How To: Druid fork links
Druid forks were used by the vast majority of bike makers in the UK from the early 19teens through 1930ish. They were a straightforward way to add an inch or two of suspension to the front of a motorbicycle, with a simple and robust design. They were offered starting in 1907, first as a retrofit to early rigid bikes, then sold as original equipment by the bike manufacturers. Here is a patent drawing dated April 24, 1917. While the patent covers several different design iterations, the one that was commonly used can be seen in the photo of the Veloce. Many manufacturers chose to fabricate their own forks rather than buy them outright, and paid licensing fees to do so. Thus you can now find many different flavors of Druids, but all use the same basic design and geometry, with each maker choosing ways to fabricate the lugs, or the size of the tubing, length of springs, etc.
Druid forks work very much like the later styles of girder forks made by Webb or Brampton. A girder carrying the wheel moves up and down, being tied to the frame with linkages and springs. Druids had the springs below the links, while Webbs, etc had the springs up higher. The links can be seen to be roughly horizontal, with the uppers slightly shorter in their length from hole-to-hole. These links, coupled with the frame and the girder, form a 4 Bar Linkage. ‘4 Bars’ are a basic type of linkage, with just four pivot points and four bodies. (the forks have double this number, as there are really two sets of 4 bars, one on each side of the forks). The designer can vary the length of the upper and lower links and and distance between them, which will force the front wheel to move vertically, or in a slightly inclined plane, or even in a figure 8. I enjoyed a year at University learning the ins and outs of such linkages and writing software programs to synthesize them. good times.
Our project bike came with a complete front end, however the Druid links were severely worn, and also bent in numerous places. Each end of the links is mounted onto a 1/2” spindle with 3/8” threads that pivots up and down with each bump in the road. If the clamping nuts are allowed to work loose, or if lubrication is lacking, the spindles and the links can wear each other out in short order. The holes in the links can be seen to no longer be round, some had grown to be ovals that were 1.5 times longer than their diameter!
Shown here are some of the old links and the raw material for the new links. I used chrome-moly steel, this particular alloy is 4142. It is a great steel for this application, strong and tough, but not brittle. The old links were probably salvageable, with straightening, then building back up the holes with weld and re-machining. But I know that the new ones will be stronger than the originals could every be. As there were no maker’s marks on the old links to replicate, once the steel is cut to size, deburred and nickel plated, they will be identical to the originals in all aspects except in their steel alloy. Don’t worry though, the old ones will be saved.
There were several methods available to fabricate new links:
Forging, CNC lasercut, CNC waterjet cut, CNC milling, conventional milling, or hacksaw and grinder. I like to do these project myself, so I skipped the CNC methods, I could never afford forging tools for two parts, and went with conventional milling operations. Once the parts were cut to rough length from the cro-mo bar, I drilled and reamed the holes to size at .375”
But I needed to do similar work for the footpeg mounts, and these cutters are not cheap. Luckily I had one more option; the rotary table on the mill. By mounting parts to the rotating table, then moving the table’s position relative to a standard end mill, I could create any radius required.
To locate the parts at the center of the rotary table, I turned a round bar to fit tightly into the center hole of the rotary table with a .3745” portion rising up for the links to fit onto. Then the table just needed to be indexed toward the cutter one step at a time. I used .020” depth of cut per pass. Then the table was rotated the full amount with the cutter spinning, and the table was fed .020” again toward the cutter and the table rotated back in the other direction. Repeat as needed for 20 minutes or so…
The parts were milled to width first, to give the cutter clearance during the rotation cut. I stacked two parts so that the machining time was halved, but I made sure to put a spacer under the bottom workpiece so that the cutter stayed clear of the rotary table top.
A note on the clamping: Be sure to clamp your parts well. I always use at least two clamps, but I removed one of them in these photos for clarity. Never clamp across two parts, clamp them in a stack (as shown above there are two parts in the stack). Use the nose of the clamp on the part. Keep the clamping stud near the nose of the clamp, not back at the foot (by the serrated adjustable teeth). That gives the clamp better leverage and minimizes flexing of the clamp setup.
here Here are the parts right off the mill. They still need deburring, cleaning, polishing and nickel plating. It is tough to see from this angle, but the two on the left are the short links that go on the top of the forks, the two the right are the longer links which mount lower. The single link at the bottom of the photo is one of the old ones, with the worn out holes.
Now the forks need new spindles, new bushings, new steering head bearings and maybe some straightening. Stay tuned.