When it comes to bike component upgrades or replacements, understanding measurements like the Bolt Circle Diameter (BCD) is crucial. BCD, or the Bolt Circle Diameter, is the diameter of an imaginary circle that passes through the center of all the bolt holes on your crankset. Knowing your bike's BCD is essential when choosing chainrings, cranks, or other drivetrain parts that must match your existing setup.

In this guide, we’ll go over everything you need to know about BCD, including how it’s measured, the tools you can use, and why it matters for various bike types.

BCD is used primarily in the cycling world to measure the spacing of bolt holes on components like chainrings. These holes connect the chainring to the crank arm, and the correct BCD ensures that the chainring will fit onto the crankset correctly. Since BCD is expressed in millimeters, a BCD of "110mm" means that the bolts lie on a circle with a 110mm diameter.

BCD is also essential for gear ratios and is often chosen based on factors like preferred riding style, terrain, and the type of drivetrain (1x, 2x, or 3x).

Common BCD Sizes

Different types of bikes and crankset designs have distinct BCD measurements. Here’s a breakdown of the most common ones:

• Road Bikes : 110mm and 130mm are the most common BCDs for road bikes, with compact cranks (50/34T) usually featuring a 110mm BCD and standard cranks (53/39T) generally having a 130mm BCD.

• Mountain Bikes : 104mm and 64mm are typical BCDs for triple chainrings, while single-chainring setups may vary based on the specific crank model.

• Gravel and Cyclocross Bikes : Often use a 110mm BCD but may vary depending on whether the bike uses a single or double chainring.

• Compact and Sub-compact : Compact chainrings generally have a 110mm BCD, while some sub-compact designs go as low as 94mm, giving more options for lower gearing, which is often preferred for endurance and gravel riding.

Understanding these common BCD sizes makes it easier to determine which measurements to look for when shopping for chainrings or cranks.

Measuring bolt circle diameter can vary slightly depending on the number of bolts in your crankset, so we’ll cover methods for both standard and non-standard bolt patterns.

Tools You’ll Need

• Calipers : Digital or analog calipers provide precise measurements and are ideal for measuring smaller diameters.

• Ruler or Tape Measure : A standard ruler or measuring tape works well for larger BCD sizes, particularly on cranksets with an even number of bolts.

• BCD Measurement App or Calculator : Some smartphone apps allow you to take a photo of your crankset and determine the BCD instantly. There are also online BCD calculators if you know the distance between bolts.

Measuring Bolt Circle Diameter for Different Bolt Patterns

There are two main types of bolt patterns on cranksets: even (4 or 6 bolts) and odd (5 bolts). Each requires a slightly different approach.

4-Bolt and 6-Bolt Patterns

1. Identify Two Adjacent Bolt Centers : Locate two adjacent bolt holes on the chainring.

2. Measure the Center-to-Center Distance : Use calipers or a ruler to measure the distance between the center of one bolt hole to the center of the adjacent hole.

3. Apply the Multiplier : Once you have the center-to-center distance, you can calculate the bolt circle diameter:

   • For 4-bolt patterns, multiply the distance by 1.414 .

   • For 6-bolt patterns, multiply the distance by 2 .

4. Resulting Bolt Circle Diameter : The resulting measurement gives you the BCD of your chainring.

Example: If the center-to-center distance on a 4-bolt crankset is 78mm, multiply 78 by 1.414, giving you a BCD of approximately 110mm .

5-Bolt Patterns

For cranksets with a 5-bolt pattern, the process is slightly different:

1. Choose One Bolt Hole : Pick one bolt and measure from its center to the center of the bolt directly opposite (across the chainring). However, on 5-bolt patterns, you won’t always find an exact opposite, so the measurement becomes more approximate.

2. Measure Center-to-Center Distance : Use calipers or a ruler to measure the distance between two bolt holes that are not adjacent. This measurement can be tricky since not all bolts align perfectly, so use your best judgment.

3. Apply the Multiplier : For a 5-bolt pattern, multiply the center-to-center distance by 1.701 to determine the BCD.

Example: If your measured distance on a 5-bolt chainring is 64mm, multiplying by 1.701 gives a BCD of approximately 109mm .

Measuring BCD Without Tools

If you don’t have calipers or a ruler, you can still approximate BCD with basic tools and methods:

1. Use a String or Piece of Paper : Lay a piece of string or a thin piece of paper from one bolt center to another. Mark the points and then lay the string or paper flat to measure with a regular ruler.

2. Use a Smartphone App : Some cycling apps let you take a photo of the crankset and calculate the BCD automatically.

BCD is crucial for several reasons beyond simply fitting the chainring on your bike. Here’s why knowing your BCD is essential:

1. Compatibility : Not all chainrings fit every crankset. Knowing your BCD ensures that any new chainring will be compatible with your existing crankset.

2. Gearing Options : Different BCDs limit or expand your options for chainring sizes. A smaller BCD generally allows smaller chainrings, ideal for low-gear setups, while a larger BCD accommodates bigger chainrings.

3. Weight and Performance : Cranksets designed with certain BCDs may offer optimized weights or performance benefits, affecting climbing, acceleration, and overall bike handling.

BCD and Gear Ratios

The BCD you select impacts your bike’s gear ratio by determining the minimum and maximum chainring sizes. Smaller BCDs enable the installation of smaller chainrings, beneficial for riders who prioritize easier pedaling on climbs or rough terrain. Conversely, larger BCDs allow for bigger chainrings, giving riders more power for high-speed rides on flat terrain or descents.

Example: A compact road bike crankset with a 110mm BCD can accommodate a 50/34T setup, which is excellent for a mix of climbing and flat riding. A standard 130mm BCD crankset typically has a 53/39T chainring, ideal for faster, more aggressive riding.

Understanding the common BCDs for different bike types can help you know what to look for based on your riding style.

1. Road Bikes : 110mm (compact) and 130mm (standard) BCDs are most common. Compact cranks (110mm) are popular for climbing, while standard cranks (130mm) suit high-speed, flat terrain.

2. Mountain Bikes : 104mm BCD is standard for many mountain bikes. 1x drivetrains often use 104mm or direct-mount chainrings, optimized for smaller chainring sizes suited to technical climbs.

3. Gravel and Cyclocross : Gravel bikes may use 110mm BCD for double chainrings or a lower BCD like 94mm for sub-compact gearing. Cyclocross bikes typically use 110mm for a balance between off-road agility and road speed.

4. Touring Bikes : Touring cranks often feature triple chainrings with BCDs around 74mm (inner ring) and 110mm or 130mm (outer rings), giving a broader range of gearing for loaded climbs and long-distance cruising.

Here are a few additional tips to keep in mind when measuring and selecting your BCD:

• Double-Check : Small discrepancies in measurements can lead to incompatible parts, so measure twice to ensure accuracy.

• Consult Manufacturer Specifications : Some cranksets and chainrings list their BCD specifications online or in product manuals, so consult these resources for exact measurements.

• Use a BCD Template : Printable BCD templates are available online. Print one, align it with your chainring, and check for an exact match.

• Consider Upgrading : If you’re limited by your current BCD, it may be worthwhile to consider a crankset upgrade. For example, moving from a 130mm BCD crank to a 110mm BCD compact crank opens up more gearing options.