4WD Frequently Asked Questions
(Prepared by Chris Gibbins December 2003)

What is a differential?

All vehicles have a differential. It allows the powered wheels of a vehicle to be driven at different speeds in order to compensate for the different distances they have to travel when negotiating a corner. 4wd vehicles require either two or three differentials in order to allow all 4 wheels to travel different distances. Therefore 4wd vehicles will have a differential between each pair of wheels and also, in the case of a vehicle which has permanent 4wd, between the back and front axles.


What does the differential do?

The differential alters the amount of power directed to each pair of wheels on an axle. More power goes to the wheel which has to travel the greater distance. This is the wheel which is on the outside of the curve being negotiated.


What is a 'limited-slip' differential?

A limited-slip dif' is a type of differential which uses either fluid or mechanical mechanisms to control the amount of power being directed to a particular wheel. When a wheel loses contact with the ground a standard differential will transfer all power to this wheel and none to the wheel which remains in contact with the ground. This means this axle is no longer effective in driving the vehicle. The limited slip dif' reduces the amount of power going to the wheel which is no longer in contact with the ground. This allows the other wheel to retain a varying degree of efficiency depending on the type of limited slip dif'.


Can I use 4wd "on the road"?

Permanent 4wd vehicles obviously use 4wd on the road. A differential on the drive shaft connecting the front and rear axles will enable this to be done safely with no adverse affects on the vehicle. In a selectable 4wd vehicle the lack of a centre differential means that 4wd should not be used on the road. When 4wd is selected the front and rear axles are locked together and therefore the front wheels, which usually travel greater distances, will have to 'lose' some power somewhere. This is done by the tyres slipping on the road surface and is obviously detrimental to the tyres. If the tyres do not slip and do the work of a differential then "wind-up" may occur.

Having said that using 4wd on the road is inadvisable it should be noted that periodic use of 4wd is recommended to avoid the transfer box, differential and wheels hubs from "drying out". If you use the vehicle regularly off-road then there is no problem but a lot of people buy 4wd vehicles and they never see a muddy track. In this case 4wd should be selected in order to circulate oil around the unused parts. If this is being done "on the road" then only do so in a straight line. This will avoid "wind up" and/or damage to the tyres. It is usually recommended to do this at least twice a year but as long as care is taken when doing so it cannot be done too frequently.


What is "wind-up"?

This is where torsion, or twisting force, is applied to the drive shaft and gear box of a selectable 4wd vehicle because the front and rear axles are rotating at different speeds and there is no central differential to equalize these differences. When driving off-road, on soft surfaces, slippage of the front types will dissipate these forces and wind-up will not occur. If the tyres cannot slip sufficiently and wind-up does occur, then the amount of torsion that can be applied to the drive train, which is fairly limited, means that eventually it will "lock up" and further progress will be impossible.


What do I do if "wind-up" occurs?

IF you find yourself unable to move the vehicle and you are in 4wd the simplest solution is to drive in exactly the opposite direction. For example, if you were driving forward and to the left, then drive in reverse and to the right (as you look out of the rear window) for approximately the same distance and the drive-train will "unwind".



Will "wind up" cause damage?

The answer is yes, either to the drive train or to the tyres. In theory you would expect the tyres to skip on the road surface before the gear box exploded or the drive shaft snapped. What usually happens, if traction is exceptionally good, is that the vehicle becomes immobilised in whichever direction the wind up has occurred. Again, in theory, if you continued to wind up the drive train and no slippage occurred then serious damage would be done to the gear box, transfer box or drive shaft. Ignoring wind up, when it occurs IS NOT recommended.

In my personal experience, I have never suffered from wind up. At least not that I have noticed. I have never heard of vehicles being damaged due to wind up unless you count a bit of extra wear and tear on the tyres. It is unlikely that most people will suffer from wind up under normal driving conditions unless serious misuse of the vehicle is happening. On the rare occasions that it does happen, damage to the vehicle is unlikely unless you completely ignore the warning signs.



How easy is it to "wind up" the drive train?

Only improper use of a selectable 4wd vehicle will cause wind up. More often it is suffered in reverse gear when manoeuvring. This is because wind up occurs only when a vehicle is turning and the outer wheels have to travel further than the inner wheels, or the front wheels have to travel further than the rear wheels. It will not occur at all when travelling in a straight line. On soft, loose surfaces it will not occur as the wheels will slip. At high speed the faster rotation of the wheels will allow natural equalisation between the distances travelled by different wheels. Slow manoeuvres, using lots of steering are where wind up is most likely.

In my personal experience it is ok to reverse in 4wd on a hard surface sufficiently to get out of an average parking space. About 20 or 30 feet. More than this and wind up will start to occur.



Will switching off 4wd dissipate the "wind up"?

If you suffer wind up, switching to 2wd will allow the differential forces to dissipate. The problem is that being "wound up" will probably mean that selecting 4wd is more difficult. On push-button 4wd models the green light will flash until such time as the transfer box is able to shift to 2wd. On a manually operated 4wd you will probably find that the 4wd selector stick is stiff. You will need to allow time for the differential force to dissipate before being able to change to 2wd. Once you have managed to select 2wd then all wind up will disappear.



Will "wind up" continue to build up over a prolonged period?

Each time wind up is dissipated you are starting from a clean sheet. No residual wind up is stored once it has been relieved.



Is "wind up" more likely to occur in 4wd Hi or Lo?

Wind up is more likely to occur in 4wd Lo ratio. This is because the drive shaft and gearbox rotate more for each revolution of the wheel. Also the reduced speed will mean that tyre slippage is less likely to occur. However, as 4wd should only be used on soft or loose surfaces, wind up will never occur regardless of which ratio is engaged.



What is the "approach angle"?

Sometimes also known as the "take off angle", this is the angle of slope which can safely be negotiated by a 4wd vehicle. That is to say, the angle between the slope and an imaginary horizontal line. It applies to the front of the vehicle and defines the number of degrees of slope that can be attempted by the vehicle without any damage occurring and the front wheels losing contact with the ground. The angle differs for various 4wd vehicles and can usually be found in the owner's manual.



What is a "transfer box"?

This is the secondary gearbox which allows power to be passed to the second axle on a 4wd vehicle. Selectable 4wd vehicles will have a second gear lever which controls the transfer box and disengages power to the second axle. Permanent 4wd vehicles also have a transfer box but it is permanently engaged in drive. Nearly all transfer boxes allow the selection of either high or low ratio depending on the condition of the terrain.



What is GPS?

Full name, global Positioning System. This is a system which uses satellites to pinpoint your exact location, to within a few feet, and displays your position on either a hand held or dashboard mounted display unit. Sophisticated units can overlay this position on a map display whilst less sophisticated units only show the longitude and latitude which needs to be cross referred to a suitable map. Other useful features are the ability to record the route being driven for later use or for backtracking. They can also display useful landmarks, average speed, current speed, altitude, sunset, sunrise and time remaining to destination.



What type of tyres should I use?

Selection of tyres depends greatly on the type of use they will be put to. Off-road tyres do not perform as well on the road as road biased tyres and vice versa. Most 4wd vehicles are used predominantly on the road and therefore road biased tyres should be selected. How much of a bias will depend on personal choice and how much off roading is undertaken. Just remember that performance of off road tyres will be substantially less than road tyres. Care should be taken not to get caught out and have an accident due to underperforming tyres. The best solution to this problem is to have two sets of wheels. One with road tyres and the other with off road tyres. The correct set of wheels can then be fitted for the type of driving you expect to do.



How should I grip the steering wheel?

When driving off-road you should grip the steering wheel lightly and with your thumbs on the outside. If you wrap your hands around the wheel with your thumbs on the inside then the next time you encounter a rut and the wheels automatically drop into it then the resultant spinning steering wheel will catch one or both thumbs and at the very least cause some pain if not a break. Trying to fight against the steering wheel is also not a good idea. The vehicle will have a tendency to "find its own route" through obstacles and there is no need to try and aggressively fight this natural tendency. Instead, maintain a light grip and get used to the feel of the wheels finding the easiest route.



How should I negotiate steep climbs?

Always try to climb a slope in as straight a direction as possible. Traversing across a slope or diagonally up a slope can cause the vehicle to slip sideways and if you also hit an obstacle whilst sliding then the potential for rolling the vehicle is very high. This applies whether going uphill or downhill.

Never negotiate hills that you do not know you can safely complete. Becoming stuck halfway up or a down a hill is extremely dangerous particularly for inexperienced off-roaders. If you do become stuck halfway, never attempt to turn around. Simply reverse back up or down the slope or better yet get a tow.



How should I traverse across slopes?

At best it is tricky to negotiate across slopes and it should be avoided if possible because your vehicle is at its most susceptible to rolling. Where you have no choice but to cross a slope then do so slowly checking the ground ahead to make sure it is as stable as possible. Passengers and any heavy objects should be positioned on the "uphill" side of the car. Watch out for ruts and bumps. If the uphill wheels hit a bump or the downhill wills drop into a rut then the maximum angle of lean may be exceeded and the vehicle may roll.



How should I negotiate a ditch or rut?

Unlike hills, these should be negotiated at an angle. This ensures that at least three wheels are in contact with the ground at any one time. Be aware of how deep the rut is. You will look foolish if the wheels drop into it and the vehicle grounds out and becomes stuck.



How should I negotiate a ridge?

These should be approached at right angles and never diagonally. A diagonal approach may leave two wheels suspended in the air and you may become stuck. If you feel that the angle of the ridge is too severe for the vehicle to get over without grounding out then it may be advisable to gain a little speed as you reach the crest in order that the vehicle's momentum may carry you over. Be aware that this may result in slight damage although due to the rugged nature of off-roaders, this is unlikely.

Remember that it is safest not to try to negotiate obstacles than you do not feel you can get through safely. If in doubt DO NOT attempt it.



How should I negotiate water obstacles?

Slow speed is best for negotiating water. This minimizes the possibility of flooding the engine and electrics. The air intake on 4wd vehicles is higher than on conventional cars. but this does not mean that any depth of water can be driven through. The air intake is normally at the highest point possible under the bonnet. This usually means around a metre, so anything less than this is usually safe. Remember though, that you cannot usually see the ground under the water and the depth may suddenly increase with no warning. If in doubt DO NOT attempt to cross.



Do Fronteras need a central diff'?

Frontera's do not need a central diff' so why fit one. It is only used to equalise the "different", hence the name differential, forces between front and rear axles on a permanent 4wd vehicle like a Landrover. Without a central diff' a Frontera cannot suffer the situation where, with one wheel suspended in the air, all drive is directed to this wheel and you become stuck. You will always have drive to the front and rear axles independent of each other. Vehicles with a central diff' and the usual front and rear diff's require a diff-lock mechanism to ensure that all power is not directed to a wheel no longer in contact with the ground.


Can I fit a central diff' to a Frontera?

Fitting a central diff' to a Frontera is unnecessary as 4wd is selectable and if used in the right conditions no problems would occur. To fit one regardless would involve changing the drive shaft for something off a Landrover. This would probably entail chassis modification and also, maybe, the transfer box, gearbox, engine, half-shafts, wheels .............. BUY A LANDROVER!