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
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.
- 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.
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
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,
- 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
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.
- 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 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.
- 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.
- 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.
- 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.
- 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.
- 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
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