vcdiag-iiThe All Wheel Drive (AWD) system on a vehicle is rather useful when driving on pavement in foul weather conditions.  It has the ability to transfer power automatically.

 

 

 For instance if you were driving on slick roads and the front wheels start spinning then a portion of the power is transferred to the rear wheels automatically.  When this happens then your vehicle is now a 4 wheel drive.

This allows the vehicle to be more controllable on slick roads due to the fact that power is distributed more evenly throughout the drive train.

Although the AWD vehicle is useful on the pavement it’s not very useful off road.  Yes it is considered a 4 wheel drive when the wheels slip but it’s not a true 4 wheel drive.  4 wheel drives have a locking differential and a transfer case. 

These components allow the vehicle to stay in 4 wheel drive mechanically until the differential lock and transfer case are disengaged.

An AWD vehicle does not use these kinds of locking components.  An AWD vehicle uses a viscous coupling device.

The viscous coupling is basically a cylinder with a shaft, extending from either end. Inside, there are plates attached to each shaft so that they spin alongside each other independently. One axle and set of plates turn the same time as the front wheels, and the other axle and set of plates turn the same time as the back wheels.

There is a fluid inside that basically connects the plates.  As long as the plates are spinning at the same speed, the fluid stays thin, and the car remains in front-wheel drive.

The moment the front wheels start to slip, the connected shaft – and therefore, the plate that is attached to it inside the viscous coupling – starts to spin much more quickly than the other one, causing friction.

At that very moment, the fluid thickens, causing the first plate – the one connected to the front wheels – to drag the other one – the one connected to the rear wheels – along with it.

Along with the plate, the axle that goes out to the rear wheels is also being turned.  The viscous coupling allows the power from the engine to be transmitted to the rear wheels, as well as the front wheels, whenever the front wheels are slipping.

There are two factors that hinder the operation of the viscous coupling:

·         High torque:  The viscous coupling is basically useless at slow speeds and climbing hills where there is a lot higher torque ratio involved.

·         Heat:  When this fluid gets hot under constant use like in off road applications it loses its viscosity; in turn loses its friction capabilities.

I hope this helps you better understand your AWD vehicle and its true capabilities.  I don’t want you to go off road thinking you have a true 4 wheel drive vehicle because you don’t have a true 4 wheel drive vehicle.

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