from more rear damping force.
4. Adjusting rebound damping should be the preferred method of adjusting car balance.
5. A car that skips or skitters over bumps will benefit from less overall damping force.
If you have been following our articles all season long, you know have the fundamentals of car control, basic Autocrossing and car setup basics. We do caution you to take in and apply this information in some systematic way. Trying to change too many things at once will surely confuse you and will only lead to frustration. Feel free to watch other drivers at each of the events and feel free to ask questions and learn from them. Other than probably at Nationals, most drivers will be happy to talk about the modifications they have made and how it affects their car.
Tom and Tom
