Feature Article

FEATURE ARTICLE 06 Four-wheel-drive increases traction, often enough The ordinary, familiar differential is an ‘open’ dif- to sustain movement and control under conditions ferential. Torque delivered through the driveshaft that might otherwise mean the driver would choose pinion and differential ring gear applies equally to stop or would get stuck in snow, mud or sand. through the spider gears to each axle halfshaft, Four-wheel-drive is not constantly advantageous, regardless of individual halfshaft speed. Aside from however, because it mechanically couples all the slight friction between internal parts, an open dif- wheels and drivetrains together; and sometimes, ferential applies equal torque to each axle – with the surprisingly enough, that connection can work consequence that the maximum torque the differen- against optimization of traction and control. There tial can apply to its drivewheels is just double what have been two quite different systems called the drivewheel with less traction can transmit alone, 4MATIC, the first hydraulic, beginning about 1990 the drivewheel that slips first. Once that first wheel and the second electronic, working through the spins, the vehicle stops progress. This is exactly service brakes, starting in 1998. We concentrate what we see when a car spins a drivewheel in a here on the earlier system since you're more likely snowpile or mudhole: Until that wheel regains trac- to see it in an independent shop. tion, a stationary car doesn’t budge. In the extremes of limited traction, an open differential gives you Locked and Open Differentials one-wheel-drive, and, worse yet, drive through whichever wheel has less traction. So we move to the locked or limited-slip differen- “Four-wheel-drive” can be a misleading descrip- tial. “Locked” is an exaggeration, as you know, tion. In principle, of course, any driveline system because inside and outside wheels must travel dif- conducting power through the transmission to all ferent speeds in a turn. Otherwise the inside wheel, four wheels is ‘four-wheel-drive.’ There are, however, usually less heavily loaded as centrifugal force significant differences arising from the distinction throws the weight outward, must skid forward along between locked and open differentials. Let’s have the pavement. that clear first. Later nonhydraulic versions of 4MATIC, employing 4ETS, use open differentials front, At the heart of the 4MATIC system is the trans- back and center, with no multidisk clutches at fer case, splitting the torque from the transmis- all. They can do this because the later system sion to each of the driveaxles. uses individual wheel brakes to keep individual drivewheels from spinning when their torque exceeds the traction available at that tirepatch. The 4MATIC differentials ‘lock’ in a way similar to The brakes have the identical effect to that of the the limited-slip differentials you may know from multidisk clutches in the hydraulic version: they domestic trucks or high-performance (at least fast- prevent overspeed of the wheel with less traction. acceleration) vehicles. In most of those, a constantly Similarly, there are two-wheel-drive brake-activat- spring-loaded multidisk clutch links the opposite- ed drive traction controls analogous to ASD. side axle shafts, through a friction material and spe- 07 cial lubricant, both requiring periodic replacement. 4MATIC AND ETS The earlier 4MATICs use hydraulics to engage the This means two wheels, one on each axle, have to clutches, as we’ll see. slip simultaneously for the vehicle’s motion to stop. There are also true locking differentials using slid- Such four-wheel-drive will, however, represent a sig- ing-dog clutches, but these engage with an immedi- nificant traction improvement over plain-vanilla ate, heavy clank, a force reminiscent of excavation or two-wheel-drive. After all, it means at least a dou- railroad equipment, an engagement force that would bling of traction, and usually more than that, assum- be quite unwelcome in a sedan. With an earthmov- ing somewhat different traction at each wheel. Just ing implement, this doesn’t matter since the grind- as two-wheel-drive through an open differential real- ing earth moves under the tire lugs and since nei- ly means one-wheel-drive, four-wheel-drive through ther driving comfort nor speed was a priority, any- two open differentials really means two-wheel-drive. way. With a sedan, that’s out of the question. On a That’s more traction, but it doesn’t use all the trac- vehicle driving mostly on hard pavement, you don’t tion available. want a truly locking differential, not only because A four-wheel-drive system that locks the front and that would abrade the tires with lower load quickly, rear axles together still has the problem that one axle but also because it would mean one or the other tire or the other must slip in every turn, because the front on an axle would have to slide in every turn, actual- wheels will invariably follow a track along a smaller ly reducing traction even under what should have radius and shorter path than the rears. A long time been ideal pavement conditions. ago, I had an ancient Jeep that never really got stuck, but took every turn with unpredictable directional Torque through the Transfer Case excitement anytime four-wheel-drive was engaged. You could never tell whether the front or the rear The earliest four-wheel-drive systems simply used axle would slide loose sideways first – that depended a second driveshaft from a transfer case to the front on everything from the load to the quality of the grav- axle differential, where another open dif- el on the road. And your corrective maneuver had to ferential did its torque- wait until the slip was clearly under- splitting work. The way, because you turned the same ‘less-trac- steering wheel in opposite tion’ limitation directions to recover still applies, depending on which axle but with this slid. Excitement, as I said. advantage: The The obvious manufactur- average speed ing fix is to include an of the front interaxle differential, which wheels would, all modern four-wheel-drives by solid do. You can’t just add a center mechanical differential and let it go at that, linkage, move at however, or you’re back with exactly the same your original problem: speed as the All the torque goes to average speed of the first wheel the rears (“aver- that slips, and the age” because each vehicle stops wheel on each there. Four- axle can still wheel-drive move at a differ- with three open ent speed from differentials its opposite num- becomes, in the ber, but each axle’s differen- extreme, one- tial ring gear turns at exactly the wheel-drive again. same speed as the other axle’s). The transfer case can engage two-wheel-drive only, four-wheel-drive or four-wheel-locked. In the last case, front and rear axles turn at the same speed. 08 4MATIC Progressive Locks supplementary front drivetrain and the transfer case, but also a detailed hydraulic system, from So the Mercedes-Benz 4MATIC system employs pump to actuator pistons, to actuate the various dif- ‘locked’ four-wheel-drive when the various sensors ferential locks that come into play under conditions indicate the need for the additional traction, but not we’ll discuss in a minute. ETS, the new 4MATIC sys- under normal driving conditions when ordinary two- tem employing 4ETS, in contrast, works mainly elec- wheel-drive through the open rear differential is suf- tronically (thus the ‘Wire’ of our subtitle), with the ficient to conduct all the forward drive moment the control unit activating specific wheel brakes to keep engine can deliver to the wheels, and not under the wheel with the least traction from spinning. ETS those special driving conditions when – paradoxi- works specifically with two-wheel-drive cars; 4ETS cally – a locked four-wheel- drive could make a low- applies to those with a front driveaxle. Somewhat traction condition worse. confusingly, 4ETS is also called 4MATIC. Perhaps the most important such contrary condi- Here are the progressive steps 4MATIC tion occurs during hard-braked, ABS-pulsed stops, employs when the wheelspeed sensors and the particularly in turns. If four-wheel-drive were still steering wheel sensor information point to drive- engaged under those conditions, it would be possible wheel spin: Driving along an ordinary road with for the interconnected drivelines either to mask a good traction and no wheelspin, the car works by wheel’s slip from the sensor’s report and thus block ordinary two-wheel-drive with an open differen- the control unit’s recognition and countermeasures, tial at the rear drivewheels. This provides the or to force a wheel with limited traction to turn at the best combination of drive and directional traction same speed as the others (this last is most noticeable in the absence of a problem. during a steep turn or rapid steering changes, The older hydraulic system provides open-dif- when wheelspeed varies at the different ferential two-wheel-drive because the multi- wheels because each follows a path with disk clutch pack for the center differential lock a different turning radius and differ- is spring loaded closed, that is, ent length (and thus, a different engaged. This couples the input speed). To accommodate turns, one shaft from the transmission to of the inputs to the 4MATIC control the transfer case planetary car- unit is a steering wheel angle sen- rier, splined directly to the dri- sor, similar but not identical to or veshaft for the rear axle. This is interchangeable with the one used the fail-safe mode of the system, with ASR, ESP and ETS.

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