Suspension Tuning, By Eric Gorr. Part 3

Eric Gorr

Engine Builder
Jun 29, 1999
The Shock Dyno
A shock dyno is a computer controlled electro-hydraulic machine that simulates and measures the damping characteristics of dampers (rear shock or front fork). A shock dyno is comprised of an electric motor, a hydraulic ram, a mounting guide, and a load cell (pressure transducer). A shock dyno quantifies how much resistance (force in lbs.) the damper produces at different shaft speeds (velocity in inches/second) and stroke lengths (displacement/travel in inches).

The load cell is connected to a PC program that plots the damping of the compression and rebound over a range of shaft speeds. The two basic types of graphic plots that a shock dyno provides is force vs. velocity and force vs. displacement. There is an optimum profile for these plots, so a suspension technician can use the results of the plots to see if there is an obvious problem with a shock or fork.

The force vs. displacement plots can show how smoothly valve shims are opening, if there is air trapped in the shock, the condition of the seals and bushings, and the condition of the oil with regards to fading over time. The shock dyno can also test the condition of the adjusters, the gas bladder, and the bearing on the top mount. In autosports it's routine to test shocks before and after servicing. It enables the professional suspension technician to test and verify his work.

Suspension Tuning for Uniquely Sized People
The one thing that most frustrates me about dirt bikes is that they're all the same size. Us riders are made in all different sizes but dirt bikes are made for skinny guys who are 5'10". Over the years my web site has been posed questions from every shape of dirt biker imaginable. These questions can be divided into two categories; too big, or not tall enough to touch the ground. Here is some set-up tips from some experimentation on real dirt bikers.

Shortening Suspension for Rider Comfort
Ever wonder why dirt bikes are built with 12 inches of travel when 4 inches of it is sag? Why don't the manufacturers offer an option of providing a kit to give a bike 8 good inches of travel? Perhaps they will after they read this book! Suspension tuners have the capability of modifying forks and shocks for shortened lengths. FMF's contractive suspension was basically a suspension whereby springs were fitted to the rebound side of the shock shaft or fork damper rods.

When the bike topped out its suspension it would rely on the springs to contract to the point where the bike normally sags to (8 inches). The magazine tested some prototype bikes are raved about the handling through turns but criticized the bike for it's vulnerability at getting grounded in deep ruts.

A suspension tuner outside Chicago, Jeremy Wilkey of MX-Tech ( ) specializes in all sorts of suspension tuning but especially shortening suspension for DTX (dirt track) or other off-road applications. When performing such modifications many factors have to be considered. When you make one change to a suspension component it affects several other things.

A suspension component can be shortened a few different ways. For a rear shock a spacer can be turned on a lathe and fitted between the rebound stop washer and the seal pack. A one-inch long spacer will shorten the rear travel by four inches because of the linkage system. Most shocks have enough threads on the shock body to accommodate adjustment of the spring. If they don't then the spring must be shortened. When a spring is shortened it becomes effectively stiffer.

When the spring is stiffer the rebound damping must be increased to compensate for the additional potential stored energy of the stiffer spring. Generally speaking if you are a heavy person, shortening the travel will adjust the spring to your weight but the shock will still need to be revalved. If you are a lightweight person you may need to switch to a progressive shock spring.

Obviously by changing the ride height of the bike the rising rate of the linkage system is going to be narrowed. One product that is available to adjust the linkage ratio is the Devol Link. The mounts of this product are fitted with adjustable lugs. For modifications to the front forks, it may be possible to just shorten the spring and place the cut-off section of spring on the rebound rod in place of the top-out spring. I did that on my XR600 to make contractive suspension.

Some types of cartridge forks don't have the space for a spring but a plastic or elastomer foam rubber spacer can be made to shorten the fork travel. Like the rear shock the spring rate is the biggest factor. You don't have to shorten the front travel as much as the rear because you can still adjust the forks at the triple clamps. Normally a bike with shortened travel will be better suited for low speed riding. Consideration will need to be given when jumping or riding through deep ruts because the lowered ground clearance will make the bike more prone to grounding-out.

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