by Jim Kerr

Concerns about the stability of SUV’s have made the news across North America. It only makes sense. They are high off the ground, have a tall body structure, and are usually designed to hold a lot of cargo. On many of the SUV’s built today, you will find warning labels notifying drivers of the SUV’s high centre of gravity handling characteristics.

2003 Volvo XC90
Photo by Laurance Yap. Click image to enlarge

My first experience with Downhill Assist Control was not in your everyday, normal vehicle. One of the Columbia Ice Field’s gigantic SnowCoaches – basically a long wheelbase monster truck with a bus body -provided the transportation, and my front row seat supplied the Ôstand on your nose’ thrill ride. Entering the first very steep downhill grade, the driver simply shifted to low gear, pressed a button, and sat back and relaxed. The bus crawled down the grade slowly and securely.

Spending several hundred thousand dollars on a Snow Coach isn’t necessary to obtain Downhill Assist Control (DAC). Two high-end European UV’s, the BMW X5 and Land Rover come with this feature, but the latest in vehicle grade controls comes in Toyota’s new 2003 4Runner.

During a downhill descent, normally a driver would put the transmission into low gear and use engine braking to slow the vehicle down. On steep hills, engine braking by itself may not be enough to slow the vehicle – and because of the attitude of the vehicle, stepping on the brakes can cause wheels to lock and the vehicle to become unstable. Now it is time for the DAC system to take over.

Toyota’s control system is easy for the driver. Turn the DAC switch on, put the transfer case in low range, make sure the vehicle speed is below 25 kph, and keep your foot off the accelerator and brake pedals. Now all the driver has to do is concentrate on steering.

During a DAC descent, the skid control computer calculates the vehicle speed, travel direction, and the gradient of the hill. Speed sensors and the yaw rate and deceleration sensor provide the information, while the computer automatically applies the appropriate wheel brakes to keep the vehicle stable and slow it down to between 5 and 7 kph.

Driving down steep grades can be tricky, but backing down one is even harder. For that occasional time when the 4Runner’s four-wheel drive system can’t quite pull it up a steep grade, the DAC system also works in reverse. When the vehicle is backing down the hill, the DAC system keeps the vehicle speed between 3 and 5 kph.

Another Toyota system helps 4Runner drivers both on and off road. It is called Hill-start Assist Control (HAC) and it works automatically as long as the transmission shift lever is in any forward gear position. If you have ever had to juggle the brake and gas pedals to start a vehicle moving while it was sitting on an uphill grade, you will appreciate HAC. The system prevents the vehicle from rolling backwards by applying the brakes for up to five seconds after the brake pedal is released. Now the driver can press on the throttle in a relaxed manner without concern that the vehicle will roll.

When HAC activates, a flashing Vehicle Stability Control indicator and a buzzer notify the driver. The rear brake lights are also turned on when pressure is applied to a caliper to notify drivers behind that the vehicle is stopped.

A new design wheel speed sensor is a key component of both Downhill Assist Control and Hill-start Assist Control. Magnetic resistance element (MRE) rotation sensors can detect wheel speed down to almost zero, compared to conventional inductive magnetic sensors that can
typically only measure accurately down to about 5 kph. The MRE sensors can also indicate the direction of wheel rotation.

Inside each wheel’s sealed MRE sensor unit, a pair of hall-effect sensors monitor the changes in magnetic field as 48 bi-pole magnets in the wheel bearings move past them. MRE sensors are used by some manufacturers as high resolution crankshaft and camshaft position
sensors, but Toyota’s 4Runner is the first vehicle to use this technology as wheel speed sensors.

Cars have long been the focus of computerised suspension, steering, and braking technology. Now, truck designers are taking advantage of technology, and using integrated systems to aid driving control. Toyota’s 2003 4Runner is an example of how high tech trucks are becoming.

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