What condition are you wanting to fix?
A hydraulic fork provides two functions.
A spring, and a damper, they are two different systems that can be adjusted to suit you. If you find the spring weak, you would either find an aftermarket set of fork springs, or shim the existing. If the damping is not to your liking, you would change the fork oil, lighter weight for less damping, heavier weight for more damping.
Motorcycle suspensions are designed so that the springs are always under compression, even when fully extended. Pre-load is used to adjust the initial position of the suspension with the weight of the motorcycle and rider acting on it. Both the front forks and the rear shock or shocks can be adjusted for pre-load on most modern motorcycles.
The difference between the fully extended length of the suspension and the length compressed by the weight of the motorcycle and rider is called "total sag". Total sag is set to optimize the initial position of the suspension to avoid "bottoming out" or "topping out" under normal riding conditions. "Bottoming out" occurs when the suspension is compressed to the point where it mechanically cannot compress any more. "Topping out" occurs when the suspension extends fully and cannot mechanically extend any more. Increasing pre-load increases the initial force on the spring thereby reducing total sag. Decreasing pre-load decreases the initial force in the spring thereby increasing total sag.
Since the weight of the motorcycle and rider are the only forces compressing the suspension from the fully extended position, preload doesn't change the forces on the springs under riding conditions. Changing the pre-load does not change the way the suspension reacts to bumps or dips in the road surface either. Two simple examples using the motorcycle's forks shows why:
1. Suppose that the bike and rider put a total weight on the front suspension of 300 lb. Suppose the spring rate of each fork spring is 50 lb per inch. Installing a 1 inch long spacer in each fork leg gives a pre-load of 50 lb per spring, a total of 100 lb. When the weight of the rider and motorcycle are loaded onto the suspension it will compress 2 inch from full extension (2 inches total sag). Now the force exerted on (and by) each fork spring is 150 lb (1 inch pre-load + 2 inch total sag = 3 inch total spring compression) for a total of 300 lb, balancing the weight of the bike and rider.
2. Suppose we now install a 2 inch long spacer in each fork leg. The pre-load is now 100 lb per spring, a total of 200 lb. The total sag will change since we still have the same 300 lb loading the forks. The total sag will now be 1 inch. The total force on each spring is the same as before, 150 lb on each fork spring for a total of 300 lb force. The front suspension's initial position is 1 inch longer than in the preceding example (1 inch less total sag).
Since the forces are the same in both examples the reaction of the suspension to bumps and dips in the road will be the same. The difference is that there is less chance of topping out in example 1, less chance of bottoming out in example 2. Motorcycle manufacturers generally provide optimal total sag settings.
Pre-load on bikes without adjusters can be changed by disassembling the fork and changing the length of the spacer between the top of the fork spring and the fork cap. Spacers can be installed under the rear shock springs similarly. A longer spacer gives higher preload, and vice-versa.
The pre-load on both forks should always be the same. Dangerous handling characteristics and possible mechanical damage can result otherwise.
Fork oil
Since forks act as hydraulic shocks, changing the weight of the fork oil will change the damping. Higher weight fork oil will give more damping, and vice versa.
Fork oil collects impurities over time and should be changed periodically. A motorcycle's manual will give guidelines on how often the oil should be changed. Decades ago, it was often recommended to use ATF (automatic transmission fluid) as fork oil. Currently most sources recommend using an oil specifically designed to be used as fork oil.
(from:
http://en.wikipedia.org/wiki/Suspens...8motorcycle%29 )