Traditional 4WD systems are still available, but as time goes on, their prevalence has lessened greatly. They work manually, and are very effective overall.
Locking front and rear drivetrains together makes the vehicle jerk and lurch in good traction, especially while turning. (This is called driveline windup or tight comer braking.) These systems will wear out quickly or break if used extensively on clean pavement.
While unlocking manual free-running front hubs saves fuel and cuts wear on the front driveline, the driver must know the hubs’ position without an indicator. When locked, they allow `shift-on-the-fly’ 4WD engagement. However, shifting into 4WD while driving with the hubs unlocked could lead to `powertrain parts on the fly!’
Many off-road drivers prefer this system. Simple and reliable, when combined with limited-slip differential options it can be nearly unstoppable in extreme conditions. But most motorists don’t encounter such extremes and prefer something more user-friendly.
Often also called all-wheel-drive (AWD), basic AWD uses an open differential at the transfer case to relieve powertrain loads (driveline windup) developed as front and rear wheels travel varying distances…