how to measure bottom bracket

How to Measure Bottom Bracket Shell Width and Spindle Diameter for Accurate Sizing

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Bottom brackets connect the crankset with the frame of a bicycle. These bike parts are typically housed in the bottom bracket shell, part of the bicycle’s frame.

There are many different types and sizes of bottom brackets available on the market today, but how do you know which one you need? Read this article to find out how to measure bottom bracket shell width and spindle diameter for correct sizing!

What Is a Bottom Bracket?

Your bottom bracket, or BB, is located on your frame’s base between the seat and your down tubes. It has two bearings, the first at any side through which the pedals are fitted, allowing chainrings and pedals to travel freely.

There are many different types or sizes of bottom brackets available for bicycles, but the bottom bracket types are divided into two categories: threaded or press-fit bottom brackets.

Additionally, there are two major types of threaded bottom brackets—English threaded and Italian threaded. The difference between these two is which type of spindle the threaded bb shell frame accepts.

How to Measure Bottom Bracket Shell Width and Diameter

If you are unsure what size or type of bicycle bottom bracket you need, the best way to determine which one you need for your bicycle frame is by measuring the BB shell width. Follow these steps to determine your correct bottom bracket size.

Step 1: Position the Bike

The first step is to flip your bike upside down using a bike stand. To make it easier for you to find the BB shell width, be sure that you are looking at the face of the BB where two facing cups look like they fit together. Make sure you are looking at the area where the crank arm meets to get an accurate measurement.

Step 2: Find out the Bottom Bracket Shell Width

To measure the width of BB shells, place the metric ruler on top of where your crank arms are mounted. You will notice that you can fit half the ruler inside with the other half hanging over past it. This is how wide your frame’s BB shell is.

Remember that when it comes to the shell width of a bottom bracket, 68mm and 73mm are the most common. Measurements are typically in millimeters.

Step 3: Determine the Spindle Diameter

The bottom brackets on different bicycle frames have different spindle diameters. Therefore, you must know the accurate spindle configurations and precise measurements.

Using a good set of calipers, measure the crank spindle diameter. You can do this by opening your calipers wide and wrapping them around the bearings on the spindle. Take note of the crank spindle diameter as this is also the bottom bracket’s overall length. It may range from 113-122 mm for most bicycles.

With both crank arms attached to the crankset spindle of your bike frame, measure the space between the two connected parts to get the effective crank spindle length.

Step 4: Measure the Inner Shell Diameter

Wrap your calipers on the BB shell’s side. Close the calipers over the opening to obtain the inner diameter. Take note of this number.

A BB shell’s diameter of 35mm or 1.37 inches means you have an English BB. You have an Italian bottom bracket if the inner diameter is 6mm or 34.92 mm (the latter is for Campagnolo cranks).

Step 5: Determine Which Type of Bottom Bracket Tool to Use

a young mechanic wears gloves using a bottom bracket wrench to lock while working in a workshop

You need a special tool to remove very specific bottom bracket cups.

These tools are available in two different styles: one for English bottom brackets and another for Italian ones. The tool is called the BBT-22 or the Park Tool Bottom Bracket Wrench/Bearing Cup Remover (BBT-30) respectively.

The first type has a 17 mm socket to remove English threaded bottom bracket cups, while the other has 19mm and 22mm sizes that will have your Italian threaded bottom bracket cups removed.

To find out the size of the tool required to remove the bottom bracket, take note of how many grooves are in the bracket’s outside rim. The number coincides with the number of notches on the bracket removal tool.

Step 6: Detach the Bottom Bracket from Your Bicycle’s Frame

Before removing, check if your bottom bracket needs servicing or replacing. If it does, you can do a quick service by adding fresh grease and oil before re-installing with a new bottom bracket cartridge for better performance.

Starting on the drive side of your English bottom bracket, slide the tool over the spindle, and, using a wrench, loosen the tool using a counterclockwise motion until the cup comes off. Repeat, but this time, do it on the left.

It should pull out easily if it is in good condition. If you are facing resistance when removing the BB, there’s a chance that your crankset needs to be replaced or was not appropriately pressed to the spindle component during installation.

Pro Tip: If the driver side doesn’t loosen when you turn in a counterclockwise direction, you may have an Italian bottom bracket. Turn it in the opposite direction.

Step 7: Determine the Type of Bottom Bracket That You Have

Some BB shells may have threading. Bottom brackets compatible with specific shells will have matching threading. Thus, you need to check the BB’s ends and determine where the threading is directed.

Step 8: Use a Bottom Bracket Size Chart

Based on your measurements for width, you can cross-reference this number with a bottom bracket size chart to find the appropriate type needed.

Frequently Asked Questions

What Size Bottom Bracket Do I Need?

The most common types available on the market today include 68mm, 73mm, and 100mm. However, this may not be the same size as your current bottom bracket – so measure properly before making a purchase!

How Many Sizes of Bottom Brackets Are There?

They come in a variety of sizes, ranging from 68mm to 100mm. Following bottom bracket standards, this measurement refers to the distance between where your left crankarm meets it and where your right crank arm connects with the spindle portion.

Which Type of Bottom Bracket Doesn’t Have a Spindle?

If your BB doesn’t have a spindle, you may have a thread-through type of bracket.

What Size Bottom Bracket Do I Need for My Mountain Bike?

Bottom bracket size is important as it determines compatibility between cranksets and bottom brackets. Mountain bikes use one of three types: 68mm, 73mm, or 100mm wide BB shell.

How Do I Determine Bottom Bracket Spindle Length?

If you’re looking for a bottom bracket with the right spindle length, measure your current one and then find out what size (length) it is. Then simply use the precise measurement as a reference point when shopping around to determine which bottom brackets are compatible with your bike!

To learn more about how to properly fit and install your new bottom bracket, head to your local bike shop!

Measuring Your Bike’s Bottom Bracket

Measuring your bike’s BB correctly, whether it is a threaded or press-fit bottom bracket, may seem like an overly complicated task. However, with our tips on how to measure bottom bracket shells and the diameter of their spindles, you will be able to get all the necessary dimensions accurately.

If you used this guide to accurately identify and measure bottom brackets on road bikes and mountain bikes, let us know in the comments. We’d love to hear about your experience!

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