This is an oversimplification, but there are two primary types of traction required for vehicles to operate safely:
- Longitudinal traction
- Required for acceleration and deacceleration, and on grades to prevent forward/rearward slipping.
- Lateral traction
- Required to prevent sideward slipping during turns due to centripetal forces, and on grades due to gravitational forces.
Vehicles lose traction and stability when the coefficient of friction between the tires and the road cannot mitigate the longitudinal and/or lateral forces acting upon the vehicle.
Despite the hype that often accompanies it, 4WD (and AWD) is
not the solution to providing all types of traction and ultimate confidence where you can drive safely in all situations where the roads are slippery.
Every driver must understand what 4WD does and does not do, otherwise 4WD overconfidence can and does lead to accidents.
4WD primarily provides increased forward longitudinal traction when accelerating. This is especially useful from a dead stop on slippery roads where maximum longitudinal traction is required. When in motion, it can provide additional longitudinal traction when accelerating or deaccelerating (via engine braking). However, when in motion at speed such as on a highway, this additional traction is not usually needed due to sufficient forward momentum. Yet, there are times when it can be very useful for accelerating or maintaining speed up a hill in slippery condtions for example. It accomplishes this simply by providing additional drive torque to more than the usual single tire. This significantly increases the overall friction between the driven tire(s) and the road which provides increased longitudinal traction. While very useful at specific times, 4WD does not provide anywhere near the overall required traction to safely drive in slippery conditions.
4WD does not provide much useful additional braking power. Other than providing some additional longitudinal traction during engine braking, it cannot ensure safe deacceleration in slippery conditions. This is where 4WD overconfidence leads to accidents, the confidence 4WD provides during acceleration quickly disappears when braking. On modern vehicles, antilock braking provides far more safety during braking in slippery conditions.
4WD does not provide much additional lateral traction to ordinary drivers. You can accelerate as quickly as you want with 4WD in a straight line, but during a sharp turn in slippery conditions, never expect 4WD to get you out of trouble unless you know advanced driving techniques. This is another situation where 4WD overconfidence can cause accidents. The coefficient of friction between the road and your tires is primarily what stops your vehicle from sliding laterally during turns. The lower the coefficient of friction, the more you're likely to slide out. What keeps you safer and on the road in modern vehicles is Electronic Stability Control, not 4WD. ESC uses the ABS system and yaw sensors to estimate the direction of the lateral movement and then applies the brakes to individual wheels asymmetrically far faster than any human could possibly respond in order to create torque about the vehicle's vertical axis, opposing the skid and bringing the vehicle back in line with the driver's intended direction. Additionally, the system may reduce engine power or operate the transmission to slow the vehicle down.
So, while 4WD has specific, limited advantages in slippery conditions, it does not provide the overall protection many people think it does. It's only one optional part of a complete package of safety features.