Monday, September 24, 2007

Fork Alignment and Bike Handling

Within bike-tech circles, there has been a growing realization that carbon fiber forks aren’t up to snuff. No, I’m not talking about the “squirrel got caught in the spokes and shattered my fork” stories. I’m referring to the unhappy discovery that many mass market carbon forks – including major brand names – are poorly aligned. So poorly, in fact, that it can have a serious negative impact on a bike’s overall handling. This quality control problem in the manufacture of carbon forks is summarized in a recent article from Calfee Design, a highly regarded builder of carbon fiber frames.

So, why does this concern someone like me, who builds and uses only steel forks? It provides a perfect counterpoint for telling you a little about my approach to building – and aligning – custom steel forks.

Building an aligned fork is more difficult than building an aligned frame. On a fork, there are
seven independent dimension and location variables, which must be satisfied simultaneously, in three dimensions. My work process for building a steel fork is based on subassemblies, similar to my approach to building a frame. The crown is brazed to the steerer, and the crown race seat is machined to specification on a lathe. The dropouts are brazed into the raked fork blades. The fork’s alignment variables all come together when these subassemblies meet in the fork jig, where I cut the fork blades to a precise final length, and pin the blades into the crown sockets. The securely pinned fork is removed from the jig and free-brazed in a specially designed rotating fixture, producing a finished fork which is free of residual stresses and very close to its final alignment standard.

Any brazed steel fork will benefit from a final alignment, simply to remove the small displacements which are a consequence of the heating cycle. This is an area where my clock-be-damned approach to framebuilding allows me to be exceptionally picky. Like most builders, I align off of a granite surface plate, using machinist’s v-blocks and gauges. But, by the time that all of the fork alignment tools are out – including custom tooling that I’ve created just for this task - I feel a little like the crazed dentist that Steve Martin played in that movie.

The Calfee article notes that “A diligent steel frame builder can align the fork blades to within a millimeter of symmetry. Certain well known builders align them to within 0.5 mm.“

I routinely confirm the alignment of my forks to within 0.5mm of specification, on
each measured variable. Apparently, I devote more attention to fork alignment than many other builders. That’s OK by me. As I see it, your fork is too important for anything but the best in craftsmanship.