Power steering systems
An effective steering system is one of the most basic requirements of any racecar. In the realm of Formula One, packaging space available for the steering components is minimal, yet the demands placed on them are considerable, both in terms of mechanical performance and providing the driver with the necessary levels of steering ‘feel’. In principle, the steering system of a modern Formula One car is not that dissimilar to those found in a roadcar, it is simply more optimised and far more compact.
Beyond the basic parameters that govern the way a car’s steering behaves, the most important contributor to a Formula One car’s steering performance is the power assistance. The regulations ban electronic or electro-hydraulic power steering, so teams have to rely on hydro-mechanical assistance, section 10.4.2 of the 2014 technical regulations stating, “Power-assisted steering systems may not be electronically controlled or electrically powered. No such system may carry out any function other than reduce the physical effort required to steer the car.”
In essence, this means teams cannot create steering systems that adapt the level of assistance given to the driver according to changing conditions. Creating systems that provide the level of consistency and feel the top drivers need to be able to extract maximum performance from a car – regardless of variations in factors such as downforce or fuel load, while remaining ‘passive’ – is not easy. Note the well-publicised problems that Kimi Raikonnen faced in extracting performance from the Lotus E20 during the 2012 season, when the car’s power steering failed to provide the level of feedback his driving style required.
Most teams use the same components as the basis for their power steering system, based around a rotary power-steering valve produced by the same company that supplies their hydraulic valves. The regulating valve is very similar to the electro-hydraulic servo valves found throughout the car for actuating systems such as gear selector mechanisms, but instead of the actuation signal being provided by a torque motor, it is created by a torsion bar or linear springs in the load path of the steering.
As the steering load increases, it creates a displacement in the torsion bar that is linked directly to the spool which, as is the case with an electro-hydraulic valve, provides a pressure differential to the ends of the steering rack, creating the power assistance. The valve’s construction features two concentric sleeves connected by the torsion bar. In operation, torque applied in either direction rotates the inner and outer sleeves relative to each other. This in turn opens flow-metering ports that direct high-pressure oil to one side of the assist actuator.
The valve uses a closed-centre circuit to minimise energy consumption and yet still offer high positional accuracy and repeatability. Due to the high fluid flow rates in the valve, the steering system remains stiff – not in terms of resistance to inputs but in relation to accurately translating driver inputs according to wheel movement. To tailor the systems to different drivers’ preferences, or to variations in car set-up, the internal valving can be adjusted to alter the response rate.
As the regulations are unlikely to change in relation to steering system design, it is probable that this method of power assistance is likely to remain in use – albeit with the inevitable refinements – for the foreseeable future.
Written by Lawrence Butcher