All-Wheel-Drive vs 4x4

 You may have seen Marty and MOOG's new project (CLICK HERE) and have questions about what the difference between all-wheel-drive and four-wheel-drive is. Funnily enough, "what the diff" actually gives you a huge hint to the answer.

There are quite a few different all-wheel-drive systems out there from various manufacturers, including Volkswagen's HALDEX and SYNCRO,  Nissan's ATTESSA E-TS used in the BNR-generation Skyline GT-Rs, plus countless others. Many of them use the same basic engineering princlples that have been used since Jensen slapped the Ferguson-Formula AWD system in their Interceptor FF model to create AWD passenger cars.  

Subaru's "symmetrical AWD" system is possibly the most famous, so let's start off by looking at this layout, which uses a north-south engine layout like a rear-drive car. However, there are a lot of cleverly engineered details hidden in there which make for a reliable, compact, grip-filled drivetrain. 

This diagram from THESE LEGENDS (CLICK HERE) shows how the Subaru transmission sends drive to the front wheels. The engine spins its power through the clutch and flywheel down a main shaft like a regular rear-drive gearbox, but the Subaru has a centre differential behind the normal gearset, which the spins a set of gears that pass the power back forwards to the front diff mounted in the front of the transmission.

In most regular manual Subarus that centre diff is a fluid viscous unit sending 50% forwards and 50% rear. STi gearboxes with the Driver Controlled Centre Diff (DCCD) technology have other modules installed to allow the driver to adjust the aggressiveness of the front and rear diff lock-up, but that is a whole other conversation for another day. 

Subaru's AWD differs from others by splitting the drive 50:50 front:rear in most of their models, while other manufacturers adjusted for more front- or rear-drive bias to fine-tune the handling of their car. This treated the car more like a two-weheel-drive which could access more grip when needed, rather than a car with all wheels being offered the same amount of grip.

Nissan's advanced ATTESSA E-TS AWD system is hydraulically controlled and has a few key differences to other AWD systems, but it was designed to work mostly like a rear-drive drivetrain, and to bring the front wheels in when more grip was needed (like, on corner exit). 

Where this all differs from proper 4x4 vehicles is in how the wheels are driven, or not. A true 4x4 has the ability to lock its diffs together to radically improve traction.

Normally the inside wheel of any car needs to turn more slowly to improve the turning radius (how tightly you can turn) and prevent tyre wear. Locking the diff so it drives both wheels equally makes that a problem, but will let you drive out of a mud bog or up a rocky trail when you'd otherwise be stuck with all the drive being sent to the spinning wheel when it needs to go to the wheel on the ground.

While the diffs in AWD cars can adjust drive to different wheels in various slip situations or by driver input, 4x4 vehicles don't typically offer a great deal of adjustment. Most 4x4s will primarily drive in a rear-wheel-drive set-up with that diff featuring a form of limited-slip centre. 

Old school fourbies sometimes have large protruding hub in the centre of their wheels, which are often how the hubs would be locked in. The theory being you'd hop out ahead of low-grip terrain (or after you're stuck), lock the lubs in and this would lock the front and rear diffs so all four wheels turn at the same rate with fixed equal drive. 

Today you can get air or electronically-operated locking diffs which can be operated by the press of a button on the dashboard. These are typically aftermarket diff centres which have been added in, and Australia is at the cutting edge here with the advanced Harrop E-locker. 

Most proper 4x4s will also feature a low-range option to allow what has been known as a "crawl gear" for creeping the car along at slow speed in tricky terrain. Older cars will have a second gear-stick to select this option at low speeds (or when stopped), while modern cars will typically have a series of buttons or switches to select 2H (two-wheel-drive high-range), 2L (two-wheel-drive low-range), 4H (4x4 high-range), 4L (4x4 low-range). 

So while AWD may be better around town or on the race track, 4x4 cannot be beaten on slippery off-road conditions.