Friday, March 2, 2007

Frame-Neutral Replacement Fork™

The recent excitement about low-trail steering geometry on road/sport bicycles has produced some wonderful new frames which reflect this design approach that was perfected a half-century ago on French cyclotourist bikes. As this interest has grown, discussions have appeared on various bicycle forums, where owners of quality frames are interested in the option of retrofitting with a new, longer-rake fork. Posted responses generally have offered encouraging anecdotal evidence, and at least one vendor has announced a one-size-fits-all retrofit fork. Generally absent, however, has been a useful discussion on the elements of frame/fork interaction, and a quantitative view of the impact of a replacement fork on the frame/steering geometry of an existing bike.

I have developed the Frame-Neutral Replacement Fork™ as an intelligent solution to retrofitting existing frames with a longer-rake fork. This fork is custom designed to provide the new low-trail steering that you seek, while preserving your frame’s original orientation. More than a simple fork fabrication, this is a comprehensive design service, working with you and your existing bike, and advising about workable options.

Let’s look at the interaction between the fork and frame. To help with this, the diagram below defines the key elements of a bicycle’s front-end geometry. The data listed for the original fork (black) and the Frame-Neutral Replacement Fork™ (red) is from a completed retrofit project.

A bicycle frame is supported by its fork, and the design of a frame begins with the fork’s length and rake. In this discussion, the fork length is measured along the steering axis, from the crown race seat to the point where a perpendicular line intersects the axle center, as shown by the FL dimension in the diagram. Fork rake is measured on a line perpendicular to the steering axis.

From the designer’s perspective, the front end of a bicycle frame is located in space by the distance along the steering axis from the lower end of the head tube (point A) to a baseline drawn through the axles (point B). As shown in the diagram, this distance along the steering axis is the sum of the fork's length-on-axis (FLA) plus the headset’s lower stack height. When the headset is a constant, this leaves us with an evaluation where the key variable, fork length-on-axis (FLA), is calculated directly as a function of the fork length (FL), the fork rake (RA), and the head angle.

In order to preserve the frame’s original orientation (head tube and seat tube angles), a longer-rake replacement fork must produce the same length-on-axis (FLA). As seen in this diagram, this requires the longer-rake replacement fork to also have a longer axle-to-crown length (FL). In the case illustrated here, increasing the rake by 22.5 mm required that the replacement fork’s axle-to-crown length be 7 mm longer than on the original fork. This may seem like an insignificant number, but leaving off those 7 mm would have made the head angle steeper by nearly 0.4 degrees. From the perspective of this bicycle’s owner, that much change in head angle would have been unacceptable.

There’s a bit more to this than simply making a longer fork. If you’re using caliper brakes, we’ll have to take a look at the longer brake reach, and how to deal with it by using various crown designs and/or an alternate front caliper. There really are a lot of neat ways to make this work. If you’re using cantilever brakes, you’re good to go with your existing components.

I have my own numerical design model, designed to solve on this specific problem. Using this model, along with a few key pieces of design data from your existing frameset, I can quickly, and precisely, define the Frame-Neutral Replacement Fork™ which best meets your needs.