Drivetrains, transmission windup, high/low range, approach and departure angles etc
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.
Drivetrain
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 Nissan Patrol is an example of a part-time 4WD, which means that it can switch between 2WD and 4WD. However 4WD can only be selected when on slippery surfaces.
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 centre differential. A centre 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 bitumen.
Axle/Transmission Windup
When a 4WD is travelling 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 4WD’s 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 centre 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 4WD’s should never select 4WD on bitumen.
Constant 4WD’s 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 centre diff and makes it like a part-time four wheel drive in 4WD mode. The centre diff lock should never be used on bitumen 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 where a limited slip or locking differential is fitted. 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.
High/Low Range
To enable a 4WD to travel at lower speeds while travelling on rough terrain it needs lower gear ratios. Not all 4WD’s have low range gearing and this restricts their ability to tackle rough terrain. However 4WD’s 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 100 km/h, then in low range at the same engine speed and the same gear, the speed would be around 50 km/h.
Some points to note about low range gearing are : -
You cannot select low range in 2WD mode.
You do not have to use low range as soon as you put the vehicle in 4WD, but only if the terrain requires it.
On most vehicles you have to be stationary when changing from high to low range, check your owners manual for your particular vehicle.
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.
Free Wheeling Hubs
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.
Rampover Angle
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.
Information on hand position, diff position, braking and cornering
The following techniques are common to all types of terrain. Techniques for particular types of terrain are mentioned under the different terrain headings.
Hand Position
When driving off road, it is important not to place your thumbs on the inside of the steering wheel. When driving over any large ruts or potholes, the wheel could suddenly turn. This may result in the thumb being bruised or even dislocated if it is left inside the rim. Remembering to leave your thumbs on the outside of the steering wheel is a very easy skill to acquire and should become second nature to you. With power steering fitted to most 4Wdrives these days, this technique is not as critical as the power steering unit dampens out sudden steering wheel movements. Owners of non-power steering vehicles will have undoubtedly experienced at some time the force at which the steering wheel turns when hitting an obstruction.
Diff Position
It is important to know the position of your front and rear differentials as they are usually the lowest ground clearance point of your vehicle. Similarly, any other low ground clearance points should be noted e.g. exhaust, spare tyre etc. When a large rock or other obstacle is on a track that you must drive over, you should ensure you avoid driving directly over it with the lowest ground clearance point of your vehicle.
Braking
When using the vehicle’s brakes hard, your vehicle’s front suspension compresses and you 'use up' most of its suspension travel, When braking sharply to avoid an obstacle e.g. pot hole or rut, and you cannot stop in time, release the brake pedal just prior to hitting the obstacle. This will allow the front suspension to return to its normal height and give more suspension travel when hitting the obstacle.
Vehicle Limitations
A four wheel drive vehicle cannot be treated like a normal car when cornering. The 4WD will roll over much easier than a car while cornering if they are taken too fast, because it has a higher centre of gravity. This applies to gravel and bitumen roads equally. Although a four wheel drive vehicle generally has better traction on gravel than a car, when safe cornering speeds are exceeded the four wheel drive will tend to roll earlier than a car.
How to correctly drive your 4WD on sand
The fundamental theme with sand driving is to conserve your momentum. Since traction is at a premium, any increase in speed can be difficult, if not impossible, and you do not want to lose any momentum, as you may not be able to regain it.
Tyre Pressures
The first thing to do before driving on sand is to lower your tyre pressures. This is done to provide better flotation by increasing the size of your "footprint" and thus dramatically improving your traction. It also reduces the amount of strain on your vehicle and minimises wear and tear on the tracks.
The optimum tyre pressure depends on your vehicle, the type of tyres fitted and the terrain. The following technique provides a good starting point to find the optimum pressure and is best performed before leaving the bitumen.
Park your laden vehicle on a level surface and place a brick 1 cm away from the sidewall of your rear tyre. Deflate that tyre until the sidewall just touches the brick and then measure the tyre pressure. Use this pressure as your starting point when initially lowering your tyre pressure for sand driving. As you become more familiar with sand driving, you con alter this pressure as the terrain dictates.
If you haven't performed the above technique before you reach the sand, don't fret. A good rule of thumb is to use a pressure of 100 kpa (15psi).
Remember though, if you are going to lower your tyre pressures, ensure you have a pressure gauge and some means of pumping your tyres back up.
As you lower tyre pressure, the tyre becomes more vulnerable to damage by stoking the sidewall or rolling the tyre off the rim. The lower the pressure, the higher the risk. However the gain in traction can be remarkable and may make the difference between becoming hopelessly bogged or simply driving away. The "correct" tyre pressure becomes a decision between better traction versus increased risk of tyre damage.
In severe cases of bogging, tyre pressure can be lowered to a minimum of 40 kPa (6psi), as most tyres require at least 6psi to remain seated on the rim while stationary. In almost all situations 10psi should be used as the minimum pressure as 6psi is likely to result in tyre damage ie. tyres rolled off rims or punctured sidewalls. Speeds should be severely restricted at these low pressures. To minimise tyre damage, it is important that these low pressures are only used on sand and tyre pressures should be increased if limestone or rocky outcrops are encountered, or when the terrain becomes more firm. Failure to do so will almost certainly result in tyre or rim damage.
Sand Driving Techniques
When travelling on sand, you should endeavour to follow in the tyre tracks of the vehicle in front as they have already compressed the sand to form a firmer surface than un-traversed ground. Never drive on vegetation as this will destroy it and lead to erosion and environmental damage.
You should avoid rapid changes in speed when accelerating or braking. Braking on sand will cause a mound to build up in front of all wheels and possibly prevent your vehicle from taking off. Rapid acceleration simply digs the wheels in and can actually lead to slower take-off speeds.
Take-off should be performed as smoothly as possible with gear changes done at fairly high revs. Sand driving requires plenty of engine power to get your vehicle "planing" on the sand. It is advisable to use low range as this multiplies the amount of engine torque available and will provide that extra gear if you encounter a particularly soft patch of sand. Check that your tyres are pointing straight ahead when taking off to reduce the takeoff effort required.
When stopping on sand, depress the clutch and allow the vehicle to coast to a stop. This will minimise any sand build-up in front of the wheels. If the terrain permits, coast to a stop, rather than braking, with the vehicle pointing downhill as this will aid take-off. Avoid the soft sand at the base of most dunes and gullies when stopping.
When turning, make the turn as wide as possible to reduce the chance of bogging. Your front wheels act more like a rudder in sand and turning too sharp has a similar effect to applying the brakes.
Steep sand dunes can be traversed only straight up or down. If you drive even on a slight angle, the weight transfer is to the downhill side wheels. If the vehicle starts to slip, the downhill wheels tend to dig in and make the angle of the dune even worse, leading to a potential rollover.
If you are travelling straight down a steep dune and the back end starts to slip sideways, it is best to accelerate slightly to try and straighten the vehicle. Never use the brake, as this will cause weight transfer to the front wheels and can increase the back end movement.
If travelling up a dune and you do not get to the top, reverse down the dune in gear, NEVER coast down the dune and NEVER attempt a U turn.
When you return home after a beach trip, it is important to hose down your vehicle to remove all traces of sand and salt. Pay special attention to areas like the mudguards where sand is sprayed around and tends to get trapped. Thoroughly hose underneath your vehicle as well, as there are many nooks and crannies where sand con also get trapped.
Vehicle Recovery - Sand
As soon as you become bogged, avoid the temptation to simply floor the accelerator as this will just make vehicle recovery more difficult. Put the vehicle in reverse and gently try to back along your tracks as they provide a compacted path. When you have reversed a sufficient distance, try going forward again while being careful not dig yourself in. Hopefully you will travel further each time you repeat this technique and eventually be able to slowly pass through a particularly soft section.
If you cannot reverse out of trouble, get out of the vehicle and let your tyres down further. A rule of thumb is to drop them by a further 15kPa (2psi). Before trying to reverse out, remove the build-up of sand from behind the tyres. See if any part of the underside is touching. If it is, clear the sand away to allow the vehicle to reverse out. You may need to try this several times.
If necessary, continue to drop the tyre pressures to 7OkPa (10psi). Also, never underestimate the assistance of your passengers giving a push. As mentioned earlier, tyres can be lowered to 6psi in extreme cases, but this should be avoided if other means of vehicle recovery are available.
If you are still stuck and your tyres ore down to the minimum pressure, you will have to resort to a snatch strap, winching or jacking to extricate yourself. The easiest method is usually by snatch strap, but this relies on another vehicle being present. If you are by yourself you will have to resort to winching (if you have one!) or jacking.
Summary-Sand Driving
How to tackle hill ascents and descents
Tips on how to drive on rocky ground and gravel roads
Techniques for tackling muddy terrain
How to safely carry out a Water Crossing
How to recover a bogged vehicle with a Snatch Strap
How to use a Hi-lift jack and an Air-bag
Tools and Spares