adjusting front forks preload

here are two files (PDF) that I find very helpful on all my bikes re: setting up suspension
Download suspension.pdf from Sendspace.com - send big files the easy way
Download SuspensionGuide-02.jpg from Sendspace.com - send big files the easy way

 

Guys he is correct, don't compare a bathroom scale with a fork spring PLEASE.
springs are rated and that is a set # when you set the preload ( like adding weight to the spring ) the spring will compress less with added weight overall then it would w/ no preload, because it is being compressed by the preload adjuster.
so say I have preload set at something that say ='s 20lbs, all that means is that I would need to add 20lbs to the forks to get them to start to compress.
There are many great books on the subject.

 

There are some valid points here but if you really are interested in the so called 'black art' have a look at an old fashioned thing called a book...and that book is The Racing Motorcycle...A Constructors Guide. Pretty much a physics text book for motorcycle engineers.
The rate of a spring does not change unless it is a progressive wound or rate spring.
Springs support the weight of bike and rider. Damping controls the oscillation of the springs with the damping medium of oil passing through valve ports and shim stacks.
OK, Pre-tension or Pre-load...shocks and forks have pre-load when built so the spring is not in a 'topped out' condition.
With all damping adjusters wound fully off lift the bike so the wheels are off the floor. One at a time is ok. Now measure form the fork dust cover to the lower clamp/triple/yoke. From the top of the spindle nut vertical to a point on the tail unit..masking tape is handy for this.
Bike back on the ground and upright, take the measurements again. That's the static sag. Consult manuals etc but I aim for about 25-30mm on the front and 10-15mm on the rear.
Do this again with rider in riding gear on-board to get the rider sag ( Difference between the two)
If you add a passenger then pre-load the shock spring so you get the same sag as the rider sag. This means the attitude of the bike stays the same as just having the rider on-board.
Ride height is a secondary effect of pre-load. Ride height is for changing the attitude or geometry in order to change weight bias and steering. Best achieved by adjusting shock length - distance from eye to eye and or pushing the forks up through the clamps/triple/yoke...small amounts make a big difference.
My '99 VFR 800 has 10grade fork oil with 130mm air gap and no external pre-load dialled in at all with a Nitron road shock in the back. I like to press on a bit and have raced for three years. This arrangement is comfortable and handles well with decent feel.