Automotive suspension design comes in all shapes and sizes, but the basics always stay the same. Whether the suspension needs lots of travel to survive rally stages, high spring rates to compensate for aero at speed, or a comfortable but composed ride to keep passengers comfortable, each component must work together to create a complete package. That may seem daunting, but it's truly not; it's easy to see how they operate and work together if you break them down into their individual components. With a greater understanding of your vehicle’s suspension, you can more readily tackle DIY repairs and optimize your car for the street or track.
Alignment:
Regardless of design, all cars work within the same physical parameters regarding suspension alignment. Your vehicle’s suspension alignment is how the wheel and suspension is poisitioned in its respective wheel well. The collection of angles is essential to how the vehicle drives and how long suspension components can last before wearing out. The three main adjustable variables are caster, camber, and toe.
Camber:
Camber has to do with the lean of the tire compared to the car’s chassis. If the top of the wheel is leaning away from the vehicle, it has a positive camber angle. If the top of the wheel is leaning towards the car, it has a negative camber angle. On a street car, you will typically see minimal negative camber, around -1.0 to -2.0 degrees maximum, while on a race car, you may see as much as -5.0 degrees. Negative camber helps to compensate for deflection in the tire and roll in the chassis when cornering.
Toe:
The toe angle is one of the most critical components of your vehicle’s alignment. It’s the angle of the wheels relative to the vehicle’s direction of travel. If the leading edge of the wheel is closer together than the trailing edge, this is called toe-in. If the leading edge of the wheel is farther apart than the trailing edge, this is called toe-out.
Toe settings can affect vehicle stability in a straight line, turn-in, chassis rotation, overall chassis balance, and tire life. Street cars will typically feature toe-in on the front axle, allowing the car to track straight, making it easy to drive at speed, and settling over bumps more easily and safely, even over rutted or rough roads. Toe-out on the front axle will make a car more unstable and ‘darty’ in a straight line, but it will turn in more quickly, making it faster around a race track.
By adjusting the steering tie rods on your vehicle to be either shorter or longer, you can adjust the toe settings on your street car the same way you would on a race car.
Caster:
The caster angle is the lean of the front strut relative to the tire’s physical contact patch. If a car has zero degrees of caster, the strut would be straight up and down. Leaning the top of the strut towards the car's rear creates a positive caster angle. Positive caster promotes self-centering and dynamic negative-camber gain as you turn the steering wheel left or right.
Dynamic camber is the amount of negative or positive camber introduced into the inside and outside front wheels when the steering is turned. All modern cars have a certain amount of positive caster designed into the front suspension. Too much positive caster can cause excessive kickback into the steering wheel over bumps and make the car more challenging to drive.
The designers typically utilize an eccentric bolt design on a street car to change alignment settings. Eccentric means that the bolt is off-center, and as it is rotated, it changes the suspension alignment of whatever component it affects. Depending on the car, the suspension design, and whether it’s front or rear, eccentric bolts adjust camber or toe.
Springs and Spring Rates:
The springs in your car's suspension provide the mechanical lift to the chassis and support the vehicle's weight. Springs come in both linear and progressive designs. You can typically identify a progressive rate spring by checking the spacing on the coils. If they are evenly spaced, this is likely a linear rate spring. If the coils are not evenly spaced and some are noticeably closer together than others, this is a progressive rate spring. Spring wire diameter, material, and coil spacing affect spring rates differently.
A progressive rate spring increases exponentially as it is compressed. It can provide increased comfort in a street car because it has a softer ‘regular’ or base rate and a much higher relative maximum rate as the spring is compressed.
A linear rate spring has a consistent spring rate regardless of the amount of compression. It provides more predictable behavior than a progressive rate spring, but may not offer the same ride comfort in a street car.
Dampers:
The energy created by the car’s weight bouncing on the spring is controlled by the vehicle’s dampers, more commonly known as the shock absorbers. Dampers utilize a piston moving through oil to control the spring movement. The damper's firmness, also known as rate, can be adjusted by altering the oil flow through the piston valve. The dampers convert the kinetic energy of the springs and the vehicle’s weight and movement into heat.
A gas-pressurized section of the damper maintains constant pressure on the oil to prevent foaming and aeration, otherwise known as bubbles. Without gas pressure, the piston valve will cavitate in the oil, creating air bubbles as it moves through. That cavitation causes inconsistent damper performance and a loss of damping rate.
Dampers come with either fixed or adjustable damping rates. Street cars and most entry-level sport dampers will have fixed damping rates. Adjustable dampers will feature one-, two-, or three-way adjustments, allowing the user to fine-tune the compression and rebound of the damper. The compression rate is how easily the damper can be compressed and shortened, while the rebound rate is how easily the damper can be decompressed and lengthened. One way to think about it is that the compression rate affects how firmly the tire is pressed into the ground, while the rebound rate affects how quickly the weight reacts to a change of direction.
A one-way adjustable damper means there's only one adjustment to change. Most one-way adjustable dampers affect the rebound rate only. On some designs, the single adjustment will affect both rebound and compression rates, although not typically at the same relative percentage.
Gas-charged dampers come in two primary types: monotube and twin-tube. A monotube design is simpler and features a larger oil and gas volume than a twin-tube design. Because of this, it can handle heat more effectively and maintain a more consistent performance, which is why it is popular on race cars and in heavy-duty applications. Monotubes usually have a higher gas pressure than twin tubes and may have a firmer ride. The examples above are of a monotube design.
A twin-tube design allows for more suspension travel in a given space than a monotube design, which, combined with lowered gas pressure, often results in better ride characteristics. Most production car dampers are twin-tube.
Most street car dampers feature a fixed spring perch and ride height. Adjustable ride height suspension, often called “coilovers,” allows setting the exact ride height of each corner of the vehicle. Aside from fine-tuning the look you want, by raising or lowering the ride height of a specific corner or end of the car, you affect the proportion of the weight that it carries. This allows you to finely tune the balance of the vehicle.
Bushings:
Commuter car suspensions feature rubber bushings at mounting points to provide compliance and, therefore, less noise, vibration, and harshness for the driver. These bushings deflect under higher loads, so engineers substitute them for spherical bearings or solid bushings in race cars and higher-performance applications. Solid bushings ensure no deflection under load so that the alignment specifications stay precisely where they need to be. That guarantees higher performance and more predictable behavior at the limit.
Sway Bars:
Sway bars, which are more accurately described as anti-roll bars, are integral to most suspension designs and can be upgraded or changed independently of the springs and dampers. The sway bar ties together the left and right sides of the front or rear suspension and comes into play when the suspension is loaded unevenly from left to right. As you increase the rate or the stiffness of the bar, usually by making it thicker, the more it will work to keep the weight load even across that axle.
All suspension components are part of the vehicle’s unsprung weight, meaning anything attached to the car but not supported by the springs. These components include the wheels, tires, brakes, suspension control arms, springs, and dampers. The lower the unsprung weight, the more nimble and responsive the car will feel.
It is pretty standard to see a divorced spring set up on the rear of many modern vehicle designs. As it sounds, the spring is mounted separately from the damper, typically inboard, for space and packaging considerations. However, moving the suspension spring inboard compared to the outer pivot point affects the motion ratio. The motion ratio is the amount of leverage the suspension arm can place on a component. In a divorced spring design, the rear suspension has more leverage, rendering the effective spring rate lower than it actually is.
The divorced design isn’t much of a hindrance for street-driven cars or even those used for autocross or occasional track days. For competitive racing use, a coilover design is a superior choice. However, for this reason, the Mk7 Volkswagen GTI TCRs use a coilover design at the rear. This results in a more accurately matched spring and damper set at the rear, easier tuning, and better performance.
With that, you have all you need to understand what’s happening beneath your car at a basic level. There are many different kinds of suspension systems with various layouts, so be sure to do further research on your vehicle’s specifics. If you’d like to read more about basic car systems, be sure to visit our blog’s home page, which is constantly updated.
