Ohlins High Speed Damping Valves

As part of a discussion about fork rebuilding and fork oil I thot it might be interesting to post a few pictures of a valve set removed from a belt drive superbike. These are from the older 43 mm S forks which have both rebound and compression damping in both legs. Ohlins forks are evolutionary so most of this still should apply. I disassembled one of the rebound dampers. The piston has six 2mm holes which are sealed by the shim stack. When the piston is moving in the direction that damper is intended to control and the pressure in the cartridge is below what is required to unseat the shim stack the holes remain sealed and all damping is provided by the low speed circuit (needles). As the pressure rises the unseated shims are creating a variable bleed. When the rebound valve is running opposite (compression stroke) to its intended control a single shim on the opposite side of the piston which is controlled by that coil spring unseats to open the three channels on the sides of the piston to ensure there is no damping contribution. This compression valve is designed pretty similarly. The compression valve just sits at the bottom of the cartridge. The compression valves I have installed in my forks are from the superbike revalve kit and are a completely different design. One of the issues with the compression valve in the picture is when the pressure rises too fast in the cartridge (like a step between slabs) the valve cannot pass enough fluid fast enough for it to dampen (you feel it). The comp valve from the revalve kit is a kind of dual step design where the entire valve unseats using an annular space between the piston and the cartridge to handle the pressure pulse. The downside is the piston is way shorter and the valve life is shorter. It is way better on choppy pavement and should be better at controlling chatter. The rebound damper in the revalve kit is essentially the same as the one in the picture but the shim stack is different.

The low speed (needles) damping is active all the time. The high speed damping only acts when the pressure in the cartridge exceeds the spring force being exerted by the shim stack. The shim stacks are engineered to act in a range consistent with the springs which are being used. Say you put in a 13 spring. The standard shim stack my not handle the rebound requirements so you can reshim to move the damping into range. Changing to a higher viscosity oil increases damping because the holes thru the valve are constant and the shim stack load is constant so its harder to pump a thicker fluid thru the holes. Downside is the fork gets harsher (particularly on the compression side) and less responsive because the pressure in the cartridge is higher. Over sharp edge bumps a thicker oil just makes it worse. I added the FG 43 service guide for reference.