What types of 4X4 can I buy?

Whether you are looking to buy a true off-road car, or a crossover/SUV will an all-wheel-drive capability, the choice is bewildering. Our previous blog on the topic gives a brief history of how 4x4s have evolved. Yet, this blog explains the technical variations that you will encounter, particularly on the used car scene, and how they are relevant.

What different 4x4 layouts are there?

Carmaker marketing departments have fostered confusion, by inventing a baffling range of anachronisms and branding to make their 4x4 systems sound technologically different to those offered by their competitors. It is so easy to be confused by them. Yet, there are no formal designations, because virtually all modern 4x4 systems are variations on only two themes.

A longitudinal (other names include inline/north-south) mechanical arrangement is the most traditional layout that you will encounter. It tends to be employed by the most effective off-road vehicles and, usually, prioritises rear-wheel-drive. Technically, these vehicles employ a transfer box, mounted alongside a manual transmission. This provides the means of powering the front wheels through a separate propshaft.

The other method is one used by many SUVs and crossovers. These vehicles tend to employ a more 'car-like' transverse-mounted engine and front-wheel-drive gearbox, which has been adapted to incorporate a second output shaft to drive the rear axle.

The most common layouts use a propshaft to transmit rotational motion to a rear differential. An electro-mechanical coupling unit tends to be used, too, to vary the amount of power sent to the rear wheels, for efficiency reasons.

Which 4x4 layout is best?

Much depends on your needs. Generally, longitudinal mechanical layouts are superior in off-road conditions but they tend to be more compromised on the road. The more modern transverse engine layouts tend to be superior on-road and cheaper to run, while still offering the on-road stability advantages of 4x4.

Yet, they tend not to be as capable off-road. This might explain why some manufacturers have offered both mechanical layouts. The Range Rover, for example, employs a longitudinal mechanical layout, whereas the smaller Evoque crossover possesses a transverse 4x4 system.

Another example is Suzuki. Its hardy Jimny, a direct ancestor to the SJ series that has remained on UK sale since 1998, employs a longitudinal transmission, with its rear wheels driven permanently. The front wheels are powered only when selected via a dashboard-mounted electrical switch.

Conversely, the company’s S-Cross crossover uses the more contemporary transverse, front-wheel-drive biased system, with rear drive controlled by a differential-mounted electrically-controlled coupling

How good is Subaru's 4x4?

Subaru is a notable exception. The company has refined its unique symmetrical AWD transmission (pictured) over the past 50 years. While a more traditional longitudinal layout is used, powered by a horizontally-opposed (‘boxer’) four-cylinder engine, front-wheel-drive is prioritised, with the transfer box gears being integrated within the gearbox casing. Virtually all power can be directed to one wheel only within a split second.

As most transverse-mounted engines and gearboxes utilise different-length driveshafts, torque-steer can become a problem, especially when the front wheels lose grip on a slippery surface. Subaru’s symmetrical solution allows engineers to employ equal-length driveshafts that help to avoid the trait.

However, not helped by the inherent inefficiencies of the ‘boxer’ engine layout, Subarus tend to suffer from higher official fuel consumption and CO2 emissions. Yet, as many Impreza and STI tuners have proven, by increasing engine output considerably without modifying the transmission and not suffering subsequent driveline breakages, Subaru's AWD hardware is immensely robust.

What about EV 4x4 systems?

While numerous mechanical arrangements exist for hybrid vehicles (which we shall cover in a future blog), Battery Electric Vehicles are different, too. Most types possess a separate motor for each axle. This reduces the need for complex mechanical transmissions but increases the electrical management complexity.

Even so, EV 4x4 layouts are very effective at maximising grip. Despite this, they are hampered off-road not because of their mechanical layouts but weight and ground clearance issues. As demand increases for off-road BEVs, we are certain that this situation will improve

Should I choose 4x4 at all?

Considering whether you really need 4x4 is not a bad idea. Regardless of manufacturers’ wondrous claims, no AWD system can function if there is no tyre grip; even the best transmissions can only make the most out of the traction available. As front-wheel-drive cars can perform impressive feats with a good set of off-road (or cold weather) tyres, you may query whether, or not, AWD is for you,

especially considering any extra running costs that may result from potentially higher fuel consumption, VED rates and insurance costs. If buying an older vehicle, seek out any specific model ownership reviews and look for particular transmission weaknesses that will allow you to consider any potential repair bill implications.

Our next blog on 4x4 transmissions looks at the maintenance requirements.

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