For decades, young boys have taken to dirt tracks or city streets racing on BMX bikes. Made even more popular by Extreme Sports popularity, there are more adults riding around on bikes that seem too small. Depending on the size and weight of the rider, a BMX bike will be customized on frame size, top tube length and tire size. Ultimately, the bike that your 8-year-old son wants is the same bike that a 6-foot professional racer uses--give or take a few inches.
Small, Micro Mini and More
Frames are sized in categories: Micro Mini, Mini, Junior, Expert, Expert XL, Pro, Pro XL, Pro XXL. Pro and larger-sized bikes are designed for ages 13 and over, weighing more than 150 pounds. Pro-sized bikes are for riders between 5 feet 6 inches and 5 feet 10 inches. The Pro XL is a mid-range customized size for heights 5 feet 8 inches to 6 feet, with a Pro XXL being for riders 5 feet 10 inches or taller.
The key area that would be adjusted based on the rider's size is the Top Tube. Pros generally ride bikes that have a Top Tube of 20.5 inches to 22 inches in length. Shorter riders, including kids, generally have a Top Tube of 18 to 20 inches in length. The Top Tube for a Pro-size bike is 20.5 inches to 21 inches in length. Pro XL sizes take this range from 21 inches to 21.5 inches, with Pro XXL being 21.5 inches or more.
The reason that BMX riders stick to a smaller frame is because of maneuverability. The demands of a race track or trick arena require a shorter distance from the handle bars to the wheel. The shorter this distance, the fast the reaction time. Control is key in BMX riding, as is speed for racing; the smaller frame, though durable and up to the beating of a dirt course, creates an essential balance for speed, balance, quick reactions and mid-air adjustments.
Tire size is also important in sizing a bike to the rider to give her maximum support and control over the bike. Wheel sizes refer to the overall diameter of the wheel and the width of the tire. For Micro Mini and Mini frames, tire sizes should be 20 by 1 1/8 inches. In the Junior and Expert frame sizes, tire sizes should be 20 by 1 3/8 inches. Pro frames have the widest tire width at 1 6/8 inches, but still have the standard 20-inch diameter that all BMX wheels utilize.
While the Top Tube helps fit the rider's torso from the seat to the handlebars, BMX bikes are not fitted for a person's height like a mountain bike or road bike. A longer bike will give you a longer wheel base, giving you stability in speed. The back-end length--the length from the pedal crank to the wheel--can be a customized length as well. When this length is shorter, it assists in pulling the back end up in an aerial but will make it more difficult to balance on the back wheel compared to a longer length. These customizations are based on a pro's comfort level and strength, and not his size or weight.