Four wheel drive vehicles come in all shapes, sizes
and body styles with different types of features and equipment. However, there are several
basic functions that all 4WDrives have in common.
A 4WD can be a constant 4WD, part-time 4WD or both.
An example of a constant 4WD is a Range Rover. It is always in 4WD and cannot select 2WD
at all. A Toyota Tacoma is an example of a part-time 4WD, which means that it can switch
between 2WD and 4WD.
A Mitsubishi Pajero is an example of a 4WD that can
be both a constant 4WD and a part-time 4WD. It can select 2WD, constant 4WD and locked
4WD. The difference between the Patrol and the Pajero is that the Pajero has a center
differential. A center differential allows 4WD to be used on normal roads (constant 4WD)
and can be locked for off-road use (part-time 4WD). The "axle/transmission
windup" text below explains why a part-time 4WD cannot be driven in 4WD on the
When a 4WD is traveling in a straight line all four
wheels rotate at the same speed, but during cornering each wheel travels at a different
speed due to the radius of the turn. All vehicles have a differential on the front and
rear axles to allow the wheels on the same axle to rotate at a different speed. Constant
4WDs have a central differential fitted to allow for different speeds between
front and back wheels, but most part-time four wheel drives do not.
When a part-time 4WD (without a center differential)
is in 4WD an attempts to corner on bitumen, all wheels need to rotate at different speeds,
but without a centre differential they cannot. This creates the phenomena called
"axle windup" or "transmission windup". High strain is placed on the
drive shafts and transmission, eventually causing one of two things to happen. Either one
of the wheels slips or spins to overcome the stress or the drive-shaft/transmission
breaks. This is why part time 4WDs should never select 4WD on paved surfaces.
Constant 4WDs have a central differential
within the transmission to overcome this problem. However once in the dirt a constant four
wheel drive can be bogged with only one wheel spinning. This is why they have a central
differential lock that stops the action of the center diff and makes it like a
part-time four wheel drive in 4WD mode. The center diff lock should never be used on
paved roads or non-slip surfaces for the reasons mentioned above.
In reality, a 4WD is only a two wheel drive with one
front and one back wheel driving when traction is lost. One wheel on each axle spins while
the other receives no drive at all due to the action of the differential. The exception to
this is when a limited slip or locking differential is installed. A limited slip diff
allows a limited amount of drive to be applied to the stationary wheel before the other
wheel on the same axle spins. A locking diff allows no slip at all and both wheels on the
same axle turn at the same speed, regardless of the amount of traction.
To enable a 4WD to travel at lower speeds while
traveling on rough terrain it needs lower gear ratios. Not all 4WDs have low range
gearing and this restricts their ability to tackle rough terrain. However 4WDs that
lack low range gearing are generally not built for severe off-road conditions or sometimes
have a "crawler" 1st gear to compensate for the lack of low range gearing.
The high range ratios in 4WD mode are the same as
the gear ratios in 2WD. When low range 4WD is selected, the gear ratios are approximately
half that of high range, although the exact ratio varies for each vehicle manufacturer.
For example this means that if an engine speed of
3000 rpm in high range fourth gear is 65 mph, then in low range at the same engine speed
and the same gear, the speed would be around 30 mph.
Some points to note about low range gearing are:
A handy hint when reversing with your vehicle while
towing is to select low range 4WD to be able to move very slowly without having to slip
the clutch. However you can only do this if you have a constant 4WD or your part-time 4WD
is fitted with free-wheeling hubs AND they are not locked in, otherwise you will
cause transmission windup.
If your vehicle is fitted with free wheeling hubs,
you will need to lock them in before selecting 4WD. The free wheeling hub connects the
front wheel to the front axle allowing it to be driven when four wheel drive is selected.
Free wheeling hubs are fitted to reduce wear on the
front diff and drive shaft, and to (marginally) help improve fuel economy when it is in
2WD. Permanent 4WD's do not have free wheeling hubs as they are always in 4WD and need the
front wheels to be permanently connected to the axle.
If you select 4WD without the freewheeling hubs
locked in, then you will only be in 2WD, even though the 4WD dash light indicator (if
fitted) will show 4WD. Even experienced 4WDrivers make this common mistake of forgetting
to lock the freewheeling hubs.
Approach and Departure Angle
The approach angle is the steepest incline that the
vehicle can approach from a level surface without touching any part of the vehicle. The
departure angle is the same thing for the rear of the vehicle. The higher the angle, the
lower the chance of impacting when climbing or reversing over obstacles.
The rampover angle is the largest peak that a
vehicle can drive over without touching the underbody. A short wheelbase vehicle
invariably has a better rampover angle than a long wheelbase vehicle. The larger the angle
the steeper peak the vehicle can travel over.