Sunday, 17 August 2008

Motorcycle Suspensions Part II


Now that we have seen what compression damping is, we should also know about rebound damping. For perfect suspension settings, these have to be syncronised.

Remember that compression (or bump damping) occurs when the wheel contacts a bump and the suspension compresses. Rebound (or tension damping) occurs as the spring forces the shock or forks to extend. Most current sport bikes have external adjustments for both compression and rebound damping as well as spring preload. On most forks, the screw adjustment at the top is rebound damping (not to be confused with the larger spring preload adjuster); the one on the bottom near the axle is compression damping. (An exception would be a Marzocchi or Paioli fork.) On the shock, the adjuster on the reservoir is for compression and the one on the shaft eyelet is rebound. These adjusters have their limits and affect only a portion of the entire damping. In other words, external adjustments can't make up for poor internal valving design; the adjustments merely fine-tune the valving action.

Of course, external adjustment can never make up for extremely worn-out dampers either, so if your bike is wallowing along like a '63 Cadillac Fleetwood Brougham with blown-out shocks, you might want to do some rebuilding or replacement before you spend the rest of your life playing
with "clickers."

Let's look more closely at rebound damping. The major trade-offs involve traction, a feeling of control and plushness. If you look at the graph you'll see they're all plotted on the same graph. There are no numbers on the y axis because these are largely subjective quantities. In other words, we are discussing "feelings." You will notice that traction starts out at a low amount at very light (quick) rebound damping settings, increases to a maximum, then decreases again. Why? At very light rebound settings, the chassis is uncontrolled. When the wheel hits a bump the shock is compressed. Then the wheel extends without any control; in fact, it extends too far. Because the sprung weight of the chassis has mass and is moving upward, it wants to pull the wheel off the ground, thereby losing traction.

If you direct your attention to the right side of the traction versus rebound damping curve, you will note that at high rebound damping, traction has suffered. This is due to the wheel not being able to follow the ground simply because it can't respond quickly enough. The suspension compresses as it hits a bump. Then, it can't follow the ground (return to its original position in the travel) fast enough after the crest of the bump to maintain traction. When this is excessive it is called "packing." Somewhere between these two rebound damping extremes, traction is at maximum.

You may have noted from your own riding experience that when rebound damping is very light, the feeling of control is minimized. The bike "feels loose." As you increase rebound damping, the feeling of control increases. The chassis isn't moving around nearly as much and the bike feels more "planted" and stable. When rebound damping is very slow, meaning there's a lot of damping, traction is so poor that the feeling of control suffers as well. Once again, somewhere between the two extremes the feeling of control is maximized.

The third quantity is plushness. At very light rebound damping, the wheel moves very quickly and the feeling is plush and mushy. As rebound damping is increased, there is more and more resistance to movement, and at maximum damping the wheel is "packing" so much, the chassis is sucked down in its travel and has not recovered for the next bump. This means the following bump has to overcome the added spring force due to this compression and the result is a jolt to the chassis upon impact.

The key thing to note here is that there is a trade-off. As you can see, maximum traction does not necessarily occur at the same damping setting as maximum feeling of control. Herein lies a problem.

Quite often riders have mistaken ideas about how much damping should be used. They think the faster they are (or the faster they want to be), the more damping they need. Nothing could be further from the truth. In fact, after a certain point, traction, control and ride quality (plushness) are all sacrificed. And, even with rebound damping settings in the ballpark, in other words, between the two peaks, there is a trade-off. Of course, there is room for personal preference, but there's not much value in having all three qualities suffer.

Here is one word of caution: The only way you will ever know if you have less traction is if you are at the limit of traction. This is a very delicate thing. If you are not at the limit of traction-i.e., sliding the tire-you can't feel the difference in traction. So street riders will want to focus on the feeling of control and save traction experiments for racetrack days.

The job of suspension engineer and suspension tuner is to make these two peaks-traction and the feeling of control-as close to the same point as possible. This is done by reshaping the damping curve internally and requires an understanding of high- and low- speed damping and valving piston design. The relationship between damping, spring forces, weight bias and all the other factors that make a bike handle are also very important. Overwhelming? thing at a time. Or should I say, one click at a time.

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