Measuring the length of a square taper bottom bracket spindle is essential for selecting the correct component for your bike's frame and crankset combination, impacting crucial factors like chainline. While the standard way to measure a spindle involves a simple physical measurement, one method involves calculation based on desired crank position relative to the bottom bracket shell.
Measuring Spindle Length Based on Crank Position (Reference Method)
According to one method described, you can determine a value related to the required spindle length by positioning the crankset and taking measurements. This method is particularly useful when trying to match a new bottom bracket spindle to a desired crank arm position relative to the frame's bottom bracket shell.
Here are the steps involved in this calculation method:
- Position the Crankset: Hold the crankset where you want it to sit relative to the bottom bracket shell. This might be against a fitted (but perhaps incorrect length) bottom bracket, or simply positioned manually.
- Measure the Distance: Use a depth tool to measure the distance from the flat inside of the crank arm to the outside of the bottom bracket shell. Ensure your measurement is parallel to the spindle axis. Let's call this measurement 'M'.
- Calculate Distance from Centerline: Add 34mm to your measured distance (M). This step is specifically for a 68mm bottom bracket shell, where 34mm represents half the shell width, theoretically giving you the distance from the centerline of the shell to the inside flat of the crank arm. The formula is:
Distance from Centerline = M + 34mm(for a 68mm shell). - Calculate "Spindle Length": Double the result from the previous step. This final number is presented as the "spindle length" by this specific method:
Calculated Spindle Length = 2 * (M + 34mm).
This method provides a calculated value based on your desired crank arm positioning relative to the bottom bracket shell edge.
Standard Method for Measuring Physical Spindle Length
The most common and direct way to measure a square taper spindle's length is to measure its total length from one end to the other. This is done when the spindle is removed from the bottom bracket assembly.
- Use a Measuring Tool: Use a ruler, tape measure, or preferably, a set of calipers for greater accuracy.
- Measure End-to-End: Place the tool against one end of the spindle and measure straight along the spindle's axis to the very tip of the other end. Include the tapered sections in your measurement.
- Measurement Points: Measure from the outer tip of one tapered end to the outer tip of the other tapered end.
Spindle lengths are typically specified in millimeters (mm), ranging commonly from around 103mm to 130mm or more for square taper bottom brackets, depending on the crankset and intended chainline.
Knowing the spindle length, either by physical measurement or calculated via methods like the one described, is crucial for selecting the correct bottom bracket replacement to ensure proper crank fit and chainline.
