Trac­tion ad­ven­tures in treach­er­ous con­di­tions

Winnipeg Free Press - Section F - - AUTOS -

drive SUVs and trucks.

When it comes to trac­tion, tires make the big­gest dif­fer­ence. Win­ter ice tires would have helped many driv­ers stay on the road or just made the morn­ing com­mute that much eas­ier and safer. The next most im­por­tant trac­tion fac­tor is how the power is put down to the road.

Hav­ing power to all four wheels will pro­vide ex­cel­lent trac­tion, and that fact is rec­og­nized by au­tomak­ers in their wide se­lec­tion of all-wheel drive ve­hi­cles. AWD spreads the engine torque out to each tire so that each can pro­vide some of the grip nec­es­sary for ac­cel­er­a­tion and steer­ing con­trol.

AWD ve­hi­cles use a trans­fer case that splits the power to each end of the ve­hi­cle, but still al­lows each axle to turn at dif­fer­ent speeds. This can be ac­com­plished sev­eral ways. Some use a plan­e­tary gearset and clutches in the trans­fer case. Oth­ers use a vis­cous cou­pling unit that uti­lizes sil­i­cone fluid be­tween two plates to drive the axles. Oth­ers use a Torsen de­sign dif­fer­en­tial in the trans­fer case.

There are other meth­ods as well, but the pur­pose of all these de­signs is to al­low the wheels at each end of the ve­hi­cle to turn at dif­fer­ent speeds.

Four-wheel drive may seem to of­fer the same ad­van­tages, but it also has a ma­jor draw­back: this sys­tem drives both the front and rear axles at the same speed. That wouldn’t be a prob­lem if we only drove in a straight line on a smooth road. But hit a bump or round a cor­ner and the wheels sud­denly turn at dif­fer­ent speeds be­cause they are not fol­low­ing the same path of travel. All-wheel drive al­lows this. Four-wheel drive doesn’t.

In four-wheel drive, the front and rear drive­shafts are me­chan­i­cally locked to­gether to turn at the same speed. This drives the tires at the same speed. Try go­ing around even a grad­ual curve and one or more of the tires has to lose trac­tion. The tighter the cor­ner, the more the tire has to slip on the road sur­face.

On dry pave­ment, this ac­tion would wear tires and place a tremen­dous load on driv­e­train com­po­nents, wear­ing them out quickly. But on slip­pery roads, the forced slip­page of the tires on cor­ners will typ­i­cally cause the ve­hi­cle to lose steer­ing con­trol. You can turn the steer­ing wheel but the ve­hi­cle won’t turn be­cause the tires are not grip­ping the road at all.

That’s why four-wheel drive ve­hi­cles are not the best on slip­pery roads. Look in the owner’s man­ual of a four­wheel drive ve­hi­cle and it will tell you that four-wheel drive should be used on loose or off-road sur­faces only.

Many SUVs and some pick­ups have an “Auto” mode for their four wheel drive. This mode al­lows the ve­hi­cle to op­er­ate in two-wheel drive (ei­ther rear or front, de­pend­ing on the model of ve­hi­cle) and the other two wheels are only pow­ered dur­ing ac­cel­er­a­tion or when the drive wheels be­gin to spin. Auto mode works fine on slip­pery roads be­cause, as soon as you back off the throt­tle, the ve­hi­cle re­turns to twowheel drive mode and the tires will get trac­tion again.

Four-wheel drive may rule the roost on off-road ad­ven­tures in mud and deep snow. But, on icy, slip­pery roads, all-wheel drive is bet­ter any time.

Just re­mem­ber: All-wheel drive may get you go­ing faster, but stop­ping isn’t any faster with an AWD ve­hi­cle. So in­stall good win­ter tires and keep your speed down for safe, se­cure driv­ing. Jim Kerr is an ex­pe­ri­enced me­chanic, in­struc­tor of au­to­mo­tive tech­nol­ogy, free­lance jour­nal­ist and mem­ber of the Au­to­mo­bile Jour­nal­ists’ As­so­ci­a­tion of


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