bicycle frame dimensions

Trail is the distance between the point where the wheel contacts the ground and the point where the steering axis of the front wheel intersects the ground.

A bike with a lot of trail will tend to ride straight and feel very stable, but will not be as responsive as a bike with less trail.

Head Tube Bottom is the distance from the bottom bracket to the bottom of the head tube. BikeCAD Pro presents this distance in X and Y components.

The Henry James jig requires a couple of other dimensions that are related to Head Tube Bottom. Head Tube Bottom (Henry James) is the dimension shown below. It is just like Head Tube Bottom except it is measured in line with the steerer tube.

The effective seat tube length is the distance between the bottom bracket and the point at which a virtual horizontal top tube would intersect the seat tube.

For a related dimension, see Effective top tube length.

Fork offset or rake is the dimension shown. Imagine a line passing through the steerer tube of the fork. Measure from there to the center of the front hub, and you've got the fork offset.

BB Height or Bottom Bracket Height is the height of the bottom bracket measured from the ground.

BB Drop or Bottom Bracket Drop is the distance by which the center of the bottom bracket lies below the level of the rear hub.

Front center is defined as the distance from the center of the bottom bracket to the center of the front hub.

Front center is controlled in BikeCAD as described at: bikecad.ca/toptube_frontcenter.

The effective top tube length is the horizontal distance from the top tube/head tube junction to the seat tube. This measurement has become significant as more and more bikes are designed with sloping top tubes.

Effective top tube length is one of three options for defining the length of the front triangle. For more on this, see here.