This is the write up on the Fork inserts I did for Gavins forks.
When looking at adapting modern cartridge dampers to our old clunkers I used the CBR600RR cartridges and down scaled them for the BSA 65 to 70 rod damper and the Triumph 63 to 70 forks. These are in prototype production and one of 3 sets is complete ready to fit. When I looked to carry these across to the OIF on the face of it this should have been much easier, the tubes are bigger so no need for down scaling. But the issue that came up was the method the forks use to hold them onto the bike. Pre OIF there are substantial fork seal holders which stop the forks from falling off over jumps, on the OIF it just has the 2 bottom bolts which retain the damper rods, lose these and then next time the front wheel leaves the deck the forks fall off. You can wheelie only for so long and then 2 stanchions hit the tarmac or earth.
You can see the problem here in this breakdown of the fork assembly.https://www.youtube.com/watch?v=ZfVUVx9P_CM
As this bottom bolt needs to be drilled for access to the compression needle the bolt would be made weaker so another path was sought.
This is the solution which will be tested on Gavin's bike, the valves are increased in size so they run not in a cartridge but inside the stanchion itself, they are also separated so the compression is in one leg and compression in the other. This allows all the adjustment to be made from both top fork nuts so the retaining bolt stays exactly as OEM or stronger (12.9 grade allen bolt).
The first build is now complete and this is what it looks like.
So how does it work. The oil flows in each leg have 2 circuits, a low speed one which flows through a hole drilled up through the centre of the valve rod, the rate of flow is controlled by a needle adjusted by the adjuster on the top of the fork nut. So the rider can control the damping he experiences when the forks are moving slowly eg on smooth roads. This is when you want higher damping forces and this includes when braking so it provides anti dive compared to the std setup.
When the road surface becomes rougher and the forks start moving faster the mid and high speed circuit comes into play, here oil is forced through holes in the valve and this oil presses against a stack of thin steel shims lifting them at the edge to allow oil to flow. The faster the forks move the more oil flows and the more bent the shims become so the bigger bumps do not increase the damping exponentially as happens in fixed hole damping. This is called Digressive Damping and the OEM damping was fixed orifice damping.
Comparison of damping types
Valve without shims and reverse flow washer
There are 6 routes through the valve, 4 for the shim stack route and 2 for the reverse flow which is a simple lightly sprung washer so it does not create any damping effect in the reverse direction as it lifts immediately.
The outside of the valves are sealed against the ID of the stanchion by low friction PTFE sealing rings, this directs the oil through the 2 circuits instead of bleeding off around the valves. This should be an improvement of the OEM O ring which can give stiction.
To get the compression bump stop back the valve rod is tapered towards the bottom to trap oil on full compression to give a ramp up instead of metal to metal contact and the top out spring is retained. The spacer above the spring is optional and to limit fork movement if the builder prefers a more normal 5" of travel instead of the OIF 6.5".
The Fork Nuts have the slow speed adjuster screws in the top and I took the opportunity to add pre load adjustment as it was on the internals I used for the valves.
As the adjuster needles for the slow circuit are fixed relative to the legs and not the fork nuts as is the normal method a telescopic adjuster rod was included to cope with the suspension movement.
The concept of splitting the compression and rebound damping between 2 fork legs is not new, its been used successfully in the past. Also the use of a valve outside a cartridge and using the stanchion as the effective cartridge is not new either, it is was introduced onto the CBR100RR in 2013 and called the Big Piston. What is new is fitting internals from 30mm cartridges directly into stanchions, so this is a much more speculative build than the scaled down CBR600RR cartridge. It will take most if not all of the summer to test, the shim stacks will need refining, oil viscosity and oil levels.