First, some terms:
Spring rate is how strong the springs are. Stronger springs take more force to compress a given amount. Heavier or more aggressive riders can benefit from stronger springs, but they also reduce the ability to absorb smaller bumps. Springs, being springs, take more force to compress as they go, thus we have spring "rate." Any force will start to compress a spring, it's how much the force needs to increase per amount of travel that determines the rate.
Progressive springs have variable spring rates and usually appear to have the spring coils closer together over part of the spring length. The spring coils that are closer together are designed to be weaker than the rest. When a bump comes along, they collapse first, until the spring coils bind against each other, and then the rest of the spring coils, which are stiffer, begin to compress. Thus, the springs can be stiff enough for more aggressive riding while still being able to absorb smaller bumps smoothly.
Preload is how much spring force the suspension has to keep the bike and rider up. Rear shocks usually have an external preload adjustment; forks are usually adjusted with spacers on top of the springs. Without preload, the bike would just sag down, collapsing the suspension. Too much preload and the suspension will be fully extended, even with the rider sitting on the bike. Basically, preload should be set such that there is some sag but not too much. Note that preload does not increase spring rate, it just preloads the initial force on the spring to shift where the bike sits on the "rate" scale.
Sag is a measure of how much the suspension settles down when the rider is sitting on the bike. When going over dips, the wheels need to extend downwards to stay connected to the ground. Without sag, even going over small bumps would let the wheels fly off the ground. This kills traction. Some sag is good. Sag also consumes your suspension travel and this will reduce the ability to take bigger bumps without bottoming. Too much sag is bad. How much is right? Lots of people will give you exact numbers but the truth is that it has to be right for you and, unless you are a serious racer, this isn't going to get reduced to a formula. You just have to try different amounts until it feels right.
Damping is the rate at which the suspension can move in and out. It is either controlled by valving or by the weight (viscosity) of the oil. Too much damping and the suspension will be too slow to absorb the bumps. Too little damping and the bike will collapse onto and spring off bumps like a pogo stick. Sometimes, you will be able to adjust compression and rebound dampening separately - this can be a good thing. Like everything else, the right amount of damping depends very much on where, how, and what you ride.
Oil height is a measure of how far down the oil level is in the fork tubes with the springs out and the fork fully collapsed. Oil height matters because forks are sealed. As they collapse, the air pressure in them increases; increasing air pressure effectively adds to the spring rate. Thus, as forks collapse, the air in them acts sort of like progressive springs. The higher the oil in the tube, the more pronounces this effect is. Oil height allows you to fine-tune how your suspension behaves in the bigger hits.
Air pressure in front forks can be adjusted with the tire-style valves at the top of the forks. Honestly, the manufactures installed these valves so you can take air pressure out, not put it in. As you ride, your forks will warm up; as they warm up, the air pressure will increase. Increasing pressure will add to the effects of your preload and oil height settings. Also, over time, forks tend to "pump up" just by riding. Serious riders with well-tuned suspension will take this extra air pressure out. However, some people use these valves to add air pressure. Now, if you are attempting to temporarily tune your suspension to some unusual set of conditions, this is fine - so long as you don't add too much. Most manufactures recommend against this. Too much air pressure is very hard on fork seals; you may start leaking fork oil. Besides, if you are using air pressure all the time, then you might want to properly tune your suspension in the first place. Air pressure changes too much as you ride and this will change how your bike handles, making it less predictable. Predictable is good; it allows you to ride closer to the edge.
Bottoming is what happens when you hit a bump so hard that the suspension collapses to its limits. If you are bottoming often while riding around then you either have too soft of a spring rate, too little preload, too much sag, not enough fork oil, too little damping, or you don't know how to ride very well. Hey, I never said suspension tuning was easy.
Lock up or binding is what happens when you screw up your suspension tuning. If you add too much preload, the spring coils will bind up before the suspension bottoms. This is a bad thing as it puts way too much force on the wrong parts of your suspension. If you have too much fork oil then you will bottom out on the oil before the suspension is finished. While this isn't quite as bad and coil bind, it's still really hard on your fork seals. Bike manufacturers usually specify limits on preload and fork oil height; it is wise to stay within these limits. If you need more - start thinking about buying new springs.
Why bother with all of this?
The object of suspension tuning is to maximise traction and control. If your suspension is too soft, and you regularly bottom out, then you will have a hard time controlling the bike. If your suspension is too hard, then the bike will stutter on small bumps and you will loose traction, and probably control. If everything is just right, you will be able to maintain control while traveling quickly over smaller bumps and still be able to absorb the bigger hits without having to slow down too much. Riding a well tuned bike makes a huge difference in a rider's ability to go fast over varying terrain. And, let's face it, if you're not going fast, your suspension doesn't really matter - just about anything will work well enough.
So, how do you do manage all those combinations in suspension tuning?
Basically, very basically, here's the order I recommend:
- Get the preload right. Once you get the preload right, move on to the next step. If you need longer spacers than the maximum allowed, then you need stiffer springs.
- Get the front damping right. Typically, this means changing fork oil weights, but your bike may be adjustable. It doesn't really matter, just get it right. The heavier the oil, the less your forks will collapse on a big hit and the less they will absorb the stutter-bumps (= loss of traction). Then, when this feels about right, go on.
- Get the fork oil height right. The higher the oil is in the tub, the faster your air pressure will increase and the stiffer your forks will get before bottoming. If the above is right, and you still bottom a lot in the front, add more oil - being aware of the maximum for your bike. Then...
- Adjust the compression and rebound dampening for you back shock. On my KLR, there is only one adjustment, so I do what I can.
So, what's right?
Riders will note that I've spent a lot of time describing what can be adjusted, and the order to adjust them, but nothing about what they should be adjusted to. This is because I can't tell you. Suspension tuning is not really about specific measurements or right and wrong, it's about the best compromise you can get for YOUR riding style. Suspension is not perfect, it's always about trading off one thing for another... you want soft suspension for traction but stiff suspension for the bigger hits. In fact, the settings you use as a learner will usually be way too soft once you really know what you're doing. On the other hand, I've also seen inexperienced riders way over-stiffen their suspension. Basically, they're hitting the larger bumps all wrong, hammering the suspension until it bottoms out. To compensate, they stiffen the suspension to avoid the bottoming, but in doing so they sacrifice traction. Without the control from decent traction, they can't ride that fast.
What I'm trying to get at is that the suspension needs to be tuned to the rider. There are just too many variables: height, weight, terrain, bike, ability, etc.. There are a lot of people out there offering specific advice. If you can find someone with the same variables as you, you might get some decent information on where to start. This is especially true if you are riding a bike outside of its intended terrain, a motocross bike on tight-woods trails for example. Generic advice for all makes, terrain, and riders, will probably be less useful than the stock manufacturer's settings. What this article is attempting to do is just provide some insight into what the terms mean and some ideas about what each setting will do. You get to take it from there.