The primary differences between BMX bikes and mountain, or MTB, bikes are in their design. A BMX bike is designed for a sprint around a short man-made track, with a smooth surface linking large jumps, while a MTB is designed to travel long distances over rough, natural terrain.
The severe impacts of BMX racing demand a simple, strong, and rigid frame. Mountain bike frames can be larger and more flexible for a smoother ride and normally have attachments for drink bottles, air pumps and even lights.
Standard BMX wheels measure 20 inches in diameter and 24 inches for the cruiser class. Rigid for rapid changes in direction, they are heavily laced for the strength to handle big landings. MTB wheels are 26-inch standard and oversized 29 inch. The larger wheels roll more easily over rocks, ruts and logs.
BMX bikes typically only use a rear caliper brake to control speed and bike attitude in cornering, jumps and whoops. MTB brakes may be caliper, disc, or a combination. They are powerful and can handle mud, water and the heat generated during a long descent.
BMX bikes have a single gear matched by sprocket size to the power and tempo of the rider. MTBs have up to 24 gears to enable a wide range of terrain from steep climbs to long flat straights. BMX pedals may be large flats with sharp teeth, or may clip to shoes. MTB pedals also appear in these forms or may add a cage or strap to hold the shoe.
BMX bikes have no suspension. Riders need maximum thrust and power as they encounter obstacles during their sprint. Their legs and body absorb impacts and pump through whoops. A modern MTB can be a hardtail, with only front suspension, or may be fully suspended. Suspension allows the tires to follow ground irregularities for better control as well as cushioning the rider.