I am not looking to copy the Landsdowne as for not much more I plan to have modern shim packs included which give difressive damping.
This is where I currently am on the upgrades, started last November but doubt this will be properly finished until July/Aug as road testing is needed to adjust the set ups to suit Brit bikes. If there is enough interest I can have these made in kit form, but got to get them working first.
I am working on 2 schemes, one for Pre OIF 64 to 70 Triumphs and 65 to 70 BSA's and another for OIF. The Pre OIF scheme is easily converted to cover Norton forks but will need springs added as the current springs will be too long. I have a set in progress for a Tiger Cub that uses Norton fork sliders which will be used for the design of the bottom pieces.
The Pre OIF is ready for fitting now, its downscaled modern Cartridge, with adjustable low speed compression and rebound damping, they use shims so the damping rate decreases as the fork speed increase so when you hit a bump you do not get a shock. The smallest modern ones are 24mm OD and do not fit pre OIF without the bottom bushes falling off (as you have to bore clearance to allow oil to pass without creating second unwanted damping). Running with no bottom bush is pointless
, so I scaled all the parts down to fit the bottom nuts with the thread still supported. They are a downsized version of the CBR600RR cartidges 2007 to 2012 so only miss out on the current fad for Big Piston Front Fork.
When I went to carry this over to the OIF I hit an issue, I need a 3mm hole to be drilled in the bottom bolt for access to compression damping adjustment, this is the bolt that on pre OIF is not critical to hold the forks together as they have seal holders. OIF have no bottom bush and so if your front wheel leaves the ground if these bolts shear the wheel falls off. Also the spring is internal so that complicates things.
See here for the issue
https://www.youtube.com/watch?v=ZfVUVx9P_CM
So my OIF scheme has the shim damper valves running directly inside the Stanchion sitting on top of a replacement damper tube, the bottom bolt will be unchanged and the damper adjustment will be in from the fork top. The Valve will have a PTFE band on the OD as the pre OIF version to get rid of stiction, this provides the seal to direct all the oil displaced by the fork movement through the centre hole for low speed movements and then as the going gets rougher through the shims for reduced damping. There were 2 possible variations and I have decided on scheme 1. If after Gavin's testing there are issues then I can revert to 2 if it can cure these issues.
1. Separate rebound and compression valves, so one rebound valve in the left leg and a compression valve in the right leg. Both adjusted for slow fork deflection speeds from the top using telescopic adjusters. (perfectly normal to have these in separate legs, lots of modern bike do this)
2. A dual compression/rebound valve in both legs, the adjuster will affect the slow fork movement rebound and compression equally but there will be a shuttle valve to make the base values different. The latest Hagon rear dampers do this single adjust affecting both rebound and compression.
I am going with version 1, its simpler and gives more control to the rider. Then you can vary the mid and high speed damping by changing the thickness, number or diameter of the shims.
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
Rebound valve
Compression valve
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 by Marzocchi . 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.
So early days yet to find out if it works and how well it can be improved.
