Measuring a BMX wheel size is typically done by checking the markings on the tire, measuring the diameter of the wheel (usually the rim), or looking up the bike's specifications online.
Understanding your BMX wheel size is crucial for replacing tires, tubes, or the entire wheel. While often stated in inches (like 20"), there are different ways to determine the exact size.
Methods for Finding Your BMX Wheel Size
According to resources like the Swytch YouTube guide, there are a few primary ways to find your bike's wheel size, which apply directly to BMX bikes:
1. Check Your Tire Markings
The most common and easiest method is to look directly at the tire's sidewall. Tire manufacturers print sizing information here.
- Inch System: You will often see a size listed like 20 x 2.35. The first number (20) represents the nominal diameter of the wheel/tire in inches, and the second number (2.35) represents the width of the tire in inches. BMX bikes commonly use 20" wheels, but smaller sizes like 18", 16", or 12" are used for younger riders, and larger 24" wheels are used for cruiser BMX bikes.
- ETRTO System: For the most accurate measurement, look for a number in the ETRTO (European Tire and Rim Technical Organisation) format, like 58-406. The first number (58) is the tire width in millimeters, and the second number (406) is the bead seat diameter (BSD) of the rim in millimeters. The BSD is the critical measurement for matching tires to rims.
Example Tire Marking: A tire marked 20 x 2.35 (58-406) tells you it's a 20-inch wheel with a 2.35-inch width, designed for a rim with a 406mm bead seat diameter.
2. Measure Your Rim
If the tire markings are worn or unclear, you can measure the rim itself.
- Bead Seat Diameter (BSD): This is the most reliable measurement. Use a tape measure or ruler to measure the diameter of the rim from the bead seat on one side to the bead seat directly across on the other side. The "bead seat" is the part of the rim where the tire's bead sits. This measurement is usually done in millimeters to match the ETRTO standard.
- Outer Diameter: While less precise for tire fitting, you can measure the approximate outside diameter of the rim flange. This gives you a rough idea of the nominal inch size (e.g., it should be close to 20 inches for a 20" wheel).
Measuring Tip: It's often easier to measure the diameter of the tire while it's inflated on the wheel, or remove the tire and measure the rim's BSD directly.
3. Check Your Bike Specifications Online
If you know the make and model of your BMX bike, you can often find the original specifications, including wheel size, on the manufacturer's website or in online bike databases.
- Navigate to the manufacturer's website.
- Find your specific bike model page.
- Look for the 'Specifications' or 'Specs' section.
- Wheel or tire size will be listed there.
This method is useful for confirming sizes or finding details if the bike is stock.
Common BMX Wheel Sizes
Here's a look at some common BMX wheel sizes:
| Nominal Inch Size | ETRTO (BSD) | Typical Use |
|---|---|---|
| 12" | 203mm | Balance bikes, very young riders |
| 16" | 305mm | Younger riders |
| 18" | 355mm | Intermediate young riders |
| 20" | 406mm | Standard freestyle/race BMX |
| 24" | 507mm | BMX Cruiser bikes |
While the inch size is commonly used, the ETRTO (BSD) measurement is the technical standard for ensuring a proper tire-to-rim fit.
