A mountain bike is a bicycle designed for mountain biking, either on dirt trails, or other unpaved environments. Mountain bikes have fat, knobby tires for extra traction and in recent years, front suspension has become the normal set-up, and full front and rear suspension is becoming increasingly common. The bikes are usually on 26” wheels, but since 2002, 29” wheels have become available on many models.
There are six main types of mountain biking and each type uses different kinds of bikes. Cross-country bikes usually have a small amount of suspension (80-100mm) on the front and back tires, and they weigh very little. They reach this small weight by using lightweight materials like titanium.
Freeride mountain bikes have much more suspension travel than cross-country. These bikes tend to have 7-8 inches (170-200mm) of travel and are built of stronger, heavier materials. For freeride bikes, a lot of weight is fine because they are usually not pedaled uphill. They are designed take on dangerous and technical downhill trails (not as quickly or effectively as a downhill bike). The frame angles are commonly steeper to be more maneuverable on skinny obstacles. They weigh from the low 30’s to upper 50 pounds. Some freeride bikes have 24” rear wheels for even more maneuverability.
Downhill mountain bikes are the heaviest type and range from over 40-50 pounds and have more suspension travel (up to 8 in). Although they seem very heavy, they are built as light as possible for quick racing. They are incredibly hardy (but not as much as freeride bikes) and because of large, high, gears and long, soft travel, are suited only for riding down dedicated downhill trails and courses.
Trials mountain bikes are set up just for trials biking. They are pretty much useless for anything else. They usually have no suspension and only one gear, making them more like an oversized BMX than anything else. Trials biking consists of trying to navigate a mad course of obstacles that can bring the rider over obstacles such as raised logs, and skinny ladders.
Dirt jumping, urban, and street mountain bikes are in between a trials bike, a BMX bike, and a freeride bike. They are usually built strong to withstand medium-sized drops and usually have 3 to 6 inches of front wheel suspension travel. They have no rear wheel suspension, and very few gears.
These are just the more popular forms of mountain biking but there are others such as single-speed, 4X racing, and Enduro biking.