Don't know your unsprung weight from your static sag, or your dual-rates from your progressives? Now you will
What do they do?
Front fork give a line of communication between handlebars and front wheel spindle, while providing suspension - absorbing irregularities in the riding surface, isolating the rider from bumps, keeping the tyres in contact with the ground and the bike level. Hit a bump with solid, unyielding forks and the force acting upwards on the front wheel can exceed that of gravity trying to keep it down, so it leaves the ground - not nice in a corner.
A spring placed between the main mass of the bike and the front wheel not only allows much of the upwards force of a bump to be absorbed while keeping the bike level but, thanks to its sotred energy, can accelerate the tyre on the downside of the bump, thus keeping contact. Without a spring, the fornt end of the bike could only fall back to earth as fast as gravity allows.
If, for example, the front of yr fuelled bike weights 150kg and the unsprung weight (wheel, brakes etc) below the spring is 10kg, then the acceleration rate can be calculated by dividing the force driving the wheel (a spring loaded by 150kg) by the mass of the thing it's acceletrating (a 10kg wheel). So we're dividing 150 by 10 to give us 15g (gravity, not grammes) of vertical acceleration, so your wheel can track the deck 15 times faster than with a rigid fork.
Spring rate - the extent to which a fork spring can absorb the upward acceleration (ignoring inertia) of the wheel - is dictated by the spring's 'rate'. Spring rate is measured according to the amount of force it takes to compress it by on inch and if the 'pitch' (the gap betwwen coils) is constant, so is the rate. A 50lb-in spring will compress an inch for every 50lb of load until there is no travel left.
Let's say our 150kg (330lb) load is fitted with two 50lb-in fork springs and hits a bump one inch high. Each spring compressses an inch or so, bearing in mind there are two springs, 100lb of force acts upon the bike. Acceleration being the result of force (100lb) divided by mass (330lb), we're feeling the bump as 0.3g of upwards acclecration. Without springs the force would likely be serveral g.
Reduce the combined spring rate to 65lb-in and you'll be only feel 0.2g, so the ride will be smoother and the tyres will have an easier time tracking the road.
There is, though, a flipside to softer spring rates. Weigt will transfer further under braking, leading to excessive fork dive to the point when there's no travel/suspension left and weight transfer can throw the rear wheel skywards.
A spring rate compromise must be struck for the type of bikes, road surface and aggression of the rider.
The life of a fork spring is tough. Not only must the spring be strong enough to cope with having the whole weight of a bike and rider being thrown on to it during hard braking, it must also be sensitive so as to track the road when lightly loaded under acceleration. Try to build a degree of compromise into the forks, most bikes now come with 'dual-rate' or 'progressive' springs as standard.
The closer the coils of a spring, for a given material and diameter, the 'softer' its rate. A dual-rate spring will have coils more tightly wound at one end than the other. Once all the close-wound coils are compressed, the wider-spaced ones come into play at a higher spring rate because the rate is inversely proportional to the number of coils in operation. So ther first half, third, or whatever, of travel can be sensitive with the last half robust enough to prevent 'bottoming' when hitting bumps under braking. Fully progressive springs have their coils wound, you guessed it, progressively further apart over the lenght of the spring for a smooth increase in spring rate as the forks compress.
Increasing preload does not, repeat not, make a spring stiffer. As the spring is already supporting the weight of the bike, pushing down on the spring simply raises the bike, increasing the wheel's available upward travel and altering the bike's stance relative to its height at the rear.
Too much preload can lead to another problem - 'topping out' - where raising the bike and upper half of the forks leaves too little downward extension. Hit a dip in the road and the fork springs accelerate the wheel to follow the contour. If, however, too much preload has been added and too little travel is left, the bike will fall harshly into the dip and the front end will leave the ground easily under acceleration as the forks fail to extend.
extracts from Bike Dec 02