If you’re new to Land Rover ownership then the choice of various gears and the peculiarities of the vehicle’s drivetrain (axles and gearbox etc) can cause a bit of head scratching. Did you know that using four-wheel drive on a hard surface could damage a Series Land Rover? Or that you can smash the transmission good and proper if you apply the handbrake while the vehicle is moving? If you didn’t then read on.
Series Land Rovers were designed to be driven mainly in two-wheel drive with only the rear wheels being driven. Only when conditions get tough or slippery was the driver then supposed to engage four-wheel drive, using a manual gear lever. On Land Rovers, the choice of two or four wheel drive was also complimented by the ability to reduce all the gear ratios, again using a manual lever, to give much lower speeds but much higher torques (turning forces) at the wheels. This choice of gear ratios is known as High Ratio (for normal road speeds) and Low Ratio (for much slower speeds).
Selecting Low Ratio gears gave the following advantages:
- The increased turning forces at the road wheels increased the pulling power of the vehicle. Much heavier loads can be towed, pulled or dragged without stalling the engine.
- The much slower speeds gave greater control and reaction time when picking a way across damaging and difficult surfaces (such as rocks and ruts)
- The greater turning forces at the road wheels enabled steep banks and hills to be climbed in a controlled and steady fashion without stalling.
- The engine applies much greater braking forces to the wheels if the throttle is decreased. In the days before anti lock brakes this was a much safer way of controlling a steep descent because the wheels cannot lock (which might happen if the brakes are touched), so the vehicle can be steered at all times.
- The greater turning forces at the wheels enabled the vehicle to power its way through thick and clinging mud that would otherwise cause a stall.
This combination of selectable four wheel drive and High or Low ratios gave the vehicle its enviable reputation for off road ability and status as a workhorse.
But, the actual design of the system used on Series Land Rovers also called for the driver to understand how and when to use these various options because getting it wrong could seriously damage the vehicle.
To understand this better we have to look in detail at how it all works.
At the front of the Land Rover is the engine and this is connected to a gearbox, called the main gearbox, with four forward gears and one reverse. A clutch separates the engine and gearbox. This clutch disconnects the drive from the engine to the gearbox to allow gear changes and the gradual take up of the engine’s power when starting off.
Not much out of the ordinary here, most car drivers will recognise this general layout as typical of most manual gearbox cars.
But unlike other cars the gearbox does not then go on to drive either the front or rear road wheels. Instead its single output shaft drives a second gearbox, called the transfer gearbox that contains the all-important High and Low Ratio gears.
So now we have two gearboxes and two gear levers (sticks). The first gearlever selects the four forward and one reverse gear and the second gear lever selects whether these gears are applied to the wheels in high or low ratio.
But we also have to remember that the Land Rover is a four-wheel drive vehicle so the transfer gearbox has not one output shaft but two. The rear output shaft drives the rear wheels (via a propshaft and rear axle) and the front output shaft drives the front wheels (via a propshaft and front axle).
Now things begin to get a little complicated. Think about the four wheels on the vehicle and the way that they turn.
Imagine you have just two wheels, one large and one small. You now fix and connect them together on a solid bar. Now try to push those two wheels forward in a straight line. Although the wheels are turning at the same speed (because the solid bars forces them to) the smaller wheel cannot travel as far in one revolution as the larger wheel. If you force the wheels along in a straight line then one or other of them actually skids a little rather than turns and this wears away the surface of the wheel and causes stresses within the connecting bar.
Now imagine that the wheels are identical in size. This time they roll along happily in a straight line but what happens when they want to turn a corner? The wheel on the inside of the turn has less distance to travel than the wheel on the outside of the turn. But we know that both wheels, which are now the same size, are connected by a solid bar and so each is forced to travel the same distance. Once again one or other wheel must skid a little to make the turn and again the connecting bar is stressed.
Right, back to our trusty Land Rover. Imagine what would happen if we locked all four wheels together with solid bars and gears. We get two very big problems;
- No two wheels will be exactly the same size because of differences in tyre wear, pressure and construction. Because of this, when travelling in a straight line, one or more wheels is forced to skid a little and stresses develop in the connecting bars and gearbox.
- When turning corners, again one or more wheels must skid a little and again those stresses build up.
The less each wheel is able to skid, the greater is the stress in the connecting bars and gearbox. If there’s no slippage at all these stresses go on increasing and increasing until something breaks! This is what Land Rover owners lovingly call transmission wind up!
Our problem now is that in order to make the engine turn the wheels, whether in two or four wheel drive, we have to lock them together with solid bars and gears connected to the engine itself!
A tricky problem then but help is at hand in the form a box of tricks called a differential. A differential allows a pair of wheels to both be driven but also allows one wheel to turn at a different speed to the other when necessary. So, corners can be turned and slight differences in tyre diameters can be tolerated without skidding or stresses - a marvellous solution! So both front and rear axle on the Series Land Rover is fitted with a differential between the wheels which is then driven by a propshaft from the appropriate output shaft of the transfer gearbox.
But this also gives rise to a problem. Because a differential allows the wheels to turn at different speeds it will let one wheel stay perfectly still and the other wheel spin. Whatsmore in this situation all the drive from the engine is directed to the spinning wheel. So if we encounter some mud or ice or even wet grass we could find one wheel spins and the other (with good grip) stays still. This means the vehicles comes to a grinding halt!
Never mind, our Land Rover has four wheel drive so if one axle suffers from wheel spin the other axle will hopefully still power us out of trouble. But remember, this will only work if there is no differential fitted between the front and rear axle, or this too would divert all power to the spinning wheel.
So it can be seen that we need a differential between the wheels on a single axle but for driving in slippery conditions we cannot have a differential between the front and rear axles themselves. This combination of a differential in each axle but no third centre differential is exactly what is fitted to Series Land Rovers.
But can you see a snag? Not having a third centre differential means that if the vehicle is driven in four wheel drive on a hard surface such as tarmac, transmission wind up will occur because the front and rear axles will try to turn at slightly different speeds for the reasons we have already mentioned.
Now when we return to the transfer gearbox we find yet a third lever, that enables us to select between four wheel drive for slippery conditions and two wheel drive for driving on normal tarmac or hard surfaces.
Thus our standard Series Land Rover has three selection levers and these are operated as follows.
Gear Changing
Standard Land Rovers have 3 gear levers:
- A main gear change lever that selects 4 forward and 1 reverse gear.
- A transfer gear lever that selects either HIGH RATIO gears or LOW RATIO gears (usually this has a red knob).
- A 4-wheel drive, high ratio selector that is depressed to select 4-wheel drive when in the HIGH RATIO gearbox (usually this has a yellow knob).
For normal on-road driving, the red lever will be pushed fully forward, and the yellow knob will be in its uppermost position. You are now in HIGH RATIO,
2-WHEEL DRIVE – drive being applied to the rear wheels only.
To engage 4-WHEEL DRIVE in this HIGH RATIO condition, stop and depress the yellow knob until it locks into the lower position. You are now in 4-wheel drive, high ratio.
To return to 2-wheel drive, high ratio, stop the vehicle, pull the red transfer gear lever fully back, thus releasing the yellow knob from its lower position, and push the red lever fully forward again. You are now back in 2-wheel drive, high ratio.
You can see that in high ratio, you have the choice between 2 or 4-wheel drive. To select LOW RATIO gears, stop the vehicle and pull the red transfer lever fully back. You are now in LOW RATIO, 4-WHEEL DRIVE. In low ratio gears, you are automatically in 4-wheel drive; you have no choice about this because there is no 2-wheel drive condition possible in low ratio gears.
You will find that low ratio gears give you much more pulling power, but your top speed is consequently reduced.
Pushing the red transfer lever fully forward will now return you to HIGH RATIO, 2-WHEEL DRIVE. If you have an overdrive fitted, this can be used in any of the aforementioned conditions, and does not affect 4-wheel drive selection.
Freewheeling hubs must be engaged when in 4-wheel drive!! They disconnect the front wheels from their respective drive shafts and are only disengaged when in high ratio, 2-wheel drive in order to save fuel. If you use the Land Rover in 2-wheel drive a lot, it is a good idea to engage the hubs for a few miles once a month to circulate the lubricating oils in the front transmission. You do not need 4-wheel drive for this.
If you drive in low ratio gears with freewheeling hubs disengaged, you will probably break a half shaft in the rear axle. This is because the rear axle is absorbing the large torques applied to the axles in these gears only (the freewheeling hubs having disconnected the front axle from its road wheels). Low ratio is 4-wheel drive only for this very reason, hence the large torques are distributed to all 4 road wheels.
Half shafts are normally the first component in the transmission system to break. They are designed to do this if the transmission is overstressed, and their replacement is fairly easy.
You should never be in 4-wheel drive if you are on a firm surface such as dry tarmac. If you drive in 4-wheel drive on such a surface, you will create enormous stresses in the transmission. Four-wheel drive is only for loose or slippery conditions, since any stresses building up will be alleviated by wheel slip.
These stresses are set up because in 4-wheel drive you physically lock the front and back axles together with a drive shaft. If one axle has tyres on with a slightly different diameter to the other (this will be caused by ordinary tyre wear), then for a given road speed, they will try to turn at different rates. Since they are locked together, this is not possible and so stresses build up.
If you do have to drive on firm surfaces in 4-wheel drive, the only way to do this is to drive a short distance – push the red transfer lever to its mid (neutral) position (thus unlocking the axles) and then return it to its original position. This needs to be repeated after each short distance driven. Dropping the clutch will not achieve the same result, neither will selecting neutral with the main gear change or overdrive gear levers.
If you gearbox does wind up, difficulty may be experienced in disengaging
4-wheel drive. If this is so, after driving forwards, then you will need to reverse to unwind the gearbox. This should free up the gearbox and allow you to return to 2-wheel drive.
Finally, when in 4-wheel drive with a full lock on the steering, some kickback may be felt through the steering wheel. This is more apparent the firmer the ground is. It is nothing to worry about and occurs because the drive shafts in the front axle do not have true constant velocity joints fitted, but simple ‘Hardy Spicer’ joints.
Next Issue we'll consider overdrives, handbrakes and how later 'permanent four wheel drive' Landrovers deal with the tricky problem of transmission wind up.
