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What Turns Your Wheels?

What Turns Your Wheels?

BANGKOK, THAILAND – Do these acronyms confuse you – FWD, RWD, AWD, 4WD? They stand for Front-Wheel Drive, Rear-Wheel Drive, All-Wheel Drive and Four-Wheel Drive – respectively. And to make it even more confusing, there’s full-time and part-time 4WD.

Vehicles act differently on loose or slippery terrain depending on whether they have front-wheel drive, rear-wheel drive, all-wheel drive and four-wheel drive. You may know which drive system your vehicle has, but do you understand how it works?

Here’s a brief “drive” through the options:

Front-wheel drive (FWD): Simply put, engine power is channeled to the front wheels to propel the vehicle. Models with this drive configuration include the Chevrolet Cruze, Sonic and Spin. FWD is the most popular and prevalent system in the market because its compact setup enhances fuel efficiency and frees up more room inside the vehicle. Plus, the weight of the powertrain is concentrated on the front driving wheels, offering good traction.

A general characteristic of a FWD vehicle is its tendency to understeer (the vehicle steering less than the amount commanded by the driver) under heavy cornering loads. In extreme conditions, the vehicle may actually plough straight on without much feedback despite steering inputs.

Rear-wheel drive (RWD): As the name implies, engine power is sent to the rear wheels to propel the vehicle. In passenger cars, RWD reigned until the advent of FWD in the early 1980s. But RWD can more effectively handle higher engine power and higher vehicle weights, which is why it’s still favored in large trucks, larger and very powerful performance vehicles, purpose-built race cars and law-enforcement vehicles designed for pursuit. The 4x2 variants of the Chevrolet Colorado and Trailblazer fall under this category. The 4x4 variants also partially fall under this category when four-wheel-drive is not engaged.

Under heavy cornering conditions, RWD vehicles generally tend to oversteer. It basically means the vehicle turning or steering more than commanded by the driver. In extreme situations, this may result in a slide or a spin. Oversteer is more difficult to control than understeer and; therefore, it is a more dangerous dynamic.

All-wheel drive (AWD): Don’t confuse all-wheel drive with four-wheel drive. Both engage all four wheels, but they’re designed and operate differently.

Generally, an AWD drivetrain operates as a FWD or RWD system – most are FWD, like the Captiva’s. An AWD system pre-emptively sends power to front and rear axles on every launch to prevent wheel slip and then reverts to FWD if no slip occurs. Power is transferred automatically via a single-speed transfer case (a transfer case connects to the transmission to split power between the front and rear wheels). The beauty of AWD is no driver effort is needed to activate the system.

AWD scores high with buyers who want excellent on-road capabilities with the added traction on grass, mud, sand or gravel in light, off-road conditions where a front- or rear-wheel-drive vehicle may get stuck. AWD vehicles tend to behave more like a FWD vehicle, in that they understeer when pushed beyond the limit.

Four-wheel drive (4WD or 4x4): Four-wheel drive typically features a two-speed transfer case with high and low ranges for maximum traction. Found in large, rear-wheel-drive trucks and larger SUVs with additional ground clearance compared to passenger cars and crossovers, 4x4 still provides the best traction and capability in off-road conditions.

4WD vehicles typically operate in RWD until four-wheel traction is required (part-time 4WD). The 4x4 variants of the Colorado and Trailblazer are considered to have a part-time 4x4 drivetrain. On full-time 4WD vehicles, all four wheels are driven all the time. A transfer case in the center of the transmission system decides how much torque is split between the front and rear wheels. Typically, more torque is sent to the rear. It changes to a 50/50 split in off-road and slippery conditions, where maximum traction is required. In full-time 4WD vehicles, the driver must still engage the low range manually.

In the pursuit of making vehicles safer, easier to operate and letting the driver focus on driving, manufacturers like Chevrolet have developed electronic systems that enhance traction in low-grip situations. These systems include electronic traction control and electronic stability programs. SUVs and trucks like the Captiva, Colorado and Trailblazer also come equipped with Hill-Descent Control and Hill-Start Assist so they are better prepared to deal with slippery and off-road conditions.