When it’s time to replace a bicycle wheel, understanding the International Organization for Standardization (ISO) measuring system is vital to ensure compatibility and performance. The ISO system provides a universal, precise standard for wheel and tire sizing, removing much of the confusion created by traditional sizing methods. In this comprehensive guide, we’ll explore how to use the ISO system to choose the right replacement wheel for your bicycle, along with practical tips and considerations for a perfect fit.

The ISO system, officially known as ISO 5775, is a standardized method for identifying bicycle wheel and tire dimensions. Unlike traditional sizing systems, which can vary in accuracy and naming conventions, ISO measurements offer a consistent and clear way to match wheels and tires.

Key Components of the ISO Standard

1. Bead Seat Diameter (BSD): The BSD is the internal diameter of the tire where it sits on the wheel’s rim. This is the most crucial measurement for wheel compatibility.

2. Tire Width: Expressed in millimeters, this refers to the width of the tire when properly inflated.

ISO Example

A typical ISO measurement might look like 37-622 , where:

• 37 indicates the tire width in millimeters.

• 622 represents the bead seat diameter in millimeters.

The ISO standard eliminates confusion caused by overlapping traditional sizing systems such as inches (e.g., 26", 27.5") and the French system (e.g., 700c, 650b). For example:

• A “29-inch” mountain bike wheel is often interchangeable with a “700c” road wheel because both have a BSD of 622mm.

• A “650b” wheel has a BSD of 584mm, despite being described in inches in some contexts.

By focusing on BSD and width, the ISO system ensures a precise match between wheel and tire, improving safety, performance, and compatibility.

1. Identify Your Current Wheel's ISO Measurements

To replace a wheel accurately, start by finding the ISO dimensions of your existing wheel and tire.

Locate the Tire Markings

• Check the sidewall of your tire for ISO markings, which typically appear as two numbers separated by a dash (e.g., 25-622 or 50-559).

• These numbers indicate the tire width and BSD.

Measure the Circumference:

If the tire markings are unclear or absent:

• Remove the tire from the wheel.

• Measure the circumference of the rim at the rim bed using a metric tape measure. If you do not have a metric tape measure, you can take the measurement in inches and multiply by 25.4 to convert the measurement to millimeters.

  
  

• You can measure the circumference of a rim by wrapping a narrow, metal measuring tape (no larger than ¼”) all the way around the rim. A fabric measuring tape can stretch and give you inaccurate measurements. Once you have the circumference, you can calculate the diameter.

  
  

• If you don't have a narrow tape measure, you can wrap a bicycle brake or shift cable around the rim and mark the points that the cable overlaps itself on the cable and measure the length between the marks.
  
  

• The measuring tape has a hook at the end. Place the hook into the valve hole and wrap the tape all the way around the rim, measuring the total circumference at the bottom of the rim bed.

• Now, divide the circumference by pi (3.142) to get the diameter.

• If your measuring tape is in inches, multiply by 25.4 to get the diameter of the bed in millimeters.

• This should get you in the neighborhood of what the BSD of the rim is on the chart.

• If the size of the rim is still unclear, you will need to measure the height from the rim bed to the bead seats. This is best done in millimeters. To do this, place a metal straight edge or ruler across the rim and use a metal object like a spoke as gauge to mark the height from the center of the rim bed to the outside edge of the wheel. You can then measure the mark on the gauge you created. In this example we will use 8mm as the height.

• The circumference of the well measured as 68 1/2", (68.5 inches). Multiplying by 25.4 gives us 1740 mm; then dividing by pi (3.142) the diameter is 553.75 mm. 16 mm additional (twice the depth of the bed) is 569.75 mm, but the bead seat diameter is usually about 10 mm smaller, and this is a 559 mm rim.

*chart credit:  sheldonbrown.com

2. Consider Tire Width Compatibility

Tire width affects performance, comfort, and fit within the bike frame.

Match the Rim’s Internal Width

The rim’s internal width (distance between the inner walls) determines the range of tire widths it can support. Use the following general guidelines:

• Narrow rims (13-17mm internal width): Suitable for 23-28mm tires.

• Medium-width rims (18-25mm): Work with 28-50mm tires.

• Wide rims (26mm+): Ideal for tires 50mm and wider, such as for mountain or fat bikes.

Frame Clearance

Ensure the chosen tire width provides adequate clearance within your bike frame and fork. Check for at least 4-6mm of space between the tire and the frame.

3. Determine Your Brake System Requirements

Brake compatibility is essential when selecting a replacement wheel.

Rim Brakes

• The wheel’s rim must have a smooth braking surface.

• Ensure the wheel diameter matches the reach of your rim brake calipers (e.g., a 622mm BSD for road bikes or 559mm for standard 26" wheels).

Disc Brakes

• Ensure the hub is compatible with your brake rotor mounting system (e.g., 6-bolt or Center Lock).

• Verify rotor diameter compatibility, as larger rotors may require more frame and fork clearance.

4. Check Hub and Axle Standards

The hub and axle system directly impact how the wheel fits your bike’s frame.

Hub Spacing

Measure the width of your current hub between the dropouts, known as the Over Locknut Dimension (OLD):

• Common road bike hub spacing: 100mm front, 130mm rear.

• Common mountain bike hub spacing: 100mm front, 135mm rear (or wider for Boost and Super Boost standards).

Axle Type

Identify whether your bike uses:

• Quick Release: A skewer system with a lever for easy wheel removal.

• Thru-Axle: A thicker, threaded axle that provides additional stiffness.

5. Assess Your Riding Style and Terrain

Your intended use significantly influences wheel selection.

Road Cycling

• Choose lightweight wheels with narrow profiles for speed and efficiency.

• Look for a rim that supports tires in the 23-32mm range.

Gravel and Cyclocross

• Opt for wider rims that accommodate 35-50mm tires for better grip and comfort on mixed terrain.

• Tubeless-ready wheels are ideal for gravel setups.

Mountain Biking

• Prioritize durability and compatibility with wider tires (2.1"-2.6").

• Check for Boost hub spacing if your bike is newer.

Touring and Commuting

• Select sturdy wheels that can support heavier loads and wider tires for comfort and stability.

6. Material Considerations

The wheel’s material affects performance, weight, and cost.

Aluminum

• Pros: Affordable, durable, and widely available.

• Cons: Heavier and less aerodynamically optimized than carbon.

Carbon

• Pros: Lightweight, stiff, and excellent for performance-focused riders.

• Cons: Expensive and potentially more prone to damage in extreme conditions.

2. Consider Tire Width Compatibility

Tire width affects performance, comfort, and fit within the bike frame.

Match the Rim’s Internal Width

The rim’s internal width (distance between the inner walls) determines the range of tire widths it can support. Use the following general guidelines:

• Narrow rims (13-17mm internal width): Suitable for 23-28mm tires.

• Medium-width rims (18-25mm): Work with 28-50mm tires.

• Wide rims (26mm+): Ideal for tires 50mm and wider, such as for mountain or fat bikes.

Frame Clearance

Ensure the chosen tire width provides adequate clearance within your bike frame and fork. Check for at least 4-6mm of space between the tire and the frame.

3. Determine Your Brake System Requirements

Brake compatibility is essential when selecting a replacement wheel.

Rim Brakes

• The wheel’s rim must have a smooth braking surface.

• Ensure the wheel diameter matches the reach of your rim brake calipers (e.g., a 622mm BSD for road bikes or 559mm for standard 26" wheels).

Disc Brakes

• Ensure the hub is compatible with your brake rotor mounting system (e.g., 6-bolt or Center Lock).

• Verify rotor diameter compatibility, as larger rotors may require more frame and fork clearance.

4. Check Hub and Axle Standards

The hub and axle system directly impact how the wheel fits your bike’s frame.

Hub Spacing

Measure the width of your current hub between the dropouts, known as the Over Locknut Dimension (OLD):

• Common road bike hub spacing: 100mm front, 130mm rear.

• Common mountain bike hub spacing: 100mm front (or wider for Boost and Super Boost), 135mm rear (or wider for Boost and Super Boost).LOL

Axle Type

Identify whether your bike uses:

• Quick Release: A skewer system with a lever for easy wheel removal.

• Thru-Axle: A thicker skewer that is threaded on one end that provides additional stiffness.

• Bolt-on : A solid, non-removeable threaded axle that uses axle nuts to secure it to the dropouts.

5. Assess Your Riding Style and Terrain

Your intended use significantly influences wheel selection.

Road Cycling

• Choose lightweight wheels with narrow profiles for speed and efficiency.

• Look for a rim that supports tires in the 23-32mm range.

Gravel and Cyclocross

• Opt for wider rims that accommodate 35-50mm tires for better grip and comfort on mixed terrain.

• Tubeless-ready wheels are ideal for gravel setups.

Mountain Biking

• Prioritize durability and compatibility with wider tires (2.1"-2.6").

• Check for Boost hub spacing if your bike is newer.

Touring and Commuting

• Select sturdy wheels that can support heavier loads and wider tires for comfort and stability.

6. Material Considerations

The wheel’s material affects performance, weight, and cost.

Aluminum

• Pros: Affordable, durable, and widely available.

• Cons: Heavier and less aerodynamically optimized than carbon.

Carbon

• Pros: Lightweight, stiff, and excellent for performance-focused riders.

• Cons: Expensive and potentially more prone to damage in extreme conditions.

Case 1: Road Bike Replacement

• Existing tire marking: 25-622 .

• Required wheel: A rim with a 622mm BSD and internal width supporting 25mm tires.

• Additional checks: Ensure the hub is compatible with 130mm spacing and quick-release axles and that the rim is compatible with rim brakes.

Case 2: Mountain Bike Upgrade

• Existing tire marking: 54-622 (29x2.1").

• Required wheel: A rim with a 559mm BSD and internal width of 25-30mm to support wider tires for improved grip.

• Additional checks: Confirm the hub spacing is 148mm with thru-axles and is for 6-bolt disc brake rotors .

1. Precision

ISO measurements eliminate guesswork, ensuring wheels and tires fit perfectly.

2. Universal Compatibility

The system simplifies the process of choosing components across brands and models.

3. Enhanced Safety

Properly matched wheels and tires reduce the risk of blowouts, poor handling, or brake failures.

1. Mismatched BSD: Always prioritize the bead seat diameter to ensure compatibility.

2. Overlooking Frame Clearance: Measure tire and wheel clearance before purchasing.

3. Ignoring Hub Standards: Verify hub spacing, axle type, and rotor compatibility.