Article by Jim Kerr; photos by Paul Williams
Suzuki has a new vehicle – the SX4, and with it comes a compact all-wheel drive system that makes this compact vehicle stand out in its class. Not only does it work well, it is also one of Canada’s lowest priced all-wheel drive vehicles, competing only with the Jeep Compass for economical all-wheel drive transportation.
All-wheel drive (AWD) is different than four-wheel drive. AWD is designed to be used on hard road surfaces such as asphalt and concrete as well as slippery road surfaces. It can operate all the time and enhances both the traction and handling characteristics of a vehicle if designed correctly. There are three basic types of AWD on the market.
Some vehicles drive all the wheels all the time and use a viscous coupling or computer controlled clutch in the transfer case to allow slippage between the front and rear wheels during cornering. This is necessary because the front and rear wheels roll around a different arc on a corner and therefore must travel at different speeds.
Another type of all-wheel drive provides power to the rear wheels and transfers power to the front wheels when necessary to maintain traction. Wheel speed sensors are used to detect tire slip so the computer knows when to transfer the power. As most vehicles on the market already have ABS braking, the data from the wheel speed sensors is already available, so it is relatively easy to detect acceleration slippage as well.
The third type of AWD system drives the front wheels and transfers power to the rear wheels as necessary. This is the type of system used on many smaller AWD vehicles and on the Suzuki SX4.
Four-wheel drive systems are designed for maximum traction on loose surfaces. When four-wheel drive is engaged, the front and rear axles are locked together and torque is divided 50/50 between the two axles. So all wheels drive all the time. While this works fine in loose sand, mud and snow, it will cause problems on hard road surfaces or ice.
On hard road surfaces, there is always a slight difference in tire speeds between the front and rear tires because of different tire diameters due to manufacturing tolerances or wear. This speed difference causes binding in the locked drivetrain and something must slip. Usually it is the tires that slip, but as they do this they create high heat in differentials and the transfer case. It is possible to destroy a transfer case in a few hundred kilometres of driving if four-wheel drive is engaged on the highway.
Ice also causes problems. Ice lets the tires on one axle slip to make up the slight speed differences, but this slipping makes the tires lose traction. This is especially noticeable when turning on an icy corner. The vehicle feels like it doesn’t want to turn. Four-wheel drive may be great at letting you pull away from an icy parking spot but isn’t great for driving on ice.
2007 Suzuki SX4. Click image to enlarge
The new Suzuki AWD system drives the front tires and transfers power to the rear as necessary. There is a switch on the console that allows the driver to select front wheel drive only, which can be useful if you have to drive with a different size tire for a short while, but AWD mode can be selected for most operation. Information on wheel speed from the ABS system and throttle position from the engine computer are used by the AWD computer to determine the amount of torque transferred to the rear wheels.
During normal driving, all the torque goes to the front. During hard acceleration or when the front wheels are slipping, up to 50% of the torque can be transferred to the rear wheels. A small electromagnetic ramp clutch (not much bigger than a jar of pickles) in front of the rear axle differential is used to vary the amount of torque driving those wheels. As soon as no wheel slip is detected, the system goes back to front-wheel drive operation again.
The SX4 AWD system also has a “Lock” mode where the computer transfers a minimum 30% torque to the rear axle all the time. The computer then monitors wheel slip and can increase the torque transfer up to a 50/50 split. Just like four-wheel drive systems, the Lock mode is useful on loose surfaces but not on ice. Drive the SX4 over 60 km/h and the computer disengages the Lock mode and reverts back to normal AWD operation.
AWD used to be reserved for big vehicles, but now it is used in sports coupes, family sedans and compact vehicles for the additional traction and safety it provides. It also enhances handling performance, moving the vehicle in the direction the driver steers the vehicle. The SX4 AWD system is compact, smooth and provides all the benefits in an economical package.