Just what does the fork do? Even good roads aren't exactly smooth. All roads have bumps, ripples, grooves, and other irregularities. Since these can cause control problems, any vehicle needs a way to deal with them.

On bicycles the only shock absorbing system is usually the tires and wheels. The inflated tire can give a little when it hits an irregularity, and the spokes will also give slightly to help compensate for bumps. As anyone who has ever ridden a bicycle will be aware, the system isn't particularly efficient. You feel every bump in a rough road, which, of course, is why the seat itself is usually mounted on springs.

While a motorcycle also has only two wheels, there is a world of difference from a bicycle. Motorcycles are much heavier. Even a relatively light street bike will weigh around 300 pounds. The heaviest cruiser may weigh as much as 1100 pounds. And this is without the weight of the rider! (Contrast this with a bicycle, where light weight is considered very important and the most expensive bikes may weigh no more than 8 - 10 pounds.)

Obviously, with this much weight, tires and spokes are not going to provide adequate suspension. Throw in the fact that a bicycle is normally ridden at speeds of 20 mph or less, while many motorcycles are capable of speeds well over 100 mph, and the importance of control is obvious.

For this reason, motorcycle forks have been designed with some sort of shock absorbing capability almost from the beginning. (Rear suspension is much more recent. The first Harley to have it was the 1941 XA, a BMW clone built for an Army test program, but production Harleys didn't get rear suspension until the Duo Glide in 1958.) There are two basic types of motorcycle front suspension, telescoping and leading link.

Leading Link Suspension:
This is the oldest suspension design, and was originally developed for bicycles. (Some still use it.) The system consists of two solid forks. The rear fork is attached to the steering head. The front fork, set a few inches ahead of the other, has the axle and wheel at the bottom and is attached to some type of shock absorbing mechanism at the top. Metal links joins the bottom ends of the two forks, allowing the wheel and axle a certain amount of vertical travel.

In early leading link suspension systems friction was used as a damping mechanism. In modern leading link designs, such as those found on a Harley "springer," a shock absorber is used.

Leading link suspension is actually the superior system in many ways. It handles steering inputs and braking better, since it is stiffer. It is not as good at handling rough roads, however, because the design dictates a fairly short amount of vertical travel.

Telescoping Forks:
The most common fork design on modern motorcycles is the telescopic type. In this design, a pair of tubes are fixed into the triple clamps at the steering head. A second pair of tubes, which hold the axle and wheel, fit over the first pair. A damper rod slides inside the inner tube, using oil as a damping medium to attenuate the up and down motion of the lower tubes. In most modern examples the springs are contained inside the upper tubes and rest on the damper rods, though you can still find some with external springs.

A more recent development in telescoping forks is the "inverted" fork design. In this type, the axle is attached to the smaller-diameter tubes, which slide inside the fixed tubes. This has the advantage of having the larger -- and therefore stronger -- tube connected to the steering head and reduces the amount of flex in the fork assembly. Since the lower tubes are lighter on an inverted fork, it also reduces the amount of unsprung weight on the bike, which is a good thing. (Don't confuse inverted forks with those found on the Kawasaki Vulcan 1500, or the Harley Fat Boy -- those are regular telescoping forks with shrouds concealing the smaller-diameter upper tubes. They are, in fact, the same fork, since both Kawasaki and Harley-Davidson use the same Japanese-manufactured Showa fork on these models.)

Special Designs:
The most obvious of these is the Telelever fork used on BMW motorcycles. This combines elements of the leading link and telescoping designs, and has the primary disadvantage of looking decidedly odd when it's sitting out in the open. (This isn't quite as obvious on the "sport" models, where most of the upper fork is concealed inside the fairing.) The Telelever design has the advantage of increased stability throughout the full range of motion, and a greatly decreased tendency to "dive" during hard braking.

How the Whole Thing Works:
The main job of the front suspension on a motorcycle is not to absorb road shocks and give you a more comfortable ride. That's just one of the side-effects. The true purpose is to keep the front wheel in firm contact with the pavement. A moment's thought will inform you that you can't steer or brake with a wheel that's off the ground. Keeping the wheel where it belongs is the result of two complimentary actions: compression damping and rebound damping. We'll look at these separately.

1.) Compression Damping: When you hit a bump, the wheel rises. Inertia makes the rest of the motorcycle want to stay where it is, so as the wheel rises it compresses the spring. This absorbs the shock of the impact. How well it aborbs this shock depends on the weight of the bike, the force of the impact, and the spring rate.

Spring rate is expressed in terms of the amount of force required to compress the spring a given distance. U.S. specs are generally in pounds/inch, while foreign specs will generally be in Kg/cm. Which you use really doesn't matter as long as you know what result you need to obtain. Standard springs have the same rate through the entire compressible range. Progressive wound springs initially have a "softer" rate but after a certain point become "harder." You can generally tell which type of spring you have just by looking at it. Progressive wound springs will have the coils closer together at one end while standard springs will have uniform coil spacing. A progressive wound spring will generally give a better ride.

2.) Rebound Damping: A compressed spring will always try to return to its resting state once the extra pressure is removed. In fact, unless restrained, it will bounce back very quickly and actually over-extend, snap back to a slightly-compressed condition, rebound again, and so forth until it achieve equilibrium. If you hit a series of bumps too close together, and have only springs in your suspension, the front end will start to bounce like a pogo stick. Springs are good at absorbing shock, but they are not good at getting things back where they belong.

This is where rebound damping comes in. Most forks use a piston working in oil for rebound damping. (Usually, the oil also attenuates the compression damping.) The oil is metered through a tiny aperature, slowing the return of the spring enough that it returns to a resting condition at the end of its rebound. More expensive forks use emulators in place of simple damping rods. These use a more precise metering setup to adjust rebound damping for different conditions.

Adjustments: Many modern forks provide some means of adjusting both compression and rebound damping to suit riding conditions and load. Others provide for some adjustment of compression damping only. Air forks are an example of this type. You can add a little air to the fork to increase compression damping when you're loaded down. (And a little is the operative word here. Air shocks in the rear may take up to 50 pounds of air; forks generally don't take more than 5 - 7 pounds.)

Rebuild or Replace, and When?
How do you know when your fork has worn to the point where it's likely to cause problems. The easiest way to tell may be to have the mechanic check it when the bike is in for regular servicing. Suspension problems are insidious. They sneak up on you slowly, so that you may never realize you have a problem until something breaks and you're faced with a major disaster. As things get looser you compensate. Put someone else on your bike for a ride around the block and he may come back scared half to death.

Some other signs of trouble:

1.) Leaks: The oil inside your fork is supposed to stay there. If it's coming out and leaking down the sliders you may presume, at the very least, that you need to replace the seals. Depending on the age of the fork you may need to do more. (And you might keep in mind that, at least as far as labor is concerned, completely rebuilding a fork generally costs exactly the same as just replacing the seals, since the tubes usually have to be disassembled to do either job.)

2.) Noise: If you ignore the oil leaks, at some point things are going to get noisy. Telescopic forks are designed to work in oil. If the oil is gone, the damper tubes will start to rattle against the fork tubes.

3.) Pogoing: When rebound damping is lost, the front end will start to bounce uncontrollably when you hit a succession of small bumps or pavement ripples. Chain-driven bikes have a tendency to shake when you accelerate hard, and this is exagerated when the fork is going bad. (The engine is trying to lift the front tire off the ground and the fork can't react quickly enough to damp the vibration.)

4.) Sticking: Very often the upper tubes on telescopic forks will develop a slight bend after years of use. This increases sliding resistance. The most obvious sign of this is when the fork doesn't instantly return to its original position when you remove the weight from the bike. If it compresses and stays that way, suspect warped tubes. (Broken springs can also cause this, of course.) A fork brace may help prevent warping, but if it's poorly designed or installed incorrectly it can actually contribute to the problem.

Another contributor to warping is the standard brake setup on most smaller bikes. A single-disk brake setup will tend to exert unequal pressure on the fork while braking. Dual front disks exert an even pressure. Since dual disks work better in any case, if you can manage it when you rebuild the forks, think about adding a second disk and caliper.

So which should I do? Rebuild it or replace it?

This depends on just what's wrong with your current fork. It can also depend on how much you have to spend, and what you think your bike is worth. If the fork tubes are warped, or so rusted and pitted that the seals give out after only a few hundred miles, but everything else is in good condition, rebuilding makes sense. If the tubes and sliders are bad, however, it may be more practical to replace the entire unit.

The ultimate replacement is a complete fork. This includes the tubes, sliders, seals, internal parts, and new triple-clamps and head bearings. Frankly, if you don't mind spending the money, this is the way to go. You get all new components. This is also the point at which you can upgrade components, since there is absolutely no rule that requires you to use the same fork that came with your bike. You can put on an inverted fork, for instance, and gain all the advantages of that more modern design.

If you do decide to use a different fork, you should be careful to preserve the steering geometry. Get a replacement with the same steering head angle as the original, and the same length fork tubes. If you want to go for a chopper look with longer fork tubes, have the work done by someone who can alter the neck angle on the frame to preserve decent steering properties. (Peter Fonda noted that the "Captain America" bike in Easy Rider worked just fine on straight roads and gentle curves, but was pure misery to steer around corners or sharp curves.)(Trivia note: Both bikes used in that movie were surplus Los Angeles Police Department motorcycles before they were customized.)

If you can't afford to buy a complete fork assembly (The one for a KZ440 lists for $1008.00 from Kawasaki.), you can consider replacing only the upper tubes. You can get these from the manufacturer, or from an aftermarket source. Again, this is a good time to upgrade by adding progressive wound springs, or retrofitting emulators, if they are available for your bike.

If you have standard (non-air-assisted) forks, and they lack any external adjustment system, then the only method of adjusting performance will be by fitting lighter or stiffer springs, and by changing the viscosity of the fork oil.

With air forks, adjustment is done with air pressure. Never use a gas station air hose to add air to an air fork. Doing so is a good way to blow out the seals. The pressure in most air forks should not exceed about 10 pounds, and is usually less. (Maximums may be higher, but most of the time maximum pressure is only needed when the fork is just about worn out.) Stock air forks generally come with a fitting (a standard Schrader valve) on each fork leg. Newer versions are factory-fitted with an equallizer tube connecting the legs. Accessory dealers sell aftermarket fork equalizers that can be fitted to the valves after removing the cores. This makes adjusting the air pressure much easier. If you don't have this sort of setup on your forks, you need to be careful to keep the pressure difference between the tubes to within about 1-1/2 pounds or performance and handling may suffer.

Do I want to do this myself?

Maybe, but probably not. On a bike with a centerstand, it may be possible to place a bottle jack under the front of the engine (use a board to spread the pressure -- it would be somewhat embarrassing to punch a hole in the crankcase) to lift the front end off the pavement. If you don't have a centerstand, you'll need a motorcycle lift to get the wheels up. You generally also need special tools to disassemble the fork tubes, and possibly to install the seals. Unless you expect to do this a lot it will probably be cheaper to have a mechanic do the work than to buy the tools. (Don't count on the local rental center having specialized motorcycle repair tools, though it never hurts to ask.)

A secondary, but equally important factor, is that only you know just how good a mechanic you are. If you screw up a fork rebuild, the result is capable of killing you.