4X4 PROF

Are you new to the off-road­ing scene? Does your head spin when you hear terms like low range and axle wind-up thrown around? We’re here to help.

Go! Camp & Drive - - Contents - Text Cyril Klopper Photo Evan Naudé Il­lus­tra­tions Owen Wil­loughby

Reg­u­lar fam­ily sedans are gen­er­ally driven by just two of its four wheels. Some­times it’s the front pair and some­times the rear. As you’d ex­pect, four-wheel drive ve­hi­cles are driven in all four wheels. Why? For the same rea­son that it is eas­ier to climb up a rock us­ing both hands and feet rather than just your feet. The first me­chan­i­cal four­wheel-drive sys­tem was in­vented in 1902 by two Dutch broth­ers named Hen­drik-Jan and Ja­cobus Spyker. Back then, race cars used un­paved dirt tracks, and the Spyker broth­ers de­vel­oped four­wheel drive to im­prove their car’s han­dling. It was a sim­ple sys­tem but it en­abled them to win sev­eral races. The com­pe­ti­tion took no­tice, and man­u­fac­tur­ers like Mercedes-Benz im­me­di­ately em­braced the tech­nol­ogy. Nowa­days there are three types of four-wheel-drive sys­tems – ac­tu­ally four, but we’ll get to the fourth kind later. The three sys­tems are: per­ma­nent, part­time, and all-wheel drive.

PER­MA­NENT FOUR-WHEEL DRIVE

The Spyker broth­ers’ race car had per­ma­nent four-wheel drive (4WD), but how did it work? Since the ba­sics re­main un­changed in over a cen­tury, we’ll use a Land Rover De­fender as an ex­am­ple: A De­fender’s en­gine is con­nected to a gear­box and this gear­box is in turn con­nected to a cen­tre dif­fer­en­tial. From here a short drive­shaft ex­tends for­ward to a sec­ond dif­fer­en­tial on the front axle. A sec­ond, longer drive­shaft leads aft to a third dif­fer­en­tial on the rear axle. Oc­ca­sion­ally it may hap­pen that none of the four wheels ro­tate at the same speed, and these three dif­fer­en­tials pre­vent dif­fer­ences in torque to cause a phe­nom­e­non called axle windup, which can dam­age mov­ing parts. The cen­tre dif­fer­en­tial also al­lows more en­gine power to be trans­ferred to the axle that re­quires it most. The two dif­fer­en­tials on the axles send power to the wheels with the least re­sis­tance. That’s all well and good but if you need to ex­er­cise more con­trol over where the power is sent, the cen­tre dif­fer­en­tial can be locked to force the sys­tem to di­vide power equally be­tween both axles. This is par­tic­u­larly use­ful when you’re ne­go­ti­at­ing an ob­sta­cle and you need the front axle to pull just as hard as the rear axle is push­ing. If you need even more con­trol over power dis­tri­bu­tion, you can fit lock­ing dif­fer­en­tials on the axles, which will send an equal amount of power to ev­ery wheel – in other words, each wheel re­ceives 25% of the en­gine’s out­put. The draw­back of a ve­hi­cle equipped with per­ma­nent 4WD is that they are heav­ier on fuel than a car driven by just two wheels. The ben­e­fits are that your ve­hi­cle is al­ways ready for 4x4 ac­tion, and be­cause the sys­tem is rel­a­tively un­com­pli­cated, the odds of the sys­tem fail­ing is low.

PART-TIME FOUR-WHEEL DRIVE

In 1911, US truck man­u­fac­turer The Four Wheel Drive Auto Co. be­came the first to de­velop a sys­tem by which the front axle could be dis­en­gaged from the gear­box when 4WD wasn’t re­quired. As a more re­cent ex­am­ple, we’ll use the Nis­san Pa­trol. A Pa­trol’s en­gine is con­nected to a gear­box and the gear­box is con­nected to a trans­fer case. From this trans­fer case a drive­shaft ex­tends for­ward to a dif­fer­en­tial on the front axle. An­other drive­shaft leads to a dif­fer­en­tial on the rear axle. When on paved roads, only your ve­hi­cle’s rear axle re­ceives power from the en­gine, and the rear dif­fer­en­tial de­ter­mines which of the two wheels will re­ceive the lion’s share of this power. If road con­di­tions de­te­ri­o­rate, you only need to tug on a short lever next to the gear lever (or press a but­ton) and a strong chain belt will grab hold of the front axle and turn it along with the rear axle – al­most like the cen­tre dif­fer­en­tial of a per­ma­nent 4WD sys­tem. The ma­jor ad­van­tage of a part-time sys­tem is that you can dis­en­gage one of the axles and thereby im­prove fuel econ­omy. The draw­backs are that the in­creased com­plex­ity raises the odds of one or more parts fail­ing. For in­stance, if you en­gage 4WD on hard sur­faces, the afore­men­tioned axle wind-up caused by >

un­equal torque be­tween the axles can crip­ple your ride. Mit­subishi found a way around this by em­ploy­ing both a trans­fer case and a cen­tre dif­fer­en­tial. It’s called Su­per Se­lect and it al­lows you to dis­en­gage the front axle and save fuel, or you can leave it in 4WD all of the time and drive it like you would an old-school Land Rover on any sur­face.

ALL-WHEEL DRIVE (AWD)

Tech­ni­cally all 4WD ve­hi­cles are also all-wheel drive, as are six-wheel-drive ve­hi­cles. But the term is gen­er­ally re­served for sedan cars that are driven in all four wheels but are not specif­i­cally de­signed to go of­froad. Think Subaru and Audi. Cer­tain AWD sys­tems are sim­i­lar to per­ma­nent 4WD in that all the wheels are pow­ered all of the time, as is the case with a Subaru. Other ve­hi­cles such as the Q5 has a part-time sys­tem that favours the front wheels and only en­gages the rear axle when wheel spin is de­tected by elec­tronic sen­sors. But why would you want a reg­u­lar car with all-wheel drive? As the Spyker broth­ers showed, four pow­ered wheels grip bet­ter than just two. An­other im­por­tant con­sid­er­a­tion is con­ve­nience. If it sud­denly starts to rain or snow you don’t have to pull over to the side of the road, en­gage 4WD, and con­tinue – the car does it au­to­mat­i­cally.

go! Drive and Camp says

4WD works great in the soft sand of the Kgala­gadi, and AWD is per­fect for slip­pery moun­tain passes.

FRONT ON DE­MAND EN­GINE DIFF GEAR­BOX TRANS­FER CASE PART-TIME 4WD AL­WAYS DRIVEN DIFF REAR

AWD REAR EN­GINE GEAR­BOX DIFF FRONT ON DE­MAND ON DE­MAND

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