4 x 4 Australia - - Gear -

A CON­VEN­TIONAL diff splits the torque equally to left and right (mi­nus some small fric­tional losses) at all times. It does this re­gard­less of whether both wheels are driv­ing you down the road at 100km/h, or whether one wheel is in the air spin­ning madly on a low range track.


CHECK out the traf­fic go­ing around the round­about. No­tice the front wheels of larger ve­hi­cles will be near the out­side of the round­about and the rear wheels will be near the in­side. This tells you that when head­ing around a cor­ner, the front wheels take a larger arc than the rear wheels.

When we lock the cen­tre diff on a full-time 4WD with a con­ven­tional diff (or en­gage 4WD on a part-time 4WD), we re­move the abil­ity of the cen­tre dif­fer­en­tial to com­pen­sate for the speed dif­fer­en­tial be­tween the front and rear when ne­go­ti­at­ing cor­ners.

This is only an is­sue on high trac­tion sur­faces, like bi­tu­men and slick rock. On loose sur­faces such as dirt or gravel the speed dif­fer­en­tial doesn’t pose an is­sue, as the sur­face al­lows the wheels to slip a lit­tle and re­lieve the ten­sion.

On high trac­tion sur­faces the speed dif­fer­en­tial go­ing through cor­ners will build up a twist­ing force be­tween the front and rear diffs, lead­ing to an even­tual driv­e­line fail­ure.

The first in­di­ca­tion of fail­ure is that the car won’t come out of 4WD. Or per­haps you’re driv­ing a part-time 4WD and you can’t push the lever back into 2WD. This is be­cause you’re be­ing pre­vented from dis­en­gag­ing 4WD by the twist­ing force in the driv­e­line bind­ing up the mech­a­nism.

To get the car out of 4WD you have two op­tions. First, you can re­verse in an arc. This will get the front wheels trav­el­ling faster than the rears and will re­lieve the ten­sion to a point where the cen­tre diff light with stop flash­ing, or you’re able to push the lever to en­gage 2WD.

The sec­ond op­tion is to head over to the side of the road, put two wheels into the gravel, and con­tinue driv­ing for­ward. The slip­page al­lowed by the loose sur­face will dis­si­pate the twist­ing force, al­low­ing you to get the car back into road mode.


A LOCKED cen­tre diff will guar­an­tee power is sent to the front and rear diffs.

An­other rea­son to lock the cen­tre diff is to avoid the prob­lems as­so­ci­ated with get­ting stuck in a ve­hi­cle with an au­to­matic trans­mis­sion. If you’re go­ing up­hill and get stuck you’ll have to per­form a re­verse re­cov­ery. If you try this with­out lock­ing the cen­tre dif­fer­en­tial, the front wheels will stay locked all the way to the bot­tom of the hill, due to the weight bias on the rear and the fact all ve­hi­cles have a brak­ing bias to the front.

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