Three new petrol engines deliver driving pleasure with a sporting edge
State-of-the-art six-cylinder unit develops excellent output and torque figures
Four-cylinder variant features unique TWINPULSE technology
Seven-speed automatic transmission available as option with steering-wheel gearshift buttons
With AIRSCARF, COMAND, a vario-roof and head/thorax side airbags, the new SLK Roadster is a remarkable car whichever way you look at it, packed with innovations no other vehicle in its class can match. The range of engines available for the new SLK is also outstanding. Customers can select from a choice of three petrol-engined variants developing between 120 kW/163 hp and 265 kW/360 hp and including - for the first time in this vehicle class - an eight-cylinder unit from the Mercedes-AMG stable.
The V6 engine in the SLK 350 is a totally new development boasting all the qualities you look for in a powerful, free-revving sports roadster engine. No other power unit in this displacement class can match the new six-cylinder unit when it comes to output and torque characteristics aimed firmly at enhancing driving dynamics and the over-all motoring experience. Added to which, the V6 newcomer also sets the pace in terms of fuel economy, exhaust emissions and comfort.
The six-cylinder powerplant develops 200 kW/272 hp at 6000 rpm from a displacement of 3498 cc. That equates with an output of just 57 kW/78 hp per litre which raises many an eyebrow in this displacement class. Torque levels are equally as exceptional, with 350 Newton metres available as low down as 2400 rpm and remaining constant up to 5000 rpm. That guarantees exceptional pulling power and rapid mid-range acceleration, but also relaxed driving in high gears. With the vario-roof down and an open road ahead of them, roadster fans could not ask for more.
NEDC combined consumption of 10.6 litres per 100 kilometres is further evidence of the state-of-the-art technology at work inside the V6 engine. Teamed up with the 7G-TRONIC seven-speed automatic transmission, the new power unit burns just 10.1 litres of fuel per 100 kilometres (NEDC combined consumption). This makes the SLK 350 some three percent more economical than the outgoing SLK six-cylinder model with automatic transmission, even though it can point to 25 percent greater output.
In the development of the new V6 engine, the Mercedes engineers trained their sights on output, torque, fuel economy, comfort and exhaust emissions in particular, and set new standards in each area. Their successful approach involved incorporating innovations into the engine construction which have a noticeably positive effect in various fields, rather than representing individual solutions as such.
Fourfold variable camshaft adjustment
By incorporating the four-valve technology and four overhead camshafts which are now staple ingredients of sports car engines, the Stuttgart-based engineers had already laid the foundations for exceptional power development. However, they refused to stop there, broadening their horizons to include a system which allows the interaction of the 24 valves to be controlled as required according to the engine load, thereby ensuring an ultra-fast charge cycle in the cylinders: continuous camshaft adjustment. The angles of the intake and exhaust camshafts can each be continuously adjusted by 40 degrees, allowing the valves to open or close at the optimum moment in any driving situation.
At low throttle this technology allows exhaust gases to flow directly from the combustion chamber back into the intake port. The camshafts are controlled in such a way as to keep the exhaust valves open for a short time whilst the intake valves are opening. During this split second, some of the exhaust gases are able to flow from the exhaust port into the intake port. The vacuum in the intake manifold assists this process. This valve overlap when expelling the used gases and drawing in the fresh combustion mixture ensures efficient internal exhaust gas recirculation. This reduces the amount of energy lost during load changes in the cylinders, leading to substantially lower fuel consumption.
With the throttle open wider, meanwhile, camshaft adjustment also serves to optimise valve overlap in line with the engine speed so that the combustion chambers are supplied as efficiently as possible with fresh mixture. The result is an increase in power output and torque. The camshafts are controlled by electrohydraulic vane-type adjusters positioned at the front end of the camshafts and controlled by means of four integral hydraulic valves. The intake camshafts are driven by a duplex chain, whilst the exhaust camshafts are moved directly by the intake camshafts via a meshed pair of gear wheels.
In addition to this fourfold continuous camshaft adjustment, a series of other measures also contribute to the exceptional power development of the V6 engine:
Flow-optimised intake ports, designed to allow optimum throughput and featuring innovative tumble flaps;
The specially developed valves, which have a shaft diameter of only six millimetres and thus only represent a minor hindrance to flow in the intake port;
The compact combustion chambers, which promote a high compression ratio (10.7:1) and impressive efficiency;
The newly developed, two-stage variable intake manifold.
As with all the latest Mercedes engines, valve clearance adjustment is hydraulic and, as such, maintenance-free.
The development team for the V6 devoted a great deal of their attention to measures contributing to the best possible engine aspiration. The engineers used sophisticated computer programs to make flow calculations, the software helping them to optimise the flow of air from the twin-chamber air filter. Here the ducts interface with a component which is of key importance for the engine functions: the hot-film air-mass sensor. Airflow-related further development work has been carried out on this component in order to optimise the air supply. The oval-shaped housing of the air mass sensor and a modified mesh with low air resistance are two of the most significant products of this detailed improvement process.
The intake module: two-stage design ensures a controlled air supply
Air supply can be adjusted according to the engine load and speed with the help of a proven magnesium intake module. The length of the intake pipes which lead to the cylinders is altered by the use of flaps: the flaps are opened at high engine speeds – from approx. 3500 rpm - and the air flows the short distance to the combustion chambers, helping to generate impressive output.
At low engine speeds the flaps are closed and the length of the intake duct increases. This creates pressure waves, which aid the intake process and ensure a fundamental improvement in torque levels at low engine speeds. Indeed, 305 Newton metres of pulling power – some 87 percent of maximum torque – is available at 1500 rpm. Turbulent airflow: tumble flaps in the intake ducts
The key features of the intake module in the Mercedes six-cylinder engine are electropneumatically driven flaps at the end of each intake port which make a significant contribution to enhancing fuel economy. Mercedes engineers refer to these as “tumble flaps”, a term which describes their role in causing the fuel/air mixture literally to tumble. This increases the turbulence of the airflow, which consequently enters the combustion chambers at greater speed and spreads out evenly. The end result is improved – i.e. more complete – combustion.
At partial load, the tumble flaps pivot up, optimising air flow and thus accelerating the speed of combustion – an advantage that becomes particularly noticeable in the extremely lean mixture produced by exhaust gas recirculation and one that helps to cut fuel consumption. Under greater engine loads, the tumble flaps are not required and fold down into the intake manifold where they cannot impede the intake process. The tumble flaps are controlled according to the situation in hand on the basis of calculations stored in the engine management system.
The tumble flaps in the intake ports can reduce the fuel consumption of the V6 engine by up to 0.2 litres per 100 kilometres depending on the engine speed, as well as enhancing smoothness.
Systematic fuel economy: heat management in all situations
In the interests of further reducing fuel consumption Mercedes engineers have developed an intelligent heat management system. For example, coolant circulation is interrupted during the warm-up phase to allow the engine to reach its operating temperature more quickly. This leads to improved oil flow and therefore significantly reduced friction in the engine. A drop in exhaust emissions is another benefit of this system. With the engine warmed up and working under full load, the heat flows are still directed in such a way that the engine oil and coolant remain constantly at optimum temperature. Here, the credit goes to an innovative, logic-controlled thermostat which is active in all operating situations.
The engine: the benefits of lightweight construction
The cylinder head and crankcase of the new V6 engine are made of aluminium. Plus, the pistons, connecting rods and cylinder liners are manufactured according to state-of-the-art construction principles which not only help to reduce weight but also noticeably enhance the responsiveness and smoothness of the engine. This is because the lower the moving masses in the crankcase, the greater the reduction in vibration and the more responsive the engine becomes to the driver’s prompting via the accelerator pedal:
The pistons are made of iron-coated aluminium. The piston crowns – developed to take into account the valve angle (28.5 degrees) – are designed to ensure a favourable combustion chamber shape.
Mercedes engineers have reduced the weight of the forged steel connecting rods by around 20 percent compared with other V6 engines, making a significant contribution to the remarkably smooth running of the new six-cylinder unit.
The cylinder liners feature low-friction aluminium/silicon surfaces, a technology which has already proved its mettle in other Mercedes-Benz passenger-car engines. Minimal distortion, exceptional heat flow and impressively low weight are among the other plus points. Indeed, these liners are some 500 grams per cylinder lighter than their conventional cast-iron equivalents.
The forged crankshaft is fitted with four counterweights. Four wide crankshaft bearings attached to the crankcase by transverse reinforcing struts also help to keep a lid on vibration.
A balancer shaft between the two cylinder banks smoothes out the characteristic free vibrations of a V6 engine to ensure exceptional smoothness. The balancer shaft counter-rotates at the same speed as the crankshaft.
The acoustics: reduced noise, but enhanced sound
In addition to outstanding output and torque characteristics, impressive fuel economy and low vibrations, the new V6 engine also boasts certain audible benefits. As part of an exhaustive programme of testing, Mercedes engineers took acoustic measurements from almost all of the engine’s 210 separate parts - from the crankcase to the engine mounts, the pistons and the injection valves – and assessed their noise output levels and frequency.
Their aim was to ensure that the engine generated a pleasing sound in any driving situation. In order to achieve this, they not only measured absolute noise levels, but also sources of annoying noise which may not be especially loud but which produce obtrusive frequencies under changes in load and engine speed - and therefore detract from the driver and passenger’s enjoyment of the engine sound.
Noise configuration is developing into one of the key areas of engine development. Here, the engineers are faced with the challenge of keeping drive-past noise suppressed whilst shaping a pleasant and sonorous engine sound. To this end, the experts charged with creating the six-cylinder unit for the SLK-Class adopted an approach focusing on reducing noise while improving sound. A series of measures were developed with the aim of restricting engine noise, from a two-cartridge air filter with integral resonators to a sound-absorbent mat under the bonnet. At the same time, the sound specialists channelled their efforts into creating sports car vocals for the SLK-Class. They achieved this by emphasising certain frequencies, predominantly through careful configuration of the twin-pipe exhaust system.
The four-cylinder engine: TWINPULSE technology brings an eight-percent improvement in fuel economy
Also making its debut in the SLK-Class is the state-of-the-art four-cylinder engine developing 120 kW/163 hp, a member of the new generation of TWINPULSE power units. This engine type combines various technologies – such as supercharging, charge air cooling, four-valve technology, variably adjustable camshafts and Lanchester balancer shafts – which push driving pleasure and smoothness to the maximum and keep fuel consumption to a minimum. This cutting-edge package allows the new SLK 200 KOMPRESSOR to get by on eight percent less fuel than its predecessor of the same power output. NEDC combined consumption stands at 8.7 litres per 100 kilometres.
The cylinder head of the four-cylinder power unit is made of high-strength aluminium alloy and equipped with 16 valves and two overhead camshafts. As in the new V6 engine, these have variable adjustment. A vane-type adjuster with integral control valve allows continuous adjustment of the valve timing according to the situation in hand, producing impressive results in terms of torque development under full load, fuel economy and exhaust emissions. The valves are controlled by means of cam followers and feature maintenance-free hydraulic valve clearance.
In the four-cylinder SLK, a supercharger with charge air cooling provides the muscle underpinning the agility and dynamism you look for in a sports car. This means that high levels of torque can even be called on at idling speed, in turn allowing a longer overall ratio. The new engines are thus impressively lean-running – i.e. extremely economical. Further enhanced performance and further reduced fuel consumption form the essence of TWINPULSE engines.
The unique combination of a belt-driven supercharger and Lanchester dynamic balancer is an integral element of the TWINPULSE system and provides a convincing solution to another conflict of interests in four-cylinder engines. Two forged, multi-bearing shafts located underneath the crank assembly counter-rotate at double the speed of the crankshaft. This allows them to balance out the inertia forces inherent in this type of engine, which are brought about – among other things – by the movement of the pistons and can cause obtrusive vibrations.
Exhaust emissions under the EU-4 limits
In common with all the latest Mercedes-Benz passenger cars, the new SLK-Class also meets the stringent EU-4 exhaust emissions standards which come into force in 2005. Like the SLK 350 and SLK 55 AMG, the four-cylinder SLK 200 KOMPRESSOR is fitted with a twin-pipe exhaust system. The two striking tailpipes on the left and right of the car’s rear end fit in perfectly with the overall design of the car.
The emission control system is based on a dual concept: sophisticated engine-specific measures whose aim is to reduce untreated emissions and highly effective emissions treatment using catalytic converters located close to the engine. These have a volume of 0.9 litres in the four-cylinder variant and 2x 1.4 litres in the six-cylinder unit. In both the SLK 200 KOMPRESSOR and the SLK 350 the catalytic converters are each fitted with two oxygen sensors – one control sensor and one diagnostic sensor – with linear control. This means that the oxygen sensors are active immediately after a cold start and supply information relating to the composition of the exhaust emissions. The electronic control unit then processes this data for use in the control of the warm-up phase, for instance. The air-gap-insulated stainless steel exhaust manifold allows the catalytic converters to reach their operating temperature more quickly.
Among the systems at work inside the engine is the continuous camshaft adjustment, which permits more efficient exhaust gas recirculation within the engine under partial loads. Plus, the adjustable tumble flaps in the intake ducts of the V6 engine, which improve the combustion process, make an important contribution to minimising the levels of untreated emissions produced by the engine. Added to which, a secondary air injection system has an afterburning effect on the exhaust emissions, causing the temperature in the exhaust ports to rise and allowing the catalytic converter to begin processing the pollutants at an earlier stage. This afterburning process also helps to reduce the carbon monoxide and hydrocarbon content in untreated exhaust gases.
Transmissions: five, six and seven-speed variants available
Mercedes engineers have further improved the six-speed manual transmission fitted as standard in the SLK 200 KOMPRESSOR and SLK 350 and given it an even sportier edge. The key to these improvements is a single-rod gearshift system, which allows drivers to make fast, sporty gear changes, whilst at the same time offering even greater shifting comfort thanks to the use of sophisticated damping elements.
By contrast to the manual gearshift system in the outgoing SLK, in which the vertical and horizontal movements of the gearshift lever were transferred to the transmission separately, a single gearshift shift rod now communicates both movements at the same time. This means an end to the indirect routes of the previous system and noticeable benefits for the driver in the form of fast and crisp gear changes.
Greater shift quality and improved precision allow the new single-rod gearshift to raise the bar in terms of driving pleasure. The driver enjoys a more harmonious shift feel, with the shift process benefiting from improved damping – and thus coming across as more rounded – thanks to the damping elements in the lower area of the shift lever and on the shift housing. In addition, the dampers cut the shift lever off extremely efficiently from the vibrations produced by the transmission and engine while at the same time reducing shift noise to a minimum.
A further change to the previous Mercedes six-speed manual transmission sees the reverse gear moved forward from its usual left rear position to the front left alongside first gear.
7G-TRONIC: seven gears for lower fuel consumption and even greater driving pleasure
The SLK 350 can be ordered as an option with the world’s first seven-speed automatic transmission (fitted as standard in the SLK 55 AMG). The 7G-TRONIC automatic transmission is brimming with technical innovations which ensure more powerful acceleration, extra-fast mid-range sprints, lower fuel consumption and a higher level of shift comfort.
The driving force behind the seven-speed automatic’s outstanding qualities is provided by a series of design features, the most important of which is the increase in forwards gears from five to seven. This allows a wider spread of ratios, as well as further reducing the differences in engine speed between the individual gears achieved by the five-speed transmission. As a result, the driver can rely on having the optimum ratio at his or her disposal for virtually any driving situation. In addition, the electronic control unit has even more scope in which to optimise the shift processes to achieve lower fuel consumption and greater comfort. For example, at 100 km/h the engine speed will be on average – depending on the driving situation - around twelve percent lower than with a five-speed automatic. This impressive engine speed adjustment system opens the door to lower noise, as well as improved fuel economy.
Mercedes engineers have achieved further advances as far as shift control is concerned. If the driver needs to accelerate quickly and therefore change down rapidly through several gears – i.e. kick-down - 7G-TRONIC avoids having to move through the gears in strict order. Instead, the transmission uses its direct downshift capability, shifting down by as many as four gears – as the situation demands – rather than just one at a time. This paves the way for direct gear changes from seventh gear to fifth, from fifth to third or from third to first, and even four-gear jumps from sixth gear to second, with only one valve opening and one valve closing in each case. The change in engine speed from the current gear to the target gear is extremely fast and spontaneous and feels more or less the same as a conventional single-gear downshift.
Where multi-gear downshift is not carried out directly – e.g. the frequent shifts from seventh into fourth or from seventh into third gear – the second part of the shift process is managed in such a way that it overlaps with the first, with the result that the driver senses nothing more than a steady change in engine speed. Shift times have been reduced significantly below the levels of the five-speed automatic transmission fitted up to now.
Like the proven five-speed automatic, which is available as an option for the SLK 200 KOMPRESSOR, the new seven-speed automatic transmission also boasts a torque converter lock-up clutch. This system is located in the hydrodynamic torque converter and largely eliminates slip between the pumps and the turbine wheel in many operating conditions. Its manages this by establishing – where possible – a virtually rigid connection between the engine and transmission shafts and thus preventing power losses.
By contrast to conventional automatic transmissions, where the converter can only be locked up in higher gears, the lock-up clutch in the Mercedes-Benz seven-speed automatic is already active in first gear. In addition, in the interests of comfort the torque converter lock-up clutch features slip control, which allows it to run extremely smoothly. This is just another way in which Mercedes engineers have achieved unbeatable levels of shift quality.
Transmissions: manual gear-change using buttons on the steering wheel
In keeping with the dynamic and agile character of the new SLK-Class, drivers can operate both the five-speed and seven-speed automatic transmissions using gear-shift buttons on the steering wheel (optional). Plus, they can limit the gear stages or select a new gear manually, allowing them to fully exploit the engines’ extensive power reserves in any driving situation. And they always have the option, of course, of switching to “M” (Manual) mode at the touch of a button on the tunnel trim. The gear currently selected is highlighted in the central display of the instrument cluster.
When manual mode is selected, the automatic transmission still shifts up to the next gear when the maximum engine speed is reached.
In addition to manual mode, drivers can also make use of the “C” (Comfort) and “S” (Sport) settings. In these programs, the gears are changed automatically but the steering-wheel gearshift buttons are also active, so that drivers can switch to manual operation at any time.
Newly developed front axle and steering ensure dynamic handling
Sporty set-up for multi-link independent rear suspension
ESP available as an option with automatic tyre pressure monitoring
17-inch light-alloy wheels standard for SLK 350
Vibrant dynamism, light-footed motoring pleasure, maximum safety - three hallmark characteristics of the SLK-Class which are moulded to a great degree by the Roadster’s chassis. The axles, steering and brakes have all been developed anew or redeveloped by the Mercedes engineers in an effort to surpass even the high standards set by the model it replaces. The advances that have been achieved make their presence felt within the first few miles and help create the inimitable motoring experience – above all thanks to the sporty handling, high-precision steering, excellent ride comfort, sure handling right up to the car’s limits and exemplary braking stability.
The previous double-wishbone front suspension with recirculating-ball steering has now been superseded by a three-link axle featuring McPherson spring struts and a rack-and-pinion steering system. This new axle technology revolves around two individual link elements which serve as torque and cross struts. Apart from more precise wheel location, the prime benefit offered by these two links is the ability to compensate for vibrations caused by tyre imbalances or brake force fluctuations more effectively than rigid wishbone designs. Added to this is the fact that, in the event of a frontal collision, the crumple zones in the vicinity of the bottom link level are larger, allowing the front end of the vehicle to absorb more of the impact energy. The torque struts of the SLK’s front axle are made from aluminium, whilst the cross struts are manufactured from forged steel.
The third front axle link is the track rod which connects the laterally positioned steering gear to the wheels. The spring struts also fulfil more than just one function, as, apart from being responsible for axle suspension and damping, they also have an active role to play in wheel location.
The spring struts consist of coil springs, twin-tube, gas-filled shock absorbers and compact head bearings. The standard-fit anti-roll bar is linked to the spring struts by means of a torsion-bar linkage, with rebound buffer springs being used to boost the effect of the anti-roll bar when the vehicle is cornering at speed.
The lower components of the front axle, plus the steering gear and the engine bearings, are connected to an assembly carrier, which is in turn bolted directly to the vehicle body. Following on from its successful implementation in other model series, Mercedes-Benz is now transferring this technology to the SLK-Class for the first time, as it offers straightforward assembly coupled with significant safety benefits: in the event of a front-end collision, the assembly carrier will crumple to absorb a portion of the impact energy. Steering: new technology for sporty handling
There were a number of good reasons for switching to the new steering system. The most pressing argument was the more direct response offered by rack-and-pinion steering which helps underline the dynamic, sporty credentials of the new SLK-Class. Furthermore, positioning the steering gear in front of the wheel centre produces a tendency to understeer, enhancing handling safety when cornering. Other aspects in favour of the new technology are its contribution to occupant protection and its lightweight design principle: whereas the gear of a recirculating-ball steering system forms a rigid block which is incapable of absorbing energy in a frontal collision, the rack-and-pinion steering can be mounted in a crossways configuration on the axle carrier, meaning that it does not interfere with energy absorption in any way. And as the system does without additional components, such as a pitman arm, drag link, steering idler arm and strengthening plate, it weighs far less than the recirculating-ball steering featured on the predecessor model.
The steering continues to be power-assisted to reduce the effort required to turn the wheel. Making its debut in the SLK-Class, however, is the speed-sensitive steering which is available as an option, and which is even smoother and more closely geared towards the driver’s needs. It is designed to reduce steering torque in relation to the current road speed by up to 50 percent.
Drivers of the new SLK-Class have a number of options for setting the steering wheel to the ideal position: by merely pulling out the handle below the steering column, the steering wheel can be smoothly adjusted for height (by +/- 25 millimetres) and for reach (by +/-30 millimetres). The electrical steering column adjustment, which is available as an option in conjunction with the memory package for the seats and exterior mirrors, offers drivers even greater ease of use.
Rear axle: multi-link independent design with improved vibration characteristics
The multi-link independent rear suspension which has already proved so successful in the past will continue to be fitted on the new SLK-Class where it combines with the newly developed front axle, the extended wheelbase and the standard-specification ESP® Electronic Stability Program to deliver a high degree of handling safety, dynamism and passenger comfort. Modifications made to the kinematics and elasto-kinematics, stemming primarily from the use of optimised bearings, both improve the vibration characteristics yet further, as well as reinforcing the two-seater’s tendency to understeer, which is in itself a guarantee of excellent handling safety. The multi-link independent suspension has also been adapted to the sports car’s increased track width, while the anti-roll bar fitted as standard on all models is attached directly to the body.
Suspension and damping of the rear wheels is the task of coil springs working together with single-tube, gas-filled shock absorbers.
Drivers with a passion for sporty motoring can opt for the SLK-Class chassis to be given firmer suspension and damping settings. With the sports chassis fitted, the body is lowered by ten millimetres at the front and the rear. In this sportier set-up, total spring travel at both axles is shortened by ten millimetres compared to the standard chassis, while the spring rates increase by as much as ten percent. Brakes: state-of-the-art technology for maximising safety
Mercedes engineers have once again made tremendous progress in the field of braking technology too, particularly when it comes to braking performance and smoothness. Each of the different models in the range comes off the assembly line complete with internally ventilated disc brakes measuring between 288 and 340 millimetres in diameter, depending on the model in question.
The SLK 350 is equipped with a state-of-the-art, lightweight braking system including perforated front brake discs. The perforations are an efficient means of dispelling dirt and water, meaning that the brakes continue to deliver excellent stopping power in inclement weather conditions. The cover plates underwent development in the wind tunnel: they have been styled in such a way that the airstream flows from the inside outwards to keep the brake discs free of spray water. At the same time, cooling of the brake discs and the wheel bearings has also been enhanced. Reliable deceleration at the rear of the new SLK-Class comes in the form of solid disc brakes. The fixed-calliper brakes used here by Mercedes-Benz feature twin-piston technology. Mercedes-Benz has used an electronically controlled solenoid valve for the brake force booster which allows brake pressure to be metered with particularly high precision. This also helps to improve the dynamics of the brake pulses emitted by the ESP® Electronic Stability Program and by Brake Assist.
Control systems: ESP with warning function to indicate low tyre pressure (optional)
The new SLK-Class, like all Mercedes-Benz passenger cars, is equipped as standard with the most advanced and most efficient driving safety systems in the world. They reduce the risk of skidding when cornering, keep the sports Roadster safely on course when braking, make pulling away on a slippery surface easier and shorten the braking distance in emergency situations. Such sophisticated technology is an important component of the safety concept applied at Mercedes-Benz and helps to prevent road traffic accidents.
ESP®, the Electronic Stability Program, is at the heart of all dynamic handling control systems. It reduces the risk of skidding and keeps the SLK safely on track. Sensors monitor driving style and the vehicle’s handling characteristics, and relay their data to a powerful micro-computer which has been programmed with a mathematical model. This allows it to compare the vehicle’s actual operating status with an appropriate target status for the current driving situation. Where there is a serious discrepancy, the system intervenes by applying a specially developed logic, triggering precisely metered brake pulses at the front and rear wheels, as well as carefully throttling the engine torque. This allows the ESP® system to stabilise the vehicle, even when it is skidding as a result of either extreme driving manoeuvres or of adverse road surface conditions, such as those caused by ice, rain or loose chippings.
In the new version of the Mercedes-Benz Roadster, ESP® is also able to offer a further function as an option which warns drivers of a loss of pressure in the tyres. ESP® constantly monitors the rotation speed of the wheels, which depends mainly on road speed, the vehicle load and the air pressure in the tyres. Because the ESP® Stability Program permanently records the wheel rotation speeds and compares the values with one another, it is able to detect any significant deviations in the readings. The control module also automatically checks other handling dynamics variables, such as lateral acceleration, yaw rate and wheel torque, in order to reliably diagnose a loss of air pressure in any one tyre. The system does not, however, measure the actual tyre air pressure. If the system detects that pressure is being lost in any of the tyres, a red warning appears in the cockpit’s central display which reads "Tyre pressure, check tyres." Wheels and tyres: extensive factory-fitted range to choose from
The SLK 200 KOMPRESSOR is shod as standard with 205/55 R 16 wide-base tyres and 16-inch light-alloy wheels sporting an attractive seven-spoke design. The six-cylinder SLK 350 model comes with 17-inch wheels featuring an even sportier five-spoke design, and is fitted with wider tyres at the rear (245/40 R 17) than at the front (225/45 R 17).
Further wheel designs are also available to order as an option; the range includes six different styles in all. The SLK 55 AMG emphasises its high-performance character with tyres measuring 225/40 R 18 at the front and 245/35 R 18 at the rear, which sit on specially styled, silver-painted AMG light-alloy wheels.
cedes customers can keep on the move, even if they suffer a puncture, the Stuttgart-based car manufacturer is also offering run-flat tyres as an option for the new SLK-Class in conjunction with the ESP® program’s tyre pressure loss warning system. These tyres use self-supporting side walls which maintain the tyre’s structure following a loss of pressure so that the vehicle can continue to be driven for a short time. The vehicle’s range under these conditions is around 50 kilometres if it is partially laden and around 30 kilometres when the boot is fully loaded, whilst top speed is restricted to 80 km/h in both cases. The TIREFIT tyre repair kit that is included with the car as standard can also be used by motorists to repair minor damage to the tyre and increase this range significantly. However, in this case too the vehicle still may not be driven any faster than 80 km/h.
Newly developed head/thorax side airbags in the seat backrests
Sophisticated all-round protection for the passenger compartment
Real-life safety – this tried-and-tested practical approach to safety as applied by Mercedes-Benz has been reinforced and thereby consistently honed over the course of recent years by a series of groundbreaking new developments from the Stuttgart-based vehicle brand’s engineers. The new SLK-Class reaps the benefits of the progress that has been made: the safety technology on board the sporty roadster is in line with the very latest findings from the field of accident research, and sets new benchmark standards for this vehicle segment.
The Mercedes concept centres around restraint systems capable of adapting to different types of collision. The result is a further decrease in the loads placed on occupants in the event of a severe collision, as well as optimisation of the protective functions triggered following more moderate collisions. It is in this key aspect that the new SLK Roadster differs noticeably from its predecessor, which was itself deemed to be one of the safest sports cars on the road. On the new model, both the driver and passenger airbags feature two-stage operation, as do the new two-seater’s belt force limiters. This means that the systems are deployed in such a way that they are able to offer the greatest possible protection for the situation at hand.
The driver’s airbag is fitted with a two-stage gas generator: in the event of a minor impact at the front, the control unit only ignites the first stage and the airbag inflates more gently. Should the control unit detect a severe frontal collision, however, it deploys the second stage of the gas generator after a delay of some 5 to 15 milliseconds. As a result, the airbag inflates at a higher pressure to provide the driver with the level of protection required in a collision of this severity.
The airbag on the passenger side operates on precisely the same principle, as well as incorporating brand new technology which further improves the way in which the airbag inflates evenly to cover a wide area. In addition to this, the initial pressure increase of the two-stage generator is reduced, meaning that the airbag inflates even more gently. This shows just how much consideration Mercedes engineers have given to the fact that children are often seated in the front passenger seat, where the newly developed airbag now offers them even more protection.
The belt force limiters, which make up part of the standard specification of the new SLK-Class, are also able to adapt to the severity of the collision. Should the control module detect a high impact load, it will reduce the restraining force acting on the belt strap after just a short time; as a result, more of the seat belt strap will be fed out and the airbag will be able to cushion the seat occupant more effectively.
Head/thorax airbag: new air cushion integrated into the seat backrest
Mercedes-Benz is also equipping the new SLK-Class as standard with the newly developed head/thorax sidebags which have already debuted in the top-of-the-range SL-Class sports cars. They are accommodated in the backrests of the seats where they are positioned between the sturdy magnesium frame and the cushion holder. Following a side impact, the head/thorax airbag transforms into an asymmetrically shaped cushion in a matter of just 30 milliseconds, with the top edge reaching high above the seat when the airbag is fully inflated. This reduces the risk of the occupant’s head colliding with either the side window or any objects that penetrate the interior of the vehicle, regardless of the occupant’s seating position.
The middle of the head/thorax airbag slides between occupant and door in the event of an impact from the side or the vehicle rolling over, so that the chest area is also cushioned.
Sensor system: upfront sensors permit quick detection of accident severity
The swift, adaptive response of the restraint system is made possible by a highly advanced sensor system. In the event of a head-on collision, fast response is taken care of by two extra upfront sensors. Their remote positioning on the radiator cross member in the front end of the SLK body enables them to detect the severity of a collision even earlier and with greater accuracy than the central crash sensor on the transmission tunnel. The information from the upfront sensors is used by the electronic control module to shorten the time that elapses between the moment of impact and the deployment of the belt tensioners, to adapt the operation of the belt force limiters to the severity of the collision and to trigger the airbags both earlier and in two stages, depending on the situation.
Rapid activation of the head/thorax airbags in the seat backrests is the task of two additional satellite sensors positioned on the outer edges of the bodyshell’s seat cross members which work together with the central crash sensor.
Quite apart from this, the new SLK-Class is also equipped as standard by Mercedes-Benz with a roll-over sensor, which reliably recognises this type of accident and relays its data to the restraint systems’ central control module. In response to this signal, the micro-computer activates the belt tensioners and the head/thorax sidebags in the SLK seats, regardless of whether the vario-roof is open or closed.
Still to be found on the list of standard equipment for the SLK-Class are a sensor pad in the passenger seat which detects whether the seat is occupied, as well as the inhouse-developed automatic child seat recognition facility. The child seat recognition facility comprises special aerials in the seat cushion which exchange data with the transponders that are fitted in the base of special child seats. Using this data, the central control module is capable of detecting when a child seat has been fitted, causing it to deactivate the airbag on the passenger side, as deployment of the airbag could otherwise increase the risk of injury. Bodyshell structure: larger crumple zones at the front end
The bodyshell structure of the new SLK-Class also offers occupants greater safety reserves than the predecessor model. The crumple zones in the front end, which absorb energy in the event of a frontal collision, have now been enlarged and the flow of forces optimised in order to offer even better protection for the passenger cell. This fact is clearly demonstrated by the results of numerous crash tests which the new Roadster successfully completed as part of a development process that lasted several years. As well as the statutory test specifications of various countries and the requirements laid down by the Euro and US NCAP (New Car Assessment Program), the SLK-Class was also subject to Mercedes’ own, extra-strict test criteria. These test criteria generally reflect the findings of the Mercedes accident research department and allow vehicles to be designed with real-life accidents in mind.
Virtually every safety-related body component has been checked and redeveloped by the Sindelfingen engineers with the aim of optimising geometry, material thickness, joining technology and material quality. The proportion of high-strength steel alloys, which ensure maximum strength combined with a low weight, has been increased to around 42 percent, an important prerequisite for achieving exemplary standards of safety. The majority of the components which determine the bodyshell’s crash characteristics are manufactured using high-strength sheet steel.
Front end: split-level side members and sophisticated firewall design
Key features of the structure at the front end include straight front side members and the twin-wall front cross member. These elements form a sturdy composite assembly, so that in the event of a frontal collision where a load is exerted on one side only at the front end (offset impact), the opposing side is also able to absorb some of the energy. Mercedes-Benz engineers have also developed a second side member level which is positioned above the wheel arches and front wheels and has been specifically designed to enhance occupant protection in the event of an offset crash at the front. This construction also takes account of the fact that a new McPherson front suspension is now being used which permits longer crumple zones compared to the dual-wishbone suspension used on the previous model.
The forwards-arching firewall is reinforced by an elaborately designed cross member, which is in turn welded to the front side members. This allows the impact forces which are released following a frontal collision to be distributed over a large area in the vicinity of the firewall. Plus, the front end of the new SLK-Class is equipped with pedal-floor cross members which also link up with the front side members and form a protective barrier in front of the footwell area.
The pendulum support on the master brake cylinder, which was already used successfully on the predecessor model, has now been redesigned; the pendulum support prevents the brake pedal from moving further backwards into the driver’s footwell. A generously sized impact absorber made from polypropylene which is positioned below the footrest is also capable of cushioning some of the impact energy.
The frame-style assembly carrier, which holds the steering gear, the engine mountings and some of the wheel location components, is also incorporated into the crash concept of the new SLK-Class and is capable of effectively dispersing energy in the event of a frontal impact. The carrier is bolted to the front side members. In the event of a crash, the wheels are braced by special impact elements positioned in front of the lateral side members.
The passenger cell of the new SLK-Class is built on a sturdy floor assembly with an inserted transmission tunnel made from thick-walled steel, plus an additional locking plate. Straight support sections running underneath the body on both sides link the front end’s side members with the rear-end structure. Lateral reinforcement for the floor is provided in the form of two solid supports underneath each seat.
The lateral side members consist of an inner and an outer wall, which are strengthened by bulkhead plates. The cross-sectional area of the side skirts increases towards the rear to produce a large connection with the (concealed) B-pillars. The bolted-in aluminium rear wall, a cross member running underneath it and the vario-roof’s solid support together form a robust composite crossways structure at the rear which protects occupants in a lateral collision.
Roll-over protection: sturdy tubular reinforcements in the A-pillars
Engineers at Mercedes also devoted a great deal of attention to roll-over protection in the new SLK-Class. In addition to the characteristic roll-over bars behind the seats, roll-over protection also comes in the form of a high-strength, A-pillar composite structure. At the core of this assembly are two oval pipes made from heat-formed, high-strength steel, which are welded together with the A-pillar’s metal walls and extend up to the top of the windscreen frame. These complement the protective effect of the roll-over bars and ensure that the passengers’ survival space remains as intact as possible should the vehicle roll over.
To enable them to withstand high loads, the oval tubes are integrated by means of high-strength steel plates into the structure of the A-pillars, which are in turn braced against the lateral side members by sturdy gusset plates. In order to give it extra strength, the material is formed at the steel works while it is still glowing hot.
Just how rigid this new construction is can be plainly seen from the results of the Mercedes-Benz roof-drop test, an internal rigidity test in which the body is dropped onto its roof at a slight angle from a height of 50 centimetres. The vehicle lands with its full weight on one of the two A-pillars, which may only deform slightly if the vehicle is to pass the test. And the new SLK-Class came through this roof-drop test with flying colours.
Mercedes engineers opted for an oval shape for the tubular reinforcement so that they would be able to keep the A-pillars as slender as possible, ensuring minimal restriction of the driver’s angle of vision. Thanks to this technique, the all-round view in the new SLK has been improved by around twelve percent compared to the model it replaces.
In the event of a collision from the rear, passengers aboard the SLK will be protected by the box-shaped rear side members made from high-strength steel of varying thicknesses. The Mercedes engineers have therefore been able to precisely define the strength and deformation characteristics of the side members, and to align these criteria with the levels of stress placed on the different body sections.
The fuel tank and filler neck are located above the rear axle. This creates a sufficiently large crumple zone for collisions from the rear. The SLK bodyshell is completed at the rear by a rigid cross member, while the steel spare wheel well forms part of the floor assembly at the rear.