All four-wheel drives are not created equal, and there are some interesting new developments that challenge the status quo.
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We’ve been asked:
What’s the difference between all-wheel drive and four-wheel drive?
Since covering off the differences between four-wheel drive and all-wheel drive, this question is the next natural step.
As the name suggests, part-time four-wheel drive means the four-wheel drive system is selectable between two-wheel drive (for on-road usage) and four-wheel drive (for off-road usage only).
This is mostly commonly seen on four-wheel drive utes and wagons, which (mostly) use a setup that is referred to as ‘shift-on-the-fly’. In these cases, the four-wheel drive system disengages the front wheels, through a simple dog clutch in the transfer case, and renders the vehicle rear-wheel drive only.
This system is simple and effective. While there are certainly benefits to having all-wheel drive on the blacktop, doing so comes with extra components and complexity. A part-time system is less sophisticated, and benefits from being durable and time-honoured. It’s easily identified by having drive modes 2H (two-wheel drive, high range) and 4H (four-wheel drive, high range), along with 4L (if there is a low-range transfer case).
Four-wheel drive can only be used on-road because when engaged, the front and rear wheels are locked into the same speeds (with no differential) and damage can easily occur.
The majority of four-wheel drives use this system, and it works. However, having only two-wheel drive in a vehicle with high levels or torque (and sometimes stiff suspension) means wheelspin and potential loss of control.
Full-time four wheel drive refers to a vehicle that can use four driven wheels on the blacktop, but can still engage a proper four-wheel drive mode. Off-road performance isn’t affected so much, but having four driven wheels on wet bitumen, dirt and through tight corners pays a clear dividend.
Full-time four-wheel drive (or permanent four-wheel drive) has been a popular choice over the years, with Toyota LandCruisers, Land Rovers and Range Rovers using full-time four-wheel drive systems. Traditionally speaking, a full-time four-wheel drive system revolves around a the inclusion of a centre differential in the transfer case.
This does the important job of allowing for slip between front and rear wheels, but the differential also needs a locking ability for good off-road capability.
One setup that plays its own game in this regard is Mitsubishi’s well-regarded Super Select system, which has the added functionality of being able to offer two-wheel drive as well, on top of all-wheel drive and four-wheel drive.
Jeep has used more sophisticated transfer cases with similar levels of functionality over the years, under the Selec Trac name.
Put simply, it gives you the option of part-time and full-time four-wheel drive in one system.
A new generation of four-wheel drive systems has emerged in recent years, which uses electronically controlled clutch packs in lieu of a centre differential. It’s used in the most recent Ford Ranger (when equipped with the V6 engine), as well as the Ford Everest and Land Rover Defender.
This can act like a true four-wheel drive system by locking the clutch packs into a proper 50/50 split, for that all-important off-road torque split.
However, clutch packs can also be partially engaged for on-road driving. One benefit here of this system is the ability of the car to vary the torque split, from anywhere between 50/50, to 90/10, or even disconnecting the front wheels for a more efficient rear-wheel drive.
It’s sophisticated and effective, but cannot match the part-time setup (or even a more basic centre differential) for mechanical simplicity. How it stands up in terms of durability and longevity will only be answered after many years of hard usage.
Have a question about your next set of wheels or just need some car advice? No query is too big, small or obscure! Call in to the radio show (Trent on 2GB Sydney 1:30pm Monday and 9:00pm Wednesday, 5AA South Australia 1:30pm Tuesday, and James on 3AW Melbourne 9:00pm each Thursday), or contact us by email here: contactus@drive.com.au.
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