AWD v 4WD

AWD v 4WD

If you're an all-weather driver or one who ends up off-road for a fair bit of time, it's almost inevitable that you'll need to consider the AWD vs 4WD debate! Although at first sight the differences would appear to be fairly minimal, the reality is that an AWD vehicle is a very different animal to a 4WD option. Here we take a look at what AWD and 4WD are, including the various types of both variant that are available on the market. Also covered is a "compare and contrast" where we take a look at the similarities and differences between the two types of drivetrain, as well as the pros and cons of each.

What is an AWD?

AWD, or All Wheel Drive cars, are vehicles that have a drivetrain which enables all four wheels (both front and rear axles) to be powered simultaneously. In a vehicle without AWD the drivetrain (the parts of the car that create power (torque) and transfer the power to the wheels) is set so that it just powers the front or back axle (rear wheel drive).

In an AWD vehicle, the drivetrain is designed to allow power to transfer in variable quantities to each axle. Sensors on the wheels transfer information about the degree of traction each one has. This information prompts the vehicle's computer to make decisions on the best division of power between the wheels, which then translates into subtle alterations in drivetrain operation. Note that the process of power transfer between the wheels is completed automatically via the computer, there isn't a manual element to the decision-making process.

What is a 4WD?

A 4-wheel drive vehicle is similar to an AWD in that the torque is conveyed to both axles in varying quantities, rather than simply to the back or front axle, as is the case with a standard car. Aside from that similarity, there are a number of key features to a 4WD system that separates it from an AWD.

The mechanism by which power is delivered to each wheel in a 4WD vehicle is different to that for an AWD, although the distinction is blurring as both 4WD and AWD technologies become more sophisticated. The other key characteristic of 4WD that sets it apart from AWD is the degree of manual control the driver has over when it's engaged and what type of 4WD is used (see further down the page for details of the various types of 4WD on the market). The degree of control a 4WD driver can wield is a key reason why 4WD vehicles have such a loyal following!

Different types of AWD and 4WD

As drivetrain technology has improved, the traditional differences between AWD and 4WD have become more blurred. There are now a number of different systems on the market, each of which offers a different driving experience. Here we consider: on-demand AWD; continuous 4WD; part-time 4WD; and low range 4WD.

On-demand AWD

Conventional AWD is engaged all the time, with the vehicle's computer continuously assessing sensor information in order to distribute torque in the most effective manner. Whilst this works well in terms of optimising traction (and therefore providing a safer, smoother ride), AWD frequently increases the amount of fuel the vehicle is using. If the AWD is engaged continuously, this can result in significantly raised running costs. On-demand AWD, as the name suggests, only kicks in when the sensors pick up that the vehicle hasn't got sufficient grip when being driven as a standard front wheel drive (the power just going to the front axle). The computer responds to a lack of grip by switching the car from front wheel drive only to AUD. With torque then able to be distributed between all four wheels, the vehicle's traction can be improved. As this process is completely automatic, on-demand AWD can increase both safety and fuel economy - a double win!

Continuous 4WD

Continuous 4WD is a drivetrain that's designed so that power is continuously distributed to all four wheels. Modifications to the 4WD drivetrain enable the axles to turn at differing speeds, ensuring cornering and steering can be completed during road driving with minimal transmission wind-up (transmission wind-up is the stress caused to the drivetrain when all four wheels are locked into rotating at the same speed, which is the default for traditional 4WD. When driving in normal conditions, a vehicle that's continuously driven in 4WD will experience enormous drivetrain wear and stress). When drivers want to enjoy the benefits of old-style 4WD (all four wheels locked to optimise traction), it can be engaged manually.

Part-time 4WD

This is 4WD that's manually engaged when necessary. When driving on tarmac roads, or other surfaces where only a modest degree of traction is needed, the power from the drivetrain will travel to the rear axle only. Manually engaging the transfer case (part of the drivetrain that switches power from one axle to both) will cause equal distribution of power to all four wheels, creating a high degree of traction.

 

Low range 4WD

Reserved for particularly difficult terrains (sand dunes, steep, muddy slopes and similar), low range 4WD can be manually engaged as an additional modification to engaging 4WD. Low range 4WD is the 4WD modality, in a low gear ratio. This combination provides a super-powerful torque to all four wheels, maximising traction and enabling a 4WD vehicle to keep going in really challenging conditions.

The differences between AWD and 4WD

Automation: AWD is automated, whereas 4WD can be engaged when the driver feels it to be necessary.

Drivetrain mechanisms: the AWD mechanism operates differently to the 4WD mechanism, which is more basic. 4WD drivetrains are notoriously robust, as the vehicles which house them are designed for regular off-road use.

Usage: AWD vehicles are designed for conventional driving on tarmac roads, with the odd bit of driving on gravel tracks of similar. They are not designed for heavy-duty off-roading. In contrast, a 4WD vehicle will be constructed for off-road use. 4WD vehicles frequently have: greater ground clearance; bigger, more robust tyres; an exhaust that isn't located under the vehicle; highly durable chassis and undercarriage; enhanced steering, different shocks and struts; and other modifications. Die-hard off-road drivers will almost invariably opt for a 4WD.

AWD vs 4WD

Both systems have their own pros and cons. One isn't superior to the other - the correct choice largely depends on the type of driving you normally do, and personal preference. If you like to let the car take the strain, and aren't a big fan of dirt tracks and muddy trails, the computerised, zero hassle AWD system is probably going to suit you better.

Conversely, if you love to be in control of your vehicle's torque distribution, and tend to spend significant amounts of time driving in tough conditions (snow or ice, for example) or traversing challenging terrain, the more durable, robust 4WD vehicle is going to probably be a better bet.

The key drawbacks of both AWD and 4WD are the same: a higher initial purchase price compared with a vehicle that doesn't have one of these drivetrain options, and higher fuel costs when torque is transferred to both axles.

Whichever option you select, there's little doubt that motoring is safer and easier when it's possible to transfer power between the front and rear axles if required.

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