In the March issue's "Technicalities," we started to delve into the "Witches' Brew" of suspension systems with damping. Specifically, we discussed rebound damping. This month we will continue to unravel the mysteries by looking at the most misunderstood, and possibly the most critical, component of suspension: compression damping.
Remember that damping is viscous friction. It turns mechanical energy into heat and is sensitive only to shaft velocity, not position in the stroke. The fundamental difference between compression and rebound velocity profiles is due to the fact that compression is forced by the shape of the bump, while rebound, though affected by other forces, is pushed mostly by the spring. This means that, for compression damping, the shape of the bump is far more important than its size. A square-edged bump results in extremely high shaft velocities, while even a big dip will typically cause fairly low velocities.
Traditionally, many have considered compression damping a necessary evil. In other words, less was better. This thought process was created because of the style of damping that was prevalent in the past. If you recall the "Technicalities" on cartridge forks [August 1994], you'll remember that damping rods have what we call orifice-style damping, which is both too harsh and too mushy at the same time. With the advent of cartridge forks and Emulators for damping rod forks, the possibilities have drastically changed because we have much more control of the shape of the damping curve.
To study the effect of altering the compression damping, let's look at damping as a whole; i.e., ignore the fact that the shape of the curve is important. The amount of compression damping affects traction, plushness, bottoming resistance and dynamic dive.
Consider bottoming resistance first. It makes sense that the more compression damping, the more resistance to bottoming (refer to the curve). It may seem obvious, but you need enough, yet not too much. The compression damping force is added to the spring forces to resist bottoming. At the same time the bottoming resistance increases, the feeling of plushness decreases. But what the heck is going on with the plushness curve on the left of the graph at minimal damping? With very little damping, the plushness can actually decrease. This occurs only on big hits when there is bottoming. On small hits, less damping still means more plushness.
Let's look at how compression damping affects traction. Imagine you're riding along and you hit a bump. If there is too little compression damping, the wheel will not meet enough resistance as it compresses the fork or shock spring. Not enough energy has been dissipated at the crest of the bump. Because the wheel itself has mass and the mass is moving upward, it wants to remain in motion and continues to move upward, compressing more than the amount required to handle the bump. This means the tire will unweight and possibly even lose contact with the ground as it crests the bump. This unweighting produces a loss of traction.
As compression damping is increased, this phenomenon decreases and traction improves. If there is excessive compression damping, there will be too much resistance to movement and the wheel will not move the entire height of the bump. This means the center of gravity of the motorcycle (the sprung mass) will be displaced upward. Not only can this be the cause of an uncomfortable or harsh ride, but this upward velocity of the chassis will tend to unweight the wheel, losing traction. In extreme cases, the wheel will come off the ground entirely and skip over the bumps. This is one of the reasons why in bumpy turns at extreme lean angles you may have experienced difficulty holding an inside line. The bike will tend to drift to the outside of the turn.
With too little compression damping, the wheel continues moving up farther than it should, while with too much compression damping, the entire chassis moves vertically. In either case, you lose traction.
The last curve on the graph is called maximum dynamic dive. This is distinctly different than static sag, which is measured with the bike standing still. Maximum dynamic dive is the amount the suspension compresses when hitting bumps or under braking. For example, under braking the front end will dive. More compression damping allows the chassis to dive less. The maximum amount of travel used is determined by a combination of the spring forces and the compression damping force.
If there is no damping of any kind and the brakes are applied, the front end will dive and begin to oscillate. If you're braking for a long time, the friction will eventually stop the oscillation and you would notice the fork is compressed. Since damping is nonexistent when there is no suspension movement, the amount it ends up being compressed is totally determined by spring forces. Maximum dynamic dive, however, is affected by compression damping as well. More damping means the forks will compress more slowly and not as much. Obviously, less damping will produce the opposite results.
If you're hitting a series of bumps with too much compression damping, the suspension will actually extend as the wheels hit successive bumps. This is the opposite of the condition called "packing," when there is too much rebound damping and the suspension is being sucked down, not having time to return fully before the next bump.
Obviously, there are trade-offs. As bottoming resistance increases, plushness decreases and maximum dynamic dive decreases. At some point between the extremes, traction is maximized. Street bikes will be best suited by being biased with less compression damping than a race bike. Remember, you will pay prices with both too much and too little. One of the biggest misconceptions about compression damping is that the faster you are, the more you need. A better goal would be to determine proper spring rates and use only as much compression damping as you need to control bottoming and dive.
Keep in mind that compression damping is dependent on movement. If there is no movement, there is no damping. Be aware also that the shape of the damping curve is critical, in not only how much damping you have, but how progressive it is as well. But that's another story.