Which four-by-four sys­tem is for you?

Winnipeg Free Press - Section F - - AUTOS - JIM KERR

DRIV­ING can be a pas­sion or a ne­ces­sity – some­times both. Good trac­tion of the tires on the road con­trib­ute greatly to our safety and can also make the ve­hi­cle fun to drive, whether it be on a dry moun­tain curve, a slip­pery Cana­dian win­ter road or in the mud on an of­froad ad­ven­ture. Pow­er­trains that drive all four wheels pro­vide the best trac­tion, and their pop­u­lar­ity has been proven by the high per­cent­age of four-by-four trucks sold and the large num­ber of all-wheel drive pas­sen­ger cars cur­rently avail­able on the mar­ket.

To sim­plify things, cur­rently, there are three ba­sic types of four-wheel drive (4WD) pow­er­trains. There are the full-time four-by-four sys­tems that op­er­ate in two-wheel drive (2WD) un­til wheel spin is ei­ther de­tected or an­tic­i­pated by the ve­hi­cle’s com­puter, which then ap­plies clutches to en­gage all four wheels. This type of sys­tem is of­ten found on smaller SUVs.

There are part-time sys­tems that can be op­er­ated in two-wheel drive or four-wheel drive high, and on many mod­els, can be shifted into four-wheel drive low range. This type of sys­tem is of­ten found on trucks or larger SUVs, and will have a man­ual con­trol inside the driver’s com­part­ment to al­low shift­ing be­tween ranges. Some shift­ing mech­a­nisms are elec­tri­cally con­trolled, while oth­ers still use me­chan­i­cal levers.

The third type of sys­tem is all-wheel drive (AWD). This is a sys­tem that is de­signed to power all the wheels all the time, and de­pend­ing on the ve­hi­cle, it may be pos­si­ble to vary the torque di­vide be­tween the front and rear axles to pro­vide both trac­tion and han­dling per­for­mance. Th­ese types of sys­tems are found on many lux­ury and per­for­mance cars and some SUVs.

So what is the best? It de­pends on what you need to do with your ve­hi­cle. Many com­pact SUVs come with full­time sys­tems be­cause they can get the best fuel econ­omy op­er­at­ing in 2WD mode, but still pro­vide the trac­tion needed to drive on a slip­pery road. Th­ese are great for every­day driv­ing.

I re­cently ex­pe­ri­enced the op­er­a­tion of a part-time four-by-four sys­tem in Ford’s F350 Su­per Duty pickup. This is the strong­est type of sys­tem. Typ­i­cal of many trucks, this sys­tem can be shifted into 4WD high or low ranges from 2WD. When in a 4WD mode, the trans­fer case phys­i­cally locks the front and rear axles to op­er­ate at the same speed. This pro­vides ex­cel­lent trac­tion on loose sand, snow or mud and crawl­ing over rocks, but doesn’t work well on dry pave­ment or slip­pery high­way driv­ing. When front and rear wheels are locked at the same speed, one or more tires is forced to slip against the road sur­face when­ever the ve­hi­cle makes a slight turn. Even slightly dif­fer­ent tire sizes can cause “bind­ing” in the pow­er­train, which is eas­ily over­come when on loose sur­faces but can cause a loss of trac­tion on slip­pery pave­ment. In my daily com­mutes, I of­ten see more pick­ups and large SUVs in the ditch when the roads get slip­pery than any other type of ve­hi­cle. This type of four-by-four sys­tem is ex­cel­lent for work, but it’s not suit­able for every­day driv­ing.

Another ve­hi­cle I re­cently tested was the new Lexus RC 350 coupe. This sports coupe came with their all-wheel drive sys­tem. I used to think Subaru and Audi had the best all-wheel drive sys­tems on the mar­ket, but Lexus has done it right and the all-wheel drive in the RC 350 pro­vides both se­cure trac­tion on slip­pery roads and a fun driv­ing ex­pe­ri­ence in the dry.

In all-wheel drive sys­tems, the trans­fer case is de­signed to power both front and rear wheels but let them turn at dif­fer­ent speeds. This al­lows max­i­mum trac­tion to the road even when turn­ing cor­ners, and it is this fea­ture that con­trib­utes to the fun driv­ing ex­pe­ri­ence. Sim­ply point the steer­ing wheel and the tires will take you where you want to go. Sta­bil­ity and trac­tion-con­trol sys­tems will help main­tain the di­rec­tion on slip­pery roads, and on dry pave­ment where there is ex­cel­lent trac­tion, the all­wheel-drive sys­tems will tend to pull you around the cor­ner as if the car were on rails. Per­for­mance ve­hi­cles like the Porsche 911 Turbo, the BMW 435i xDrive and the Subaru WRX STI use all wheel drive to en­hance their dry-weather per­for­mance, and now Lexus joins them with the RC 350. Fun, trac­tion, per­for­mance and safety.

There are many choices in four-by­four sys­tems. De­ter­mine your needs and there will be a ve­hi­cle that can meet them.

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