Measuring your bottom bracket (BB) is a crucial step in replacing or upgrading it on your bicycle. A correctly sized bottom bracket ensures proper crankset installation, smooth pedaling, and optimal performance. This comprehensive guide will walk you through the process of accurately measuring your bottom bracket, covering different bottom bracket types and providing step-by-step instructions.
H2 Why Measuring Your Bottom Bracket Matters/H2
The bottom bracket connects the crankset to the bicycle frame, allowing the pedals to turn smoothly. Incorrectly sized bottom brackets can lead to several problems:
* **Poor Shifting:** Misalignment of the crankset can negatively affect front derailleur performance, leading to slow or inaccurate shifting.
* **Crank Arm Interference:** The crank arms may rub against the frame if the bottom bracket is too wide.
* **Bearing Damage:** An incorrectly sized bottom bracket can place undue stress on the bearings, leading to premature wear and failure.
* **Frame Damage:** In extreme cases, an incompatible bottom bracket can damage the frame itself.
* **Inefficient Power Transfer:** Improper crankset alignment can decrease the efficiency of your pedaling, resulting in wasted energy.
Therefore, precise measurement is essential for a successful bottom bracket replacement or upgrade.
H2 Understanding Bottom Bracket Standards/H2
Before you start measuring, it’s helpful to understand the different bottom bracket standards. These standards define the dimensions and interface between the bottom bracket and the frame. Here are some of the most common types:
* **Threaded Bottom Brackets:**
* **English/BSA (British Standard):** The most common type, found on most older and many modern bikes. It has a 68mm or 73mm shell width (mountain bikes often use 73mm, while road bikes tend to use 68mm). Some older Italian bikes use a 70mm shell. The drive-side (right) is left-hand threaded, and the non-drive side (left) is right-hand threaded. Older Raleigh bikes sometimes have a 76mm shell.
* **Italian:** Less common than BSA, Italian threaded bottom brackets have a 70mm shell width. Both sides are right-hand threaded. Found on some older Italian bikes.
* **French:** An obsolete standard with a 68mm shell, right-hand threads on both sides. Very rare.
* **Press-Fit Bottom Brackets:**
* **BB30:** Features a 30mm spindle diameter and a 68mm shell width. The bearings press directly into the frame. Early versions were prone to creaking.
* **PF30:** Similar to BB30, but uses cups that are pressed into the frame, providing a more secure fit and reducing creaking. Typically has a 68mm shell width (road) or 73mm (mountain).
* **BB86/92:** Designed for Shimano Hollowtech II cranksets. BB86 is used on road bikes (86.5mm shell width), while BB92 is used on mountain bikes (91.5mm or 92mm shell width).
* **BB386EVO:** A wider version of BB30, with an 86mm shell width. Offers increased stiffness and compatibility with a wider range of cranksets.
* **T47:** A newer threaded standard that uses a larger diameter thread than BSA. It’s available in various widths and allows for larger diameter spindles and bearings. Designed to be stiffer than BSA while being more tolerant than Pressfit standards.
* **BBright:** Found on Cervelo bikes, this uses a pressfit system that is asymmetric. The drive side of the bottom bracket is larger than the non-drive side. The shell is 79mm wide.
Understanding these standards will help you identify your current bottom bracket and choose the correct replacement.
H2 Tools You’ll Need/H2
* **Caliper:** A digital or vernier caliper is essential for accurate measurements. Digital calipers are generally easier to read.
* **Ruler or Measuring Tape:** A ruler or measuring tape can be used to get an approximate measurement, but a caliper is recommended for precision.
* **Bottom Bracket Removal Tool (if removing the BB):** The specific tool depends on your bottom bracket type. Common tools include a spline tool for threaded bottom brackets and a bearing press for press-fit bottom brackets.
* **Pencil and Paper:** To record your measurements.
* **Cleaning Supplies:** Clean the bottom bracket area before measuring to ensure accurate readings.
H2 Measuring a Threaded Bottom Bracket/H2
This section focuses on measuring English/BSA threaded bottom brackets, as they are the most common. The process for measuring Italian threaded bottom brackets is similar.
**Step 1: Identify the Bottom Bracket Type**
Visually inspect the bottom bracket shell. Look for threads inside the shell. If you see threads, it’s a threaded bottom bracket. Determine if it’s BSA or Italian by looking for markings on the bottom bracket cups or by consulting your bike’s specifications.
**Step 2: Measure the Shell Width**
1. **Remove the Crankset:** If the crankset is still installed, you’ll need to remove it to access the bottom bracket shell. Use the appropriate crankset removal tool.
2. **Remove the Bottom Bracket Cups:** Use the correct bottom bracket tool to remove the cups. Remember that the drive-side cup is left-hand threaded on BSA bottom brackets.
3. **Clean the Bottom Bracket Shell:** Wipe the inside and outside of the bottom bracket shell to remove any dirt or grease.
4. **Measure the Shell Width:** Use the caliper to measure the width of the bottom bracket shell. Place the jaws of the caliper on the outside edges of the shell. Ensure the caliper is perpendicular to the shell.
5. **Record the Measurement:** Write down the measurement in millimeters (mm). The most common widths are 68mm and 73mm. Some older Italian bikes use a 70mm shell, and some Raleigh bikes use a 76mm shell.
**Step 3: Determine the Spindle Length (for replacement)**
The spindle length is the distance between the ends of the bottom bracket spindle. This measurement is crucial for ensuring proper chainline and crankset alignment.
1. **Check the Existing Spindle (if available):** If you’re replacing an existing bottom bracket, you can measure the spindle length directly. Remove the bottom bracket from the frame.
2. **Measure the Spindle:** Use the caliper to measure the distance between the ends of the spindle. Ensure the caliper is aligned with the spindle.
3. **Record the Measurement:** Write down the measurement in millimeters (mm).
4. **Consult Crankset Specifications:** If you don’t have the old bottom bracket or spindle, consult the manufacturer’s specifications for your crankset. The specifications will typically list the recommended spindle length for different frame types and bottom bracket shell widths. Online databases and the crankset manufacturer website can be helpful resources.
**Important Considerations for Threaded Bottom Brackets:**
* **Thread Direction:** Remember that BSA bottom brackets have different thread directions on the drive and non-drive sides. The drive-side is left-hand threaded, while the non-drive side is right-hand threaded. Italian bottom brackets have right-hand threads on both sides.
* **Cup Material:** Bottom bracket cups are typically made of aluminum or steel. Steel cups are more durable but heavier.
* **Bearing Type:** Bottom brackets use either cartridge bearings or loose ball bearings. Cartridge bearings are generally more sealed and require less maintenance.
* **Chainline:** Correct chainline is crucial for optimal shifting performance. The chainline is the distance from the center of the frame to the midpoint between the chainrings. The recommended chainline varies depending on the type of bike (road, mountain, etc.). Consult your bike’s specifications or a mechanic for the correct chainline.
H2 Measuring a Press-Fit Bottom Bracket/H2
Measuring a press-fit bottom bracket requires a slightly different approach than measuring a threaded bottom bracket. You’ll need to measure the shell’s inner diameter (ID) and width.
**Step 1: Identify the Bottom Bracket Type**
Visually inspect the bottom bracket shell. Look for the absence of threads. If there are no threads, it’s a press-fit bottom bracket. Identify the specific type (BB30, PF30, BB86/92, BB386EVO, T47, BBRight) by looking for markings on the frame or by consulting your bike’s specifications. If your frame has a T47 bottom bracket, you can measure the outside diameter of the thread directly.
**Step 2: Measure the Shell Inner Diameter (ID)**
1. **Remove the Crankset:** Remove the crankset to access the bottom bracket shell.
2. **Remove the Existing Bottom Bracket (if necessary):** If you’re replacing an existing bottom bracket, you’ll need to remove it using the appropriate bearing press tool. Be careful not to damage the frame during removal.
3. **Clean the Bottom Bracket Shell:** Clean the inside of the bottom bracket shell to remove any dirt or grease.
4. **Measure the Inner Diameter:** Use the caliper to measure the inner diameter of the bottom bracket shell. Place the jaws of the caliper inside the shell and measure the distance between the inner edges. Ensure the caliper is perpendicular to the shell.
5. **Record the Measurement:** Write down the measurement in millimeters (mm). Common inner diameters include 42mm (BB30, PF30), 41mm (BB86/92), and 46mm (BB386EVO).
**Step 3: Measure the Shell Width**
1. **Clean the Bottom Bracket Shell:** Wipe the outside of the bottom bracket shell to remove any dirt or grease.
2. **Measure the Shell Width:** Use the caliper to measure the width of the bottom bracket shell. Place the jaws of the caliper on the outside edges of the shell. Ensure the caliper is perpendicular to the shell.
3. **Record the Measurement:** Write down the measurement in millimeters (mm). Common widths include 68mm (BB30, PF30), 86.5mm (BB86), 91.5/92mm (BB92), and 86mm (BB386EVO), and 79mm (BBRight). T47 will vary depending on the frame, but is easily measured. Most T47 shells are 68, 73, 86 or 92 mm wide.
**Important Considerations for Press-Fit Bottom Brackets:**
* **Proper Installation:** Press-fit bottom brackets require precise installation using a bearing press tool. Incorrect installation can lead to creaking and premature bearing failure. It is recommended to have a professional mechanic install press-fit bottom brackets.
* **Creaking:** Press-fit bottom brackets are known for creaking. Using a retaining compound during installation can help reduce creaking. Applying grease between the BB cups and the frame is often recommended.
* **Bearing Quality:** Use high-quality bearings to ensure smooth performance and long life.
* **Frame Compatibility:** Ensure that the press-fit bottom bracket is compatible with your frame. Different frames have different shell dimensions and standards.
* **Tolerance:** Frames built for Pressfit bottom brackets need very tight tolerances. Variations in the internal diameter can lead to creaking or bearings being too tight.
H2 Measuring Spindle Length (If Not Marked)/H2
Sometimes, the spindle length isn’t marked on the existing bottom bracket, or you may not have the original BB. Here’s how to estimate it:
1. **Consult Crankset Documentation:** This is the best option. Find the technical documentation for your specific crankset model. It usually lists the recommended spindle length for various frame BB shell widths. This information is invaluable.
2. **Online Databases & Forums:** Search online cycling forums and databases. Experienced cyclists often share information on compatible BB spindle lengths for specific crankset/frame combinations. Be sure to verify the information with multiple sources.
3. **Measure Chainline Offset:** Accurately measure the chainline offset of your *existing* crankset setup (if applicable). This is the distance from the center of the frame to the chainrings. Note that this method is useful if you know you liked the fit of your old crankset.
* Remove the rear wheel.
* Stretch a string from the center of the headtube, past the seattube, to the rear dropouts. Mark the halfway point of the rear dropouts on the string.
* Measure the distance from the string to the middle chainring on the crankset.
* Compare the measured chainline to the Shimano chainline specification. Shimano chainline spec is typically 43.5mm for road, and 48.5mm for mountain, but may vary.
* Use the measurement to guide your spindle length choice. If the chainline needs to move outboard, a longer spindle is needed. If the chainline needs to move inboard, a shorter spindle is needed.
4. **Trial and Error (Not Recommended):** As a last resort, you can try installing different BBs with varying spindle lengths until you achieve optimal shifting and crankarm clearance. However, this is time-consuming and can potentially damage your components if you choose an incorrect size. It is better to consult with a professional mechanic.
H2 Common Mistakes to Avoid/H2
* **Using the Wrong Tools:** Using the wrong tools can damage the bottom bracket shell or the bottom bracket itself. Always use the correct tools for the job.
* **Over-Tightening:** Over-tightening bottom bracket cups can damage the threads in the frame. Use a torque wrench to tighten the cups to the manufacturer’s specifications.
* **Forcing Parts:** If a bottom bracket cup or bearing doesn’t want to install easily, don’t force it. Check for obstructions or thread damage. Forcing parts can damage the frame or the bottom bracket.
* **Ignoring Thread Direction:** Always pay attention to the thread direction of bottom bracket cups. Incorrectly installing a cup can damage the threads in the frame.
* **Skipping Cleaning:** Failing to clean the bottom bracket shell before installing a new bottom bracket can lead to creaking and premature wear.
* **Assuming Compatibility:** Don’t assume that a bottom bracket is compatible with your frame or crankset. Always check the specifications before installing a new bottom bracket.
* **Not Using Grease or Anti-Seize:** Applying grease or anti-seize compound to the threads of bottom bracket cups can prevent corrosion and make removal easier in the future. Apply grease or anti-seize between the cups and the frame.
H2 Seeking Professional Help/H2
If you’re not comfortable measuring or installing a bottom bracket yourself, it’s always best to seek professional help from a qualified bicycle mechanic. A mechanic can accurately measure your bottom bracket, recommend the correct replacement, and install it properly.
H2 Conclusion/H2
Accurately measuring your bottom bracket is essential for ensuring proper crankset installation and optimal cycling performance. By following the steps outlined in this guide, you can confidently measure your bottom bracket and choose the correct replacement. Remember to take your time, use the right tools, and consult a professional if you’re unsure about any step in the process. With a correctly sized and properly installed bottom bracket, you’ll enjoy smoother pedaling, more efficient power transfer, and a more enjoyable riding experience.