Four-wheel drive (4x4)

Part-time four-wheel drive is the heavy-duty system found on ladder-frame SUVs and pickups. It starts as a rear-wheel-drive layout and adds a transfer case behind the gearbox.

• In 2H the transfer case simply passes drive to the rear propeller shaft; the front shaft, differential and hubs idle.
• Select 4H and a dog clutch (or a chain and sprocket) inside the transfer case locks the front propeller shaft to the rear one. Both axles now turn at the same speed and the front wheels pull as well.
• Select 4L and an extra reduction gearset in the transfer case, typically between two and three to one, multiplies torque to every wheel and slows the vehicle to a crawl, for steep climbs, descents and deep ruts.
• Many systems add a locking rear differential, and some a front locker too, so one wheel in the air cannot stop the vehicle.

Because 4H and 4L lock the axles together with no centre differential, the front and rear wheels cannot turn at different speeds. On a tight turn on tarmac the drivetrain winds up, the steering goes heavy and something eventually has to slip: the tyres, or a part. So 4WD is engaged only on loose or slippery ground and switched back to 2H on the road.

What you notice: a lever or dial for 2H, 4H and 4L, a slight thump as the front axle engages, the ability to crawl up a riverbank at idle in low range, and the heavy, separate rear axle that gives these vehicles their truck-like ride. The Mahindra Thar and Scorpio-N 4XPLOR, Force Gurkha, Maruti Jimny, Toyota Fortuner 4x4, Isuzu V-Cross and Tata Yodha 4x4 all use this type.