The Girder Fork

With the exception of a small minority of mountain bike suspension forks (Lauf being an obvious example), nearly everything on sale today relies on telescopic legs to enable the front wheel to travel up and down. But it hasn’t always been this way. One such alternative, known as a Girder Fork in the world of motorbikes, takes its name from the classic structural shape of the girder truss used primarily in bridge or roof construction, which was commonly used to create the fork legs. Back in the 1930s this was a popular choice for motorbike manufacturers, with some still continuing to develop variants of this approach, such as BMW's Duolever arrangement.
 
When adapted for use with mountain bikes these forks are more commonly referred to as linkage forks, which is probably a more useful description. The most famous example that I’m aware of was seen gracing the front of many Proflex bikes back in the 90’s, going under various names that included Noleen, Girvin and K2. This probably goes a long way to help explain my subsequent fascination with their design, having lusted after them as a teenager. But other examples have been produced by the likes of Amp Research, Look and Leonardi Racing.
 
Linkage forks have come in various different arrangements over the years. Some have both linkages below the head tube and the shock mounted somewhere between them (such as the Amp Research fork). While this arrangement improves frame compatibility (as the fork isn’t limited to a minimum headtube length), this layout generally has to rely on pretty high shock leverage ratios to achieve anything approaching a useful amount of travel on a mountain bike. However, forks like the Leonardi Racing Alike partly get around this by placing the shock above the linkages. Even so, having both upper and lower linkages below the head tube also limits the forks structural efficiency. For this reason, designs like those found gracing Proflexes all those years ago have the potential to be much stiffer (stiffer even than conventional, single crown telescopic suspension forks) by being supported both below and above the headtube.
 
Furthermore, linkage forks can be designed in such a way that dive under breaking is significantly reduced, enabling them to use their travel far more efficiently. This has further benefits, such as reducing the need for as much low speed compression damping, enhancing the forks potential sensitivity. Linkage forks also side step the issue of binding, something that becomes increasingly important for telescopic forks as head angles get slacker. This occurs when the entire fork tends to flex along its length rather compress due to friction between the forks lower and upper stanchions.
 
It is because a linkage fork pivots on bearings that they can be designed to be much more sensitive than a telescopic fork. As a slight segue, my perception is that most mountain bikers who have an opinion on the matter, would always view cartridge bearings as superior to bushings, regardless of the application on their bike. Somehow, cartridge bearings have simply become known as ‘the best’ with bushings normally thought of as something seen on cheaper bikes. But this over-simplifies the situation. While cartridge bearings have a number of positive attributes, these tend to only apply when they spin through full rotations, something that, for example, rear suspension pivots just don’t do. Instead, some rear suspension pivots would be lucky to rotate through just a handful of degrees under normal operation. It is not uncommon for the balls within a cartridge bearing to slide momentarily before they start to rotate within their races. In something like a hub this isn’t an issue, but in a rear suspension pivot these balls may never do anything other than slide backwards and forwards. Not only will this result in the balls ending up with flattened surfaces resulting in premature wear, they also introduce undue friction making suspension less sensitive. Bushings avoid these issues, but the mountain bikers blind lust for cartridge bearings seems unassailable.
 
Anyhow, given that I’m building my own frame in the New Year, I thought that it might be fun to see if I could design a linkage fork to fit it. Now, this is just me humouring myself at this stage – the bike will be fully rigid, but I like to have something to play around with in my head. I’ve designed the fork around a standard rear shock (200 x 51mm) to produce just over 100mm of travel. The drawing below illustrates what it might look like. My plan is to utilise the long, damp autumn evenings to build a full-size mock-up of the fork out of wood to see if it will actually fit the bike and cycle through its travel in reality. It’ll be pretty rough, but enough to hopefully prove that the idea is viable. Fingers crossed I’ll have something tangible to show for my efforts in the not-too-distant future.