Transmission systems for off-road vehicles are unique. Unlike a normal road vehicle where the gearbox is a single unit, off-road vehicle gearboxes comprise three, four and sometimes five units.
1. Main gearbox
Similar to a normal road vehicle’s gearbox but built to withstand heavier torque loads. Many 4x4s are made with a choice of manual or automatic transmissions.
2. Transfer gearbox
Power from the engine is transmitted via the main gearbox to the transfer gearbox which is a two-ratio unit reducing the overall gearing. The result is two individual sets of forward and reverse gears. The high ratio is used for normal driving and the lower gear ratio is used for off-road work or starting off on a steep slope when towing a heavy load. From the transfer gearbox power is transmitted to the front and rear prop-shafts. In the case of full-time four-wheel drive vehicles it first goes through a centre differential or centre viscous coupling. In the case of part-time four-wheel drive vehicles this ‘centre’ component does not exist. For the most part, soft-roaders do not have a transfer gearbox.
3. Centre differential/viscous coupling
Located between the front and rear prop-shafts in full-time four-wheel drive vehicles only, this component distributes the power to the front and rear prop-shafts. Because the front and rear wheels rotate at different speeds when turning a corner this component must permit a differential in rotation speeds. A differential unit is fitted between the front and rear prop-shafts to do this. For off-road driving this differential can be locked, preventing differential rotation, locking the shafts together. This differential lock must not to be confused with differential locks found on axles as the job they do is entirely different.
A viscous coupling, in brief, does the same job as the centre differential but locking is done automatically.
A full-time four-wheel drive with its centre differential locked is the same as a part-time four-wheel drive vehicle engaged in four-wheel drive, the front and rear prop-shaft are attached, as if it were a solid shaft. These shafts drive the wheels via the axle differentials.
4. Axle differentials
These components, one on the front and one on the back, distribute power from the prop-shafts to the wheels. Again, because of the different rotation speeds of the wheels when the vehicle turns, this differential permits this speed differential. (Hence the name differential) All wheeled vehicles have differentials for this purpose. Axle differential locks are fitted to some vehicles and are discussed later.
5. Overdrive unit
Fitted as optional equipment to some older vehicles, the overdrive is a gearbox that adds an additional high gear ratio. Overdrives are built for the relatively light duty of motorway cruising and are not intended for use with low gear ratios.
Manual versus Automatic transmission
There is much debate as to which transmission system is superior for a 4×4. Assuming that the vehicle will have the dual role of city vehicle and off-roader, here are my findings:
Advantages of manual transmission:
• Engine braking down steep slopes is far superior where vehicle control is easier and safer.
• Easier to drive in very uneven terrain where as auto gearboxes tend to surge and can be difficult to control.
• The vehicle can be pull or push started.
• Manual gearboxes are easier to repair and more mechanics understand them.
• Manual transmissions are more economical to run and often less expensive.
• They run cooler when worked hard in heavy sand conditions.
Advantages of automatic transmission:
• Allows very gradual application of power to the wheels which would only be possible by slipping the clutch (with a manual gearbox).
• Technique of rocking, as a method of getting a vehicle out of a near-bogged situation in mud, is easier.
• Sand driving is altogether easier with auto gearboxes.
• More relaxed driving on road and on winding, rough bush tracks where there is a lot of slowing down and speeding up.
• Auto transmissions stress the vehicle less and are often a better choice when purchasing a used 4×4.
Electronic Hill Descent Control (HDC)
Electronic Hill Descent Control first appeared in the Land Rover Freelander and Discovery Series-2 and is now commonplace in many modern 4x4s. HDC in conjunction with the anti-locking brakes (ABS) is used to slow a vehicle on steep descents.
I do not consider HDC a ‘must have’ and while it is useful in some situations it is more of a sales gimmick than a useful off-road tool. The trouble with it is that it engages at a speed that is too fast for most steep slopes. So the driver must use the brakes and transmission to slow the vehicle. In a well controlled descent it does not engage. HDC should therefore be considered as a kind of parachute should things go wrong; Release the brakes, hold on tight and steer the vehicle. HDC will assist in steering control albeit at a potentially unsafe speed. It should also be engaged when driving up steep slopes as it also engages in reverse and in the case of an uncontrolled rearward slide it could arrest a fast, dangerous descent.
Traction Control (TC)
Various systems have been developed to cancel out the wheel-spin that results from tractionless wheels on open differentials, normally accomplished by axle differential locks. These range from electronic traction control working with the anti-lock braking system, first seen in the Mercedes M-class, Discovery Series-2 and Range Rover. Jeep’s first Quadra-Drive is hydraulic powered and does a similar job. These systems are beyond the scope of this book to illustrate in detail, however it is enough to say that they assist traction when wheels leave the ground or spin when the surface gets slippery. They do not, as advertised, make off-road driving easier but instead change the techniques required. Early traction control systems are harder on the vehicle and environment than axle differential locks, which in effect, do the same job.
Some modern TC systems are miracles of engineering. They are so effective that it is sometimes absolutely impossible to even spin a wheel. Either the vehicle moves or all four wheels spin out and the vehicle sticks; there is no in-between. What these systems do is take away the challenge of difficult off-road driving and drivers of vehicles like the 2006 Jeep Grand Cherokee and Land Rover Discovery-3 are rarely tested as their vehicles do all the thinking. See Chapter-5.
Permanent/full-time versus selectable four-wheel drive
Full-time four-wheel drive
Full-time four-wheel drive has been an option for the off-road motorist for many years but only in the last 20 years has it been recognised as the most user friendly type of four-wheel drive transmission. It has been fitted to vehicles such as the Jeep CJ-6 and
CJ-7 and Range Rover since the early 1970s, the Land Rover 110 since the mid eighties, and the Mercedes-G and the Toyota Land Cruiser in the 1990s. The Mitsubishi Pajero/Shogun’s transmission offers the options of part-time, full-time and true four-wheel drive with a system called ‘Super-Select’.
Most full-time four-wheel drive vehicles have a centre differential located between the front and rear prop-shafts to prevent wind-up caused by the different rotation speeds of wheels on sealed surfaces. (In the case of some vehicles with automatic gearboxes it is a self-locking hydraulic viscous coupling).
The advantages of full-time four-wheel drive transmissions are numerous and include safety, even tyre wear and better control and handling. Its only disadvantage is frequent misuse by those who operate it.
The trouble is that a full-time four-wheel drive vehicle with the centre differential unlocked is not operating in true four-wheel drive and drivers operate the vehicle as if it is. Hundreds if not thousands of high-speed roll overs on gravel roads around the world could be avoided if drivers lock the centre differential and drive in true four wheel-drive! More about this in chapter-5.
Contrary to popular belief, the full-time four-wheel drive system decreases tyre wear and does not affect fuel consumption greatly. Although there is no rule for the increase in fuel consumption caused by four-wheel drive while cruising, from my own experiments, I doubt if it is as much as 2% – hardly significant considering the increase in safety it provides.
Selectable/Part-time four-wheel drive
This system is less expensive to produce owing to the absence of a centre differential, which is not required, since the front prop-shaft is disengaged when driven in two-wheel drive.
When a vehicle with part-time four-wheel drive is engaged in four-wheel drive, it is equivalent to a permanent four-wheel drive vehicle with its centre differential locked. With part-time systems, because the rotation of the front axle side shafts and prop-shaft do not serve any purpose when travelling on firm surfaces, free wheeling hubs disconnect these components and will improve fuel consumption.
Part time 4WD vehicles pay a penalty in that the rear tyres (those used for driving the vehicle when in two-wheel drive) wear out before the front. This is especially true of vehicles driven in rough conditions where four-wheel drive should have been engaged but was not, often because the driver did not feel it was necessary.
Super-Select four-wheel drive
Super-Select four-wheel drive is found in the Mitsubishi Shogun/Pajero. This system gives the operator the full range of traction options: Part-time four-wheel drive, full-time four-wheel drive with a centre differential unlocked and then true four wheel drive when this differential is locked. In some respects this is the ideal system. Again, its only disadvantage is drivers not using the system to its best advantage and not engaging full-time and true four-wheel drive when they should. Unless this expensive and complex system is used properly, the buyer has spent his money on nothing more than a gimmick.
Hydraulic viscous coupling
The hydraulic viscous coupling solves all of the problems of axle wind-up while at the same time operating as a non-slip differential. It works like a centre differential which is permanently locked but still absorbs all differential wheel speeds caused when driving on firm surfaces.
DIFFERENTIAL LOCKS: CENTRE, AXLE LOCKING AND LIMITED SLIP
The subject of differential (diff) locks is one of the most confusing and misunderstood aspects of four-wheel drive vehicles. This is illustrated by the way many magazine buyer’s guides indicate this in their expansive charts; Indicating a ‘yes’ or ‘no’ is too simplistic and confuses the issue because not all diff locks have the same function. What a diff lock does depends on which diff is being locked AND what kind of 4×4 transmission is fitted.
For example let’s compare a Defender’s full-time 4WD with a diff lock and an Isuzu Frontier’s part-time 4WD, also with a diff lock. When both of these vehicles are in four-wheel drive with their diff locks engaged, the configuration of the drive to the wheels is different. This is because the Land Rover’s diff lock is locking a centre diff, locking the front and rear prop-shafts together while the Isuzu’s diff lock is located on the rear axle locking the left and right rear wheels together. With the Land Rover, although its ‘diff is locked’, the wheels on the rear axle remain unaffected, driven by an open, unlockable differential. It is in effect in the same configuration as the Isuzu with its diff unlocked.
Differential locks on individual axles
An axle diff lock prevents differential wheel speeds on that axle, preventing wheel-spin on opposite wheels. They help tremendously in sticky situations particularly when two wheels on the same side drop into a trough and the axle is grounded, or when opposite front and back wheels leave the ground when traversing a ditch at an angle. Without axle diff locks, two airborne wheels, one on the back and one on the front, spin helplessly and the vehicle stops.
Axle diff locks can be a hindrance when engaged on flat ground where the surface is slippery but traction is similar on all four wheels. This is because a locked axle differential always causes under-steer. Under-steer causes disturbance and therefore increases the rolling resistance of the tyres which can cause a vehicle to bog down. Typical terrain on which this occurs is on a flat beach. It is not uncommon for the inexperienced driver, who tends to use every tool at their disposal to prevent difficulty, to create more problems for themselves by locking an axle differential. In this case only the centre diff (if you have one) should be locked.
When diff locks are fitted to both the front and rear axles it is imperative that the rear lock is operated first. A vehicle moving over slippery ground with a locked front axle and an unlocked rear diff will want to spin out and may become very difficult to control. Front diff locks severely inhibit steering control.
Limited-slip differentials (LSD)
A limited slip rear differential does the same and gives the same advantages as a lockable differential but, as the name suggests, the advantage is limited. There is some slip, which can be an advantage and a disadvantage (see the table below).
In most cases limited slip differentials are fitted on the rear axle only. This is usually advisable, for when fitted on both front and rear axles, some limited slip differentials can alter the vehicle’s handling characteristics and cause instability at speed.
Vacuum/Pneumatic differential locks
Until fairly recently the most common type of locking device was
the air-locking diff, so called because it required a compressor to actuate the locking mechanism. These systems are still available
and come from the USA, Australia and Great Britain. The ARB air-locker is one of the best available. Engine-vacuum powered differential
locks are fitted to many vehicles as a standard fitting.
Post-delivery differential locks
Don’t fall into the trap and believe that a four-wheel drive vehicle must have an axle diff lock before it will be effective off-road. It is true that there are some obstacles that only vehicles with a lockable diff will negotiate with ease, but these can in so many cases be overcome with driving skill. However, if you intend tackling the very toughest off-road conditions then axle differential locks are essential.
A rear axle diff lock should be regarded as a ‘nice to have’ item and a front axle diff lock, a ‘I want to be unstoppable!’ item. Steering a vehicle with both axles’ diff locks is almost impossible and the average driver, even with front diff locks fitted, rarely ever uses both.
Automatic locking differentials
Auto-lockers such as the Detroit Locker are automatic locking
differential devices that lock when traction is needed, and disengage when a wheel needs to rotate at a different speed due to the vehicle turning on firm ground. No conscious decision has to be made to lock the differential – maximum traction is permanent. Automatic diff locks are a disadvantage in soft sand when the vehicle is turned, as the locking rear axle tends to cause drag on the outside wheel hampering progress. Contrary to what the manufacturers claim, I do not advise fitting an auto diff lock to a front axle as it can cause severe handling difficulties on slippery surfaces. Because they cannot be manually disengaged when steering becomes difficult, I must assume that they are unsuited to front axles.
Fitted to part-time (selectable) 4×4 vehicles, free-wheel hubs fit on the front wheel hubs and enable the side shafts and prop-shaft to be disconnected from the wheels. The one and only purpose behind free-wheel hubs is to prevent these components from rotating unnecessarily and thereby reduce fuel consumption when driving on a firm surface.
“Can free-wheel hubs, if engaged and operated on the road, damage the transmission?” This is a very common question. The answer is no. However, the opposite is true: if hubs are left unlocked for long periods the following damage can result:
On some vehicles the lubrication of the front hub bearings depends partly on axle rotation which sends oil to the bearings. With the front hubs disengaged, the axle remains stationary and the hub is not effectively lubricated.
Spline shaft damage
Spline shafts are located in the side shafts (in the case of vehicles with independent suspension) and in the prop-shafts (in the case of vehicles with solid axles) that allow for suspension travel as the vehicle moves over uneven ground. In conditions where the drive shafts are rotating, wear will be spread evenly over the splines. Should the drive shaft or prop-shaft remain stationary for long periods, as will occur if the hubs remain disengaged, the splines wear on a single plane. If serious uneven wear has occurred, drive shaft vibration will result. It is therefore important that, should you have free-wheel hubs fitted to your vehicle, drive with them engaged once in a while. If free-wheel hubs are not offered as standard equipment and you wish to fit them, do not skimp – cheap units fail when the going gets tough.
Automatic free-wheel hubs
Automatic free-wheel hubs engage the front wheels automatically when the front prop-shaft rotates under power, i.e. when four-wheel drive is selected in the cab. Old types of automatic free-wheel hubs did not lock when compression braking (descending steep slopes) or moving in reverse. Modern auto free-wheel hubs do operate when moving in reverse and down steep slopes.
Modern auto hubs are engaged simply by engaging four-wheel drive. Auto-hubs have improved and have become as reliable as the manual types. For this reason many manufacturers are fitting these in preference to the manual types. Many serious off-roaders still prefer manual types.
Reduction gearboxes fitted at each wheel hub serve to increase axle ground clearance. While it increases ground clearance it also means that once the vehicle has bogged, it is very much deeper and therefore much more difficult to extricate.
Four-wheel drive transmissions – summary
1. Part-time four-wheel drive transmissions have two differentials; one on the front axle and one on the rear axle.
2. Full-time four-wheel drive systems have three differentials. One on the front axle, one on the rear axle and one in the centre, driving the front and rear prop-shafts.
3. A differential lock on an axle prevents differential rotation between the two wheels on that axle. (Left and right).
4. A differential lock in the centre prevents differential rotation between the prop-shafts. (Front and rear)
6. It is possible to have all three differentials lockable (full-time 4wd), or two differentials lockable (part-time 4wd). This is the ultimate traction configuration.
7. Free-wheel front hubs are used to save fuel. They are fitted to part-time four-wheel drive vehicles only. They cannot be damaged by leaving them locked.