Girder Fork - Update

​Following on from my November blog post, where I set-out my plans to make a full-size model of a mountain bike girder / linkage suspension fork as well as listing some of the supposed benefits of this design, I’ve been spending a number of long nights in the shed trying to turn my CAD plan into a reality. Well, I've now finished it and thought that I'd present the results of my labour.
 
To try to help explain how the fork works I have colour coded the drawing below. Those parts shaded blue would be attached 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.

After trying to make the fork out of plywood (which kept ‘chipping’ around the pivots) I ended up making it out of MDF. My coping saw skills leave a lot to be desired, but this was never meant to be a thing of beauty. Rather, a practical test of an idea. For that it has been really helpful.

The fork is designed to work specifically with the headtube length of the Full-Moto frame that I will be making in February, but with some relatively simple alterations to the top shock mount and the girders / fork legs the fork can be adjusted relatively easily to fit any frame with a headtube length down to 125mm. Start playing around with the linkages and/or shock length and this could be reduced further. So, this general layout can be made to work with a wide range of frames, but, unlike a more conventional single crown telescopic fork, it will always need to be tailored for a bikes headtube length. There is some wiggle room if you are happy to accept a slightly altered axle path, but I suspect that this is a big reason why this type of fork has not had the commercial success of its telescopic cousins. Essentially, if you wanted one then it would need to be customised (to a greater or lesser extent) to your specific frame.
 
My 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’’ steerer). The nature of a linkage fork means that its offset varies through its travel with a minimum of 38mm and a maximum of 50mm. It 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 inherent tune-ability of an air shocks spring rate 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 after some tuning to give a decent wheel rate.
 
Although I have no immediate plans to make a fully functioning version of the fork, I’m hoping that the brazing and fabricating skills that I will gain on my frame building course in February will at least make it a possibility. While I’m not aware of any other steel mountain bike full suspension fork, I still think that it can be made to work without being too heavy – think of a Jeff Jones truss fork with some pivots, linkages and a shock thrown in! Alternatively, I could try to find a friendly CNC machinist who can convert my CAD files into shiny pieces of aluminium. Or maybe even try to make the girders out of carbon fibre…

Anyhow, really I just wanted to prove to myself that it could be done and that there wasn't some practical reason (other than the aforementioned adjustments required to accommodate different headtube lengths) why this style of fork hasn't ever really challenged telescopics for sales. Plus it was pretty good fun making it! Let me know what you think.