Thump. We’ve all missed spotting that speed bump or pothole in time to brake during our motoring journeys. Your ego takes a bruising, but spare a thought for your car’s suspension. Not only does it have to deal with bumps and holes in the road, it must also provide your car with satisfying handling dynamics.
The theory behind suspension is more than 100 years old, but engineers are still fine-tuning the hardware and adding technology to improve vehicle performance. Since the days of horse-drawn carts, designers have striven to isolate the passenger compartment from road imperfections. Initially, they suspended the cabin on top of the fixed-cart chassis using leather straps. Oh how far we’ve come…
Automobiles isolate the cabin by allowing the wheels to oscillate and, to achieve this, several suspension configurations are employed. Bumps in the road are essentially forces acting on the wheels in a vertical direction. To provide a smooth ride, the suspension must absorb the vertical displacement of the wheel while dissipating the energy of the moving components. For handling performance, it is important that the tyres maintain an optimal contact patch on the road because this is where cornering forces are generated.
A good suspension system allows the tyre to stay in contact with the road surface under most road conditions. The vertical orientation (camber) of the wheel is important during cornering, as vehicle body lean or weight transfer skews the contact patch and negatively impacts the maximum lateral force. Here, suspension geometry plays a vital role.
Springs: These store the kinetic energy generated by the wheel’s upward movement and release it by exerting a force in the opposite direction of the spring compression. This force is proportional to the displacement of the spring and not the speed at which it’s displaced. A softer spring rate results in a comfier ride, but it can compromise the vehicle’s body control, especially during cornering.
A stiffer spring rate results in better body control (or load-carrying ability in commercial vehicles), but affects the ride quality. The springs’ secondary function is to support the vehicle-body mass and set the correct ride height. The types of springs include coil, leaf and air units.
Anti-roll bars: When a vehicle corners, weight transfer compresses the suspension on the outside wheels just as the suspension on the inside wheels is extended. This is especially true if the vehicle is fitted with independent suspension and has a high centre of gravity. The resultant body roll has a negative impact on handling and wheel camber relative to the road surface. Fitting an anti-roll bar minimises this movement.
The bar is a mechanical device in the shape of a “U” and runs between the suspension components of both the wheels on a single axle. The compression of one wheel on the axle results in a torque moment in the anti-roll bar that also tries to compress the other wheel’s suspension. As a result, the vehicle stays flatter in corners.
Shock absorbers/dampers: These items dampen the body oscillations relative to the wheels when crossing undulations. That’s why they’re also called dampers. Shock absorbers provide a resistance force in the opposite direction of suspension movement. The key fact is that the force is proportional to the speed of the movement and not the displacement. So, you can easily compress or extend a shock absorber if you do it slowly. The contrary is true if the operation is repeated at speed.
Without shock absorbers, a vehicle continues to bounce after hitting a speed bump. The shock absorber dissipates the kinetic energy (mostly through heat) and stabilises body movement.
Most suspension systems are reactive and respond to a road imperfection or bump only once it has been hit by the wheels. Future technology will employ active suspension where a camera (and/or radar) scans the road surface ahead and primes the suspension. Mercedes-Benz’s Magic Body Control is an example of such a system; it adjusts the damping on each wheel in a fraction of a second to compensate for speed bumps – even when you don’t see them.
Land Rover is working on a similar system, but has taken it a step further. The scanned road information is uploaded to the Cloud where all other vehicles can benefit from the GPS-based information, altering their suspension settings for a certain road. Active suspension also allows the normally mutually exclusive suspension characteristics of sporty handling and comfort to merge into one setup…
For more technical features, head on over to our friends at CAR Magazine.