The Physics of Off-Roading

One of the most important factors for success in off-roading is traction, or the grip your tires have on whatever surface you're driving on. Traction is largely affected by the type of tires you're using, their size, and their air pressure, as well as whether you're using four-wheel drive (4WD). For instance, some tires have bigger treads meant to give you better traction when driving in mud. And using 4WD gives all four of your tires better grip and control on the ground. Lowering the air pressure of your tires also increases traction because it allows more of your tire surface to grab onto whatever you're driving over.

Momentum also factors into off-roading. Momentum -- the mass of your vehicle multiplied by its velocity (speed) -- gets you to the top of a hill. Although the mass of your vehicle is fixed (mostly), you can control your speed. Friction with the ground and the force of gravity acting on your vehicle kill your momentum. And you don't want that to happen, unless you like being stuck on a hill.

Before going off-roading, you'll want to be familiar with three different angles on your car: the approach angle, the departure angle and the break-over angle. Knowing your vehicle's angles will help to keep you from scraping it on rocks and other obstacles or getting stuck.

If you're going off-roading, you first need to understand some basic principles about how your car interacts with the outside world. Traction, momentum and your car's angles each play a part in getting you over, or through, the obstacles in front of you.

Picture yourself driving a car toward a ramp. If the ramp is too steep, your front bumper will hit the ramp, like a wall, before your tires are able to reach it. The maximum angle (from the ground) that a hill or obstacle can have and that the front of your car can still clear is called the approach angle. The same principle applies to the rear bumper and wheels on your car; this is the departure angle.

Likewise, when coming down off, say, a rock, you have to know how much clearance you have in your car's midsection so that it won't scrape the rock. The angle between your tires and the middle of your car's underside is dubbed the break-over angle. If you don't know the break-over angle of your car, you can wind up balancing on a rock like a teeter-totter with all your wheels off the ground.

All of these factors are important to off-roading, but they differ depending on where you're driving. We'll address that next. We'll also find out what kind of off-roading you're into.

Measuring Your Angles

You don't need a protractor to measure your car's angles. If you don't want to risk damaging your car through trial and error, try this: Take a long, straight object such as a yardstick, broomstick or board, and place one end where the front of your front tire touches the ground. Lift the other end up until it hits the lowest point of the front of your car, usually the bumper. The angle of the object is your approach angle. You should never try to climb a hill steeper than this, or attempt to go over a rock higher than the point at which the board makes contact with your bumper.