4WD really?

As if the topic needs any additional points, I still find a need to post. I suppose it's a personality defect.

If we are going to get specific, one point for people to understand is that most 4wd systems are engineered specifically so that not all wheels receive equal torque. Typically there is a bias engineered into the drive train that causes more power to be sent to the rear of the vehicle than the front. It does depend on the suspension balance of the vehicle though, as manufacturers usually try to cause a bias that creates an underdrive scenario (rather than overdrive) at the limits of steering control. Power application plays a roll in this.

There are a few different 4WD designs common in vehicles and trucks and usually when you discuss such systems you can't discount the method of sending power to both ends of the vehicle, namely the transfer case.

There are generally two types of transfer cases. One type is more common and is made up of gears and shafts. Another type uses a synthetic material surrounding steel plates in a case. Considered a viscous transfer case. A viscous transfercase is self locking, what this means is that as the difference in speed increases from front to back, the silicone fluid tries harder and harder to drag the slow end along. Thus transferring up to the maximum engineered torque to the end that isn't slipping. The significant attribute of a viscous transfer case is that it allows for some difference in rotation speed between the front and the rear. Thus tire scrub and binding don't occur. These transfer cases rarely have a low range. (I'm unaware of any that do)

Mechanical transfer cases come in three flavors. Those that are for Part-Time use, those that are for Full-Time use and those that are for Anytime Use. Since steel gears are involved in connecting the front of the vehicle to the rear, if these connections are made directly, and you go around a corner, you'll have the front of the vehicle turning the wheels faster than the rear. Since these are connected directly together by the gears, these gears fight letting the two ends spin at different rates. This is what happens in a Part-Time transfercase. These transfercases are Part Time as they should only be used in slippery situations. The slippery situation allows the transfercase to force one end of the vehicle to catch up (by slipping a tire) this preventing damage to the gears from binding. A Part Time transfercase (or a transfercase with a part time option) is putting the max engine power it's designed for to the front and rear, regardless of which end is slipping.

A Full-Time transfercase, or transfercase option, adds a Differential to the connection between the front and the rear. This differential is installed to allow the front of the transfer case to spin at a different speed than the rear thus to prevent gears from binding on dry or paved roadways. The downside to such a differential is that while there is traction, the max engine power is that is engineered into them is put to both the back and front of the case, but when one end of the vehicle slips, a majority of the power is sent to the slipping end of the vehicle. (and if you have an open rear or front end, that means all that power transfer goes to just one wheel)

An Anytime Use transfercase uses a unique type of mechanical differential. Known as a Torsen design, this is a mechanically designed limited slip differential. It senses torque and works to ensure that if an end of the transfercase begins to slip, that no additional power is sent to that end, and instead more power is sent to the end of the transfer case that has traction.

Mechanical transfercases often have the option of Low range, to help when trying to get unstuck or pulling a heavy load.

Thus you want to know what kind of transfer case you have when figuring out what you want to do as well. I installed a limited slip in my Ranger this past fall and it's made driving around in slush and snow much easier without having to use 4WD. My Wife's Durango has a limited slip rear, with multi-setting mechanical transfercase that supports 2WD, 4WD Full-Time, 4WD Part Time, and 4WD Low.

You didn't read the entire point. Torque is applied evenly until one tire breaks loose, then all of the torque is applied to that tire. You just proved the point, thanks.

You picked up on half of my statement and pointed out the precise variance that I described in the second half.

Here's some advice. Don't ever discuss horsepower and torque on USENET. Most people don't really understand anything about it, and all they'll do is argue and cause confusion. Very few car enthusiasts can correctly explain differential gears, gear ratio math, or anything about limited slip. This is a good example of that truth.

Here's why the statement above is false. Ordinary differential gears apply torque evently to both sides. They have to. They're not capable of doing anything else. They can't make decisions, nor can they change mathematically what they are doing. When a tire breaks loose, you will see the SPEED going to that tire. In this case, the poster didn't know the difference between speed and torque. Even though speed is a very simple concept, once you bring a car axle onto the scene, nobody can grasp it.

Most people just can't handle the concept of torque, so I say just let it lie. I promise I won't read any replies to this, so be sure and make them really long.

Well what it all boils down to is the wheel with the least amount of traction will spin first if it's not a posi or LS. To try and say the torque is applied this way and that way is just rhetoric.

I'm with you, most people, including a good number of technicians don't understand the tork and differential thing. Or try to make it something it's not.

I haven't put a lot of thought into this one because I've never really cared. But here are my thoughts:

The gearbox itself is speed reducer therefore a torque increaser.

Power in = power out less parasitic losses.

The differential is a torque splitter. The sum of the two tires' moments (torques) must sum to the input torque multiplied by the gear ratio.

The MAX torque on any drive tire is tricky. The traction force is the reaction to the moment and equal to the weight on the wheel multiplied by the friction coefficient (one for static - not slipping, lower for sliding friction - burnin' rubber). The torque is the friction force multiplied by the tire radius.

Power = torque x rotating speed (note...no spinning, no power) (if you "build it on the brakes" the power is eaten up in the transmission...not the gear box or tires)

For an open diff, the sum of the tire rotating speeds (two) equals two times the input speed decreased by the gear ratio. So, if one tire is not rotating, the other is rotating twice the speed it should if they were both turning the correct speed. e.g. if one not turning and the other is, if the speedo shows 20 mph, the one spinning is 40mph.

Clutches and slipping in the differential is a little tricky...but the features still comply with the laws of physics.

for the vehicle, the sum of the forces = mass*acceleration

skimmer