The critical information for the correct spring rate is the relationship between static sag and rider sag. So you should have measured your rider and static sag. If not please got to Motorcycle suspension setup and find out how to measure and adjust those two figures by adding or removing preload.
As explained in the motorcycle suspension setup BOTH figures must be in the indicated range. If that is not the case you will be faced with one of the following scenarios:
For a quick reference check out the table below.
Spring rate is normally measured in Newton millimetres (N/mm). 10N/mm means you need a force of 10 Newton to compress the springs by 1mm. (don't worry, we won't go deeper into physics)
It is determined by the following variables:
Most dirt bike springs are made of steel but you also get titanium springs. They are lighter, look trick and are more expensive.
Progressive springs vs. linear springs
The coil diameter and the number of ACTIVE coils determines the spring stiffness = spring rate. If the coil diameter is the same, the spring with fewer active coils will be harder. When the coils touch each other they are rendered inactive. Why is that important?
There are 2 types of springs: linear springs and progressive springs. Linear springs have a constant spring rate and are often referred to as "constant rate springs". They have evenly spaced coils when you look at the spring from the side.
Progressive springs have coils spaced closer together on one side of the spring than on the other side. As the progressive spring compresses the coils that are closer together will touch each other and become inactive (coil bound). If a portion of the coils are inactive, the spring has fewer active coils and becomes harder.For various handling reasons it is not recommended to fit progressive springs on dirt bikes.
The only bikes where progressive springs will improve the handling are adventure bikes because of the load change (pillion passenger, bags etc.)
Changing preload only compresses the spring more or less. This does not make the spring harder or softer it only changes the bikes balance. Unless you physically remove the spring and fit another, the spring rate will not change.
The preload only raises or lowers the portion of the bike above the corresponding wheel. It affects the bikes balance which means it makes the bike higher at the rear and lowers the front or vice versa. This in turn changes the geometry of the motorcycle and that will change the handling.
That is why preload can only help to balance the bike when the correct springs are fit. If the springs are too soft and you try to get the sag right by cranking the preload your handling will be way off.
Changing springs is not a difficult job so any dealer/mechanic should be able to do that. It is a bit costly though as you must change front and rear at the same time. Otherwise you will have exactly those balance problems we have discussed above.
You don't have to buy original springs though. You get very good aftermarket springs like Eibach, Yacugar or Fox Racing Shox.
That is often not so easy so say. The dealer should be able to help
you out but often they know little about suspension. If you weight is not more than 10kg above or under the "normal" rider weight of 70-85kg you should be able to go one spring harder or softer front and rear.
Otherwise race tech has a spring rate calculator on their web site but it is sometimes simply wrong. I am not sure how they get to the values but it can help as a guideline.
It actually needs human judgement and experience to get it right not just a formula. Why? E.g. in some years the damping in the suspension was made rather hard by the factory. Then even if you would need to go 2 steps harder you can get away with only one. It also depends on your riding style. The faster you ride the harder your suspension has to be etc. So the suspension technician of your trust will take all the variables into consideration and use his experience to chose the right sprins for you.
Therefore best is to ask a suspension specialist.
I can recommend
The Dirt Bike Garage Manual
HOW TO GUIDE for "do it yourself" riders