The Active Car

This article was first published in 2003.

The first step in the advent of the active car has already occurred. Electronic Stability Control and - to a lesser extent ABS and traction control - are active systems that alter the way in which the car drives on the basis of inputs from sensors and the action of programmed internal logic. But these are only at the very beginning of what is possible.

Imagine being able to actively change wheel alignment settings on the go - with different alignments automatically selected on the basis of how fast the car is being driven and the sort of cornering it is undergoing. What about active steering, where panic inputs can be speeded up and jerky steering can be automatically smoothed?

In this story we look at how actively altering the way the car drives could change the whole feel of a car, allowing manufacturers to easily 'program' the active functions of a model for specific markets and driver profiles. Prodrive Automotive Technology go even further, suggesting in an SAE engineering paper that a car brand could be defined by software instructions differentiating versions of a common platform.

You want sportier handling with faster responses and less interventionist electronic systems? Or do you want a car that will look after a less skilled driver? The same platform can achieve both aims if it has sufficient active systems and the programming is carried out with that 'branding' in mind.

Active Systems

1. Four-Wheel Steering

While currently out of fashion (Honda and Mazda - amongst others - have produced four-wheel steer cars), active rear-wheel steering has the ability to change the driving experience. (Note this is not the same as passive rear-wheel steering, obtained in many cars through suspension bush deflection.) Four-wheel steering improves steering feel around centre because the delay before the rear wheels start generating a cornering force is lessened. This gives a driving perception of improved agility and better steering feel. Parking is also made easier. However, four-wheel steering will not improve outright cornering grip and may in fact degrade it.

2. Steer-by-Wire

If the direct mechanical linkage between the steering wheel and the front wheels is replaced with an electronic interface, changes can be made to the magnitude and/or speed of the effective steering input.

(Note that even current mechanical steering systems can be quite non-linear, with steering inputs around centre having a much lesser effect than the same input when a turn of lock has already been applied. However, with these systems, the variation in characteristics remains the same - it doesn't alter as you drive along! For more on traditional non-linear steering see "The New Breed of Controls - Part 1".)

With steer-by-wire, clumsy steering can be ignored by the car, steering corrections (eg in a tail-slide) can be added automatically, and panic steering inputs can be accelerated. Prodrive see steer-by-wire as the next big step in vehicle dynamics, suggesting that it "has the potential for dramatically improving the ease of limit handling by 'building-in driver skill' to the vehicle".

The main obstacle to the widespread implementation of active steering is buyer resistance - few like the idea of a car not reacting to steering inputs, for example.

3. Active Toe Control

The amount of toe that is used on the front and rear axles can have a dramatic affect on how a car handles. From pronounced throttle lift-off oversteer (especially in a front-wheel drive) to the sudden turn-in experienced with front toe-out, to (in some cars!) a complete lack of directional stability with a slightly incorrect front toe, the amount of toe-in or toe-out is very important.

Changing it actively to suit the driving being undertaken has the potential to alter driving behaviour in a way that is imperceptible to the driver in its implementation. For example, if a front-wheel drive car could detect that a driver was repeatedly attempting to gain lift-off oversteer, a measure of rear toe-out could be introduced. That same amount of toe-out would make the car potentially dangerous in a swerve-and-recover manoeuvre in an urban area but there would be no need to have it still present - when the car was being driven conservatively, rear toe-in could be automatically used.

4. Active Camber Control

While it looks very exciting, Prodrive suggests that active camber control is actually less useful in the real world than active toe control. The pictured DaimlerChrysler F400 Carving (covered at "DaimlerChrysler's F 400 Carving") gained a tremendous increase in cornering grip levels by using tyres with different compounds across the face of the tread - when it used radical camber angles, it was in fact applying a different rubber compound to the road.

The main advantage of active camber control is improved tyre wear when cornering near the limit. The disadvantage is that if a large camber variation is to be employed (eg 20 degrees) very complex mechanical systems need to be put into place, especially when the wheels are also steered.

5. Stability Control

Stability control brakes individual wheels (or sometimes a combination of wheels) to yaw the car. For example, if the car is understeering, the rear inside wheel is braked which causes the car to tighten its cornering line. An oversteering car has the outside front wheel braked.

A major advantage of the technology is that the major actuators (the brakes) are already in place and the system required to allow individual wheel braking (ABS) is also already installed. A marketing advantage is that the system can be clearly demonstrated as improving safety in that it only intervenes when the car is no longer heading in the direction requested of it by the driver. The disadvantage is that unlike the other active technologies mentioned here, because of their primary function of saving drivers, stability control systems tend to intervene early and harshly.

6. Active Torque Distribution

Four wheel drive cars that can actively distribute the torque front-to-back and side-to-side are able to affect understeer and oversteer without slowing the vehicle in the way that stability control does. The Lancer Evo is one of the best known cars that can actively change both the front/rear and the rear side/side torque distribution.

The advantages of taking this approach include the fact that the system can work with a sporting driver (rather than against them), and rapid and largely imperceptible interventions can be made. The disadvantages include the high cost (eg the car needs to be four-wheel drive to start with) and the fact that off-throttle corrections to the car's handling are much more limited - changing the torque split when the driver is not on the throttle makes little difference to handling.

7. Active Damper Control

Dampers which can be varied in their behaviour while on the move have been available for some years, but like four-wheel steering, at the moment the technology is relatively out of favour. The primary benefit is that ride can be improved for a given level of handling, although some handling benefits can also be gained (eg to turn-in) if the dampers are asymmetrically altered in their characteristics (side to side or front to rear). To have a more major impact on handling, complex control strategies need to be developed.

8. Active Suspension

Of all the 'active' technologies perhaps it is active suspension which has had the longest gestation period. In fact, it seems to have been 'about to be introduced' for perhaps 20 years...

One way of overcoming the major power demands of fully active suspension is to limit the speed and functionality of the system. If the system is to control only front/rear balance and the main vertical movements, it needs to work only up to a maximum speed of 5 movements per second. DaimlerChrysler's Active Body Control is of this type. High bandwidth active suspension systems work at speeds of up to 25 movements per second. They have better control of the suspension but at vastly increased cost.

The advantages of active suspension include the ability to alter ride height for aerodynamic benefits, improve ride, and change handling balance real time. The disadvantages include cost and complexity.

9. Active Anti-roll Bars

Much cheaper than active suspension, active anti-roll bars can have their relative stiffness altered while on the move. This allows the handling balance to be altered real-time. The current BMW 7 series has an active anti-roll bar system, and a simple hydraulically controlled system has been developed for the aftermarket (see "Active Suspension For The Masses").

The Best System?

So what combination of active systems is best? Prodrive believe that a system incorporating active toe control and active torque distribution would be best for driving pleasure. "Steering and throttle are major driver inputs," they say. "It is important to have both under the supervision of on-board active systems."

Conclusion

The widespread implementation ABS and electronic throttle have set the scene for car systems that do not always follow the driver's wishes. ABS allows the wheels to continue to turn when the driver is standing on the brakes; electronic throttle cars frequently open the throttle blade to an extent which does not match the driver request - for example, when compensating for a low engine torque output (it opens more than requested) or in order that wheelspin not occur (it opens less than requested).

So a car that alters the steer angle of the wheels without being instructed to do so (by actively altering toe) or which won't allow a large steering angle to be input at 100 km/h (steer-by-wire) is not a radical change in philosophy - those philosophical changes have already begun... But the potential of active systems - especially for enthusiastic drivers - is simply huge.

SAE paper 2003-01-0097, Brand-by-Wire - A Possibility?, Damian Harty, Prodrive Automotive Technology

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