BMX Bike Basics
Part 2 - What Size?
Bicycles come in sizes. Not just wheel sizes. When most people see sizing, they think of the wheel size. This is how bikes are sold at department stores. Those frames are sized generically, and these bikes aren’t meant to do much more than pedal sidewalks or easy paths. The BMX sizes also differ from road or mountain bike sizing. BMX sizing is done with the top tube (TT). The top tube is the uppermost tube of the frame. The one you step over. This length determines the amount of space you’ll have in your cockpit. A difference of half-an-inch may not sound like much, but it can be. To measure the top tube, measure from the center of the seat tube to the center of the head tube. Digging deeper another determining size factor is the distance from the head tube (where your handlebars are) to the bottom bracket (where your crank and pedals are). This measurement is called reach. That’s getting into math, so here’s what you need to know when you go look for a bike.
Let's start with the 20” wheel freestyle bike. They are simpler and come in two basic sizes. Pro and XL. The Pro size has a 20.5” top tube. The XL has a 21” to 21.25” top tube. The majority of freestyle bikes are Pro size. Unless you’re very tall, or have arms like a chimpanzee, the XL will be comfortable for riding, but the best tricks will come from the shorter top tub. This will give the most maneuverability in tight spaces and allow quick manipulation of the bike mid-air. In the freestyles you’ll also find a few 18” wheel models great for younger kids, and also ‘big wheel’ bikes with 26”, 27.5”, or 29” wheels. These non-20” wheel bikes come in whatever frame size they come in. All are capable of tricks in the skatepark, but to pull off the best ‘Nitro Circus’ style stunts the standard 20” wheel with 20.5 top tube is the preferred choice.
Now for the complicated part…. The race bike. Sit down. Relax. You’ll be here for awhile. Currently here are all the sizes for the BMX race bike: Micro, Mini, Junior, Expert, Expert XL, Pro, Pro XL, Pro XXL. On race bikes from about 2000 to 2012 you may encounter Mini XL and Junior XL. In the early 2000’s the race bike was really splitting off in design and lack of standards and experimentation led to some manufacturers doing their own thing. Longer sizes such as Pro XXXL do exist, but these are manufacturer specific and are used to break out the Pro XXL size into smaller top tube increments. It is important to size a race bike correctly. Too big and it is hard to get over obstacles and learn skills like jumping and manualling. Too small and it is unstable and dangerous. Max performance often comes from using the smallest frame a rider fits comfortably and adjusting the reach. This combination will be a little unstable, but that is the fine line to walk at the highest levels of competition – relaxed stability where the rider can control the bike but is near the edge of losing control. It takes time to get to those levels and amateurs should find what feels best to them. Definitely try out the rental bikes at your local track. They should have multiple sizes available.
The ages mentioned below are assuming someone in the majority percentile for height/weight of their age group. The top tube sizing between the brands can vary +/- 0.5” in some cases. One of the reasons why is the geometry of the seat tube – the tube the seat goes into too. The angle of this can give a bike a shorter or longer top tube measurement. This can also affect the wheelbase, which affects… and that affects…. and…. There are so many little differences in measurements that the race bike builders invest a lot of money into research in search of the perfect race bike. Up until rider’s hit the Pro size bikes they’ll probably go through a frame size every two or three seasons. Also, you can customize race bikes to fine tune them. That’s the next article. Here's the basics of race bike sizing:
Micro: 15 to 16.5” TT. This typically has an 18” wheel (a few may have 20”) and is for 4- to 5-year-olds.
Mini: 17 to 17.5” TT This is usually the first pedal bike for most riders. Ages 5 to 7 or 8.
Junior (some older makers called these Mini XL): 17.25 to 18.75” TT From about ages 7 to 10.
Expert (some older makers may call it Junior XL): 19 to 19.5” TT Ages 10-13. From here kids start to develop differently.
Expert XL: 19.5 to 20” TT This size is good for ages 10 to 13. This is a in-between size. Some kids in the lower part of this age range may be growing faster but not quite big enough for a pro size. Kids at the upper end of this age range may not be growing as fast, and not ready for a pro size. The Expert XL is unique since it often uses a blend of pro size parts and expert size parts.
Pro: 20.25 to 20.5” TT Ages 12 and up. This is the size that will carry teens through their remaining seasons. (at 16/17 many teens exit racing for the next decade or two, and we call this “aging out” or the “16X coffin”). Experienced adults from heights of 5’5” to 5’9” do well on Pro sizes if they are seeking that relaxed stability and max turn performance.
Pro XL: 20.75 to 21.25” TT Ages 12 and up, height from 5’7” to 6’0”. Most adult riders find this to be their preferred size due to increased stability without much reduction in maneuverability. For many adults coming into 20" class racing the Pro XL is a good starting point.
Pro XXL: 21.25 to 22” TT Height from 6’0” and up. Riders under 6’ will enjoy the XXL because it is very stable, but for this under 6' crew the 2X may turn like a bus and jump like a dump truck.
The 20” wheel size race bikes are called ‘class’ bikes; 24” wheel size race bikes are ‘cruisers.’ The cruiser frame is typically 21.25 to 22” TT. In the cruiser sizes there exists such things as Junior, Expert, Pro, and Pro XL, but these are brand specific. The majority of cruisers use Pro XL or XXL sizing. The 24” wheel is much more comfortable riding on the track at the expense of turn performance. The cruiser bikes were aimed at adult riders. If your younger kids are wanting to ride cruiser you already know it is niche and you’ll likely know what you’re doing when picking a size. It sounds complicated but it really isn’t. If you’re new to racing don’t be afraid to ask questions on bike sizing.