Dowty Oleomatic forks
They are an “upside-down” design with the outer legs clamped in the yokes, and with smaller diameter sliders – very modern!
The sliders carry a piston at the top with a seal working on the inner surface of the outer legs. With the legs filled to the correct level there is a short column of oil above the piston and a small volume of air above that.
I agree with Keith about the appeal of using an air suspended fork, with its ideal progressive action, from 60 plus years ago – provided you can make it work as designed. I have been down a long road in an effort to reach this desirable conclusion, so far without success. When acquired, my ’49 FS – off the road for 30 plus years – would not hold air at all. We polished the sealing surface and fitted a new set of seals ( cost £65 ) with no improvement. I concluded it would have to be a spring conversion. I bought one from Gerry Howard which I believe was based on the Brum design. It worked after a fashion but bottomed out on potholes far too readily and the plain spring was not progressive. Very disappointing. The damping method was unchanged from the rather primitive Dowty original.
Later I discovered that the top slider bearing on one side was wearing unevenly and this could only be due to a bent outer leg. I had this sorted out by Abbarepair and optimistically returned to the air system. At first my static test looked good but it only flattered to deceive. Oil running down the legs again.
I consulted John Bould of Lansdowne Engineering, who makes hydraulic dampers of a superior design for classic Nortons etc, for help. He designed and built a modified spring suspension, firmer and with a degree of progressive action, and put in his adjustable dampers. A bit OTT for a Scott I have to agree!
After an adjustment to the spring rate, initially too firm, it now works far better and saw its first outing at Abbotsholme this year. At one point we hit a nasty little bump or pothole that I failed to spot on an undulating but rather nice straight stretch, when doing about 60mph. The bike took off and bounced me out of the saddle, causing a bad fright, but it came down again safely in a straight line. Point is, even though it was a severe test the forks did not bottom.
I would still like to make the air suspension work, just for the satisfaction, especially with the modern damping, but I’ve had enough hard work for the time being. The next step would have to be a honing job on the inside sealing surface of the outers ( I understand Eddie has such a hone ) and maybe investigate a modern garter seal design to substitute for the simple Dowty cup.