Suspension Tuning, By Eric Gorr. Part 2

Eric Gorr

Engine Builder
Jun 29, 1999
Internal and External Adjustments
Suspension dampers can be adjusted two ways, internally and externally. External adjustments are limited to the riding circumstances and the adjustment range on the compression and rebound clickers. Internal adjustments are virtually unlimited because it encompasses revalving and re-porting of the damper piston and valve shim stack.

The external adjusters, low speed compression and rebound, can only affect minor changes in handling. Typical low speed compression or rebound riding situations might include far-spaced shallow whoops, tabletop jumps, braking and accelerating around tight turns.

All compression and rebound clicker adjusters are marked S and H, meaning soft and hard. That can also be interpreted as soft fast and hard stiff. The focus of a professional suspension tuner's work revolves around internal adjustments. When a suspension component is revalved it is also rebuilt, meaning that the bushings and seals are checked for replacement and the oil is changed. Revalving is the discipline or repositioning, removing, or replacing valve shims in such an order as to affect a change in the damper's performance.

Aftermarket Piston Kits
The latest trend is to combine a piston design with a valve shim pack so as to affect a greater change. There are two main types of piston/shim systems. The manufacturers are Race Tech (Gold Valve) and Pro-Action (3-Stage Incremental). The main difference between the two is the port design of the piston.

The Race Tech set up relies on a high flow piston with a large series of shims that can be rearranged in set patterns to adapt to the needs of set number of rider profiles. The Pro-Action set up relies on a piston with smaller ports and a multi-stage shim arrangement that separates the circuits of passive and active to give the damper a wider tuning range. The piston works at the edge of the spectrum and provides a hydraulic lock capability during riding situations where all the suspension travel is used quickly.

From a marketing standpoint, the Race Tech Gold Valve is simple and can be installed by inexperienced technicians. The support provided by Race Tech is excellent. The kits come with detailed instructions, a video is optional, as well as training seminars geared towards amateur race tuners and home-based mechanics. The Pro-Action 3-Stage Incremental valve isn't available over the counter. It can only be installed by a Pro-Action franchise.

The reason is that the valving must be set-up for the individual, and there are a wider variety of valving patterns to suit virtually any rider profile. The Pro-Action approach also relies on matching the proper spring to the valving. Pro-Action's set-up is more expensive than a typical revalving job, but its more comprehensive and produces a truly custom result.

How Incremental Valving Works
The rear shock valve stack is comprised on a series of steel washers with a variety of outer diameters and thickness, mounted on two sides of a piston. This is called a bi-directional valve. One side handles the compression damping and the other handles the rebound damping.

The valve shim stacks have different arrangement patterns because the compression stack aids the spring and the rebound stack controls the stored energy release of the spring. With regards to the sizes of the shims, the larger the diameter and the thinner the thickness, the more easily the shim will bend and increase oil flow through the piston.

The faster the oil flow the less the damping. Stock Japanese dampers use high flow pistons with a complicated series of shims that aren't very sensitive at slow shaft speeds. The shims don't open at slow shaft speeds and mostly the clickers control the damping. However that can cause some potential handling problems when accelerating out of turns. The bike is riding at a point on the rear spring where the clickers don't provide enough damping and the piston valving isn't in the response range, so the bike chatters. The Pro-Action incremental valving concept separates the three main damping phases of low, mid, and high. They do this by using a special piston and a valve stack with transition shims to separate the three circuits. The incremental valve stack is more sensitive at low shaft speeds so the clickers don't have to carry the damping load.

The mid speed valve helps make the transition from low to high speed damping modes to give a plush ride especially under an acceleration load. The piston has smaller ports, which provide a hydraulic lock affect at high shaft speeds. That reduces the load on the nitrogen charged gas bladder and the elastomer foam bumper.

On the front forks, the evolution of design has changed at a greater rate than rear shocks because riding techniques have changed over the years. Riders tend to do more front-end landings so the manufacturers have concentrated on cartridge fork design. Modern forks contain a bi-directional valve on the rebound rod and a mono-directional valve for the passive compression. Pro-Action is the only company that uses a two-stage passive compression circuit with a low flow piston that handles a greater share of the active compression damping. In some cases they even remove the active valve stack from the rebound rod.
Likes: Timmy Leyh