Motorcycle Classics

A Brief History of Girder Forks

Back when girder forks were status quo, various types were tried and developed. With most following the same basic principals, they were, not surprisingly, very similar in design. The types of girder forks found on motorcycles fall broadly into two main categories: parallelogram and leading link.

Probably the earliest example of the parallelogram type is the Druid, patented posthumously in 1917 by the estate of one Arthur Drew. Druid forks are recognizable as looking like a bicycle fork with a strengthening brace at the front and a couple of coil springs behind. Though some models included rebound springs, there was typically no provision for damping.

Most popular in terms of usage on pre-WWII British motorcycles was the Webb fork, which again used a parallelogram linkage to allow for suspension travel, usually controlled by a large central spring. Advantages over the Druid included the option of a rotary friction damper and tapered coil springs for progressive compression. Whether Mr. Webb ever made any money from his invention is unlikely, as just about every British motorcycle manufacturer built its own fork following the basic Webb design.

The Brampton fork, later fitted to Vincents, was similar in concept to the Webb. Vincent’s own Girdraulic fork used forged alloy blades for extra strength instead of the welded tube construction of the Brampton, and added hydraulic damping.

The leading-link fork fitted to Harley-Davidsons carried the front wheel axle ahead of the spring leg, which slid against a coil spring carried in the main fork. The British Castle fork fitted to the Brough Superior closely followed the Harley design, but added large rotary friction dampers. The “modern” Harley Springer front end is essentially a development of the first leading-link design but with progressive compression and rebound springs.

Though not fitting into either category, Triumph fitted some of their 1920s motorcycles with a girder fork that paired a hinged lower link and a slider at the top, meaning the fork would move backward and forward during compression, altering the steering geometry. My father rode a 500cc Triumph so equipped in the early 1930s and declared it to be an evil handling bike. He traded it for a 250cc BSA — but not before the Triumph had twice pitched him off!

Whatever the style, all girder forks have pivot points and/or sliders that need to be kept well greased. Bushings wear over time and may need to be replaced, especially if maintenance has been neglected. Spindles are made of special steels, and only replacements designed for fork applications should be used.

Regularly inspect fork legs for cracks and signs of rust. The slender tubes used on Webb/Brampton forks are especially prone to rusting from the inside. And before fitting a set of used girder forks, make sure they’re up to the job. Many manufacturers produced forks using different size tubing for different applications. Norton, for example, built a heavyweight fork from tubes that tapered from about 7/8 inch O.D. down to 5/8 inch, and a lightweight fork tapering from about 3/4 inch down to 1/2 inch. MC

  • Published on May 6, 2011
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