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.

Saturday, 16 August 2008

Suspension Introduction

Recently I was facing a few issues with my suspension. Its working fine but I was not sure how to set it up. What is Compression Damping or Rebound Damping and other terms. I am going to share with you what I am learning. Please feel free to comment with anything you know.

Today we will talk about: 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.