Off-roading is anything but predictable. When your favorite hobby consists of driving a souped-up Jeep Wrangler over piles of jagged rock and up 60-degree inclines, the only certainty is that tires will blow out, rims will bend and vehicles will roll. Since there are few service shops in the middle of the Australian outback or Utah desert, off-roaders travel with their own extensive tool kits.

Air compressors are an essential tool in the 4x4 lover's arsenal. Compressors are small electric motors that pump out large quantities of pressurized air. The power that drives an air compressor -- the pressure it produces -- is measured in pounds per square inch (psi). A typical air compressor generates between 100 and 200 psi (690 and 1,379 kilopascals). Another useful measurement for air compressors is cubic feet per minute (cfm) or how much air the compressor can displace in a minute.

Air compressors usually run off of the vehicle's 12-volt power supply. The compressor can be permanently mounted to the vehicle or stowed away in a portable toolbox. For maximum power and air displacement (over 250 psi/1,724 kilopascals and up to 8 cfm/0.23 cubic meters per minute), you can modify an engine-mounted air conditioner pump to work as an air compressor.

Many people use air compressors in conjunction with on-board air tanks. The tank can be filled with compressed air before a drive to have 5 or 10 gallons (19 to 38 liters) of on-demand pressurized air without waiting for the compressor to power up.

Air compressors have a wide variety of uses in off-roading, but the most common is inflating tires. The compressor hose can be attached to a tire pressure gauge to achieve the desired psi rating. Air compressors are also essential for re-seating a tire that has slipped off the rim.

Off-roaders also travel with pneumatic tools like impact wrenches, ratchet wrenches and screwdrivers powered by compressed air (think of the nifty pneumatic wrenches used by NASCAR pit crews to remove tires in seconds). Extendible blow guns can shoot targeted blasts of compressed air on filthy, hard-to-reach engine parts.

For serious off-roaders, compressed air is used to power air lockers. These devices can instantly lock the differential on all four tires, creating intense traction over extreme terrain.

And what if you just want to take your 4x4 on an enjoyable camping trip with the family? Air compressors are perfect for inflating camping equipment, river rafts and even powering an on-board margarita blender.

For more information on off-roading, take a look at the links on the next page.