How to read a BMX spec: what matters when choosing a bike?
TT 20.5", 4130 Cr-Mo, MID BB, 9T cassette hub, integrated headset… If you are reading a BMX specification for the first time, some of those terms may mean very little to you.
You do not need to know every detail. Understanding the key terms will help you work out what is worth comparing before you buy.
Here we explain TT, Hi-Ten, Cr-Mo, crank types, MID bottom brackets, integrated headsets, cassette hubs, freecoasters, 9T, rise and Gyro, one at a time.
1. What does TT mean on a BMX?
TT stands for top tube: the length of the frame's top tube. On BMX bikes, it is given in inches, for example 18", 20", 20.5" or 21".
It is one of the key measurements for choosing a BMX size, especially on bikes with 20-inch wheels. Two 20-inch BMX bikes can have very different top tube lengths and suit riders of different heights.
- A shorter top tube gives a more compact frame and will generally suit a shorter rider.
- A longer frame gives a taller rider more room.
- A frame that is too long can make the bike harder to control.
- A frame that is too short can feel cramped for a tall rider.
‘20-inch BMX’ is not enough to tell you the bike's size. Check the top tube length too.
If you are unsure which frame length to choose, read our BMX size guide.
2. Hi-Ten and Cr-Mo: what is the frame made from?
Two terms appear regularly in BMX descriptions: Hi-Ten and Cr-Mo.
Hi-Ten is high-tensile steel, used in many entry-level and lower-priced BMX bikes. It can be a suitable option for a beginner.
Cr-Mo, or chromoly, is chromium-molybdenum steel. It allows strong components to be made with a useful balance of strength and weight, and is often used on BMX bikes intended for regular, more intensive riding.
Seeing Cr-Mo in the specification does not mean the whole bike is made from it. You might see:
- Hi-Ten frame — a frame made from high-tensile steel.
- Cr-Mo top tube — a chromoly top tube.
- Cr-Mo down tube — a chromoly down tube.
- Full Cr-Mo frame — the whole frame is made from chromoly.
Look beyond the word ‘Cr-Mo’ and check exactly which parts use that material.
3. BMX cranks: one-piece, two-piece or three-piece?
Crank construction is worth comparing, particularly when you are looking at two bikes at a similar price.
Simpler bikes may have one-piece cranks. BMX bikes intended for regular riding often use three-piece cranks, usually made from chromoly steel.
Three-piece cranks are common in freestyle BMX and can be a useful starting point when choosing a bike for learning tricks regularly.
You will also see two-piece cranks. Their construction differs from a conventional three-piece design, but the number of pieces should not decide the choice on its own. Material, spindle diameter, the way the cranks fit together and the bottom bracket matter too.
For more detail, read our guide to choosing BMX cranks.
4. What does MID BB mean?
BB stands for bottom bracket.
The bottom bracket is the bearing assembly that supports the crank spindle and lets it rotate.
MID BB is a common standard in freestyle BMX. Its bearings press directly into a suitably machined bottom bracket shell in the frame.
When buying replacement parts, ‘MID’ is only part of the information you need. The bearings must also fit the crank spindle diameter, for example 19, 22 or 24 mm.
Our BMX bottom bracket guide explains MID, Spanish, Euro and American standards.
5. What is an integrated headset?
The headset sits between the frame and fork and allows the fork and bars to turn smoothly.
With an integrated headset, the bearings sit directly in matching seats in the frame's head tube. There are no separate external cups pressed into the frame.
This is a common design on freestyle BMX bikes.
6. Cassette hub or freecoaster?
These terms describe the rear hub.
A cassette hub is a common option. When you ride fakie — rolling backwards — the drivetrain normally turns the cranks backwards too.
A freecoaster lets you roll backwards without having to turn the cranks. Riders who do plenty of tricks involving fakie often choose one.
On many freecoasters, there is a gap in pedal movement before the drivetrain engages again. This is called slack. The amount and feel depend on the hub's design.
A cassette hub is a straightforward, popular starting point for a beginner. A freecoaster becomes more relevant once you know which riding and tricks you want to develop.
7. What does 9T mean?
T stands for teeth.
If a rear hub is listed as 9T, its driver has nine teeth.
A common freestyle BMX setup is 25/9: 25 teeth on the front sprocket and nine on the rear hub's driver.
This should not decide your first BMX purchase on its own. It is useful when comparing drivetrains and, later, when replacing parts.
8. What is BMX handlebar rise?
Rise is the height of the bars, usually given in inches on a BMX.
Smaller children's BMX bikes generally have lower bars. On a 20-inch BMX, you might see an 8", 9", 9.5" or taller bar.
Bar height affects your position and how the bike handles, but it should not be the only measurement you use to choose a bike.
For a closer look at width, rise and diameter, read our guide to choosing BMX handlebars.
9. What is a Gyro or brake detangler?
Gyro, also known as a brake detangler, is a system that lets the bars spin without winding up the rear brake cable.
It is useful for tricks that involve spinning the bars, such as barspins.
It is not essential on a first BMX. You do not need a Gyro for bunny hops, manuals, basic jumps or learning to ride.
10. How much should a BMX weigh?
Weight matters, especially for younger riders. A lighter bike can be easier to lift and control while learning.
That does not mean you should always choose the lightest BMX. Weight alone tells you nothing about the quality of the cranks, frame, hubs or other parts.
For a child, consider whether the bike is manageable for their height, build and skills.
What should you check in a BMX specification?
If you do not want to analyse every detail, start with these:
- Wheel size and TT — to find a bike that fits the rider.
- Frame material — Hi-Ten, selected chromoly tubes or full chromoly.
- Cranks — construction, material and spindle diameter.
- Bottom bracket — the standard and bearing type.
- Headset — conventional or integrated.
- Rear hub — cassette or freecoaster.
- Driver — for example, 9T.
- Bars — including their rise.
- Weight — especially for a younger rider.
You do not need to memorise the whole specification. Understanding the abbreviations will help you compare two bikes that look very similar at first glance.
Updated: 2 September 2026
