A car’s handling characteristics are based on many different factors. The vehicle’s overall design, weight and weight distribution all play a role, but so too do individual chassis components, the wheels, tyres and steering. During the vehicle design phase the engineers initially use a combination of collective experience and specific design principles to put all parts together: a Mercedes-Benz, for example, always aims to offer the highest levels of comfort and safety – factors also taken into account in chassis design.
Preliminary prototype vehicles then give an indication as to whether the measures chosen are along the right lines. This is generally the case, thanks to the engineers’ experience, but with a prototype there is always room for improvement. For example, springs and dampers have to be adjusted in such a way as to absorb bumps in the road surface and reduce transmission of any unpleasant effects to the passenger cell, i.e. to make the car more comfortable. At the same time, road-holding ability has to be outstanding if it is to deliver a high degree of active safety. Tuning a purely mechanical chassis is always a compromise between various necessary factors. To give just one example, more firmly adjusted springs and dampers result in reduced ride and tyre comfort – and vice versa. Previously developers had to decide which priority to pursue; nowadays, electronically adjustable chassis components allow for individual adjustments, even while driving. If present, the test driver includes these components in his assessment.
A new model will go through many different phases on the traditional route to production standard. Initially there are various prototype stages, the external appearances of which are usually far removed from the eventual form of the new model. Often, for example, the chassis of the new development is fitted underneath the body of the predecessor model, which in many cases has a similar ambition. These prototypes provide vital clues at a very early stage as to the orientation of the new vehicle, and at the same time give an indication of the potential for future development work.
The next stage sees the construction of pre-prototypes. These are equipped with the body of the future car and in many aspects with technology that has already moved a long way towards production readiness. The pre-prototypes also serve as a basis on which the final decision is taken before proceeding to series development. This then involves the construction of numerous prototype vehicles with a view to testing all components under a wide range of conditions, as well as their collective operation in the complete car. These cars already show a marked resemblance to series specification.
Test drives are conducted at all stages of development. Tuning the chassis, to use just one example, is a lengthy and painstaking process. If testing reveals that something must be modified or a new adjustment is necessary, the work is carried out immediately and the car is returned to the test track. Each test drive helps to refine the product, until eventually it achieves the required series production configuration.
During testing, the drivers assess the car against a number of objective criteria in the form of a checklist. This is particularly important if comparability is to be maintained between different vehicles and different drivers. But subjective criteria are also recorded, since not everything that comes to light can be assigned to a single narrow assessment criterion. Useful in this regard is what is sometimes referred to as the “backside test” – in which top engineers and test drivers are able to pick up various driving conditions merely by sitting in the car. It is the same instinctive ability often reported by aircraft pilots, although enhanced in their case by a third dimension. Finally, of course, experience is indispensable in automotive testing, for a technician refines his criteria with every drive.
But driving and registering the potential for improvement are not the only key qualifications required. A tester also has to be able to implement these in the form of concrete instructions, so that the designers and mechanics can carry out the required modifications.
In order to achieve the perfect end-product, a test driver considers the vehicle during each drive in terms of an overall system, from the first metre of the tour to the last. The chassis developer pays close attention for example to noises and ergonomic factors, as well as to the appearance and haptics of surfaces in the interior.
Test driving and safety
Test drives conducted on public roads are of course subject to the full range of traffic legislation. Consequently driving manoeuvres that may endanger other road users are prohibited. The driver moves within permissible parameters. A good test circuit is one that provides insights into the vehicle while remaining within legal limits. Test driving is about automotive development not sporting competition.
One of the most important assessments that can be made about a chassis concerns the reactions a road surface draws from the car and its occupants. Any unevenness, for example, is transmitted to the vehicle interior. Nearly all reactions are described by six terms: roll, throw, tilt, pitch, lift and drop.
Roll: When minimal stimuli (experts refer to these as long wave or low frequency) from the road act reciprocally upon the car in a crosswise direction, the vehicle is caused to roll or lean laterally.
Throw: The term throw is used when strong reciprocal stimuli act crosswise at rapid intervals (shortwave or high frequency).
Tilt: Tilt is the reaction caused when weak reciprocal stimuli on the vehicle act lengthwise and are long wave (low frequency).
Pitch: If reciprocal stimuli acting lengthwise are strong and shortwave (high frequency), the vehicle’s reaction is said to pitch.
Lift and drop: Forces that work both horizontally and equilaterally (crosswise and lengthwise) on the vehicle cause it to lift and drop. The effect is dependent on speed, load and road surface.
Every car has what engineers call understeer/oversteer characteristics. These refer to the reactions of the car under limit handling conditions and can only be described in terms of the laws of physics. Rear-wheel drive vehicles, for example, tend to push the rear outwards on cornering, thus steering the entire vehicle into the curve (oversteer). Front-wheel drive cars on the other hand tend to push the front wheels towards the outside of the corner (understeer) when cornering at speed; the front wheels therefore do not completely follow the driver’s steering angle.
Another aspect of oversteer/understeer is the phenomenon by which a vehicle develops independent reactions through spring travel at the wheels causing an additional steering effect that must be avoided and corrected. If necessary, the tracking angle can be rendered more neutral by adjusting spring travel.
Assessment also involves inspecting vehicle behaviour on braking. How powerful is the braking effect, does it provide the required stopping distance? Is there the correct distribution of braking force between front and rear wheels, and between right and left wheels? Is the braking effect “blunt”, as experts put it, in other words is the brake pressure docile and easily dosed? Or are the brakes very responsive to pedal pressure, reacting very aggressively? Does the vehicle maintain reliable directional stability under emergency braking? Do driver assistance systems (e.g. the anti-lock braking system ABS, the electronic stability programme ESP® and brake assist BAS) function correctly and have they been set up accurately?
The test driver tests the car’s steering in tight and long corners, as well as in rapid series of bends. Does the steering transmit the driver’s commands directly or indirectly to the wheels? Does it convey the feeling of being in close touch with the road? Is it light or heavy? For vehicles with front-wheel drive, how strong an impact does the driveline have on the wheels? The size of the turning circle is also established; Mercedes-Benz enjoys a reputation for offering vehicles with small turning circles, which is achieved in part thanks to complex axle design and steering kinematics.
The nature of the asphalt surface also creates reactions in the car in terms of rolling behaviour. Noise level is a good indicator of this. On smooth asphalt surfaces, interior noise level drops; rough surfaces result in a higher noise level, and may additionally be accompanied by humming and booming noises. In each case the test driver establishes the noise level in the interior in order to check the effectiveness of the vehicle’s sound insulation, including the isolation of the passenger cell from chassis components and the effectiveness of insulating mats, and calls for improvements where required.
An airstream flows around a car when it is moving. Fuselage noise is referred to as “clean” by the testers; its origins lie in the design of body and window surfaces. Nuisance noise such as whistling or fluttering, on the other hand, can be caused by factors such as poorly fitting door seals or protruding parts, such as A-pillar trim, rear view mirrors or windscreen wiper blades. The assessor’s job is to isolate the cause of the nuisance noise and implement appropriate improvement measures.