Volvo S80 Active Safety. Makes driving both more fun and safer

A rigid body is a prerequisite for stability, safety and comfort. The front and rear suspensions must be rigidly connected to one another through the body in order to achieve top-class road characteristics.

Almost half the strong Volvo S80 body structure is made of high strength steel (HSS) The choice of HSS and the actual design of the body both help to make it extremely rigid. Therefore all the substructures have also been designed to be very strong.

By working in an integrated way and consistently using computer-aided design engineering coupled with advanced calculation analyses (Finite Element Method), the body of the new Volvo S80 is the most torsionally-rigid Volvo to date. At the same time, it has also been possible to reduce the weight of the body.

In figures, this results in torsional stiffness of 18.6 kNm/degree, or, in layman’s terms, the equivalent of this hypothesis:

If another Volvo S80 (approx. 1,500 kg) were suspended on a wire from one of the front wheels of a Volvo S80, the body would turn itself less than one degree around its own longitudinal axis. One degree, which is not visible to the naked eye or hardly noticeable when examining the car.

The torsional stiffness of a four-door car body usually lies within the range of 6–14 kNm/degree; this clearly demonstrates the extreme rigidity of the Volvo S80. This is also an extraordinary figure, bearing in mind that the car features a split rear seat, since the rear seat frame plays an important role in this case.

This very high degree of torsional stiffness makes an important contribution to the fine driving characteristics, especially when cornering, and provides benefits in terms of improved safety and lower noise.

The chassis geometry of the Volvo S80 has been carefully developed using both new knowledge and old skills. The experience acquired from the high-performance versions of the Volvo 850 and subsequently the S70 and V70, has played a major role in the development of single suspension components and the tuning of the suspension.

The S80 features a choice of two optional active chassis systems – STC and DSTC –both of which actively assist the driver by counteracting the laws of nature and increasing stability and traction.

They cannot, however, fully do away with the laws of nature and should not be regarded as speed boosters, but they will most certainly help to give the car reliable, safe behaviour in situations in which many other cars abandon their drivers.

STC
Stability and Traction Control (STC) can be described as a combination of two existing Volvo systems, the low-speed, brake-activating TRACS system in the S70/V70/C70 models and the speed-independent, torque-limiting DSA system in the S40 and V40.

STC is designed to prevent the driving wheels from spinning when starting and accelerating, as well as counteracting wheel spin if the driving wheels hit slippery patches when driving.

The system improves tyre grip on slippery surfaces and provides the optimum combination of traction and side stability which in turn enhances the high level of active safety.

The traction function works at lower speeds, up to 40 kph, by transferring the engine power to the wheel with the best grip from the one with the least grip. This is done using the ABS sensors and the brake system.

The stability function reduces engine torque, by cutting down on the fuel supply, if one of the driving wheels starts to lose its grip on the road, both during acceleration and when driving on a slippery road. This minimises wheel spin and brings the car back to full stability.

The system reacts and is activated within milliseconds and driving characteristics are never affected. An indicator lamp on the dashboard flashes if and when STC is engaged.

STC is a standard feature on the Volvo S80 t6 and is available as an option on both six- and five-cylinder cars.

DSTC
Dynamic Stability and Traction Control (DSTC) is an even more sophisticated stability system to improve active safety by also intervening and counteracting any tendency to skid.

If the car makes a sudden movement and loses its directional line, or enters a corner too fast and the rear end starts to turn the car inwards, DSTC gently returns it to the right line. This is done by applying the brakes on one or more wheels.

DSTC works through the ABS system, which has been further developed.

A number of yaw angle sensors monitor the rotational speed of all four wheels, the steering wheel angle and the directional behaviour of the car.

The DSTC processor receives the signals and compares the actual behaviour of the car with the desired behaviour. Any deviation from the norm, such as the rear wheels starting to break away, and the system intervenes and makes corrections, activating the brakes on the wheel that will bring the car back in line. If necessary, the system also reduces engine torque, like STC.

In a way, it would be true to say that DSTC is able to compensate for the driver’s mistakes up to a certain point, as it counteracts and prevents skidding when the driver has already started to lose control.

The system is particularly effective if the driver has to brake hard and steer away at the same time, like the well-known elk test. The system counteracts any tendencies to skid and returns the car to its original course.

DSTC works through the ABS system which has been further developed to incorporate this system. In addition to more relief valves and a more powerful microprocessor, a brake reinforcement unit with electronic activation, sensors for side acceleration, yaw rate and steering wheel angle have been added.

The DSTC system will initially only be available on six-cylinder cars, Volvo S80 t6 and Volvo S80 2.9.

Brakes
Needless to say, the brake system is also both powerful and sophisticated, performing as it does so many more functions than simply stopping the car safely depending on driver input. Brake pedal feel has been further improved to provide a distinct yet comfortable feeling at all speeds and the pulsating ABS action is less pronounced. The stability and feeling of being in full control when braking on uneven surfaces is top-class, especially if there is a difference in friction between the left and the right sides.

The system features large-diameter discs, 286 mm with 15” wheels and 305 mm with 16” wheels with ventilation at the front, and 288 mm at the rear. All the wheels also feature sliding callipers.

Full use of the braking system is always possible as a result of Electronic Brake Force Distribution, EBD. The brake systems in modern cars are usually of two types. Either the front brakes are much more powerful than the rear ones or the braking force of the rear brakes is reduced by pressure-limiting valves to keep the force down to a certain level.

EBD, on the other hand, carefully monitors the behaviour of the rear wheels in relation to the front wheels and regulates the braking force electronically to match the degree of braking and load. In this way, perfect balance is always maintained by making the most effective use of the available braking force. Even when the driver brakes very hard, full directional stability and steering ability are maintained. The system is particularly effective when driving with heavy loads.

EBD permits far more braking force to be applied to the rear wheels as it always ensures full braking stability by regulating the pressure at an early stage of the braking process.

The entire EBD function is integrated in the ABS system, thereby enabling the function of the system to be monitored. Moreover, possible faults can be indicated by a warning lamp, as different from conventional pressure-limiting systems.

Should a defect occur in the ABS/EBD systems and be indicated to the driver, the system automatically changes to passive function. The car still has a normal braking system but without the ABS or EBD functions.

The most vulnerable part of a brake system is the brake lines. In order to minimise the risk of corrosion and brake circuit failure, the pipes are made of heavy-duty, long-life cu-ni-fer (copper-nickel-iron) alloy.

Front suspension
The design and geometry of the front suspension play a vital part in giving a powerful front-wheel drive car like the Volvo S80 the best possible dynamic properties. Here, too, it is a question of combining space and function in the optimum manner.

The front suspension features spring struts of the same design as the ones in the S70/V70/C70 but with modified geometry. The shock absorbers and coil springs are one unit and are asymmetrically positioned in relation to the strut.

Below the springs struts, there are triangular lower links and an anti-roll bar is directly attached to the spring struts via a link. This design produces very good directional stability when accelerating and braking on uneven surfaces. The suspension is supported by both anti-lift and anti-dive functions to prevent lifting tendencies when accelerating and diving tendencies when braking. It is important that the car maintains a horizontal position whenever possible for both safety and comfort reasons.

A small offset makes the front end less sensitive to acceleration forces and vibrations from the wheels. So a small front-wheel centre offset has been chosen, only 43 mm with wider tyres, 49 mm for the others.

Rear suspension
The rear end plays an important role in a front-wheel drive car. In a car with a pronounced orientation towards dynamism and comfort, the need for a first-class rear suspension was obvious.

The independent rear suspension is of the multi-link type and is mounted in a chill-cast aluminium subframe which is attached to the car at four points by means of insulating rubber bushings to keep noise and vibration to an absolute minimum.

The choice of aluminium gives the installation low weight and the low design enables the subframe to be installed without encroaching on the passenger compartment or luggage space.

The movements of the wheels are controlled by four links on each side (multi-link) – two trailing arms, upper and lower links, a track rod and an anti-roll bar.

This system permits a certain degree of rear wheel steer when cornering, as it provides assistance and helps to stabilise the car when braking.

The multi-link design combines a very high level of comfort with fine handling and controlled behaviour. There is also a safety aspect; the subframe is part of the rear impact protection system, protecting the fuel tank and creating the space for the spare wheel to lie flat, on top of the frame but in the boot, as it absorbs and helps to dissipate the impact forces.

The coil springs and shock absorbers are integrated to form one compact unit. There is also the option of an automatic levelling system, Nivomat. This system keeps the car horizontal and stable even when it is heavily laden. This also results in improved rear visibility and reduces the risk of dazzling oncoming motorists in the dark.

The steering is using the rack-and-pinion principle which is the most space-efficient and precise way of moving the wheels. The system has a very direct response and does not transmit any vibration or torque force. The power steering is available in a speed-sensitive version which adjusts the degree of amplification to match the speed and needs of the driver – light steering at low speed, more stable steering at higher speeds.

The nimble steering, which responds even to low input from the driver, has only three turns of the wheel, lock to lock.

Optimal optics
Active safety also includes seeing and being seen in poor visibility or at night. The light provided by the headlights is a very important safety factor. Being able to see along a long stretch of the road ahead could be a decisive factor in certain situations, just like being detected from the rear from far away. The Volvo S80 has very effective headlights and large tail-lights.

The headlights, double, so-called free-form headlights for both full and dipped beam, are equipped with wipers/washers, and can also be supplemented with elegantly integrated foglamps.

When full beam is applied, the dipped beam function remains in operation, thereby generating a very powerful beam and considerably increasing the driver’s field of vision in terms of both length and width.

The purpose of a headlight is to compress the light to obtain maximum effect and then to distribute the light to achieve the desired light image. In a conventional headlight, the reflector performs the former function and the optical pattern of the glass the latter.

In free-form headlights, the reflector is designed to perform both these functions and thus permits the use of clear glass without optical patterns. In other words, it is not the actual glass that forms the light image. The light is collected and distributed directly through the reflector surfaces which are designed in such a way that they produce the final light image.

As the surface of the reflector has a number of orientation points which no longer comply with any simple mathematical formula for the condition of the surface, they are called free-form surfaces - hence the name free-form headlights. As a result, the size and shape of the headlight opening is less important. The pure appearance of the free-form headlights helps to give the front of the Volvo S80 its very characteristic appearance.

Characteristic in appearance is almost an understatement when it comes to the large tail-lights. Clearly recognisable from the ECC environmental concept car, the light clusters are not simply aesthetic. They each house twin tail-lights. So, if one bulb blows, there is still one left.

It goes without saying that active safety is far more than this. It is a concept which everyone interprets in different ways. The areas mentioned above are important to Volvo and are just some of the points that are prioritised when developing a car, from concept to finished product.

Related page: Volvo S80 Original Press Release

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