Do not cut the springs! bike forks are so close to coil bind so you'll loose useable travel.
Don't just buy X brand springs talk with a supention house. Ask questions etc. you don't need to spend a ton to have it set up right.
Talk with Peter he's more then happy to help you figure this stuff out.
Peter Kates
G.M.D. Computrack Boston
11 Summer St.
Bellingham, MA 02019
508-876-9407
peter@computrackboston.com
Here are a couple of general rules about springs. First there is no such thing as, " the spring for my weight." Each rider may have a different preference for the feel they like. Typically there are three spring rates that will work for any given rider weight. If lets say the calculated spring rate for a 180lb rider for a particular bike is a 5.4 kg/mm, you should test a 5.2 kg/mm and a 5.6 kg.mm. Only the rider can make the final descision. Second MX and Off road springs seem to work best with 6-12mm of preload. To find this you will need to measure the spring free length before it is installed on the shock. I set the preload at 8mm and check the free sag and the rider sag. You should get 24-30mm of free sag and 95-115mm of rider sag with the preload at 8mm. If this checks out you have a good starting point.
Remember that springs are only position sensitive. There is no speed factor here. A 5.4 kg/mm spring will take 5.4 kg to compress the spring 1mm, 10.8 kg to compress it 2mm etc. Damping force is speed sensitive. If the shock or fork is not moving there is no damping force being generated, reguardless of where in the stroke it is. The position sensitive component refers the amount of force that can be generated at that point in the stroke. For example, lets say that you have a leverage ratio curve with a 3:1 ratio to start and 12% positive progression. If you move the wheel at a given speed, we'll say 75 mm/sec., from full extension through the first 25mm of travel, it moves the shock about 1/3 the distance, and about 1/3 the speed. So the shock sees about 8 mm/sec. The piston, valving and clicker postion will dictate the damping force generated at that speed, but for this example we'll say it's 100 lbs of force. Now if you move the wheel at the same speed (25mm/sec.) but it's in the middle of the stroke you will have about 106 lbs of force. Then at the last 25mm of wheel travel you will get 112 lbs of force. What is happening is the shock is being moved more per mm of wheel travel as the suspension is being compressed. Since the shock must travel futher in the same amout of time, the shaft speed is increased and produces more damping force. Some systems, i.e. KTM's PDS system, also adds a second piston that kicks in at a specific point in the shock stroke. The damping force is still dictated by the shaft speed, but the shock can now generate more force at the same speeds.
I took the time to explain this because the spring rate and preload determin where in the stroke the bike will ride, and thus what part of the leverage ratio curve is used. This is why putting on a softer spring can make the bike feel stiffer. The softer rate will make the bike sit further into the leverage ratio curve, which is generaly more progressive than the top. As a general rule spring rate and preload don't make the suspension feel stiff, the damping rates do. A spring rate that is too firm will make the bike feel "tight", but not stiff, or harsh.
Spring rate and preload also affect the dynamic geometry. Let's say we have a good set up to start with. Now if you put a softer spring on the back, the rear of the bike will sit lower and slow down the rake and trail at the front. This will make the bike steer slower and be harder to "flick" left to right, as well as feel heavier through the bars. If you fit a stiffer spring the bike will sit higher in the travel and steer more sharply, flick easier, and turn faster. The same priciples apply to the fork springs, but softer springs make the bike sharper and stiffer srping make it slower steering.
The damping forces, both rebound and compression, are to control the speeds at which the springs can move. The key to off road suspension is to allow the wheel to move fast enough, but with losing control of the chassis. Balancing spring rates, preload, and damping curves are the goal here.
I hope this helps you, and I hope I explained it so you can understand what's going on with the suspension system. It's very complex and difficult to convey with out visual demonstrations. The only way to really get the spring rate right is for you to test the bike where you will ride it. Suspension set up is a never ending maze of compromises, you'll never get it perfect, just really good. As you get faster your needs for spring rates and damping will change too, so don't just set it and forget it. Take good notes and keep track of changes and you will come up with a good base line for most conditions.
Hope this helps, feel free to call, email, post etc, if you need help.
