771 Comparison of two and four wheel drives

The total force that a tyre can transmit to the road surface resulting from tractive force and cornering for straight and curved track driving is limited by the adhesive grip available per wheel.

When employing two wheel drive, the power thrust at the wheels will be shared between two wheels only and so may exceed the limiting traction for the tyre and condition of the road surface. With four wheel drive, the engine's power will be divided by four so that each wheel will only have to cope with a quarter of the power available, so that each individual wheel will be far below the point of transmitting its limiting traction force before breakaway (skid) is likely to occur.

During cornering, body roll will cause a certain amount of weight transfer from the inner wheels to the outer ones. Instead of most of the tractive effort being concentrated on just one driving wheel, both front and rear outer wheels will share the vertical load and driving thrust in proportion to the weight distribution between front and rear axles. Thus a four wheel drive (4WD) when compared to a two wheel drive (2WD) vehicle has a much greater margin of safety before tyre to ground traction is lost.

Transmission losses overall for front wheel drive (FWD) are in the order of 10%, whereas rear wheel drive (RWD) will vary from 10% in direct fourth gear to 13% in 1st, 2nd, 3rd, and 5th indirect gears. In general, overall transmission losses with four wheel drive (4WD) will depend upon the transmission configuration and may range from 13% to 15%.

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