How to Drive on Rocks
Driving on rocks is more aptly called "rock crawling." Because of the extremely uneven surface that rocks present, anything faster than a crawl will toss everthing in the vehicle (including the people) around violently, as well as probably damage suspension or other components. Rock crawling requires low gears, good ground clearance, and some skill. Large soft tires, locking differentials, skid plates, and greater suspension flex will also help a lot.
Rock crawling is a situation where selecting low-range in the transfer case is usually required. Besides making the vehicle go very slow, low-range provides the torque needed to power it up and over obsticles such as rocks. Although some trucks have a "granny" first gear, first gear in most vehicles is just is not low enough. Most four wheel drives with a two-speed transfer case cut the over-all gear ratio in half or better.
For instance, a Jeep's low-range ratio is often 2.72 to 1 which cuts the speed by nearly a third and increases the torque by a factor of almost three. The Jeep Rubicon model comes from the factory equipped with a 4:1 low-range and locking differentials making it the first production rock crawler. Aftermarket transfer cases with wild ratios up to 6:1 or greater are available for builders of extreme rock rails or rock buggies. Use caution not to exceed safe engine RPM's when using extremely low gears.
Modifications for rock crawling
One can navigate rocks up to some size with a stock four-wheel-drive but if rock crawling is to be done to any great extent then vehicle modifications are recommended. Here are some recommended mods and what they do: (Many of the suggestions apply to off-road vehicles in general but are especially important on rocks)
- Oil pan skid plate - Mentioned seperately becasue of it's importance. This one item will save you from catastrophic failure due to oil pan damage done by straddling tall rocks in the center of trail. (Sometimes you have to)
- Suspension Lift - Better approach and departure angles and clearance for larger tries.
- More flex - Flex is the ability of the suspension to handle large dips and rises in the terrain under one or more wheels at a time. The intent is to keep the tires on traction surfaces without tipping over. For instance one tire is up on a rock while the other is in a deep hole. This usually depends on the type of lift kit installed but some items can be added later. Lift kits with longer upper and lower control arms, sway bar disconnects, long travel shocks, and other features appropriate to the particular vehicle's suspension geometry will increase the flex. You may find an "RTI ramp" for measuring flex at off-road events or off-road stores.
- Tires - Only taller tires can give you more ground clearance otherwise the axles stay at the same level. Obviously more ground clerance means you can clear more stuff with less maneuvering. Also taller tires tend to roll over obsticals easier but there is a catch. A taller tire changes your over-all crawl ratio (The combined effect of all your drive train components' gear ratios) for the worse. As long as you have low enough gears in your transmission, transfer case, and axles to turn the taller tire then taller tires help. Wider tires may or may not be to ones advantage depending on the spacing of the rocks but tend to help in many situations. Low pressure and an agressive tread will help to grip rocks.
- Wheels - Tough steel wheels are used in rock crawling. Aluminum and chrome look nice but are subject to rock rash or cracking. Steel can be simply repainted unless it is bent.
- Skid plates - Beside the essential oil pan skids many other parts of the vehicle can be armored. This helps prevent damage from scraping and banging exposed parts on rocks. Here is a list of some popular skid plates.
- Differential - Front and rear differentials are pretty tough to begin with but many people add upgraded cover plates since they are essentially sideways oil pans that are easily damaged by rocks. Various plates and sliders are available to both protect the differential as well as help it slide over things.
- Gas tank - Many 4WD's come with a gas tank skid plate of some sort. Many off-roaders upgrade it because of the severe consequences possible with any breach of a fuel tank.
- Belly skid - Again many 4WD's come with some kind of protection for the transmission and transfer case but many solutions and products for this protection exist.
- Steering Box - The steering box is up pretty high but also often right out there waiting to be damaged. It is a relatively inexpensive and easy mod to add.
- Lower control arms, etc. - Many other vehicle specific protection plates exist for critical suspension components and other things under the vehicle.
- Body Armor - One can expect body damage on boulder strewn trails. Diamond plating the rocker panels, and corners will help minimize damage. The rocker panel can also be protected by adding rock rails which also help the edges of the truck slide over obsticles. Many rock rails include a step for getting in and out of the vehicle. Headlamps, tail lights and other lights can be protected with cages that bolt on. Grill gaurds fall into this category as well.
- Winch and other recovery gear - You can get really hung up in a rock garden. Items such as straps, d-rings, snatch blocks, Hi-Lift jack, etc. may come in handy.
- Locking differentials - "Lockers" as they are known, defeat the intended ability of the differentials to allow the tires to turn at different speeds for cornering. The result is that both wheels on an axle (or all four if the front and rear are locked) will turn in unison at the same speed. The effect on traction is incredible and when a wheel loses contact with the ground the other ones will still turn. Don't expect to steer though. Use lockers sparingly, only when they are really needed.
- Heavy duty tie rod - The tie rod that causes your front wheels to work in parallel is subject to being bent by rocks. Aftermarket heavy duty tie rods and associated tie rod ends, drag links, and other components are available.
- Drive-line - Rock crawling puts extreme stresses on the u-joints and axles, these stresses are caused by increased angles, larger tires and the large changes in torque caused by grabbing and slipping on rocks. Heavy duty parts can include high performance cv joints, cro-moly axles, and larger u-joints.
- Steering box upgrades - The poor steering box is put to the real test. In some situations it can be torn from the frame. Various solutions are out there on the market that range from simple heavy duty brackets up to complete super-duty hydraulic steering systems.
- Neutral safty switch override (for manual transmission vehicle only) - This allows the starter motor to be run when the vehicle is in gear and the clutch is engaged. In late model Jeeps it is as simple as removing fuse #20. For usage info see tactics section below. Of course put things back to normal after trail use for safety's sake.
Tactics for rock crawling
- Lower the tire pressure - Lower pressure allows the tire to deform around the shapes of the rocks and increases traction. It also smoothes out the ride a little. Typically a great improvement is found but lowering to somewhere between 15 to 25 psi but experiment and see what your vehicle and terrain really needs. Extreme rock crawlers may go as low as the single digits of tire pressure.
- Pick a line - Look at the trail ahead and consider the size and type of obsticles you are about to encounter. Also consider the camber of the trail. An off-camber trail's slope added to the effect of having a tire up on a rock could tip the truck over. Try to pick a line that uses the obsticles to improve an off-camber situation. Decide whether to go around an obsticle or over it. Consider where your differentials are and attempt to steer so as to avoid hitting things with them. Avoid straddling big rocks if you can. Be aware of the width and length of your wheelbase and use it to your advantage.
- Use a spotter - Have a person (preferrably an experienced spotter) direct your moves. Their view of the situation can help them tell you a lot. Only one spotter should be the one talking to the driver. Others may interject information to the primary spotter but a driver gets little advantage from several people shouting directions at once.
- Start in gear (manual transmission only) - This technique makes use of the nuetral safety switch override listed above in mods. Normally a vehicle is prevented from starting in gear to avoid inintentional motion and the possible negative results. However, when confronted with a steep situation where you don't want to lose ground and the engine is off it can be to one's advantage to start the engine in gear. Just put it in first gear and with the clutch engaged turn the key. The starter does have the power to move the truck from a complete stop in very low gears and the engine will start. Just be absolutly sure to return the neutral saftey switch to normal operation after trail use.