High-speed vs. low-speed
Damping, damping and more damping. Whenever the subject is suspension theory, damping is always a major topic of discussion. The terms high- and low-speed damping are commonly used but often misunderstood. To help clarify this somewhat muddy subject, let's look at what low- and high-speed damping actually are.
Damping is sensitive to velocity, meaning the amount of damping force created is affected by how fast the fork or shock is compressing or extending. Low-speed damping refers to the damping created at lower velocities.
It is important to make the distinction that we are talking about shaft velocity, not bike speed. It makes sense that if you double the bike speed while going over the same bump, you will double the shaft velocity on compression. However, what may not be so obvious is that the shape of the bump is far more important than its size when it comes to the velocity produced. A square-edged bump that is one-inch high is going to produce far greater velocities than a long rounded bump that is also one-inch high.
Damping is not a linear function. In other words, when you double the velocity, you don't necessarily get twice the damping; you may get as much as four times as much damping. A typical compression damping curve for damping rod-type forks looks something like Figure 1. Notice that each time the velocity is doubled the force increases by a factor of four. Point A at 1 m/s (meters per second) shows a force of 10kg. At Point B, at 2 m/s the force is 40kg.
So what part of the curve is low-speed and what part is high-speed? Good question. These are not specific terms defining specific velocities, they are relative terms. If the maximum compression velocity of the fork is 6 m/s, we could arbitrarily say that anything less than 1 m/s is low speed and anything above that is high speed. It would also make sense that there is a mid-speed damping and even a low-low-speed damping and high-mid-speed damping, etc. The point is, even though the terms high- and low-speed damping are not specific velocities, they do allow suspension tuners and designers to communicate with the rider when discussing sensations or symptoms. Our dyno tests have shown that velocities as low as 0.05 m/s are very important when making changes to a damping curve.
Remember that compression (or bump-damping) occurs when the wheel contacts a bump and compresses, and rebound or tension damping occurs as the spring forces the shock or fork to extend. The fact that the compression velocity is forced by the size and shape of the bump means it sees a very wide range of velocities. Rebound, on the other hand, is largely controlled by the spring force and therefore sees a much smaller range of velocities. Typically, rebound may see one third of the velocity of compression. For this example, where compression sees 6 m/s, rebound may see 2 m/s. This means the transition to high-speed rebound damping might best be thought of, in this example, at around 0.2 m/s (see Figure 2). The rebound graph is inverted because we call compressive forces positive and tensile forces are considered negative.
To get a more complete picture of the entire damping curve, you would simply put the two curves together as in Figure 3. Each time the shock goes through a complete cycle, it passes through zero twice-once at the bottom of the stroke and once at the top of the stroke. When the wheel hits a bump, the shock accelerates to a maximum velocity, slows down and then stops compressing. At this point, the vertical velocity of the shock is zero. It then changes direction, accelerates in the negative direction, slows down and stops again when it's done extending. As you can imagine, every stroke of the shock does not use all the travel and certainly does not reach maximum velocity.
Be aware that high speed at the rear shock on one bike may not be high speed for another. This is because of another factor: leverage ratios. If the velocity at the wheel is the same for two models of motorcycles with different linkages, the shaft velocity can be radically different. A shock linkage with high leverages will have shorter shaft-travel and lower shaft velocities than a bike with low leverages.
If you refer to Curve A on Figure 4, you will recognize the compression damping curve of a damping rod-style fork. Curve B is more preferable because it has more low-speed damping for a good, firm controlled feeling, and less high-speed damping for a more comfortable ride on square-edged bumps. You would expect this type of ride out of a well-tuned cartridge fork or a damping-rod fork fitted with emulators or gold valves.
Most current sport bikes have external adjustments for both compression and rebound damping, as well as spring preload. We've covered spring preload in the October '95 "Technicalities," so where and what do the damping adjustments do? On forks, the screwdriver-actuated adjustment at the top is typically rebound damping, the one on the bottom near the axle is compression damping. On the shock, the adjuster on the reservoir is for compression and the one on the shaft eyelet is rebound. These adjusters are generally low-speed adjusters with the exception of the shock compression adjustment, which is usually high-speed (and fairly ineffective). Low-speed adjusters typically affect the entire range of damping, meaning they raise or lower the entire curve. They do affect high-speed damping, but they're much more noticeable at low-shaft speed because their percentage change is much greater.
Once you recognize what adjustment you have and its effect, you can begin to fine-tune your suspension. But remember just because you have external adjusters doesn't mean you have much control of the damping curve. In Figure 4, for example, you could never make Curve A look like Curve B with any kind of standard external adjusters. Remember also that with adjusters you can make an improvement in one area and actually make it worse in another with the same setting change. When playing with adjusters, record your initial settings, make one change at a time and record all your results.
This article was originally published in the December 1996 issue of Sport Rider.
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