Jul 15, 2008
The GLK-Class boasts a high level of flexibility thanks to its AGILITY CONTROL suspension, and reconciles the conflicting suspension design aims that particularly affect the SUV class: chassis engineers seek to create a vehicle that is both sportily agile and comfortably smooth, yet one which can also cope with off-road terrain. If the focus is on sporty, active handling, the suspension and, above all, the shock absorbers need to display a certain firmness, which precludes the desired levels of suspension comfort associated with the brand and limits off-road prowess. If, on the other hand, the vehicle is purpose-built for ride comfort and off-road expertise by fitting softer dampers, its driving dynamics will of course suffer. "Amplitude-responsive damping" provides the solution: here, the shock absorber's smoothing forces are of a flexible nature, rather than having a rigid setting. During normal driving along moderately contoured roads or off-road excursions at low speeds, the system is tuned for soft response to the benefit of the occupants' sense of wellbeing and the vehicle's off-road abilities. In order to keep the occupants feeling at their ease when driving at a brisker pace or performing abrupt evasive manoeuvres, the dampers switch to a firmer setting
in such situations, permitting a high level of handling stability.
in such situations, permitting a high level of handling stability.
The steering systems in the GLK
At the same time, the driver is assisted by the speed-sensitive power steering that comes as standard in the V6 models and provides the optimum level of steering assistance for the situation in hand. The system therefore makes light work of parking or off-road manoeuvres by maximising power assistance. As the speed increases, however, assistance is reduced in favour of greater directional stability. The precise electrohydraulic steering is used in the GLK 220 CDI 4MATIC. Steering comfort has been further enhanced in both steering variants, thanks to meticulous attention to detail. For instance, the engineers have decoupled the vibrations associated with the airbag module in the steering wheel. Reducing the unsprung masses in the steering system in this way has given steering comfort an additional boost.
17-inch wheels and tyre pressure loss warning system as standard
All GLK models come as standard with mixed-sized tyres which, together with the AGILITY CONTROL suspension and asymmetric power distribution between the front and rear axle, form the basis for a consummate transfer of power between the wheels and the road. And the more effective this power transfer, the less frequently the electronic control systems need to intervene.
Two chassis set-up options are available. If the "off-road styling package" is specified, the GLK comes with longer spring travel and a comparatively softer basic set-up to optimise off-road handling characteristics. Should the customer opt for the "exterior sports package", on the other hand, the specification includes a sports suspension lowered by 20 millimetres with shorter springs, firmer dampers and stronger transverse stabiliser bars. This sharpens the vehicle's driving dynamics for sporty driving without significantly impairing the balanced ride comfort.
The road-oriented "exterior sports package" features size 7.5 J x 19 light-alloy wheels with 235/50 R 19 tyres at the front and size 8.5 J x 19 wheels with 255/45 R 19 tyres at the rear. If the vehicle is specified with the "off-road styling package", 7.5 J x 17 light-alloy wheels shod with 235/60 R 17 tyres are fitted at the front, while 8.0 J x 17 wheels with 255/55 R 17 tyres can be found at the rear. As an option, All Terrain tyres specially tailored to the GLK are available as accessories. All models are equipped with a TIREFIT system for repairing tyre damage. Alternatively, a compact spare wheel which can be used on both the front and the rear axle can be specified as an option.
All GLK models are fitted with a tyre pressure loss warning system to ensure potential tyre damage does not have serious consequences. The system detects any sudden drop in tyre pressure and alerts the driver to the situation by flashing up a warning message in the instrument cluster. The optional tyre pressure monitoring system improves safety yet further. Incorrect tyre pressure or a gradual drop in pressure is indicated by the "Check tyres" warning message; the "Caution Tyre defect" warning appears in the event of a rapid loss of pressure. The tyre pressure monitoring system automatically detects when new tyres or wheels are fitted.
ADAPTIVE BRAKE: sophisticated brake control system with additional new functions
The all-new ADAPTIVE BRAKE control system incorporates the basic anti-lock braking system (ABS), acceleration skid control (ASR) and yaw control functions. ABS and ASR monitor and control the driving dynamics along the vehicle's longitudinal axis, while yaw control looks after the lateral dynamics. If ADAPTIVE BRAKE diagnoses critical driving situations, precision application of the brakes and control of drive torque is used to maintain or restore traction and directional stability as far as is physically possible.
New additional braking functions make ADAPTIVE BRAKE safer and more convenient. Hill-start assist prevents the vehicle from rolling contrary to the intended direction of travel. The enhanced functionality also includes brake 'priming'. Should the driver suddenly release the accelerator, the system prepares for possible panic braking by pressing the brake pads lightly against the brake discs. If the brakes are indeed applied with full force, the stopping distance is therefore shortened noticeably by building up pressure the instant the brake pedal is pressed. The ability of ADAPTIVE BRAKE to precisely generate even minimal brake pressure means the film of water that accumulates on the brake discs when driving in the wet can be removed by briefly applying the brakes lightly, further reducing the brakes' response time in the wet and, in turn, the stopping distance. This function is triggered automatically once a certain number of windscreen wiper cycles has been reached and the driver has not applied the brakes in the meantime.
The GLK also comes with standard-fit adaptive brake lights. This system warns the traffic behind when the driver applies the brakes in a critical situation by causing the brake lights to flash if the vehicle suddenly decelerates, and, in the case of emergency braking from speeds in excess of 70 km/h, automatically switching on the hazard warning lights once the vehicle comes to a standstill.
4MATIC: high-performance all-wheel drive with sophisticated control systems
The 4MATIC powertrain at the heart of the GLK-Class is one of the most capable all-wheel drive systems on the market, incorporating control systems that set new standards. The basic concept, with the engine installed longitudinally and the transmission and transfer case designed as a single block, produces a compact and lightweight arrangement that minimises frictional losses, as well as heralding a number of advantages over its counterparts where the drive unit is installed transversely. The upshot of this is that fuel consumption is on a par with that of a comparable model with conventional two-wheel drive, while the minimal vibration and noise levels rival those of today's luxury-class models.
The basic 45:55 split of drive torque between the front and rear wheels combines with the ESP®, ASR and 4ETS dynamic handling control systems to produce supremely assured and perfectly predictable performance. When tuning the control systems, the engineers at the Mercedes-Benz Technology Center (MTC) made a defined level of understeer a priority. All GLK models display these characteristics under all manner of road conditions, come rain or shine, snow, ice or unsurfaced tracks.
The newly developed multiple-disc clutch in the centre differential comes to the system's aid when friction between the tyres and ground is particularly low, for example when driving over snow or ice. A basic interaxle lock of 50 Newton metres gives a major boost to traction – setting the benchmark for vehicles with no additional differential locks without any loss of handling stability.
"Off-road engineering package" provides superlative off-roading
The "off-road" button on the centre console makes GLK models equipped with the off-road engineering package even more assertive in rough terrain. Pressing the button activates a special driving program which varies the shift points of the 7G‑T RONIC transmission, "softens" the accelerator pedal characteristics and activates the ESP® off-road functions including off-road ABS and a custom off-road 4ETS. In this mode, there are fewer interventions involving the engine manage-ment system; the system is also designed to tolerate a higher degree of wheel slip. This control strategy improves traction off-road, particularly on low-friction surfaces such as sand, gravel or stone chippings. The off-road ABS on the GLK was premiered in the current M‑Class and detects a wide range of different terrains by permanently referencing the wheel-slip models. This produces the best possible deceleration strategies for off-road adventuring over rocks, sand, stone chippings, gravel or slush.
On models with the off-road engineering package, the automatic transmission features a Manual drive mode (M) in addition to the Comfort (C) and Sport (S) modes, which enables the individual gears to be selected with shift paddles on the multifunction steering wheel. This operating mode promises maximum fun at the wheel when venturing off-road.
A further switch activates the Downhill Speed Regulation (DSR) system, which automatically maintains a pre-programmed speed between 4 and 18 km/h on steep downhill inclines. The GLK driver can vary the speed at any time during the descent using the cruise control lever. Mercedes-Benz is the only car maker to offer this important equipment detail for off-road motoring. Conventional systems offered by competitors can only maintain the speed programmed at the outset, meaning the speed cannot be altered while the system is in action. A heavy-duty, weight-optimised underguard and underbody protection round off the all-terrain specification of the GLK with off-road engineering package.
If the COMAND APS system is specified, the navigation system has a useful extra feature designed specifically for off-road vehicles. If the vehicle enters an area not covered by digitised maps, the driver can activate a tracking system to automatically save the vehicle's route. This means that GLK drivers can always find a way back to their starting point, based on the principle of Ariadne's thread.
The off-road-related body dimensions are equally impressive. The ground clearance of 201 millimetres and the short body overhangs (front 816 mm, rear 957 mm) make for favourable angles of approach and departure – 23 degrees and 25 degrees, respectively; the breakover angle is 19 degrees. The GLK can negotiate inclines up to 70 per cent, while the maximum tilt angle is 35 degrees. Meanwhile the relatively compact wheelbase length of 2755 millimetres and the vehicle weight of 1830 kilograms, which is comparatively low for an SUV, allow the GLK to make good headway in even the most topographically challenging terrain.
As well as a tyre pressure loss warning system, the Electronic Stability Program (ESP®) incorporates a vehicle/trailer stabilisation function as standard, which can defuse precarious handling situations resulting from towing a trailer at an early stage by applying the individual wheel brakes as and when required. The permitted towing capacity is 2,000 kilograms.
Vehicle development under extreme conditions
Fine-tuning the dynamic handling control systems is one of the major challenges when developing an SUV. Compared with conventional passenger cars, this vehicle category has to reconcile an even wider range of requirements: active safety, ride comfort and driving pleasure not only need to be guaranteed on-road but also off-road, on rudimentary tracks, unsurfaced roads or in demanding terrain. The systems also need to work reliably under all possible (and impossible!) meteorological conditions experienced in the world's many different climate zones. In view of this, the test engineers select the world's most gruelling onditions to test SUVs like the GLK up to their physical limits, thus ensuring that the subsequent series-production vehicles will operate reliably for many years, regardless of operating and environmental conditions.
Development testing therefore takes place under punishing desert conditions in the baking Namib Desert, while the sub-zero conditions of the Arctic Circle provide the opposite extreme to measure and test the GLK prototypes. An ongoing comparison between "Fire & Ice" is designed to ensure that the various findings do not have a reciprocal negative influence. The same principle of harmonisation is applied when resolving other conflicting aims during the GLK fine-tuning process, as illustrated by the "Climb & Speed" development programme: if the chassis tuning sessions on the racetrack show the GLK to be directionally stable and safe at high speed, while the vehicle delivers the hallmark Mercedes ride comfort on torturous bone-rattling stretches, the question is then whether the set-up can produce the desired results when clambering through off-road terrain. Overall the developers clocked up some 4.5 million test kilometres worldwide. The field tests using prototypes are supplemented by extensive measurement programmes on a wide range of test rigs and the detailed simulation programmes conducted as part of the Digital Prototype. In the course of the development phase, the experts tested around 1000 different vehicle variants in virtual simulations and conducted over 220,000 driving manoeuvres in the process.
While the fine-tuning of control systems such as ESP®, 4ETS, ASR, ABS is based on objective measurement criteria using state-of-the-art technology, the finesse and experience of the test engineers continue to play a decisive role. For instance, the developers in the MTC managed to optimise the control quality and interaction of the individual systems in such a way as to substantially improve powertrain performance. At the start of the development test drives, the individual control systems still had a reciprocal effect on one another in specific driving situations that resulted in part of the drive torque going to waste, for instance on demanding off-road terrain. After completing the tuning sessions, the systems were harmoniously networked to such an advanced degree that the wheels had another 40 Newton metres or so of traction-enhancing torque at their disposal.
During the "Fire & Ice" development testing – as the fine-tuning drives staged in desert conditions in southern Africa and in the wintry wastes of the Arctic Circle have come to be known – two interviews were conducted which give a special insight into the development work performed by the engineers.
First is Thomas Merker, Head of Development for the Mercedes-Benz M-, GL- and GLK-Class, speaking during the heat trials in the Namib Desert:
Why do you come to Namibia to carry out testing? What particular challenges does Namibia pose for the GLK?
Within the context of our global development process, Namibia covers a portion of the challenges we wish to overcome with our GLK. And the prevailing conditions found in the Namib Desert especially are precisely what we want for our torture programme under extreme desert conditions. On the one hand, there are the extreme temperatures, with the air heating up to 50 degrees Celsius and surface temperatures that can even reach 80 degrees. On top of that you have extremely fine sand and the notorious corrugated tracks through hostile rocky terrain. A vehicle that can withstand the conditions here is ready for any desert in the world. Naturally, the findings made in the Namib are always considered in the context of our test results under other operating conditions. Any enhancements must also satisfy completely contradictory requirements, of course, meaning they must also work in the bitter cold of the Arctic, the humid heat of the tropics, as well as in routine day-to-day operation in Central Europe.
What tests do you carry out here?
For one thing, we inspect the thermal stability of the vehicle as a whole, and check for instance whether the AGILITY CONTROL suspension or the electronic control systems continue to operate flawlessly when exposed to the extreme environmental and track conditions. We also conduct "sand testing", an endurance test specially designed for the GLK.
Is there actually any need to expose the GLK to such extreme conditions? After all, the vehicle will mostly be driven under normal road conditions in Western countries.
Compared to the G-, GL- and M-Class, the GLK is certainly the most road-oriented model series within the Mercedes-Benz SUV and off-roader line-up. Yet we still attach great importance to the vehicle's performance abilities off the beaten track, and even under the most extreme conditions. That's part of the SUV philosophy at Mercedes-Benz: regardless of the fact that only a few customers venture off-road, we must still prepare the vehicle for this eventuality and guarantee a certain level of off-road performance.
Plus, of course, we also need to think of the car markets in countries where conditions are no less out of the ordinary, such as Russia, for example, or the Middle East states. There are also the requirements of the world's developing automotive markets, such as India or China, to bear in mind. At the end of the day, our customers in Europe also stand to benefit from these developments, as there is no difference between the models delivered to a whole array of markets.
Are there are conflicting requirements? What sort of conflicting aims need to be reconciled to ensure that the GLK is able to perform to Mercedes' high standard both in Artic climes at minus 30 degrees and in a desert environment with temperatures of 45 degrees?
We must of course ensure that optimisation of a particular parameter to safeguard resilience to high ambient temperatures does not take place at the expense of the vehicle's performance capabilities when faced with Arctic conditions. To take an example: if we were to optimise the engine's cooling air supply solely with a view to operation in extreme heat, problems would occur during the warm-up phase at low outside temperatures, and vice versa. Painstakingly detailed work needs to be carried out in order to cover all bases. And this applies to virtually all areas of development. Does the heating respond fast at minus 30 degrees, does the THERMOTRONIC quickly cool down the interior rapidly when the sun is at its most intense and temperatures are around plus 45 degrees. Or the fuel supply to the engine: does diesel fuel continue to flow freely at extremely low temperatures, is the formation of vapour bubbles in petrol engines prevented at extremely high temperatures? Do the filter systems for the engine and passenger compartment operate reliably? Do all of the different sealing systems perform their tasks efficiently?
Looking at it from another point of view, can any parallels be drawn in the loads exerted on the GLK when driving in the extremes just mentioned?
As incredible as it may sound, there are indeed parallels. The loads exerted on the vehicle when driving over dunes or through deep sand, for example, are certainly similar to the demands experienced when driving in deep snow or up Alpine passes. Powder snow and desert dust likewise behave in a similar fashion. We are experience this for ourselves when driving in a long line of traffic: in winter when the road is covered in snow, you find yourself driving through a fine cloud of snow when following a vehicle in front; in summer on a dried-out track the effect is the same, but the cloud is of dust instead. In both cases, we have to prevent the fine particles from being drawn into the vehicle, where they could clog up the filter systems of the engine or passenger compartment ventilation, for instance.
In the second interview, Wolfgang Keller, who is Head of Overall Vehicle Testing for the Mercedes-Benz M-, GL- and GLK-Class, gives his thoughts on the specific test demands experienced under Arctic conditions:
Now that the series of testing in the Namib Desert is over, the car is due to be put through its paces in a polar environment close to the Arctic Circle. The contrast between the conditions could not be more stark. What particular findings do you glean from this extreme climatic change?
Just like the fine-tuning drives in the Namib, the test drives near the Arctic Circle form part of our approval procedure prior to the start-up of series production. In both instances, the test vehicles are the same, meaning we can be sure that the future customer vehicles will meet the stringent Mercedes requirements. It is true that very few of our customers will be driving their GLK both in the Namib and near the Arctic Circle; but anyone who makes extensive use of their GLK – for example, for skiing in the Alps in the winter then a beach holiday in Spain in the summer – will come close to experiencing the conditions we simulate.
Don't you verify such characteristics in an earlier phase of the development process?
Naturally we inspect all vehicle components back in the early stages of development, both in special test vehicles and on test rigs, as well as by conducting simulations as part of our digital prototyping. Yet, as the development process draws to an end and all assemblies are performing at their optimum, in line with our specifications, there comes a point at which the vehicle needs to be precision tuned. This brings the system as a whole into perfect harmony and ensures that it operates reliably and to the best of its abilities under all conditions. There is a clear comparison to be made with the making of a grand piano: here again, the individual components are developed with utmost care and manufactured with absolute precision. Following its assembly, however, minutely detailed work is required to tune the instrument and produce an optimum sound. And that is precisely what we seek to give our customers – the best possible "sound"!
What fundamental differences must the dynamic handling control systems overcome in Namibia and in Sweden?
In Namibia, there is a low but relatively constant level of grip, both on the sand tracks and in the dunes. The idea here is to distribute the drive or braking torque evenly between all four wheels, thereby achieving maximum traction combined with high handling stability. Things are far tougher at the Arctic Circle though. Here, we are faced with the problem of tremendous fluctuations in friction: you can have more powdery or more compacted snow, sheet ice and areas of clear tarmac, all within the space of just a few metres. Handling stability and, consequently, driving safety must be absolutely guaranteed, both when accelerating or braking and when cornering. Propulsion is of secondary importance in this case.
Does the fine-tuning of the control systems under Arctic conditions throw up any other major challenges?
When driving in wintry road conditions, it is especially important that the ESP, ASR and 4ETS control systems act not just effectively, but smoothly too. Smooth, harmonious intervention is most important from the point of view of driver-fitness safety. Hectic, inharmonious interventions by the control systems may assist the driver, but they would also distract his attention from what's happening around and prove wearying in the long run. We signal critical driving states by means of the ESP warning lamp, which prompts the driver to drive more cautiously.
What benefits does the sophisticated 4MATIC powertrain have to offer when driving over ice and snow?
The latest-generation 4MATIC system boasts the ideal credentials for driving in wintry conditions. The friction-optimised construction means we have substantially less frictional losses in the powertrain than other systems where the engines are installed transversely, for instance. The basic split of the drive torque enhances traction and agility. The basic locking torque in the transfer case furthermore enables the GLK to pull away with ease, even on surfaces completely covered in ice.
The GLK chassis is seen as breaking new ground in the world of SUVs. What specific benefits does it have to offer under Arctic conditions?
The amplitude-dependent damping of our AGILITY CONTROL suspension keeps the tyres in optimum contact with the ground under all conditions, even on hard-packed snow or black ice. The result is effective transmission of longitudinal and lateral forces. And the better the power transfer between the tyres and the slippery road surface beneath, the less frequently the electronic control systems need to intervene. AGILITY CONTROL lays the foundation, as it were, for outstanding driving dynamics and safety, whilst at the same time guaranteeing the excellent standard of ride comfort that Mercedes is renowned for.
Complete range of diesel and petrol engines
The state-of-the-art engine line-up ensures an exceptionally high level of drive smoothness and impressive performance right across the GLK range, not to mention low fuel consumption and emissions figures when compared to its rivals. Customers have a choice of four model variants: diesel aficionados can opt for the GLK 220 CDI BlueEFFICIENCY or GLK 320 CDI, while those with a preference for petrol can select the GLK 280 or GLK 350, both of which feature a V6 powerplant.
The GLK 220 CDI 4MATIC BlueEFFICIENCY comes with a brand new generation of diesel engine under its bonnet that redefines standards for power output, torque, emissions characteristics and fuel economy. The compression-ignition engine also delivers impressive environmental performance and, like all diesel engines for Mercedes passenger cars, features exhaust gas recirculation, an oxidising catalytic converter and a maintenance-free diesel particulate filter as standard. The smooth-running four-cylinder unit limits itself to 6.9 litres of diesel for every hundred kilometres, whilst emitting 183 grams of CO2 per kilometre. The Mercedes-Benz engineers have also succeeded in further reducing levels of untreated engine emissions. Even without an active denoxification process, the new four-cylinder diesel already meets the future EU5 emissions standard. The state-of-the-art powerplant propels the GLK 220 CDI 4MATIC from 0 to 100 km/h in 8.8 seconds and on to a top speed of 205 km/h.
Cutting-edge technology was deployed in order to attain these class-beating figures. The principal features of the new Mercedes diesel engine:
- Two-stage turbocharging ensures high power output and optimum torque delivery.
- Fourth-generation common-rail technology with a rail pressure that has been increased by 400 bar to 2,000 bar, plus a new piezoelectric injector concept featuring direct injector needle control creates the ideal basis for more flexible injection timing, leading to smoother engine running.
- The maximum ignition pressure is 200 bar, which also contributes to the
- Both the oil-spray nozzles and the water pump are activated in accordance with requirements to save energy.
- The camshaft drive is positioned at the rear in order to enhance running refinement and achieved high standards of pedestrian protection.
- The engine block is made from cast iron, the cylinder head from aluminium.
- Two water jackets guarantee maximum cooling even at the points of greatest thermal radiation; it is this that enables an ignition pressure of 200 bar and such a high power-to-displacement ratio.
- The aluminium pistons slide up and down in cast-iron barrels for minimum frictional resistance.
- The connecting rods are made from forged steel, and their weight has been optimised by the Mercedes engineers.
- In the interests of vibrational comfort, the forged crankshaft with its eight counterweights turns supported by five bearings. The radii of the crankpins are rolled and are notable for their high strength.
- To compensate for the free vibration moments which are inherent to four-cylinder inline engines, there are two Lanchester balancer shafts at the bottom of the engine block running in low-friction roller bearings rather than conventional plain bearings.
The new 125-kW/170-hp drive unit has a powerful feel to it, its response is agile, it delights with its tremendous pulling power and impresses with exemplary levels of smoothness for a four-cylinder engine. Besides its exceptional output, the new power unit offers breathtaking torque delivery from low revs along with the best torque characteristic curve in its displacement class: peak torque of 400 Newton metres is on tap over a wide rev band from 1400 up to 2800 rpm. This means that the engine can be run extremely economically at low rev speeds in everyday driving situations.
Supreme six-cylinder comfort comes to the compact-SUV segment
The diesel line-up is augmented by the proven V6 powerplant in the GLK 320 CDI, which develops 165 kW/224 hp and delivers a peak torque of 540 Newton metres. This propels the GLK to even more admirable performance, with a top speed of 220 km/h and a time of just 7.5 seconds for the sprint from 0 to 100 km/h. The V6 engine also features exhaust gas recirculation, an oxidising catalytic converter and a maintenance-free diesel particulate filter, burns 7.9 litres of fuel per hundred kilometres and complies with the EU4 emissions standard.
The two smooth-running V6 petrol models – the GLK 280 4MATIC and GLK 350 4MATIC – develop 170 kW/231 hp and 200 KW/272 hp respectively. Both make their mark with nifty performance combined with moderate fuel consumption. The 3.5-litre V6 in the GLK 350 4MATIC is particularly noteworthy, achieving figures similar to those of a sports car: it has a top speed of 230 km/h and races from 0 to 100 km/h in 7.1 seconds. Both engines fulfil the Euro 4 limits and consume 10.4 litres and 10.6 litres per hundred kilometres respectively.
7G-TRONIC seven-speed automatic transmission as standard
All of the engine variants are matched with the 7G-TRONIC 7-speed automatic transmission as standard, which includes a Comfort (C) and a Sport (S) shift mode. The "C" drive mode prioritises drive smoothness and economy combined with high power reserves. Switch to the "S" mode selects a significantly sportier set-up, with a modified driving strategy and accelerator characteristic curve. In conjunction with the optional off-road engineering package, the automatic transmission also comes with a manual drive mode (M), complete with shift paddles on the multifunction steering wheel.
The 7G-TRONIC come with a special function that decouples it when stationary. This technical feature reduces the transmission absorption torque when the vehicle is stationary with a gear engaged. This eases the burden on the engine, which has the effect of reducing fuel consumption, as well as improving vehicle acoustics, passenger comfort when stationary and the idling characteristics. The slip-controlled clutch is engaged the instant the driver releases the service brake pedal, enabling the vehicle to pull away without any delay or loss of comfort. This operating state is achieved by controlling a mechanical clutch so as to produce a defined level of slip. The clutch itself was designed for operating with constant slip.