Bicycle forks… how hard can they be to make…
Well, there is a lot more to forks than you would probably think; they do after all have a big affect on a bike’s stability and how a bike handles and rides. I knew all of this theory or ‘science’ before the course and wondered how that theory would translate into the practicalities of building forks.
There are 4 main components to forks, the steerer, the fork crown, the fork blades and the front drop outs. These were all waiting for me at the start of day 3.
Before I could start to get busy with these, I still had the frame seat stays to complete and fasten to the frame which was waiting in the jig.
There are no jigs for attaching the seat stay caps to the stays but Dave is full of tips and techniques to help this happen. Once the stays were brazed against the seat lug, the stays were tacked to the dropouts and the whole thing was left to cool – it could then come out of the jig!
I have a lovely 2nut Chris King headset which is for a 1″ threaded steerer, so that is what I wanted to build. Dave doesn’t use many machines in his process, but cutting fork threads is one thing he does on a lathe. Cutting a British 1″ x 24tpi thread is effortless and produces a perfect result. I was also now a dab hand at slotting and shaping the ends of stays and blades so I finished the slots for the dropouts with no effort at all.
Brazing the dropouts to the blades was exactly the same as the rear stays and dropouts, same process, same method. It’s actually a little scary to know that my safety on this bike depends a lot on how well I braze these forks, especially the crown and steerer joint. The steerer is a large chuck of steel, so too is the crown – both absorb and take lots of heat from a hot flame to get them to temperature but it is an easy joint to see that the brass has penetrated and flowed through the entire joint.
Like most other things, these now needed to cool, so it was back to the frame. The excess seat tube and head tube could be cut off (with no great accuracy) so that the tubes could later be filed and faced.
So this is the first thing about forks; as simple as it sounds, the hole for the brake caliper needs to be in the middle of the crown – my crown didn’t have a hole so I needed to drill one. Even the hole has 3 diameters, 6 mm at the front and 8 mm at the rear with a 10 mm/2 mm countersink. A Sharpie pen was used as a substitute for Engineer’s Blue and with some accurate measurements using a surface table, I got the centre of the crown marked and punched ready for the drilling. Dave also machined the crown race seat to 26.4 mm which is the standard for 1″ headsets.
Now this is where you need to measure and look to your frame design before you can fit the blades to the crown. My forks need to accommodate a certain size tyre, fit a specific length brake caliper and have enough clearance for mudguards. Not only that, they must also provide my design with the correct rake to combine with my chosen head tube angle to give a fork trail that will handle well. Dave’s experience means he knows all these measurements in his head and has jigs set up ready.
Rake is the distance or ‘offset’ of the centre of the hub axle in front of the steering axis. For my bike to handle well, Dave wanted a rake of 45 mm – my wheel would be held 45 mm in front of the centre of the steerer. My blades come with a curve pre-formed into them. Some blades will come straight. For me to get my rake set, I needed to add more bend to the blades. Naturally, Dave has the perfect ‘tools’ to achieve this. Some careful placement and brute force together with accurate hack sawing get the results we needed, the right length and bend to the blades.
A dry ‘test’ fit into the fork jig to check alignment and the blades were brazed into the crown.
At the end of day 3 I had a set of bicycle forks on my workbench with a set or tubes brazed together that resembled a bicycle frame.
It was the first point in the course where I thought I could actually end the week with a complete frame and forks. Dave’s confidence seemed to be correct and we were on track. Tomorrow, day 4, would see some more brazing to the rear dropouts, some frame checks and alignment and adding those all important braze-ons such as cable guides and bottle bosses.
This was definitely enough to justify having a celebratory pint of beer.