Lotus’ 99T chassis for the...
Lotus’ 99T chassis for the ’87 Formula 1
season featured its electro-hydraulic fully active suspension system, a design that worked so well that a lack of tire-slip angle meant the car couldn’t put enough heat into its tires to generate maximum grip.
Automotive suspension design has traditionally had to strike a difficult balance between three conflicting needs: road holding or handling, load carrying and passenger comfort. At its most basic level, your car’s suspension must be able to support the vehicle’s weight (and its cargo) while providing adequate handling capabilities and at the same time provide reasonable ride comfort for its passengers. With passive shock and spring suspensions, you could either have a cushy grandpa-approved ride quality or a firmer setup that’s great in the corners but hard on the kidneys. But that all started to change back in 1981 when some clever chaps at Lotus (as part of its Formula 1 team) began to develop the first active suspension system capable of adapting to changing loads and driving conditions.
Lotus’ cutting-edge approach to true active handling meant replacing the passive shocks and springs with double-acting hydraulic actuators at each corner of the car. As the wheel and tire meet a bump, the wheel’s acceleration and vertical load is measured by load sensors, travel sensors and accelerometers and this data is transmitted to a computer that calculates the required wheel velocity and displacement to counteract the bump and triggers the servomechanism at each wheel accordingly. This high-bandwidth electro-hydraulic dance takes place hundreds of times per second, allowing each wheel to accurately follow the contour of every bump, dip and curb it encounters, protecting the body structure against unwanted forces and virtually eliminating unwanted weight transfer.
This electronically controlled...
This electronically controlled damper is a typical example of the system used by Nissan on the GT-R and Porsche on all its PASM-equipped sports cars, where damper response is continuously adjusted by the ECU based on input from vehicle load and speed sensors.
Successfully testing this system at the Brazilian F1 race in 1983, Lotus proved the concept had merit, but for a number of reasons it never quite lived up to its potential. In theory, the system could raise cornering speeds considerably (Lotus engineer Peter Wright was famously quoted as saying its active-suspension F1 car could go around “any corner at any speed”), but in practice it reduced tire-slip angle so much that it was nearly impossible to get enough heat into the tires to allow them to function properly. The hydraulic system was also a bit of a horsepower pig, robbing 4 to 4.5 hp on a smooth road and up to 9 hp on a rough road. Add to that the computational complexity of the system and the need for aerospace-quality hydraulic actuators and you can start to see why this type of fully active suspension has yet to make it onto a mass-produced road car.
But as an engineering challenge with the potential to decouple the relationship between load carrying, road holding and ride quality, it should come as no surprise that many of the world’s major automakers have continued to look for ways to implement active suspension technologies on their vehicles. Nissan, for example, began developing a hydraulically controlled suspension system in the late ’80s that actively adjusted shock absorber damping (referred to as skyhook damper control) using a pressure control valve in combination with a small accumulator and a hydraulic cylinder to go along with onboard accelerometers, front and rear vehicle height sensors and a vehicle speed sensor (along with a computer to control the system, of course). Although not a fully active system like Lotus’ F1 invention, Nissan’s semi-active approach proved effective at smoothing out bumps and irregularities in the road via its frequency-sensitive damping while also controlling body roll when cornering by increasing hydraulic shock pressure on the outer wheels and reducing it on the inner wheels. Similarly, nose dive under braking could also be controlled by stiffening the front shocks and softening the rear shocks. The system was even sophisticated enough to make damping adjustments for crosswinds and adjust to high speeds by stiffening up slightly for improved roll control and yaw stability.