I’ve tried to include in the section a list of terms and a discussion of topics that are either unique to 4-wheel drive vehicles (such as transfer cases), or which affect off-road performance (such as various types of differentials).

4-Wheel Drive (a.k.a., 4WD): This is what places your vehicle head and shoulders above most others on the road; the mechanisms that allow you to send traction-seeking power to all 4 tires, enabling you to go where others only dream of!   Okay, perhaps that’s just a little over dramatic.  Most 4WD vehicles have a second gear shift lever that you use to put your vehicle into 4-wheel drive (some of the whimpy vehicles have a push button or something other silly mechanism).  This particular gear shift lever is not attached to your transmission.  Instead, it controls your transfer case (see "Transfer Case" below for more information).  

Air Down: To lower the air pressure in your tires to improve traction off road.  The key is to know how much to air down; it depends on several factors, including tire size, driving style, and type of terrain you will be driving.  When mudding, you will be using lots of throttle and spinning your tires.  You don't want to air down very much at all in this situation, cuz you will quickly spin a tire off of the rim.  When traveling on graded Forest Service Roads (typically between 20 - 40 mph), airing down "moderately"  will soften the ride.  For rock crawling, air down about half your street pressure (or lower, depending on tire size).  As a rule of thumb, air down 31" tires to around 12 - 14 psi; 33" tires to 10 - 12 psi; and 35" tires to 8 - 10 psi for maximum traction.  Remember, the more you air down, the more likely your tire will separate from the rim!  No tire spinning and fast cornering at these extremely low pressures!

Crawl Ratio:  This is the lowest gear combination possible for your truck.  It is calculated by multiplying your First Gear (transmission), times your Transfer Case low gear, times your differential gears.  For example, a stock TJ AX-15 manual transmission first gear is 3.83-to-1.  The NP231 transfer case low range is 2.72-to-1.  And the differentials are typically geared at 3.07-to-1.  Therefore your crawl ratio would be 3.83 x 2.72 x 3.07 = 32-to-1 crawl ratio.  This is fine for muddin' and scenic drives along Forest Service Roads, but for rock crawling, most Jeepers strive for higher crawl ratios.  The most practical ways to accomplish this is to install lower gears in your differentials and your transfer case.  For more information on gears, visit my Gears Page.

Differential (a.k.a., "diff" or "pumpkin"): the spherical-shaped object in the center of your axles, between the two wheels, which the drive shafts connect to. It houses the ring and pinion gears, which convert power from the drive shaft (which rotates in one direction) to the axles and wheels (which rotate perpendicular to the drive shafts). There are basically three "types" of differentials: Open Differentials, Locking Differentials, and Limited-Slip Differentials (read about differences in the order listed).

High Centered: When a vehicle is stuck on an obstacle near its center (usually on a part of the frame, or the skid plate), it is referred to as being "high centered".  This situation is obviously more common with stock-height vehicles, but can happen to anyone.  Recovery can be accomplished by several big guys "rocking" the vehicle, or by winch or tow strap (pulling either fore or aft).  To avoid getting high centered, always try to drive your tires over the big rocks, instead of trying to straddle them.

Limited-Slip Differential (a.k.a., "posi-traction", or just "posi"): a mechanism that acts like something in between an open diff and a locked diff (much closer to open than locked, however, as far as its performance on the trail). So if you haven’t read about the two other types of diffs, go read those first, and then come back to this one. Limited slips, in my opinion, are better suited for street use than for trail use. Cuz if one tire is in the air, and the other on the ground, a limited slip will act just like an open diff (which is a bad thing). If both tires are on the ground, it will help somewhat. And on the street, it has much better "manners" than an automatic locker. All limited slips are automatic. You can’t turn them on or off. They also require a special gear lubricant, which contains a "friction modifier", in order for them to operate properly.

Locking Differential (a.k.a., "locker", "locking diff", or just "locked"): a mechanism inside your differential that "locks" both wheels together so that they always turn at the same speed, regardless of whether you are going straight or turning. If you make a sharp turn under high-traction conditions (such as on dry pavement) with a locked differential, the inside tire (not being able to turn slower than the outside tire) will "chirp" or skip across the pavement. This will cause your tires (and your drive train) to wear out much quicker. On the trail, however (under low-traction conditions), a locker can be the difference between conquering an obstacle and becoming hopelessly stuck. With a locker, as long as you have at least one wheel with good traction, you can continue forward progress. There are different types of lockers: automatic, air-operated manual, and a new cable-operated manual). With an automatic locker, you don’t have to do anything. The mechanism can sense when it is needed, and locks the differential automatically. On the downside, automatic lockers often have undesirable street manners. The manual design requires (or "allows") you to turn your lockers on and off at will. Because of this, they are usually more expensive. However, you will know when they are locked and when they are not. And you can turn them off for driving on the street and they act just like open diffs (which is a good thing). Also see "Differential", "Open Differential", and "Limited-Slip Differential".

Open Differential (a.k.a., "open diff" or just "open"): the mechanism inside your differential that allows the inside wheel to turn at a slower speed than the outside wheel while your vehicle is making a turn. Almost all vehicles come from the factory with open diffs, cuz this is the ideal setup for driving on the street (which provides excellent traction under most conditions). Even most off-road vehicles come with open diffs, cuz most people who buy off-road vehicles never take them off road anyway. In poor traction situations (such as ice, mud, etc), an open diff will direct power to the wheel with the least amount of traction. That means if one of your tires is on dry pavement, and the other one is hopelessly slipping on a patch of ice, the tire that’s slipping on the ice will receive all of the power and continue to spin. The tire with "good" traction won’t be doing $hit, and you won’t make much forward progress. On the trail, the same thing will happen if you "catch some air" under one of your tires; meaning one wheel is on the ground, and the other is in the air. The tire in the air will be turning, the one on the ground won’t do anything, and you won’t be going anywhere! Also see "Differential", "Locking Differential", and "Limited-Slip Differential".

Transfer Case (a.k.a., "t-case"):  The transfer case is a gearbox, which is located just behind the transmission (underneath your vehicle).  Connected to the back of your t-case is the rear drive shaft, which extends to the rear axle.  Conversely, attached to the front is the front drive shaft, which powers the front axle.  Your t-case also has a shift lever, which comes up through the floor of your vehicle and into the cab.  The t-case basically does two things.  First, it allows you to shift between 2-wheel drive and 4-wheel drive.  Second, it allows you to shift between 4WD "high" and 4WD "low".  You will use 2WD most of the time while you are traveling on the road.   When conditions become slippery on the road, such as snow or ice, you will want to shift into 4WD "high".  When you shift into 4-Hi, all you're really doing is engaging the front axle, which gives you traction on all 4 tires.  The gearing doesn't change between 2WD and 4WD "high".  That's why most late model 4WD vehicles feature "shift on the fly", which simply means you can shift into and out of 4WD without depressing the clutch, or slowing down, or anything else.  However, 4-Lo is a whole 'nuther story.  Shifting into 4-Lo dramatically lowers your gearing, anywhere from 2 to 4 times, depending on your t-case.  This lower gearing also drastically reduces the speed (and torque) of your vehicle.  Suppose you're traveling at 50 mph in 4th gear.  What do you think would happen if you downshifted into 1st gear and let out the clutch at that speed?  You would prolly loose control of your vehicle.  Shifting from 4-Hi to 4-Lo would have a similar effect.  Therefore, prior to shifting into 4-Lo, you must slow down to just a couple of mph, depress the clutch, and then shift into 4-Lo.  Shifting into 4-Lo is usually easier if you are moving very slowly (vs. coming to a complete stop).  

Unlock:  Usually refers to unlocking one's front hubs.  As in, "As soon as we get off of the trail, I'm gonna stop to unlock."  It could also refer to unlocking a selectable locker.  Vehicles with front lockers sometimes have a hard time turning.  So if you have a selectable locker in the front (such as an ARB), and you are having trouble turning, you may hear someone recommend that you "unlock your front".

To Be Continued . . .