4x4 everyting

[mandh2010]

Re: unknown technical terms
i'm looking into a truck and need some good straight help. PLZPLZPLZPLz
what is the very technical difference between these?
-Part Time 4WD
-4x4
-full time 4WD
-A4WD
-Shift on the fly
how are they all different? when is the best scenario for each? is installation possible without throwing obscene amounts of money at the car? how expensive is upkeep? recommendations? is there a good resource for more inforation on these? any information about the different systems would be great!
thanks,

Mandh2010

[KiwiBacon]

It's difficult to sift through the BS sometimes to see what each system actually is.

Basically there are two main systems.

Part-time 4wd.
This runs in 2wd (rear wheel drive for a pickup truck) and the front wheels can be engaged mechanically (use a lever), electrically (switch) or pneumatically (also usually controlled by a switch).
When in 4wd they don't like being used on hard surfaces, you scrub up the tyres and risk breaking an axle, cv or other drivetrain parts.

Full-time 4wd.
This is in 4wd the whole time and has a centre differential to drive both front and back wheels without the hard surface problems that part-time 4wd vehicles have when in 4wd.
There is usually a way to lock the centre differential to make front and back axles turn at the same speed when traction is likely to be lost. But a lot of modern vehicles rely on computer control of the brakes (traction control) instead of locking differentials.

Then we have the variations of these systems.
A lot of four wheel drive cars drive the front wheels the whole time and switch in the back when needed. This can be a computer controlled clutch or as simple as a coupling which allows enough slip to stop the problem of breaking stuff (i.e. a viscous coupling).
Some makers have a full-time 4wd system but with a centre diff that will only allow a certain amount of slip. Some also split the torque differently front to back.

Some makers (mitsubishi comes to mind) have a system that used to let you switch between fulltime 4wd and part-time 4wd. Why I have no idea as the fuel economy wasn't much different to full-time 4wd.


Some cars have enough commonality between 4wd and 2wd versions where it's possible to simply bolt the 4wd drivetrain in (I think I own one like that, but I wouldn't know until I actually tried to do it, could be lots of small differences). But other times everything can be different including the body not having clearance for driveshafts.

[MagicRat]

Some of your terms are synonomous.

The key to understanding these terms is to understand a fundamental principle of four wheel drive:
When a car or truck turns a corner, the front wheels travel in a wider arc than the rear wheels.
Therefore, the front wheels will travel a further distance than the rear wheels.

In a normal 2-wheel drive vehicle, this characteristic is not a problem, since only the front or rear wheels are driven. The others just 'coast'.

But, if all 4 wheels are driven, then there must be some way of allowing both front and rear wheels at different speeds, so one can go around corners.

Full-time four wheel drive (aka all-wheel drive) does this by having a special gearset called a "differential" located in the transmission, or in a seperate gearbox called a transfer case. This gearset allows the front and rear driveshafts to turn at different speeds, thus the front and rear wheels also can turn at different speeds.

Usually, this differential is a 'limited slip' type, which allows for better traction in slippery conditions.

There is a less sophisticated, more simple system called 'part-time four wheel drive', aka 4x4. This system does not have this special differential. The front and rear driveshafts turn at the same speeds, This forces the front and rear wheels to turn at the same speeds.

This is not a problem when conditions are slippery, such as in snow, gravel, dirt, mud etc. The individual wheels can spin a bit to compensate for going around corners, uneven terrain etc. These systems can be placed in a 2 wheel-drive mode for use on high-traction surfaces, like dry pavement. In this case, either the front or rear wheels are no longer driven, so the car can go around corners etc with no ill effects.

Full-time systems can drive the 4 wheels all the time. Part-time systems drive only 2 wheels unless conditions get slippery. Usually a driver must push a button or pull a lever manually to engage part-time four wheel drive. But some newer systems do so automatically, as soon as wheel slippage is detected.

Also, some full-time systems can be placed in 2 wheel drive mode to save fuel; and some also have a part-time mode for very slippery conditions.

Also many truck-type 4 wheel drive systems have a "low-range", which is a special gearset to get more pulling power, but less speed, for rough terrain and steep hills.