Front Fork Dampening

Jul 27, 2010

While we don’t spend a lot of time thinking about our suspension while riding, it happens to be the one element that can and does have the most profound affect on our comfort and satisfaction as a rider. (Granted if your a Horsepower fan you’ll say “power” but hey this is our forum and we’ll write what we believe!) Anyway, standard front forks are rather simple objects to work with and/or even understand. They are just like emulsion shock absorbers with straight wound springs, except the springs are on the inside instead of the outside. Since the forks get shorter when you hit a bump, all forks are effectively air-assisted. (Because of this, it is important to get the oil level set correctly when you change your fork oil, or the spring rate will be wrong).

Here is a standard fork leg, using a dampening rod. On compression, the fork spring is compressing and storing energy. The Rebound Washer is pushed by the fork oil up against the upper stops. These stops have holes in them to let oil through. Oil is collected in the pocket between the Damping Piston rod and the fork tube. Also, oil flows through the Compression Holes, up the center of the Damping Piston rod, and out the piston. The size and number of the Compression Holes determines the compression damping. During rebound damping, the Rebound Washer is forced by the fork oil down against the lower stops. These lower stops form a seal. The oil trapped between the Damping Piston rod and the fork leg must travel through the Rebound Hole, down the Damping Piston rod, and out the Compression Holes. Since the Rebound Hole is much smaller than the Compression Holes, the rebound damping force is almost completely controlled by the size of the Rebound Hole and the oil thickness. Also, another aspect of your front fork decision is in regards to Spring-rates. This selection is fairly straightforward: starting with the weight of the bike and the rider and skewed by intended use. Racing demands maximum chassis control and comparatively stiff springs, while touring dictates lighter springs. A spring is rated by how much weight is required to compress it a certain distance. Also, motorcycle fork spring systems come in a variety of rates and three main styles. The first is called a straight-rate spring. Second is called A dual-rate system would have one light spring and one heavy spring placed end-to-end. (This type of spring system responds like the softer-rate spring for the first part of the travel. Occasionally, there’s a third spring in some fork systems, for three distinct rates). Finally, there’s the progressive option. This kind of spring is wound with its coils closer together on one end than the other.

Your forks can be modified by changing the fork springs, changing the fork oil, and drilling new and/or larger holes in your damper rods. But that’s a whole other article. Later!

(portions of this article were retrieved from the Tech section of MotorcycleCruiser.com.)

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