You measure bike fork offset, also known as fork rake, by determining the horizontal distance between the center axis of the steerer tube and the center of the front axle dropout.
Understanding and Measuring Fork Offset/Rake
Fork offset (or rake) is a critical geometric measurement that influences how a bike steers. It's the distance the dropout is offset from the steering axis (an imaginary line through the center of the steerer tube). A larger offset generally results in less trail and quicker steering, while a smaller offset increases trail and contributes to more stable steering.
Measuring this precisely requires a flat surface and a way to support the fork accurately. Here's a common method, incorporating the provided reference:
Measurement Procedure
To accurately measure fork offset using the specified method, follow these steps:
- Prepare the Surface: Ensure you have a perfectly flat surface, such as a workbench or large plate of glass.
- Position the Fork: Rest the steerer tube of the fork on a V-block. The V-block helps hold the cylindrical steerer tube securely and consistently. Place the fork on the flat surface so it is stable.
- Take Measurement A (Dropout Height): Measure the height of the fork dropout (where the wheel axle would sit) off the top of the flat surface. Record this measurement as Dim A.
- Take Measurement B (Steerer Height): Measure the height of the center of the steerer tube off the top of the flat surface. Record this measurement as Dim B. The V-block helps define the point on the steerer tube to measure from or to.
- Calculate the Offset: Subtracting Dim B from Dim A will result in the fork offset or rake.
This calculation works because the steerer tube is held at an angle relative to the flat surface by the V-block. Dim B is the height of the steering axis above the surface at the point of the V-block. Dim A is the height of the axle center (which is offset horizontally from the steering axis) above the surface. The difference between these two heights directly corresponds to the horizontal offset when measured this way.
Here is a summary of the measurements:
| Measurement | Description | Notation |
|---|---|---|
| Dropout Height | Height of the fork dropout off the flat surface | Dim A |
| Steerer Height | Height of the steerer tube center off the surface | Dim B |
| Fork Offset (Rake) | Dim A - Dim B | Result |
Using a digital caliper or a precise ruler is recommended for accuracy. Ensure the fork is resting naturally and is not twisted or strained during the measurement.
Understanding your fork's offset is helpful if you are replacing a fork, building a new bike, or simply trying to understand how different frame and fork geometries affect handling.
