True BMX bikes started it all, back in the late sixties. They were knockoffs of motocross motorcycles and were designed for racing over jumps and around berms in the dirt. Pretty soon kids everywhere had them, racers or not. BMX bikes are still designed for racing, although you don’t have to race to enjoy the light weight, speed and dirt worthiness of these machines. They usually have 20-inch wheels (24-inch-wheel “cruisers” are the exception), knobby tires, upright handlebars with crossbars, small saddles, long cranks and rear hand brakes. The frames are light and sturdy, and the higher the price, the lighter they get.
BMX bikes are generally made of chromoly steel or aluminum. Chromoly frames are a bit heavier and more economical. Aluminum frames are lighter and are often made of oversize or exotically shaped tubing. Besides weighing less, aluminum is also rustproof. So, if you scratch your frame, there’s no need to rush to touch it up.