Fea­ture: 4x4 sys­tems ex­plained

Not all 4x4 set­ups are cre­ated equal. We in­ves­ti­gate the cru­cial dif­fer­ences and how they im­pact your driv­ing on-road and off

Car (South Africa) - - CONTENTS - BY: Ni­col Louw Ni­col­l_­car­mag

FOUR-WHEEL DRIVE. ALL-WHEEL DRIVE. PART- OR FULL-TIME SYS­TEMS

The ter­mi­nol­ogy can be con­fus­ing, es­pe­cially when you add brand­spe­cific names such as Qua ro, 4Motion and 4Matic. Ba­si­cally, the one shared goal of all these sys­tems is to pro­vide mo­tive force (torque) on each of the four wheels to in­crease trac­tion. Here we look at the dier­ent meth­ods of trans­fer­ring en­gine torque to the wheels.

The first four-wheel-drive ve­hi­cle with an in­ter­nal-com­bus­tion en­gine was also the first hy­brid. Fer­di­nand Porsche de­signed and built the Lohner-porsche for Lud­wig Lohner in Vi­enna in 1899 and it fea­tured four elec­tric hub mo­tors pow­er­ing each wheel, with the petrol en­gine ful­fill­ing a gen­er­a­tor func­tion.

The first four-wheel-drive car with an in­ter­nal-com­bus­tion en­gine pow­er­ing the wheels me­chan­i­cally was the Spyker 60HP de­vel­oped for the Paris to Madrid race in 1903 by Ja­cobus and Hen­drik-jan Spi­jker of the Nether­lands.

Al­though a cou­ple of mil­i­tary four-wheel-drive ve­hi­cles were also de­vel­oped, the first masspro­duc­tion, four-wheel-drive ve­hi­cle was the 1945 Jeep CJ-2A (“CJ” name stands for Civil­ian Jeep).

It took a long time for all-wheel drive to reach mass pro­duc­tion in pas­sen­ger ve­hi­cles and this claim goes to 1972’s Subaru Leone. The Audi Qu­at­tro fol­lowed in 1980 and rev­o­lu­tionised ral­ly­ing; no twowheel-drive ve­hi­cle has man­aged to win the World Rally Cham­pi­onship since the Qu­at­tro be­came the first four-wheel drive champ in 1983 (driven by Hannu Mikkola).

The torque mo­ment for propul­sion is pro­vided at the crank­shaft of the in­ter­nal-com­bus­tion en­gine. This torque is then sent through the trans­mis­sion, where the re­duc­tion ra­tios (lower gears) serve as torque mul­ti­pli­ers (with low range adding another re­duc­tion ra­tio to the com­plete gear set). The mul­ti­plied torque value is now avail­able at the trans­mis­sion-out­put shaft be­fore reach­ing all four wheels of the ve­hi­cle. That im­plies that they are some­how all con­nected to the out­put of the trans­mis­sion, but we’ll clar­ify this point later.

The prob­lem is that all four wheels do not ro­tate at the same speed, es­pe­cially when round­ing a bend, where the inside wheels travel a shorter dis­tance than the out­side ones. The com­po­nent on the front and rear axles that is de­signed to cope with the speed vari­a­tion is known as a dif­fer­en­tial. There­fore, all four-wheel-drive ve­hi­cles need at least two dif­fer­en­tials; one fore, and one aft.

Lastly, the two prop­shafts (or drive­shafts) that send the torque to the front and rear axles may also spin at dif­fer­ent speeds de­pend­ing on the av­er­age of the wheel speeds on each axle. There­fore, ei­ther another de­vice is needed to cater for this dif­fer­ence, or the ve­hi­cle can be driven only on low-fric­tion sur­faces in four-wheel drive where slip can oc­cur be­tween wheels and the sur­face to avoid driv­e­train wind-up and sub­se­quent dam­age.

Per­ma­nent four-wheel drive

When the sys­tem de­liv­er­ing drive to all four wheels is per­ma­nently ac­tive, there must be a cen­tre dif­fer­en­tial to cater for the speed dif­fer­ences be­tween the two axles. This is be­cause the driver does not have the op­tion to dis­en­gage AWD when op­er­at­ing on high-fric­tion sur­faces such as tar.

KEY AT­TRIBUTES Three dif­fer­en­tials, one on each axle and a cen­tre unit. The en­gine can be trans­versely or lon­gi­tu­di­nally mounted.

TAR USE Can be used on tar roads as long as all three dif­fer­en­tials are “open” and not locked (see What is a diff lock?).

OP­TIONS Diff locks can be present at all three dif­fer­en­tials. Low range is a pos­si­bil­ity for off-road ve­hi­cles.

EX­AM­PLES Subaru is known for its sym­met­ri­cal all-wheel-drive sys­tem cou­pled with boxer en­gines and equal­length drive­shafts. An off-roader ex­am­ple is the de­funct Land Rover De­fender, also with three dif­fer­en­tials and low range. In­ter­est­ingly, the De­fender has a diff lock only on the cen­tre dif­fer­en­tial as stan­dard, al­though many own­ers fit a lim­ited-slip diff or lock­ing diffs to the front- and rear-axle units.

Selectable four-wheel drive

This type of sys­tem is more suited to proper off-road use, with a me­chan­i­cal cen­tral trans­fer case and low range send­ing drive equally to the front and rear axles in the four-wheel-drive modes.

KEY AT­TRIBUTES Usu­ally only two dif­fer­en­tials (one on each axle); lon­gi­tu­di­nally mounted en­gine; trans­fer case with low range; de­fault rear-wheel drive when 4x4 is not se­lected. Fuel is saved in two-wheel drive ow­ing to less fric­tional losses.

TAR USE Not rec­om­mended in 4x4 modes, ex­cept if the ve­hi­cle is fit­ted with a cen­tre dif­fer­en­tial that is open.

OP­TIONS Diff locks on the front and rear axles (and cen­tre diff, if fit­ted). Low range is usu­ally stan­dard fit­ment;

EX­AM­PLES The Toy­ota Hilux is a good ex­am­ple of a ve­hi­cle with a trans­fer case that in­cludes low range but no cen­tre dif­fer­en­tial. An ex­cep­tion in this cat­e­gory is the Mit­subishi Tri­ton that has a cen­tral Torsen dif­fer­en­tial that is open in four-wheel-drive high range (4H) and can there­fore be driven on tar in this mode. Do not at­tempt this in 4HLC (or low range; 4LLC), where the cen­tre diff is locked.

On-de­mand four-wheel drive

This sys­tem is found on many per­for­mance­ori­ented road cars and tar-bi­ased SUVS. The idea is for the ve­hi­cle to run as de­fault two-wheel drive – FWD in most cases – and then (au­to­mat­i­cally) en­gage four-wheel drive only when needed.

KEY AT­TRIBUTES Usu­ally trans­verse mounted en­gine; two dif­fer­en­tials (one on each axle); multi-plate clutch or vis­cous­cou­pling cen­tral unit. As the sys­tem is ac­tive only when needed, fuel is saved as fric­tional losses are less. Low range is rarely found in this seg­ment.

TAR USE The sys­tem has no prob­lems op­er­at­ing on tar, as it shifts to four-wheeldrive solely when slip is de­tected on the driven (mostly front) wheels. When en­gaged, the clutch or vis­cous cou­pling can still take up slight speed dif­fer­ences by al­low­ing a de­gree of slip (as well as al­ter­ing the torque dis­tri­bu­tion front to rear).

OP­TIONS Diff locks are usu­ally not of­fered and the driver has lit­tle con­trol over the four-wheel-drive sys­tem’s op­er­a­tion.

EX­AM­PLES The Mercedes-amg A45 has a Haldex cen­tral mul­ti­plate clutch to en­gage the rear wheels for trac­tion pur­poses when all 280 kw is let loose. The Nis­san X-trail, mean­while, has a sim­i­lar setup, but al­lows the driver to choose be­tween FWD (open clutch), auto (Ecu-con­trolled clutch) or 4x4 (clutch per­ma­nently en­gaged) modes.

WHAT IS AD IF FLOCK?

A dif­fer­en­tial con­sists of a plan­e­tary gear set that also acts as the fi­nal re­duc­tion ra­tio to mul­ti­ply the torque value while de­creas­ing the ro­ta­tional-in­put speed to the wheels by the same fac­tor. An open dif­fer­en­tial al­lows an un­lim­ited speed dif­fer­ence be­tween the two wheels on the driven axle for cor­ner­ing pur­poses. This can lead to a trac­tion prob­lem when, for ex­am­ple, a wheel is sta­tion­ary on solid ground and the other spins in mud. A diff lock forces both wheels to ro­tate at the same speed, im­prov­ing trac­tion but neg­a­tively im­pact­ing cor­ner­ing dy­nam­ics. Driv­ing with a diff lock en­gaged on tar can lead to driv­e­train wind-up and se­vere dam­age. A lim­it­ed­slip dif­fer­en­tial al­lows a cer­tain amount of wheel-speed vari­a­tion on an axle be­fore lim­it­ing slip and im­prov­ing trac­tion.

ELEC­TRIC AWD

Engi­neer­ing four-wheel drive on an elec­tric ve­hi­cle (EV) is much sim­pler than the me­chan­i­cal set­ups ex­plained in these pages. If four elec­tric mo­tors are em­ployed – one a wheel – it negates the need for trans­mis­sions, dif­fer­en­tials and trans­fer cases. The tech has been in use for a long time in min­ing, where large trucks (and other earth movers) run with an elec­tric mo­tor pow­er­ing each wheel (or axle). These trucks em­ploy an in­ter­nal-com­bus­tion en­gine as a gen­er­a­tor, sim­i­lar in con­cept to the Lohner-porsche. To­day, many per­for­mance­ori­ented EVS, such as the Tesla Model S, use an elec­tric mo­tor per axle to achieve per­ma­nent four-wheel drive.

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