How Does The Toyota Rav4 Four - Wheel Drive System Work?



Toyota RAV 4The Toyota RAV4 was never designed to compete with the off-road capabilities of its Hilux and Land Cruiser siblings. Indeed, the model name is a contraction of Recreational Active Vehicle with four-wheel drive – a description that identifies the RAV4 as a lifestyle vehicle with the additional benefit of some all-wheel drive performance.




But to dismiss the Toyota RAV4 as an urban-only SUV undersells the sophistication of its chassis. Nowhere is this more evident than in the highly accomplished current generation model, which uses its optional all-wheel drive system to keep you safe, under control and mobile in conditions that would be impossible in a traditional car.

The latest RAV4 features a programme that co-ordinates every function of the four-wheel drive system, stability control system and electric power steering. Compared with the all-wheel drive setup in the previous generation RAV4, this new management system does not always need to rely on slip detection before deciding on the distribution of torque to each wheel.

The driver now has the option to come out of this default setting and increase the level of the system’s interaction by engaging Sport mode, activated by a button in the centre console. Like a heightened level of awareness, this mode sends data to the relevant ECU from sensors monitoring vehicle speed, steering angle, throttle input and yaw rate, which the programme then uses to determine the proportion of torque to send to the rear wheels.

This is physically actioned by varying the strength of electric current heading into an electro-magnetic solenoid in the control coupling located ahead of the rear differential (see image above). Depending on driving conditions, the amount of torque being directed into the differential can vary by as much as 50 per cent, from the car being entirely front-wheel drive for optimum fuel efficiency to an equal sharing of torque between front to rear axles.

So accurate is the monitoring of vehicle behaviour that the shuffle of torque between the axles will have already started before any slip or understeer is detected. From the moment the steering is turned, the system automatically adjusts to a 90:10 front/rear torque distribution, priming the car to deliver optimal cornering poise and grip. This simultaneously reduces the load on the front wheels and the likelihood of experiencing understeer.

Should the cornering force go on to exceed the car’s adhesion to the road surface, however, yaw rate controls react within milliseconds to instruct the system to apply up to 50 per cent of the available torque to the rear wheels to improve grip.

There are times, such as in snow or on steep gravel tracks, when the driver will know before even setting off that grip levels will be poor. In this instance, it is possible to maximise grip by locking the torque distribution in a 50:50 ratio at speeds up to 25mph. This locked function is activated simply by pressing a button to the left of the steering column, which then illuminates a discreet warning light above the fuel gauge (see images below).

Thanks to a minimum ground clearance of 187mm and relatively short overhangs, the current RAV4 is then capable of riding over heavily rutted tracks and clambering over relatively large obstacles.

Although the RAV4 may be categorised as a modern ‘soft-roader’, its adoption of Toyota’s technology and four-wheel drive expertise means the RAV4 is anything but ‘soft’.
 

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