Undercuts
An undercut obstacle typically consists of a ledge in which the lower portion is cut in more than the top of the ledge. It's only a problem if the undercut ledge is taller than half the height of your tire. What happens is your Jeeps tire pushes into the lower cut-out portion. In order to get up to the top, the tire has to roll up, under the ledge, and in the opposite direction of the Jeep. This is where axle U-joints, suspension mounting points, and steering components are under huge amounts of stress and become prone to failure. A front locker aggravates the situation further. There is no good line through an undercut. Back up and take a different line through the obstacle.
When To Gas It
We'll admit it. There's nothing cooler than a driver really throttling a Jeep and spinning tires to climb to the top of an obstacle. We love to see it 'cause it makes for great photos. It also makes for a broken Jeep, especially if it's done carelessly. If you have to throttle your Jeep, always do it with the front wheels pointing straight. The front-axle U-joints are less likely to explode when they aren't required to turn while you're making power with the right foot. Also, go light on the thin pedal when there are surfaces of varying traction. For example, if the right side of the Jeep is firmly planted on dry rock and the other side is in greasy mud, you're likely to pop some parts on the right if you're into the throttle too hard. Go easy on the throttle in loose or slippery terrain where rocks or terra firma may be hidden underneath to suddenly provide traction to one or more wheels, shock-loading the drivetrain.
Farmington Bump
We had never heard of this until we visited Farmington, New Mexico, many years ago. Harold Off of Off-Again Off-Road introduced us to the "Farmington Bump." Some ledges here can't be crawled at slow speeds. You need to grab a higher gear and give it a little bump to get up. The key is to get the front tires on the obstacle or ledge. Start with the rear tires a foot or so away from the climb, get some momentum, and bump it up. Let out of the throttle slightly while the suspension is unloaded, and then roll back into it once the rear tires are in full contact with the climb. If you made it up, you successfully completed a Farmington bump. If you didn't make it, try a little more lead-in space (maybe 2 feet) and go again. This is not a fun driving technique in overly tall, narrow, unstable, or short-wheelbased Jeeps.
Front-End Up
The most common over-throttle usage and resulting breakage stems from the front end of your Jeep catching air while you're climbing a slick ledge. An inexperienced driver will stay in the throttle. When the front tires come back to earth it's not uncommon for a driveshaft, axle U-joint, ring-and-pinion, or other front-end component to fling out. When the front end sees airtime in this situation, let out of the throttle slightly or all the way, just until the front tires settle, then get back into it. This method, when timed properly, will get you up the climb and keep your Jeep in one piece.
Know When To Stop
Secretly, spectacular breakage is our favorite. When a whole axle assembly or other driveline part scatters and turns into an oily fog of fragments, we swoop in with cameras in hand like vultures looking to pick meat off of a dead carcass. Know when to let out of the throttle. If you hear an unusual banging or metallic pop, it's probably a good time to stop and evaluate the breakage. A continued throttle attack with a broken and flinging front driveshaft can lead to a torn up and leaking engine oil pan. Likewise, a busted axle U-joint can lead to a separated steering knuckle when the broken axleshaft ears try to pass each other. It's all too common that we see spectators telling an unaware driver to get off the gas. Know your Jeep and what it's supposed to sound like. A half second after this photo was taken, this Jeep pitched the front driveshaft; the driver stayed in the gas, broke the rear axlehousing, and then pitched the rear driveshaft as well.