4x4 (four by four)
vehicle with 4 wheels powered by 4 wheels - 4WD or AWD (four wheel drive or all wheel drive).
4x2 (four by two)
vehicle with 4 wheels powered by 2 wheels - 2WD (two wheel drive).
6x6 (six by six)
vehicle with 6 wheels powered by 6 wheels - 6WD
In 2WD the traction (resistance) of only two wheels is used for forward movement. The other two tires have traction as well, but they are just rolling along. In rear wheel drive vehicles the front tires' traction is used to steer the vehicle. If more torque is applied than there is traction available, the 2 tires will break lose and start spinning. Once the tires are spinning, we say "we lost traction". So, if in need of more power/torque to move more weight or to go faster more traction is needed.
That can be achieved by sending some of the torque to the other two wheels (4WD) and using their traction as well. So, in 2WD torque is equally applied on 2 tires and the traction of those two tires has the burden of supporting the engine's power - each powered tire has deal with 50% of the available torque.
In 4WD torque is directed towards 4 tires. In 4WD the traction at each powered tire has to deal only with 25% of the torque created by engine, transmission, transfer case, and axle. Since each tire in 4WD has to carry a much smaller torque load (25% instead of 50%), it is much much less likely for the tires to break lose. That is why a 4WD can climb much steeper grades than a 2WD.
4WD was invented to use the traction of all 4 tires to either move more weight or drive on surfaces with marginal traction, or both without wheel spin.
Historians seem not certain who invented four wheel drive. According to what I found on the internet, the first 4WD vehicle was designed by Ferdinand Porsche (the founder of Porsche cars) for the Austrian truck manufacturer Jacob Lohner in 1900. The first vehicle in the history of 4WD had electric hub motors on each wheel - the engine powered a generator for power supply.
First mechanical 4WD (transfer case, drive shafts etc.) was built by the Dutch company Jacobus Spyker in 1902. Since the Lohner/Porsche four wheel drive has such an unusual status with its electric hub motors the Spyker should be considered the world's first four wheel drive.
The world's first automaker, Daimler Benz has a solid place in 4WD history as well. Mercedes started building 4WD vehicles 1903 - some of them already with all wheel steering.
Full time four wheel drive (not to be confused with: part time 4WD ) is a system that powers all four wheels at all times. Each tire gets about 25% of the available torque when the ground is level with a consistant surface. Driver has a choice of a "4-high" (that's your every day setting) and "4-low".
When "4-low" is selected the wheels receive substancially more torque than in "4-high" - at the same time the vehicle moves at substancially slower speeds.
The low setting is an advantage for drivers who need to tow and maneuver a heavy trailer and for drivers who at one point or another may want to negotiate difficult off-road terrain.
All wheel drive (AWD) is a system that powers all four wheels of a vehicle at all times as well. Difference to full time 4WD is that a "4-low" setting is not available. Due to the lack of "low range" AWD vehicles are much less capable in off-road settings than 4WD vehicles.
Part Time 4WD
Main mode is 2 wheel drive for everyday pavement use. Usually only rear wheels are powered (In some cases front wheels are powered instead).
When needed (usually beyond pavement) 4WD can be engaged. 4WD is used part of the time. When 4WD is engaged front wheels are powered as well. There are two different settings for 4WD - "high" and "low". 4WD "high", sometimes called high range, cannot be used on dry pavement with a "part time system. For extreme situations 4WD "low"is available, it cannot be used on dry pavement either. 4WD "low", also called low range, does not provide more traction, however, it provides two to three times more torque at about half or a third of the speeds in high range. For off-road use part time 4WD system is perfect.
Part Time 4WD requires:
· a transfer case
· two drive shafts
· two axle differentials
· two powered axles
· has NO center differential
Full Time 4WD
Also called permanent 4WD and constant 4WD. Main mode is 4WD. 4WD is used full time. No 2WD mode available. All four wheels are powered at all times. Operates well on dry pavement due to a center differential or equivalent device (planetary gears, etc). Part Time 4WD does not have a center differential.
A center differential is essential for on-road use but can be detrimental for off-pavement use. When leaving pavement the center differential needs to be disabled (locked). It either locks automatically or it has to be locked (disabled) manually.
Normal setting for on-road use and light duty off-road use is 4WD "high", also called high range. For more torque 4WD "low", also called low range, is available. Low range provides substantially more torque to the wheels and allows slower speeds than in high range. 4 low does not provide more traction - it only provides more torque.
Full Time 4WD requires:
· a transfer case
· a center differential
· two drive shafts
· two axle differentials
· two powered axles
All Wheel Drive (AWD)
Not to be confused with automatic 4WD. Only mode is 4WD. Full time. No 2WD available. No 4WD "low" available. AWD is almost useless beyond pavement.
Requires:
· transfer case
· center differential
· two drive shafts
· two axle differentials
· two powered axles
1 flange (attaches to drive shaft), 2 pinion gear, spider gears, 4 ring gear, 5 left axle shaft, 6 right axle shaft
Here is what a differential is supposed to do: Always distribute equal amounts of torque to both wheels - always react to resistance (traction) to allow a wheel with more resistance to rotate less and a wheel with less resistance rotate more (needed in turns where the inside has to rotate less than the outside wheel). The rpm difference created by the diffrential is always proportional. If the inside tire rotates 15 rpm less in a turn than going straight - then the outside tire will rotate 15 rpm more than going straight.
Locking Hubs were invented to engage/disengage the front wheels from the front axle shafts. Idea was to keep parts that were not needed in 2WD mode from rotating in order to reduce mechanical drag (less wear and tear of front axle parts) and thus save gas.
Leave your locking hubs disconnected when there is no need for 4WD in sight. You'll achieve a quieter ride on pavement with less vibrations with hubs unlocked. Lock them as needed for 4WD operation (doesn't matter whether you lock them before you shift into 4WD or right after) and unlock them when going back to 2WD. Leaving them locked in 2WD will have no negative effects.
I would also recommend you leave your hubs locked during bad weather season. This will allow you to shift in and out of 4WD whenever the need arises. Good news is that you will not need to stop for shifting into 4WD - with the locking hubs engaged the drivelines are synchronized. Leaving you hubs locked will cause no damage to your vehicle and will not influence it's handling.
However, driving with part time 4WD engaged (if you have hubs - you have part time 4WD) on dry sections of pavement you will not only create unsafe driving due to understeer - you may also cause expensive repairs due to "driveline bind".
Automatic Locking Hub Manual Free Wheeling Hub
If the vehicle is equipped with free wheeling hub, they are located on the two front wheels. Ensure that the free wheeling hub is set at “LOCK” position when the shift lever is in either “4H” position or the “4L” position. This is to ensure that the drive power is transmitted to the front wheels.
For automatic locking hub fitted vehicle, the hubs are automatically locked or unlocked according to the position of the transfer shift lever during driving.
Automatic Locking Hub
To Lock:
· Bring the vehicle to stop
· Engage 4H (transfer shift lever)
· Resume driving – the hub is now locked
To Unlock:
· Bring the vehicle to stop
· Moves to 2H
· Reverse slowly (straight back for 2 meters)
· The hub automatically unlock – resume driving.
In 2WD with locking hubs disengaged rear drive shaft, rear differential and rear axle are rotating. Front tires are rolling along - only rear tires are pulling.
In 2WD with locking hubs engaged front drive shaft, front diff and front axle, rear drive shaft, rear diff and rear axle are rotating. Front tires are rolling along (since connected to the axle, they make the front drive line parts rotate) - only rear tires are pulling.
In 4WD with hubs disengaged front drive shaft, front diff and front axle, rear drive shaft, rear diff and rear axle are rotating. Front tires are rolling along (the transfer case makes the front drive line parts rotate, but since the wheels are disconnected the powerflow does not reach the tires) - rear tires are pulling.
In 4WD with hubs engaged front drive shaft, front diff and front axle, rear drive shaft, rear diff and rear axle are rotating. Front tires are pulling - rear tires are pulling.
Automatic Locking Hub Manual Free Wheeling Hub
If the vehicle is equipped with free wheeling hub, they are located on the two front wheels. Ensure that the free wheeling hub is set at “LOCK” position when the shift lever is in either “4H” position or the “4L” position. This is to ensure that the drive power is transmitted to the front wheels.
For automatic locking hub fitted vehicle, the hubs are automatically locked or unlocked according to the position of the transfer shift lever during driving.
Automatic Locking Hub
To Lock:
· Bring the vehicle to stop
· Engage 4H (transfer shift lever)
· Resume driving – the hub is now locked
To Unlock:
· Bring the vehicle to stop
· Moves to 2H
· Reverse slowly (straight back for 2 meters)
· The hub automatically unlock – resume driving.
