THE AVERAGE 4X4 HAS AT LEAST TWO SHIFTERS. SOME HAVE THREE AND OTHERS HAVE FOUR OR MORE. THESE CONTROLS ARE SOME OF THE MOST USEFUL ITEMS YOU HAVE IN THAT BOX OF TOOLS YOU CALL A 4X4. AT THE RISK OF GETTING A LITTLE TOO BASIC, LETS TALK ABOUT HOW TO USE THEM WITH THE BEST RESULTS
The Transmission
The transmission is a cornerstone to "wheeling finesse. Choosing the right transmission can be a key as well, but we"re not going to get into the manual vs. automatic thing again as we did back in the February "06 installment of Finesse. I"ll leave it at this: Drive what you like, but be tuned into the limitations of, and techniques for, your choice.
Manual
A manual trans provides a direct link to the engine, which leads to more efficiency and more engine power converted into traction. The driver of a manual trans must always be on the job and making gearing choices, picking the right gear with enough leeway for the engine to go a little slower or a little faster without strain.
The old standby rule for manuals is; "When you go up, shift up. When you go down, shift down." There are lots of caveats that go with that. As the driver, you must make all the key choices in gearing selection, and in some situations, you may not have an opportunity to make a change. On a hillclimb, for example, you may be unable to make a shift midway up the climb. If you can, it"s probably an easy hill or you"re doing a power-shift that"s mondo hard on the drivetrain. Again, you want an engine speed that allows you to slow down a little, as needed without stalling, or speed up a bit without revving to the moon. A low gear will multiply engine torque, and if you climb a hill revved up, you may put too much torque to the ground and spin the tires. Using a higher gear and a bit of momentum, you can go up without spinning a tire ("Go up, shift up!").
Descents are the manual trans" best area. You can use a low gear to apply engine braking and supplement with the footbrakes as needed. If your lowest gear is too low and the tires slip (just like if you were using too much footbrake) you need a higher gear... but you don"t want to shift up on the face of a hill. What you do is rev up! Use a little throttle and bring the engine speed up a little (repeat (a LITTLE!) until the tires aren"t slipping. Better to go down a little faster, but under control, than to go down with the tires sliding and half sideways.
One of the big lessons to learn with a manual is not to use the clutch as an intermediate gear. Yeah, clutch slipping works kinda like a torque converter by letting you increase your engine"s speed without increasing the vehicle"s speed. It"s a technique that works. Use it when necessary, but use it sparingly or you"ll be buying lots of clutches and flywheels. The ideal place for your left foot is flat on the floor, unless you"re declutching to make a shift. Sometimes that"s even true in a stalled engine situation. In some situations its safer to restart in gear... assuming your rig doesn"t have a clutch lockout switch.
Another key element to "wheeling a manual is backlash management. There is always a little slop in the drivetrain, and because there is no "cushion" in a manual, as there is with an automatic, you can induce more shock loading throughout the running gear and break more parts. The trick is to avoid stuff like speed shifts (especially on rocks) and sudden, heavy torque loads. Yeah, sometimes you need to put the hammer down, but load the drivetrain with the clutch first, taking up all the slop, and then slam the hammer down.
Automatic
An automatic lets you concentrate on other stuff and makes most of the gearing choices for you. You have to be smarter than it is, however, and recognize the times when you need to override its choices. In most cases, you "wheel an automatic in "D" (for three-speeds) or "3" (four-speeds), or, if you have more than four gears, use the setting that goes no higher than the direct (1:1 ratio) gear.
The most common override moment would be a descent, where you shift into the lowest gear. The overdrive parts in most automatics are generally not robust, so use OD only in easy areas, if you use it on the trail at all.
One nice slushbox trick for very slippery situations, where the ground will not hold much torque, is to start in second. Many automatics are configured so that if you manually select 2nd, the tranny will start off in that gear, thus reducing torque multiplication and wheelspin. If yours has this feature, it should be mentioned in the owner"s manual. Or, you can just try it on the street someday and see for yourself
Many electronically controlled trannies have a "Tow/Haul" mode, which changes the manner in which the tranny operates. It usually cancels out overdrive and locks the converter clutch faster, as well as changes shift points and pressures. More sophisticated rigs may use a completely different set of transmission parameters when in four-wheel drive. Your owner"s manual will fill you in on how these tools can be used.
Left foot braking is a primary tool with automatics. You can finely control speed over rough terrain by using your left foot on the brake and your right foot on the gas. By working these two pedals, sometimes against each other, you are essentially controlling the converter ratio and thus the application of torque to the ground.
Another tool is the ability to shift gears in places you couldn"t, or couldn"t easily, with a manual. Downshifts on an uphill (which the automatic will usually do on its own), or a descent are both possible because you are never uncoupling the engine from the drivetrain (well, you are, but it happens really fast) as you are when depressing a clutch.
The main performance downside of automatics versus manuals is reduced downhill engine braking. You compensate for that with a touch of the footbrake as needed. Lower gears improve engine braking performance on autos, just as with a manual. Another downside is that there are situations where you cannot control shifts, regardless of the position of the shifter. The governor (or the electronics) may override the position of the shifter and shift up or down. Extremely high throttle openings and rpms are one such place. A high output shaft speed, such as on a descent with an extremely lowgeared rig, is another.
That wonderfully flexible torque converter also comes with a price in terms of oil temperature. The harder you work it, the hotter it gets. A trans temp gauge, which you must monitor closely in hard spots, and an auxiliary cooler, are must-haves for any hard worked "wheeling automatic. Keeping it at, or below, 200 degrees is ideal. In reality, if you can keep it below 250, you can call it good. Above 250 is in "harms way" for most ATF.
The Transfer Case
The old adage goes, "Shift into four-wheel drive when you get into terrain where you wouldn"t drive the family Buick." That"s still basically true... and we"re not talking about rental Buicks, OK! We all know that a rental car is the best off-highway vehicle in the world!
Part-Time
As most of you already know, a part-time t-case in 2WD only supplies power to the rear output until four wheel drive is engaged. You can engage or disengage four-wheel drive (high-range) at any reasonable speed. Recognizing there are always exceptions to the rules, it can be said that part-time transfer cases are more robust than fulltime cases.
You would choose a part-time t-case over a full-time for a rig that"s intended to be used on the trail more than just occasionally. On the other hand, the part-time t-case is a less useful street tool in inclement weather, such as snow. While it"s good for pulling out of a snow drift or off sheet ice, when it comes to turning, a part-time rig, locked into 4- wheel high, is actually much more squirrelly than a full-time rig (with its center diff unlocked), or even a 2wd.
When a vehicle is turning with fourwheel drive engaged, its front axle will be working at a slightly different speed than its rear. With the t-case locked into a 50/50 split, this difference forces one or more tires to slip. Put your rig into 4-hi in a parking lot and make a few tight turns and you"ll see. A slipping tire delivers no grip and if you lose the cornering grip of one or more tires on an icy road, it tends to put you into the snowbank sooner. The best advice for most part-time rigs is... use 2wd in the snow unless you are stuck.
High range is only marginally useful on the trail. Many rigs are not high powered or low-geared enough to be able to use high in many trail situations. Anything hard enough to need fourwheel drive usually needs low-range too. High can be used for anything more or less level that has traction issues and needs the extra pull of another axle. Sometimes high range four-wheel drive is used in mud to help get the tires spinning faster.
Low-range is used when you need the extra torque multiplication on rough ground. Low-range is about low speed too. Many times you need to go slow to avoid beating the truck on rough ground. Low-range allows for slow descents as well and supplies equal engine braking to all four wheels.
Full-Time
The full-time four-wheel drive transfer case realm has a few subcategories. They all share the characteristic of having a center differential that allows the front and rear outputs to operate at different speeds. Most work the same way as an axle diff and the same rules apply. The main one is that the average of the speed of the two outputs always equals the speed of the carrier, whether both outputs are turning at the same speed, or one is stopped and the other is turning at double speed.
Very few full-time t-cases do not have a locking device for the center diff. This lock "locks" the center diff and splits the output 50/50 just like a part-time rig. Some of these locks are automatic (using a viscous coupling, Gerotor pumps or electronic controls). Others are manually controlled, mechanical or otherwise. With the center diff unlocked, the full-time tcase is just like an open diff... torque will take the path of least resistance. With open axle diffs and an open center diff, you can end up with one tire spinning and three stationary.
Using a full-time system on the trail is much like using a part-time unit. The versions that lock automatically will do so when they sense a speed differential between the two outputs. On the manually controlled types, you lock the diff when the going gets tough. Avoid tire spinning in this mode... overworking a center diff can cause them to fail in short order.
With the center diff locked, the fulltime rig is operating just like a part-time t-case. The element to keep in mind is that most full-time t-cases are not as stout as their part-time brethren. That"s partly due to the extra complexity of the unit and the use the engineers predicted for it.
Where full-time four-wheel drive shines is in maneuvering on low traction surfaces. As mentioned in the part-time section, the front and rear axles will want to turn at different HIGH PERFORMANCE LIGHTING SYSTEMS Another Bright Idea from a Company Accustomed to Lighting the Way. PIAA"s new 410 Driving Lamps are absolutely sensational! Now you can replace your factory fog lamps with a driving lamp that clearly outshines them all. The new 410 Driving Lamp features PIAA"s new Intense White beam color and is specifically designed to fit in a square opening in your grill, such as the Ford Super Duty (above) and a variety of other vehicles. PIAA"s status as a global leader in lighting remains unchallenged. Which isn"t surprising considering our unmatched record of product innovation. www.piaa.com 410 SERIES speeds and the center diff will allow that. This will keep the tires rolling over the surface rather than skidding over it, and more cornering grip and traction will be retained.
A study done by Dana some years back showed that with two otherwise identical full-time vehicles in shallow, slippery mud, one with the center diff locked and the other open, the locked one would get stuck when making a hard turn and the unlocked one would not. The reason was grip. The locked rig began slipping a couple of the tires, lost enough traction to be slowed significantly and got stuck. Bottom line, the full-time setup is better for maneuvering in slippery situations and is your best choice for a 4x4 used in snow country and on some types of trails.
Splitters/Super-Low Low Range
The aftermarket offers transfer cases with extra low low-ranges, or conversions for existing t-cases. These are a good option for getting the overall gearing low without having to run super low axle gears. After a swap to bigger tires, the axle gearing can be dialed in to offer good street performance and the extra low gears in the low range give you the deeper gears needed for more gnarly trail work. This is ideal for a dual purpose machine. It also allows you to keep stronger gears in the axles. As you may know, low ratios reduce pinion gear size and going really low can make the axle weaker.
devices also known as underdrives. The basic concept is that the unit offers an additional range of gears that are lower than the standard low-range. Often, the device is sandwiched between the trans and transfer case. The splitter multiplies the torque and divides the speed. If you add a 2:1 splitter between the trans and t-case, it doubles the overall ratio. With a 4.03:1 first gear and a 2.72:1 low range with 4.56:1 axle ratios, you have a combined ratio (crawl ratio) of 49.98:1. Engage a 2:1 splitter and you go down to 99.96:1.
The big advantage to a splitter is gearing flexibility. In that way, it"s sometimes a better option than a tcase with an extra-low low-range. People with that option sometimes find themselves caught between ratios that are too low and ratios that are too high. With a splitter, you can have a "Three Bears" gearing choice just right for every situation.
A splitter is especially useful for terrain that requires slow going. It"s good for gearing down for no-footbrake descents. It"s a good option for rigs that want to keep good highway gearing but need lower ratios to operate effectively in the dirt. For the more hardcore types, the option of an extra-low ratio gives them that super slow option.
Most people use a splitter for rockcrawling. They are of more use with manual trans rigs that cannot go slowly enough to avoid constant stalling. They"re also of most use in rigs with smaller engines. My experience with a splitter in a rig with a very torquey diesel and an automatic trans was that I seldom needed it, though it was handy on occasion.
There is a downside to a splitter, and super low gearing in general. There"s such as thing as too low and too slow. In low traction situations, momentum is a better tool than low gears. Remember that momentum can be as little as just a couple of mph. A vehicle operating at 100:1 with 35-inch tires needs its engine spinning at 4800 rpm to go 5 mph. Not only that, its multiplying the torque by 100 too. That means a 230 lbs-ft engine is delivering 23,000 lbs-ft to the tires 230x100=23,000). Your tires and the ground surface won"t hold anywhere near that amount of torque, so they will spin out. However, in the right circumstances, they might momentarily hold enough to overload a drivetrain part, and then something breaks. When the vehicle is in a lower gear than it really needs, the tires will lose grip sooner than in a higher gear.
