Girder Fork
With the exception of a small minority of mountain bike suspension forks 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, commonly used 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 a small number of companies, including BMW, continuing to develop non-telescopic forks today.
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 like those seen on Proflexes have the potential to be stiffer than conventional, single crown telescopic suspension forks by being supported both below and above the headtube. They also side step the issue of binding, an issue 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 than compress due to friction between the forks lower and upper stanchions.
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. One benefit of this is that they potentially require less low speed compression damping, enhancing the forks sensitivity. But the bit that really interests me is their ability to minimise the change in a bikes geometry. Given that the primary driver behind the geometry of my Full-Moto frame has been to place the rider in a more stable body position (relative to most conventional mountain bikes), being able to avoid steepening the head angle and lowering the handlebars under heavy braking clearly compliments what I am trying to achieve.
So, I thought I’d see if I could build a full-sized mock-up of a linkage fork to begin testing how viable the idea is. To try to help explain how the fork works I have coloured coded the drawing below. The parts shaded blue are fixed to the frame (steerer tube as well as top and bottom fork crowns), those in green are the girders that move with the front wheel, while those in orange are the linkages that control the movement of the green elements relative to the blue elements as well as drive the shock.
The fork pivots on four 12mm axles that rotate within eight nylon bushings. It has been made to accommodate a 110mm Boost front hub with a 15mm axle and uses a 28mm diameter wooden dowel as a proxy for a steerer tube (the closest that I could get to a 1-1/8’’). The fork has a little over 100mm of travel controlled by a 200 x 51mm RockShox Monarch RT air shock that I bought second hand.
Just like many single pivot rear suspension layouts, the forks leverage ratio starts off being progressive (rising rate) for around the first two thirds of its travel, before going regressive (falling rate) at the end. Given the tune-ability of air shocks and the fact that they naturally ramp up towards the end of their travel I’m fairly confident that the fork and shock can perform happily together with some tuning.
When time and finances allow, I will attempt to make a fully-functioning prototype.
Just like many single pivot rear suspension layouts, the forks leverage ratio starts off being progressive (rising rate) for around the first two thirds of its travel, before going regressive (falling rate) at the end. Given the tune-ability of air shocks and the fact that they naturally ramp up towards the end of their travel I’m fairly confident that the fork and shock can perform happily together with some tuning.
When time and finances allow, I will attempt to make a fully-functioning prototype.
