All-wheel drive

All-wheel drive sends torque to all four wheels without the driver having to select anything. The most common form on Indian SUVs starts with a front-wheel-drive layout and adds a route to the rear.

• A power take-off unit on the transaxle turns the drive through 90 degrees into a propeller shaft running to the rear axle.
• Just ahead of the rear differential sits a multi-plate clutch coupling in an oil bath.
• In normal driving the coupling is open or lightly loaded and the car runs on its front wheels alone, which saves fuel.
• When the control unit sees the front wheels start to spin, or detects a hard launch, a steep incline or a chosen drive mode, it squeezes the clutch plates in milliseconds and the rear wheels join in. The split between the axles varies continuously.

Because the coupling can slip, the system can stay engaged on dry tarmac without binding in corners, unlike a part-time 4x4. Full-time systems such as Audi's quattro take a different route: a centre differential, often a Torsen, splits torque permanently between both axles. Electric cars get all-wheel drive more simply still, with a second motor on the other axle and no shafts at all, as in the Kia EV6 and BYD Sealion 7.

What you notice: nothing, most of the time, which is the point. On a wet ghat road, a muddy track to a farmhouse or a slushy climb you get traction without touching a switch. The cost is extra hardware and weight, slightly higher fuel use and price, and usually no low range, so it is for grip, not for serious rock crawling. The Grand Vitara and Hyryder AllGrip, XUV700 AWD and Skoda Kodiaq 4x4 are typical.