Most people are familiar with mountain bikes these days, but just in case you're not sure exactly what they are, let's have a quick review...

For many years one kind of bicycle dominated the scene: racing bikes. With narrow seats, downward curving handles, and a sleek look, they were what everybody rode. Now, mountain bikes are everywhere.

There are several good reasons for that, not least of which is that mountain bikes greatly expand the territory for cyclists. To see how, we have only to look at their unique design features.

The phrase ATBs or 'All Terrain Bikes' sums up the idea. It signals a bike that can go anywhere - dirt trails, unpaved mountain roads, and more. Part of what makes this possible are the large, knobby tires. Another key element are the wider wheels. But, in recent years, the addition of new front and rear suspension designs has helped enormously.

That suspension comes in four basic types today. Fully rigid models are just straight forks with no cushion or shock absorbers. A hard tail lacks rear suspension. A soft tail, by contrast, has basic rear suspension in which the frame flexes. Dual suspension offers front and rear cushion, along with a rear shock that permits wheel pivot.

All but the first make possible a much smoother, better controlled ride on uneven surfaces. Even hardy cyclists suffer less fatigue and can extend their riding time on a mountain bike, thanks to the different designs. Suspension that moves anywhere from 2-8 inches is one of the basic reasons. That makes possible much less stress on the joints.

Wider, knobby tires are another essential element. The inner dimension of racing bike tires may be as small as 18mm (0.8 inches). Mountain bikes are typically 35-50mm (1.5-2.2 inches) or more. That extra width helps increase control and reduce the force on the frame. Both make for a smoother ride in rough terrain.

Add to that set of attributes a handlebar design that differs radically from its racing cousin. The wide, flat, straight geometry is far superior for controlling the bike on bumpy, sharply curving roads. An alternative design, riser handlebars allows for even more control.

Another key area of difference is higher ground clearance. Sprockets and other gear on racing bikes can be low enough to the ground that the pedals clear only an inch or two. Mountain bikes have much more ground clearance both on the pedals and the frame as a whole. Rocks, uneven ground, vegetation, and more make that a necessity. A 13" clearance is not uncommon.

Some contemporary designs have even eliminated the chain, a bigger problem with mountain bikes than racing models. In both types, getting pants greasy is a common problem. But the issue goes farther with mountain bikes. Dirt, gravel, and plants can more easily make their way into chains and sprockets in the wilderness. Chainless models solve that problem.

Mountain bikes have come a long way since their widespread introduction in the 1980s. But they still provide the one thing that is essential in any bike: a fun ride in terrain away from the crowds.