7/18/2017 5:09 PM
Edited Date/Time: 7/18/2017 5:13 PM
All modern forks utilize the same basic two systems to produce damping, the base valve and mid valve. Although they both work to generate damping forces, albeit differently, the application of those forces is slightly different and in my opinion this results in a different feel to the rider.
I’d describe mid valve damping as having a more firm/sticky feeling which tends to resist small amplitude or high frequency bumps but allows the forks to move on lower frequency higher energy bumps using a minimum of travel. In general the bike will use less travel, stay higher in the travel, stay more level (less dynamic pitching), and react faster but will transmit higher forces onto the rider. This damping favors control over comfort.
Due to hysteresis I’d describe base valve damping as having a more spongy or springy feeling with the forks more willing to freely move in response to the terrain (small bumps and ripples). In general the bike will use more of its travel, will rock forward and rearward easier (pitch), and react slower due to all this movement thus losing some feedback to the rider. However, the suspension will absorb better, and will transmit lower forces onto the rider. This damping favors comfort over control.
The mid valve bleed defines the starting point at which the two systems start to blend. The starting point being defined as a specific fork speed which is dictated primarily by the shape of the bump and secondarily by the speed of the motorcycle. Above this rate and the mid valve shim stack begins to act and restrict flow creating damping along with the base valve. Below this rate the base valve is doing all the damping work. An analogy with two stroke power band can be used. Think of the bleed as a tool which defines at what RPM (fork speed) the engine will come on the pipe. Think of the mid valve shim stack stiffness (resistance) as the tool which defines how hard (damping) the engine will hit once it comes on the pipe.
In general, the damping of a fork with a relatively large rod piston and a small rod will be overwhelmingly biased to mid valve type characteristics. The magnitude of this overall feeling being determined by the mid valve shim stack stiffness. Tuning can reduce the influence but never change it. A kit forks tend to be designed with mid valve bias dimensions.
In general, the damping of a fork with a relatively large rod and a small piston will be overwhelmingly biased to base valve type characteristics. The magnitude of this feeling is determined by the base valve shim stack stiffness. Tuning can reduce the influence but never change it. Forks purposed for normal off road riding tend to be designed with base valve bias dimensions.
To a degree the two systems are opposite ends of the spectrum in terms of how they’ll feel to the rider with the system being a zero sum game. In other words, increasing one usually means a decrease in the other. The key to making any modern fork damping system work is to:
1) Chose the fork which dimensionally is bias towards the type of riding to be done.
2) Properly blend the two systems (they are never fully decoupled as one affects the other) so that the rider feels the right combination of comfort and control.
This will vary from rider to rider, bike to bike, fork to fork, track to track, so there is no one solution. Even within the constraint of one rider/bike/track it’s going to be a compromise.
Retired Mechanical Engineer, published technical writer, mscperformance.com, Bisimotoengineering.com,
