Rocket Science, voodoo, black art...suspension in general, and damping specifically, have been shrouded in mystery for years. Enhanced control, better traction and more compliance translates into lower lap times as well as improved safety and comfort. Many race team managers consider their best investment to be in suspension, both in knowledge and in better hardware. All the horsepower in the world does you no good if you can't apply it to the ground.
FORCES
So far, in previous "Technicalities" articles, we've looked at the difference between cartridge forks and damping rod forks, introduced Gold Valve Emulators, talked about spring sag, ride height, spring rate and preload. A lot of answers were discussed in these segments but never tied together, and we haven't even considered damping. Let's begin by looking at the forces involved in suspension action. What are they? Spring forces, damping forces and frictional forces. Of course, there are forces due to acceleration of the masses involved, but we'll ignore them for the purpose of this discussion.
The key thing to note about spring forces are that they are dependent on position only. Springs only care where they are in the travel, not how fast the suspension is compressing or rebounding.
Damping is viscous friction. It is caused when liquids are forced through some type of restriction. The key thing to remember about damping is that it is dependent on fluid movement. This means a shock creates no damping force unless there's movement-movement of the damper unit in compression or rebound as opposed to bike movement. Damping cares about vertical wheel velocity, not bike speed.
The third type of force is frictional force. Frictional forces depend on the perpendicular load on the surfaces in question and the materials involved, including lubrication, if available. The higher the load, the greater the friction. More slippery materials, better surface finishes, more sophisticated lubricants and better design can minimize friction.
The other factor when considering friction is whether there is movement between the surfaces. The two conditions are known as "static" and "dynamic" friction: static friction (stiction) when there is no movement between the surfaces, and dynamic friction when there is movement. Stiction can be felt in forks, for example, when you push down on the handlebars. The breakaway resistance, or stiction, is higher than the friction once there's movement. We won't get too in-depth about this aspect except to note that static friction is always higher than dynamic friction. Suffice it to say that, as far as frictional forces are concerned, less is better.
Before go any further, let's define a couple of terms. The compression stroke, sometimes called the bump stroke, occurs when the wheel contacts a bump and the suspension compresses. The rebound or tension stroke occurs when the suspension is extending.
ENERGY
Those are the forces involved, but what about energy? Springs store energy when they're compressed. They release energy when they recoil. Damping, on the other hand, turns mechanical energy into heat. Why is this knowledge of energy important? A number of reasons. First, when a shock gets hot, it's OK. It's designed to. Fade, on the other hand, is not good. Fade is when the shock loses damping. A well-designed shock with good fluid can get hot and still not fade perceptibly. Second, an understanding of energy helps make the subject of suspension simpler.
Let's consider a bump that's two inches high. When the tire contacts the bump, the suspension compresses. As it does, the spring stores some of the energy while the damper creates compression damping and dissipates some of the mechanical energy into heat. The wheel slows down, stops compressing, changes direction, and starts extending, or rebounding. The spring is releasing energy and the damper is creating rebound damping, once again turning mechanical energy into heat. If everything goes perfectly, the center of gravity of the motorcycle goes along in a straight line with the wheel moving up and down beneath it, maintaining constant traction.
The suspended motorcycle system can be modeled as we have shown in the accompanying figure. The center of gravity is separated from the wheel by the spring and the damper unit. Each of these components has mass. Everything above the spring is considered "sprung" mass and everything below the spring is considered "unsprung" mass. Half the spring is sprung and half is unsprung. The portions of the damper attached to the sprung mass is considered sprung, and the portions attached to the unsprung mass are unsprung.
If there is too much compression damping or the spring is too stiff, there will be too much resistance to movement and the wheel will not move the entire two inches. This means the center of gravity of the motorcycle (the sprung mass) will be displaced upward, giving an uncomfortable or harsh ride and, due to the upward velocity of the chassis, possibly pulling the wheel off the ground, losing traction.
If there is too little compression damping or the spring is too soft, the wheel will not have enough resistance as it's being compressed. Not enough energy has been stored or dissipated at the crest of the bump. Because the wheel itself has mass and that mass is moving upward, it wants to remain in motion, continuing to move upward, compressing more than the two inches required to handle the bump. This means the wheel will lose contact with the ground as it crests the bump, and lose traction in this situation as well.
If the compression damping and spring are perfect and the suspension gets compressed, the spring still stores energy. If there were no rebound or too little rebound damping (the suspension extends too fast), the spring would recoil and release this stored energy unchecked. This would cause the center of gravity of the motorcycle to move upward quickly. As a result, this movement of the chassis would cause the tires to unweight and lose traction.
If the rebound damping is too high (comes back too slow), the wheel crests the bump and can't follow the back side of the bump, once again at the cost of traction. This is compounded when there is a series of bumps as opposed to just one bump. This condition is called "packing." As the suspension is rebounding, it is extending too slowly after it crests the first bump. By the time the wheel hits the second bump, it is still partially compressed. As it hits the second bump, it has to overcome the additional energy stored in the spring. This makes the ride harsh and, in extreme situations, excessive rebound damping can suck down the center of gravity of the motorcycle, adversely affecting chassis geometry.
One thing you might have noted is that excessive rebound damping can give a sensation similar to excessive spring rate or excessive compression damping. This could get interesting. In fact, we've just barely started to cover the subject. It's important to have a well-defined testing procedure as well as a more thorough understanding of damping and it's practical effect on handling and ride. Until then, one step at a time.
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