This month we look at the fundamental principles behind the way a vehicle driven by more than one axle actually works. When driving off-road it really does help if you know what the vehicle is trying to do and how your driving techniques can help to get the best performance out of it.
Driving all the wheels on any motor vehicle dramatically improves performance off-road. You only have to tow a trailer across a field with a Land Rover to appreciate the extent to which an un-driven axle hampers progress, and if the trailer is heavily laden the drag can easily overcome the available traction provided by the towing vehicle.
Early vehicle designers soon realised this problem and it wasn’t very long after the initial emergence of motor driven vehicles that the concept of driving all the wheels came under consideration. Military and farm vehicles would be the main beneficiaries if this could be achieved.
But there are certain fundamental problems that arise when you try to drive more than one axle, particularly if it includes a steering axle. As we know the problem of getting a vehicle to go round corners was solved by the inclusion of a differential in the axle between the halfshafts. This allows the wheels to rotate at the different speeds required to negotiate corners successfully. On a four wheel vehicle with rear wheel drive for instance, another differential would have to be added to the front axle, as all four wheels travel at different speeds when cornering. The first problem is to maintain the vehicle’s ability to steer. This was solved by chopping the halfshafts and fitting a universal joint in each halfshaft to allow them to articulate. This was a simple but effective solution that is still in use today.
When one axle is driven this is usually referred to as two wheel drive, but we know that it only needs one wheel to spin to halt progress, so this could possibly be more accurately described as one wheel drive. When two axles are driven it still only needs two wheels to spin to get stuck, although manufacturers still call this four wheel drive, true if you are on a firm surface! This resulted in the principle of differentials sometimes being termed ‘lazy’, this means that they would stop turning as soon possible. In early years some dodges were discovered to help the problem of a single wheel spinning stopping the vehicle. Applying the brakes slightly fools the diff into ‘thinking’ that both wheels have traction so it continues to supply drive to both halfshafts. Some have even resorted to tying one wheel with rope to stop it turning, this forces the diff to turn the opposite wheel to hopefully provide enough traction to extract the vehicle.
On a two axle vehicle an extra gearbox is used to transfer the drive from the main engine-driven gearbox to both the front and rear axles, and as you may expect, this is called the transfer gearbox. This simple system is found on most early Land Rovers before 1985, Suzuki, Jeep, Daihatsu, Isuzu, Toyota, and Mitsubishi etc. If you lifted one of these vehicles off the ground with four wheel drive selected and turned one of the wheels you would also see wheel rotation on the other axle. This is useful because if one of the axles loses traction completely the other axle is still being driven. However, there is one disadvantage with this system, a consequence of this mechanical principle often referred to as transmission wind-up. As mentioned earlier, the axles need the differentials to allow all the wheels to rotate at different speeds when cornering. When both axles are linked by a transfer gearbox this is not possible. When four wheel drive is engaged the gears within the transfer box are ‘meshed’ and this does not allow any variance in speed between the front and rear propshaft. On loose or slippery surfaces this is not a problem as this build-up in torsional forces in the transmission is released by a wheel spinning slightly. In use this is not usually noticeable but can be detected through steering wheel judder. On hard surfaces this torque build-up cannot be released and forces increase until the wheel eventually spins, or occasionally a mechanical failure takes place. The ‘fuse’ in the transmission system is often a halfshaft which usually breaks at either end just behind the splined section. This system obviously allows only two wheel drive to be used on the road.
This can be avoided by including differential in the transfer gearbox. But the more differentials you have the more likely it will be that you are potentially denying drive somewhere. Since the emergence of the Range Rover in the early 1970’s we have had what are termed ‘permanent four wheel drive’ vehicles. This allows the front and rear propshafts to rotate at different speeds, therefore overcoming the wind-up problem. A lever in the vehicle enables the differential to be locked. When this is done the principle of the system reverts to that of the system described earlier for the other vehicles. What some users of this system don’t realise is that when this centre differential is unlocked the vehicle is really only one wheel drive. That is to say that it only needs one wheel to spin to halt the vehicle in mud. However, one advantage is that low range can be selected on firm surfaces without any transmission wind-up. This is useful for slow driving on good surfaces such as when driving through public areas such as shows, or when manouevring trailers without having to slip the clutch.
One important point to remember with this system is that not only is it easier to get stuck with the centre differential is unlocked but, if you forget to lock the diff before a descent, it only takes one wheel to lose traction for the vehicle to run away down the hill, whereas it would take two wheels to lose traction for this to happen with the diff locked. A common misconception is that the ‘Diff Lock’ as found in Range Rovers, Discoverys and Defenders actually locks the differentials in the axle, this is certainly not the case.
Modern vehicle electronics have taken over to some extent. Electronic traction control works like anti-lock brakes in reverse. On Land Rovers it is controlled by the same circuitry. Instead of releasing the brakes as they lock as with ABS, the brakes are lightly applied as the wheel stops turning, this maintains drive to both halfshafts in each axle. This facility is very effective in use and keeps three wheels turning in the event of one wheel losing traction or lifting off the ground.
So is your vehicle really four wheel drive, probably not, unless you have a Mercedes G Wagon, or Unimog, which have ‘cross-axle’ differential locks fitted at the factory, just don’t try cornering with them locked! Or you may have aftermarket locking differentials fitted. More of those later in the series.
