What Is Four Wheel Drive
What is four wheel drive? Surely its having all four wheels driving a vehicle isn’t it? Well, yes and no is the answer as very few four wheel drive vehicles have four wheels driving despite many people thinking they actually do, to understand this statement we have to look at four wheel drive and how it works.
Most four wheel drive vehicles have a longitudinal engine configuration in which the engine sits front to back in the vehicle and has the gearbox attached to the rear, on a normal front engine, rear wheel drive vehicle it has a propeller (prop) shaft connecting it to a rear axle to propel the vehicle. On a 4X4 it has a transfer case attached to the rear of the gearbox to give one prop shaft to the rear axle and another prop shaft going forwards to drive the front axle. Many true 4X4’s have an additional set of gears in the transfer case commonly called “low range”, these are an additional set of gears which decrease the speed of a vehicle to give more pulling power and much more control when traversing rough terrain, but at much lower speeds.
4X4’s have a driven axle at each end, it’s the axles which give the problems as they contain something called a differential (diff) and this differential is there to allow the wheels to rotate at different speeds, but why. As a vehicle turns a corner the wheels turn at different speeds and the differential allows them to disconnect so the inner wheel turns slowly and the outer wheel travels faster because the outer wheel has to travel a further distance than the inner wheel when cornering. Power always takes the line of least resistance, it’s important to remember this as anyone watching a stuck vehicle will only see one of the driven wheels spinning and the other will be stationary, this is because of the diff and power taking the line of least resistance. Basically the differential is unlocked and all the power is transferred to the wheel with the least resistance which makes it spin so we can say the differential is working correctly and the wheel with the least traction is getting the full power the engine is developing which makes it spin.
Now we consider we have a 4X4 and it has two driven axles, one at the front and one at the rear, we have two differentials as we have one in each of our two driven axles, and if we have one wheel in each axle with less grip then the other wheel on the same axle the differentials will work. This basically means the wheel on each axle with the least grip will spin and the remaining wheels will be stationary and we will be going nowhere fast.
We can now realistically say we have two wheel drive but with a choice of four positions as we know we only have two wheel drive unless the vehicle is moving forwards in a totally straight line and each wheel has equal traction, then we have four wheel drive. If we apply the same philosophy to a standard road car we can say it has one wheel drive but with a choice of two driving positions. It is this ability to have any two wheels driving, but with a choice of four positions which gives our 4X4 added traction.
Basic Four Wheel Drive
In a basic four wheel drive system we have two separate gearstick’s instead of the normal one gearstick; this is the traditional gearstick to control the normal gearbox and one to control the transfer case. Most transfer cases have the following settings, 2H, this is traditional two wheel drive in high range, 4H, this is four wheel drive in high range, N or neutral, this is the neutral position which disconnects drive in the transfer case and is used if accessories are engine powered, and 4L which is four wheel drive in low range. Our normal gearstick is used as normal, irrespective of which range setting the gearstick on the transfer case is in because all the normal gears can be used irrespective of whether 4H or 4L is selected. When four wheel drive is selected in basic four wheel drive systems the front propshaft is engaged by solidly connecting it to the rear propshaft which essentially means both front and rear axles are driving at the same speed, and if we refer back we can see this can cause problems. We know the two wheels on the same axle turn at different speeds when cornering, but both axles turn at different speeds also, the front axle travels further than the rear axle so we NEVER engage four wheel drive on any solid surface such as tarmac as this creates a situation called transmission wind up. Transmission wind up is when both axles need to turn at different speeds when cornering, but cannot, and something must give; wind up does not occur on a slippery surface such as ice, snow, mud or other slippery surface because the slippery surface allows an axle or wheel to fractionally slip. This slip may only be for a minute fraction of a second and not normally noticed by a driver but it is sufficient to relieve the transmission of any stress or indifference created and prevents any transmission failure, and any transmission damage from wind up will incur a very substantial repair cost.
Many vehicles come with permanent four wheel drive, Land Rover’s current range are nearly all permanent four wheel drive, and Mitsubishi Super Select systems can run in either permanent four or two wheel drive on the roads, so how do they do this? When a vehicle has permanent four wheel drive, it’s fitted with a centre differential between the front and rear prop shafts to allow them to operate without damage to the four wheel drive system. Basic centre differential (abbreviated to diff) units are simply that a differential that is similar to that in an axle and works in exactly the same way. Basic centre diff’s have another trick, this is the centre diff lock, and is merely a mechanism which locks this up so both front and rear prop shafts are solidly locked together and equal power is transmitted to each axle. Electrically operated centre diff locks are available on many vehicles and simply use an electric solenoid for manual operation, and in many cases they are automatically monitored and lock or unlock as required during normal road or solid surface operations; these often come with a switch to lock them. Electronic centre diff’s are the ultimate and most complex and work in the same way as electrically operated units, in addition many have something called power transfer capability or torque control, this basically means they can shuffle power between both axles within certain limits. If a permanent four wheel drive vehicle is fitted with one of these units it can use the other electronic systems to sense traction, wheel slip, and many other things, this then shuffles power to the axle with the most grip, but will normally bias most power in a fixed ratio to one axle, usually the rear axle. Electronic centre diff’s can also be permanently locked with a switch.
Many newer four wheel drive vehicles may have many of the functions of the transfer case gearstick replaced with switches; many now use a switch to engage four wheel drive and only have three positions on the transfer case gearstick. These would be H, N, and L; high, neutral, and low. There are many configurations and layouts and each vehicle is individual.
Many vehicles are equipped with a limited slip differential, what is this and how does it work? We know how an axle differential works and how it unlocks to allow the wheels to travel at different speeds, limited slip diff’s limit the maximum power to each wheel on the same axle. Basically they are an axle differential with a set of clutches in them, they are described as a ratio and the most popular set up is 70/30 or 70% maximum power to one wheel and the remaining 30% of power to the other. Instead of power taking the line of least resistance it limits the power to each wheel, in this configuration only 70% of power goes to the wheel with the least traction, and the remaining 30% goes to the wheel with traction to ensure the vehicle will still move forwards to some degree.
If we can have locking centre diff’s, can we also have them in the axles? Yes, many vehicles come with them fitted to the rear axle and most of the popular 4X4’s will find aftermarket suppliers offer them for their vehicles. What are their problems? Basically if you have an axle differential which is locked and equal power is going to each wheel it makes steering very tricky as the rear wheels will drive forwards only, having an unlocked diff allows the wheels to rotate at different speeds which aids steering. If the diff is locked it makes steering tricky as the axle and steering work against each other and it’s almost impossible to steer a vehicle in any terrain with the rear diff locked. So why have them? In many slippery conditions you need to go forwards and if the differential is unlocked the power will shuffle from wheel to wheel on the same axle and essentially you will only have one of the two wheels driving. With the diff locked you have at least three wheels driving as both wheels on the axle with a locked diff are driving with equal force, and at least one of the wheels, possibly both will be driving on the other axle, giving much more drive and traction.
Many vehicles are fitted with front disconnecting hubs, what are these? Basically they are a fuel saving device used on many current 4X4’s and come in manual and automatic variants. In a traditional off road vehicle they have a splined driveshaft to each wheels hub, its hub has a matching splined female spline and the wheel attached directly to the hub. On vehicles with disconnecting hubs an alternative hub is bolted to the outside of the hub and is visible on the outside, centre of the wheel, the splined shaft passes through the main hub and into the disconnecting hub to drive this instead of the main hub. Inside the disconnecting hub are the female spline and basically a simple locking/unlocking mechanism and the locking/unlocking mechanism can be manually or automatically operated according to its type? With manual front hubs you have to get out of the vehicle every time you engage four wheel and lock both front hubs manually, this is usually a flap which lifts up and you twist it, likewise you have to get out of the vehicle every time you disengage four wheel drive to unlock them. Manual hubs are often (and incorrectly) seen as the most reliable type of hubs and many people replace troublesome automatic hubs with manual versions.
Automatic hubs were developed with more leisure of lifestyle 4X4’s as often their drivers did not like getting out of their vehicles in their nice clothes and lock the manual hubs, they didn’t want to get themselves or their vehicles covered in mud, so automatic hubs became standard fitments. Automatic hubs work simply on the principle of relative motion, when disengaged they are held in a central position by a spring, when 4H is engaged the front propshaft is locked in and driving, this in turn drives the front axle. When the front axle’s drive shafts turn they press against the spring and turn a few degrees, as they turn they engage by a locking mechanism which engages the hub in forwards or reverse gears, when the vehicle is stationary, even with the front axle engaged the spring returns to the central position and the hubs disconnect. Automatic hubs work by either the driveshaft moving a few degrees and turning them, of the wheels turning and the body holding against them as would happen when going down a steep hill, either way all that is needed is the driver to engage four wheel drive from inside the vehicle.
Automatic hubs get a lot of problems and many regard them as troublesome, often it is these and previous owners who fail to maintain them which cause the problems, correctly maintained front hubs do not suffer from any problems. All automatic hubs contain some very fine components and these components either wear or rust, this is because automatic hubs often become filled with water, mud, or sludge which causes the rust and abrasive damage. Maintaining a front hub is as simple as unbolting it, removing it, cleaning it, and packing it fully with grease, if its full with grease no water or dirt can get into them, so ignore the bad press they receive as this is often from ignorant owners who fail to maintain them. My oldest 4X4 has automatic front hubs and they are the original items supplied with the vehicle when it was new in 1979, they have been maintained every year by repacking them with grease, and despite some arduous working conditions, they are as good as new and work perfectly.
Soft roaders are vehicles which look like a 4X4, some are based on 4X4 designs, and others look like estate cars; these have four wheel drive but lack the low range, and not being a traditional 4X4 means they have different layouts for ease of use. These soft roaders are not intended for operating in the same harsh off road environments as traditional 4X4’s and many drivers of these vehicles do not understand how a traditional 4X4 system works so the manufacturers use more foolproof systems to compensate. Many soft roaders use a selector switch with multiple positions; a typical one may have two wheel drive, automatic, and permanent four wheel drive. If we select the two wheel drive position it will remain in two wheel drive, if we select the four wheel drive it will remain in permanent four wheel drive, but if we select automatic it will switch between both two and four wheel drive as the vehicle requires it without any driver input. This is to make it as foolproof as possible, but the same rules of when to engage four wheel drive apply.
Why bother with soft roaders? Basically because they fill a gap in the market as many people may live in a semi rural or rural location and occasionally need four wheel drive to cope with wintery conditions such as snow. Soft roaders fill the gap between the cost of a normal road car and a true 4X4 and offer a user four wheel drive with simple controls, and without the user having to understand the complexities of a four wheel drive system, and how to correctly use it.
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