Which four-by-four system is for you?
DRIVING can be a passion or a necessity – sometimes both. Good traction of the tires on the road contribute greatly to our safety and can also make the vehicle fun to drive, whether it be on a dry mountain curve, a slippery Canadian winter road or in the mud on an offroad adventure. Powertrains that drive all four wheels provide the best traction, and their popularity has been proven by the high percentage of four-by-four trucks sold and the large number of all-wheel drive passenger cars currently available on the market.
To simplify things, currently, there are three basic types of four-wheel drive (4WD) powertrains. There are the full-time four-by-four systems that operate in two-wheel drive (2WD) until wheel spin is either detected or anticipated by the vehicle’s computer, which then applies clutches to engage all four wheels. This type of system is often found on smaller SUVs.
There are part-time systems that can be operated in two-wheel drive or four-wheel drive high, and on many models, can be shifted into four-wheel drive low range. This type of system is often found on trucks or larger SUVs, and will have a manual control inside the driver’s compartment to allow shifting between ranges. Some shifting mechanisms are electrically controlled, while others still use mechanical levers.
The third type of system is all-wheel drive (AWD). This is a system that is designed to power all the wheels all the time, and depending on the vehicle, it may be possible to vary the torque divide between the front and rear axles to provide both traction and handling performance. These types of systems are found on many luxury and performance cars and some SUVs.
So what is the best? It depends on what you need to do with your vehicle. Many compact SUVs come with fulltime systems because they can get the best fuel economy operating in 2WD mode, but still provide the traction needed to drive on a slippery road. These are great for everyday driving.
I recently experienced the operation of a part-time four-by-four system in Ford’s F350 Super Duty pickup. This is the strongest type of system. Typical of many trucks, this system can be shifted into 4WD high or low ranges from 2WD. When in a 4WD mode, the transfer case physically locks the front and rear axles to operate at the same speed. This provides excellent traction on loose sand, snow or mud and crawling over rocks, but doesn’t work well on dry pavement or slippery highway driving. When front and rear wheels are locked at the same speed, one or more tires is forced to slip against the road surface whenever the vehicle makes a slight turn. Even slightly different tire sizes can cause “binding” in the powertrain, which is easily overcome when on loose surfaces but can cause a loss of traction on slippery pavement. In my daily commutes, I often see more pickups and large SUVs in the ditch when the roads get slippery than any other type of vehicle. This type of four-by-four system is excellent for work, but it’s not suitable for everyday driving.
Another vehicle I recently tested was the new Lexus RC 350 coupe. This sports coupe came with their all-wheel drive system. I used to think Subaru and Audi had the best all-wheel drive systems on the market, but Lexus has done it right and the all-wheel drive in the RC 350 provides both secure traction on slippery roads and a fun driving experience in the dry.
In all-wheel drive systems, the transfer case is designed to power both front and rear wheels but let them turn at different speeds. This allows maximum traction to the road even when turning corners, and it is this feature that contributes to the fun driving experience. Simply point the steering wheel and the tires will take you where you want to go. Stability and traction-control systems will help maintain the direction on slippery roads, and on dry pavement where there is excellent traction, the allwheel-drive systems will tend to pull you around the corner as if the car were on rails. Performance vehicles like the Porsche 911 Turbo, the BMW 435i xDrive and the Subaru WRX STI use all wheel drive to enhance their dry-weather performance, and now Lexus joins them with the RC 350. Fun, traction, performance and safety.
There are many choices in four-byfour systems. Determine your needs and there will be a vehicle that can meet them.