Suspension Tuning, By Eric Gorr

Discussion in 'Eric Gorr's Moto Tech Articles' started by Eric Gorr, Aug 9, 2014.

  1. Eric Gorr

    Eric Gorr Engine Builder

    Joined:
    Jun 29, 1999
    Messages:
    384
    Likes Received:
    9
    Suspension Revalving

    How Damping Works
    Suspension fluid (oil) flows through the ports of the piston and up against the shims. The shims pose a resistance to the oil flow, which provides a damping effect. The damping effect is directly related to the diameter and the thickness of the shim. The shims act as a series of tiny springs, flexing to increase the flow area for the oil. The greater the flow area, the greater the oil flow and less the damping effect. The first shims that the oil encounters are the ones that affect the low-speed damping. These shims are large in diameter and thin in thickness.

    The oil deflects these shims easily because of their large surface area and the relatively thin steel poses low spring tension. The shim stack or valving is arranged in a taper shape. The large-diameter low-speed shims are positioned closest to the piston and the small-diameter high-speed shims are positioned farthest away from the piston.

    The low, mid, and high-speed circuit shims are separated by transition shims. Think of the valve stack as gears in a transmission, and the transition shims as shift forks. The more tapered the valve stacks, and the thinner the transition shim, the suspension becomes plusher in it's handling. Less plush suspension is typically too stiff to absorb the small bumps on acceleration, and too soft for square-edged bumps at speed. Much of the problem has to do with a mismatch between the piston's port arrangement and the overall valve stack.

    Why Revalve?

    The term revalving is often tossed around in the dirt bike magazines, but have you ever wondered what suspension tuners do to revalve a set of forks or a shock? The answer ranges from not much to a whole lot. Some unscrupulous tuners just power wash the outside of the components, turn the clickers, and charge you a lot of money. Other tuners replace the pistons and valve stacks, carefully crafting the arrangement of the valve shims to suit your riding demands and compensate for the idiosyncrasies of your model bike.

    Tuners need information about you and the way you ride in order to revalve your suspension. If they don't give you a survey form to complete or interview you, then be suspicious about the work they are asking to be paid to perform! Revalving can be defined as the removal, reposition, or replacement of shims in the valve stacks of the compression and rebound pistons of a cartridge fork or rear shock.

    Revalving should be performed when you've exhausted the basics like setting the sag, making sure your bike has the right springs, and the forks and shock have fresh oil, seals, and bushings. Only then can you make a determination whether your bike needs revalving in order to make it handle better. The main reasons why you need good handling suspension on a dirt bike is:

    1. To keep the wheels in contact with the ground to provide traction and drive for the rear wheel and steering for the front wheel.
    2. To minimize the impacts and vibration on the motorcycle.
    3. To minimize the stress loads on the rider and prevent fatigue and injuries.

    The rear wheel must stay in contact with the ground in order to provide driving force. The front wheel needs to stay in contact with the ground in order to provide steering control. Impacts on the motorcycle can cause all sorts of problems like loose bolts, foaming of the fuel in the carb's float bowl, long-term damage to the bearings that support the suspension components, and long term damage to the electrical components.

    The chronic problems to a rider from a poor handling bike are much more obvious. Forearm pump-up is probably the most common. Long term damage to a rider's neck and spine may take years to manifest but some people might be immediately sensitive pain. Having a professional suspension tuner re-valve your suspension might seem expensive (Average cost of total rework with parts $600) but what price do you put on pain?

    The main things that a suspension system is affected by are:

    1. Changes in the sprung mass from moving up and down.
    2. Changes in motion like acceleration, braking, and turning.

    The sprung mass of a moving dirt bike can be hard to define because the entire motorcycle leaves the ground! Technically the sprung mass includes everything except the wheels, swing arm, lower fork tubes, and the rear shock. Those parts are considered unsprung mass. Because dirt bikes are capable of jumping, gravity and the weight of the rider affect the sprung mass. The movement of a motorcycle's suspension going up is termed rebound and the movement down is compression.

    Changes in the motion of a motorcycle can cause it to roll, pitch, yaw or any combination thereof. When a motorcycle accelerates the bike pitches backward. The driving chain forces try to wrap the swing arm underneath the bike. Of course that cannot occur because the shock is a finite length and connects the swing arm to the frame, but it causes a transfer of force.

    The rear wheel pushes down into the ground, transferring force up the swing arm and causing the front end to lift. The natural tendency of the rear wheel is to hop because the damping isn't enough to compensate for the spring force. When a motorcycle is braked for a turn the bike pitches forward, shifting the weight to the front. The rear end tends to kick because of the torque reaction of the brake caliper on the swing arm and the weight shift. When a motorcycle is turned it rolls, pitches, and yaws at the apex of the turn.

    A complicated motion! The front end is forced to either compress or change the fork angle or extend and plow out of the turn. Meanwhile the rear end tries to make a radial motion without loosing traction and spinning out.
     

    - SPONSORED GEAR DEALS -




Share This Page