Most modern motorcycles are equipped with adjustable suspension of sorts. Adjustable suspension allows us to fine tune the bike’s handling and comfort levels according to our weight, riding style, load, road conditions, and tyres.
Having an adjustable suspension system is fine and dandy, but the first place to begin with is spring preload.
Where the suspension sits between fully compressed and fully extended is called sag. Generally, the front sag should be between 30 to 35% of the forks’ travel, and 25% for the rear shock/shocks.
"Adding more preload" by turning the adjustment collar on the shock's body exerts more force on the spring. The spring pushes back and raises that end of the motorcycle. Conversely, reducing preload will let that end of the bike sit/sink lower.
The main purpose of preload is to adjust your motorcycle’s chassis balance, hence its handling. For example, a lighter rider requires less rear preload, while a heavier guy needs more to maintain the bike’s balance.
With this in mind, we can use preload to tune our motorcycle’s handling. Less preload in front and more preload in the rear will lower the front and lift the rear for an “aggressive” stance, since there is more weight on the front wheel. The bike will steer quicker, although the front feels heavier. But going too far will have the rear tyre feeling unplanted and may weave about under hard braking.
Conversely, a bike with more front and/or less rear preload will steer a little slower but feels more planted especially in long, high-speed corners. Go too far with this adjustment and the bike will feel like a cruiser with a large rake (slower to respond to steering inputs). The front tyre may also feel unstable under hard acceleration and around long, long high-speed corners.
You should also increase the bike's rear preload if you carry a passenger and cargo regularly to avoid the rear end squatting.
Additionally, adjusting the preload (especially the rear) changes the bike’s seat height. Reducing preload lowers the bike and vice versa. However, there is only so much you can do to alter the seat height via preload adjustment, since it influences the suspension’s sag setting. So, you need shorter springs if the height is still too tall for you. This is why certain manufacturers sell suspension lowering kits - they lower the bike but do not affect the overall balance and handling of the bike.
This is all that preload does. Preload does not change the suspension’s rate, contrary to popular belief. The suspension feels harsh or too “comforty” because extreme preload adjustments lets the suspension move too little or too much into its travel. So, again, preload adjustment DOES NOT CHANGE the spring rate.
NOTE: The baseline settings should be provided in your owner’s manual. These are the basic factory settings for you to use as the starting points, from which you can add or detract to find the settings that you prefer. Also, note that the same settings may not suit two different riders as each person rides differently.