Most people think that a girder front end is for looks only, and does not help in making the bike handle. Not so with this one!
When selecting a custom front end for your next project, there are three main styles to choose from, the Glide or Hydraulic, the Springer, and the Girder. These three are the most commonly used by custom bike builders, because they are proven designs offering good looks and performance for road handling. The glide is currently the most popular, to the point of being boring and is sometimes thought to be the best performer of the three. This may or may not be true because if designed properly, a Girder offers some benefits that a Glide or Springer can't -- yet gives outstanding performance and great looks, which is what most bike builders seek in the first place.
The Durfee Girder is actually based on the girders used on the Indian motorcycles of the 1930's and 40's which were a good design, being rugged enough to be used by the U.S. Army while out chasing Rommell and gang in the Sahara Desert during World War II. Imagine extreme heat, no roads, blowing dust and sand, hills, valleys and craters that could swallow a tank and you start to get the picture of how rugged these front ends were and how good they can be today with advances in materials and processes unknown 70 years ago. Many others besides Indian used the girder design, in fact a lot of the other American and most of the European motorcycle manufacturers of that period used girders in their production motorcycles.
The Durfee Girder like all true girders gets a lot of its strength from its triangular design. It has been proven that the triangle is one of the strongest structures for support as can be evidenced by looking at almost any bridge, rail road trestle or skyscraper frame. The triangle in the Durfee Girder is incorporated into the design by using two legs, with the rear one bent. A long triangle is then formed from the axle hole to the triple trees. The triangle is then divided into two smaller triangles with the addition of the cross member at the bend of the rear leg.
We use 4130 chrome-moly, seamless tubing on the round girder because chrome-moly is one of the toughest metals available for this kind of job, it is normally used in making airframes or race car chassis. Chrome moly is best welded using TIG (also known as heli-arc) for proper joining of the parts
The triple combination of triangular design, TIG welded, chrome-moly tubing and proper mating of the parts before they are welded together add up to benefits a glide or springer cannot match
An additional benefit of the girder design is that the length of the forks and the rake in the forks or frame does not determine how well a girder works -- the pivot points still pivot, and the spring compresses and rebounds as it should. In the glide design and to a lesser extent in the springer, your frame rake angle and front end length dictates how well the "suspension" part of the design works. Changes in rake and length from stock dimensions and geometry can diminish suspension action commensurate with the amount of change. The telescoping feature of the glide and the rocker arm in springers was designed to work in an up and down motion. As you increase the angle (as in additional rake) you have less and less "suspension" and more bending of the legs. In long front ends with extreme rakes, your glide or springer morphs from telescope or springer actions into a suspension action that only exists from the flexing of the legs. The girder, especially ours, because of the materials used, fit, and expert welding makes it all but impossible to bend or flex the legs. The triangles mentioned above and the fact that a bunch of German Rocket Scientists (seriously, just see their quote below) said that it would take 58,000 pounds of force to start to bend one of our legs.
Beyond being a superior design, our girder is light weight. A complete, ready to bolt to your next custom Durfee Girder weighs less than 29 pounds in stock length. A "billet" glide can easily weigh twice that amount. But don't let the girder's light weight mislead you, because it possesses extreme strength for the weight that no glide or springer can duplicate.
"We can safely conclude that the 4130 girder of the Durfee design has no obvious deficiencies and has a considerable safety margin for all normal riding. For really severe abuse it will fail in a relatively safe manner. The same cannot be said of the same design that would be made exclusively out of (mild steel) 1020. Both fatigue problems and dangerous modes of failure would be present in such a fork. If you want to put yourself in the hands of shady operators then either material can be extremely dangerous. We have found no evidence of cheap shortcuts in the Durfee Girder."
--Otto & Sichling Inc.
To help the front end hold up under many miles of wear, we don't use bushings! No bushings at all. This sounds like the wrong way to go, but after some extensive research, we found that the needle bearings or bushings would wear unevenly, creating egg-shaped holes in the frame cross tubes. In place of the bushings, our pieces use some metallurgy science along with precision cut and machined parts that ride on each other's surfaces. Six grease fittings are used to keep everything properly lubricated. Now, thirty+ years later, we are hearing from you guys (about one or two a week) who are still riding our originals! This is evidence that the way we do our bushing-less pivot points was and is a good idea.
This is the mechanical end of the girder, but the real trick part lies in how it absorbs the shock for the front end of the motorcycle. The Durfee "Classic" girder uses a single coiled spring at the top of the fork legs. The spring oscillates with the pivot motion of the two legs, which share two common cross members. The secret to the good handling of the front end is the barrel spring. By barrel, we mean that it is shaped like a barrel: large diameter in the middle and narrows towards the top and bottom. This gives it a two-way, multi-stage, progressive wind, helping to prevent any pogoing of the spring under shock absorbing conditions. This effect is a result of non-linear dynamics, which is physics beyond the scope of this article, but it all boils down to the fact that the different diameters of the coils in the spring acts as a team. When one section is constricting, the rest is preparing to do the same when asked to. If compression is released, then the spring will go back to its original state, with no effect to the rest of the spring. In the end, the entire spring will be working in harmony with, instead of against, itself.
The Durfee Girder may not be a new design in motorcycle suspensions, but it is a refined system, combining old and new methods to make a strong and lasting front end. However, although we have a tried and true product that is easy on the eyes, we are not resting on our laurels. Durfee Enterprises was founded on innovation and we'll keep looking for new ways to make our products even better, stronger and more distinctive. We're just glad you've let us know we've got a pretty good thing going.