Forks on the Road

My favorite part of building frames and forks is getting to see the finished products in use. Sometimes I get to see my work rolling down the road next to me, but more often I just get to see photos. I’ve been receiving photos from distant customers of their installed Frame-Neutral Replacement Forks, so I thought that I would share a few.

These customers share a common story. They all had nicely built, good fitting bikes, outfitted with carefully selected components. They all had tried carrying small-to-moderate loads in a front bag, and were unhappy with the resulting handling of the high-trail steering geometry. Each wanted to obtain a low-trail replacement fork, either as a long term fix, or as an experiment prior to committing to a full new frameset.

Randy rides his Rambouillet (shown below) on all surfaces with 700Cx30 tires. The FNRF was designed with a 62mm offset, yielding about 40mm of trail. This fork has lighting wire guides up the right blade, and separate under-crown mounting points for the fender and the VO randonneur front rack. Matching the original orange pearl paint finish would have been a bit pricey, so this fork was painted in the cream color used for Rivendell’s contrasting head tubes. As the lovely young Maia shows us, the installation was completed with a big front bag, and a nice centerpull brake to handle the new 67mm reach.

Bill recently bought the new Surly Traveler (below), which is the well known Cross Check frame with S&S couplers. He also owns a Kogswell P/R with a 40mm trail fork. While he generally liked the Surly, he found that he really preferred the way that the P/R handled with a front load. Bill decided to modify the Surly with a FNRF with 68mm of offset, which yields about 41mm of trail with his favorite 700Cx33 tires. He also requested an under-crown fender mount, and special through-hole bosses on the fork blades for the Tubus Duo lowrider rack. Bill selected a color from the DuPont paint chip book, and reports a “very close to perfect” color match. The fork carries a Nitto M12 front rack, and Jitensha medium bag.

Orin also rides a Rambouillet, as a randonneur and urban commuter, and wants to carry things in a front bag. His FNRF (shown below) was built with 62mm of offset, yielding about 40mm of trail. He also requested a full set of rack/fender mounts, generator hub up-and-over wire guides, a dedicated light boss low on the left blade, and a switch to a threadless steerer. With the photo, he noted that this light wire previously had been wound tightly around a support, and needs to relax to look neater in the fork’s guides. Although the fork was built to use the VO randonneur front rack, Orin later decided to design and fabricate his own stainless steel rack. This bike already was using a front Tektro R556 long reach (55-73mm) caliper brake, which was able to handle the longer reach on the new fork.

Fork It Over

What happens if receive your Frame-Neutral Replacement Fork™ and really like it? This is the story of a customer who recycled his low-trail fork into a new bike designed around that geometry.

I originally built a low-trail replacement fork for Jay’s Rivendell Canti-Romulus, designed with a 60mm offset that produced about 44mm of trail. Soon after receiving this new fork, Jay telephoned to say that he wanted to commission a new frame, incorporating this fork, if possible. He had some fit issues which could be addressed with a made-to-measure frame. He also had a wish list of small details, including a kickstand plate, which would be easy enough to get just right on a custom frame.

No problem. Every frame that I design begins with the fork. There certainly wasn’t anything to prevent Jay’s fork from being recycled, as long as it passed my inspection for damage or misuse.

Jay planned to strip the components from his Romulus to use on this new frame, which would be built with cantilever bosses. Before those rear cantilever bosses had been brazed into place, he sent an e-mail saying that he’d unexpectedly sold the Romulus complete, and would be buying all new components. I pointed out that brake technology now was an open option, and asked if he might like to consider using extra long reach (55-73mm) dual-pivot sidepull caliper brakes. Jay already was familiar with the beautiful “Silver” model from Rivendell, and jumped at the chance to use brakes which would match so nicely with the style of bicycle he envisioned.

But wait … doesn’t the fork have cantilever bosses? It sure did, but those were removed with some very careful hand work with files and sandpaper. No additional heating on the fork blades, and no trace that the canti bosses ever were there. Spoiling the fork’s paint wasn’t a concern since the new frameset was going to be painted in a different color.

So there it is, a second life for a Frame-Neutral Replacement Fork™ and a happy customer with a new bicycle designed to fulfill a dream. That, I propose, is why I do this.

And, finally, a few details, starting with that mounting plate for a fancy Pletscher kickstand.

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.