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This is how engineers achieve a balance between ride and handling…
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, but 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. We’ve come a long way since…
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. It’s important to note that 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. Think here of typical American vehicles in movies and how much they lean in corners during chase scenes.
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. Vehicles fitted with independent suspension all-round usually have anti-roll bars on both axles. 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.
The downside is that you lose comfort inherent in independent suspension, as the anti-roll bar forms a link between the suspension setups. Although an anti-roll bar is most common and inexpensive, hydraulically linked suspension can achieve better results. McLaren, for example, uses this system in its topflight supercars.
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 shock absorbers work on the principle of oil displacement by a piston in a cylinder. The piston has a couple of small holes that allow the pressurised oil to flow through when the piston moves, thereby providing a damping force.
Main suspension types
A suspension system consists of springs, dampers and links that control the wheel-movement geometry relative to the vehicle body. Although many suspension systems (and combinations) exist, we’ll look at the main types.
Type 1 – independent double wishbone
Main uses: sportscars and racecars.
Pros: excellent wheel-camber control while cornering, especially if the bottom and top triangular lengths are tuned for the application. The setup has an excellent wheel-travel range.
Cons: although compact in height, it does require a wide space between the wheel and vehicle body. It can be expensive to manufacture and needs an anti-roll bar for good body control during cornering.
Type 2 – independent MacPherson strut
Main uses: production vehicles, especially on the front axle of front-wheel-drive cars.
Pros: compact in width, and cost-effective to produce, as it has few parts.
Cons: lots of vertical space is needed. Wheel-camber control is compromised during cornering, and it requires an anti-roll bar for good body control.
Type 3 – torsion beam
Main uses: rear axle on less expensive front-wheel-drive passenger vehicles.
Pros: cheap to produce, and no anti-roll bar needed as the torsion beam fulfils the same function.
Cons: Comfort is compromised, as this suspension type is not independent and the camber control of wheels is affected.
Type 4 – solid rear axle
Main uses: commercial vehicles and bakkies (rear axle mostly).
Pros: strong and durable, and therefore able to operate in harsh environments.
Cons: it’s heavy and comfort is compromised, as this suspension type isn’t independent. The clearance below the differential can be a problem and camber control of wheels is compromised.
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.
Jaguar 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…
Author: Nicol Louw