Rocks
We used to think that rocks were native only to the Southwest, but the reality is that rocks are everywhere. And rockcrawling is popular coast-to-coast. So here's the short list of driving techniques that will maximize your fun and minimize the damage to your 4x4.
A rocky section is the one type of terrain where speed is not your friend. That's why it's called rockcrawling. It's all about finesse - choosing the right line and getting your tires to go exactly where you think they ought to. If you attack rocks with speed and/or wheel-speed, you can't control where your vehicle will end up, and it's nearly always a loud and painful breaking experience.
Remember our discussion about...
Remember our discussion about tire pressure? Rocks: the one type of terrain where you'll experience the biggest improvement in performance with lower air pressure. This photo shows the Mickey Thompson Baja MTZ Radial at 28 psi. The bulge is actually pretty good for a tire with this much air, but check out the next photo.
We'll oversimplify rocks for now. The driving technique for rocks is picking a line - where your tires will go - that will get you through. This is why it's called technical. You have to estimate exactly where all four of your tires will go as you drive over an obstacle. You also have to figure out if your back tires will have the same line, or if they will veer off to one side or another as your front tires steer.
Remember geometry from middle school or high school? Use it to quickly imagine triangles under your truck. Do this in the front and rear (approach and departure angles) to see if you have enough clearance to get the metal parts of your rig over the rock so your tires can meet them. You'll also need to become aware of what point in between the front and rear tires will contact, or clear, the rocks that you drive over. This is called the break-over angle. This will likely be a trail-and-error deal. You don't have to actually measure any of these angles, but you do have to get a feeling for how much space and what angles your truck has so you can estimate from the driver's seat whether you'll be able to clear trail obstacles.
This is the same tire, same...
This is the same tire, same rock, same everything, except the air pressure has been dropped to 16 psi. You can see how much more of the tread now contacts the rock, dramatically increasing the ability that the tire has to grip the rock. The added sidewall bulge gives even more grip, and side lug design of the MTZ makes great use of this.
And then there's a bit of physics. Will certain rocks you drive over teeter totter as you traverse them? If you use the sidewall of the tire, will it push your 4x4 to the side, or climb the rock?
Finally, there's the back up plan. If you your tires slip, which way will the truck slide? And how far? Will you be able to drive out of it? There's a lot to think about, but that's why it's so challenging. And the less likely you are to make it through something, the more rewarding it is when you do.
 Typically, you'll want to...  Typically, you'll want to put your tires on top of the rocks to keep the rocks from hitting your axles, steering components and anything else under the 4x4. Hitting these things can stop forward movement, and can damage components. This rule is balanced by the ability to climb the rock, and the potential sheetmetal damage when the tires come down off the rocks. |  One drawback to lowering the...  One drawback to lowering the air pressure is that you reduce your ground clearance. This is an example of what you have to mentally plan for as you drive over rocks. If we could have moved toward the driver's side three inches, the rock would have missed the suspension mount, and sat under the rocker panel. Of course, the frame and suspension mount could take the impact, and if the rock was much taller, it could have damaged the rocker panel. |  There's another type of rockcrawling...  There's another type of rockcrawling you might encounter while off-roading. Giant granite hills or mountains, or ones made of sandstone, can be a lot of fun and can be done in much less extreme vehicles than boulder hopping requires. But traction is still the name of the game. A word about gleaning your line from what others on the same trail choose: it works if you have the same wheelbase, track width, ground clearance and tire size. The Wrangler and Grand Cherokee shown here require different lines over the exact same obstacles. |