The series 1 Moulton fork/swingarm has a reputation for cracking or twisting, and it’s generally agreed that the design was not up to the job when combined with indifferent manufacture. The series 2 rear fork is extremely strong but is also heavier and raises the rear end of the bike (and the saddle) to such an extent that getting a toe down at traffic lights can be impossible. Most people also fine the series 1 fork to be more elegant in appearance.
I believe a well-brazed series 1 rear fork will give perfectly good service as long as the rear pivot is free to move and the bike isn’t abused in any way (some aggressive mounting techniques are associated with rear fork twist). However, a common approach is to have the rear fork strengthened by adding a second shaped steel plate underneath the existing brake mounting plate.
These photos show a typical repair before repainting. I don’t know who did the brazing as I bought the fork ready-strengthened. In engineering terms, strengthening a part by adding pieces doesn’t always work as it can create new stress rises, but this particular repair has a good track record on Moultons. Its main benefit is to create a box section at the root of the blades to reduce flex and therefore reduce the risk of fatigue, rather than increasing ultimate strength.
The repairer has also “topped up” with brass where joints were previously welded, probably at the Kirkby factory. The new metal has been neatly formed but the brazing is crude, and I will improve it by filing the brass smooth and filling the holes. To be fair, the factory brazing and welding on an F-frame is usually pretty appalling too.
The repair is more or less invisible when the fork is fitted to a bike, being on the underside. I have seen some cruder jobs where the extra plate is flat (as it is on a series 2 fork) and is visible from the side of the bike. The extra plate here has been shaped, which is not best for stiffness but does give a neater appearance.
I’m going to have this fork painted but it will be kept as a spare in case the unmodified rear forks on one of my F-frames cracks or twists. The unmodified forks I have are free from cracks and quite well brazed, so I don’t expect them to cause problems. They have also survived many decades in their unmodified form.
Moultons and other endangered species 