I had a very interesting introduction to preload. My friend had gotten a new bicycle with a rear monoshock. As he proudly explained this spring made the bicycle go faster, i examined it closely.
I found that i could make the spring longer or shorter by turning it.I made the spring as long as possible [ minimum preload ] and rode the bicycle.It offered a comfy ride on bad roads but moved far too much on tarmac.Trying to pedal hard meant most of the force was soaked up by the spring and i just bobbed up and down in the seat.
I made the spring as short as possible [ maximum preload ] and rode the bicycle. The ride was harsh and bouncy on bad roads but conducive to high speeds on tarmac. I could....put the power down so to speak and not bob up and down as before.
Of course, i has not heard of the term preload as a kid but have come to the following conclusions now -
1. Every rider weighs a certain amount, and the springs that hold him up must be set to work accordingly. If the springs are set to bear a rider weighing 50 kilos and a 100 kilo man sits on the bike, the springs act as if a 50 kilo rider is on the bike, and an extra 50 kilo force is being exerted by the bumps or whatever.
He therefore feels a lot of movement when riding around, the forks are diving on the brakes, the suspension feels soft etc.To eliminate these problems, the springs must be set such that - A 100 kilo rider upon the bike produces no movement. When extra force over and above the rider weight is exerted, the springs must compress and rebound as usual. Therefore preload tells the springs how much load will be put on them initially.
2. All straight rate springs produce equal amounts of compression for equal amounts of applied force. If a spring compresses 1mm for every kg force, you can take it for granted x kgs produce x mm of compression.
Progressive rate springs are those which produce varying amounts of compression for the same applied force. They will compress x, 0.8 x , .065 x etc for each kg force applied. These are usually not used in bike suspension.
Therefore, no matter how much preload you add or remove, the maximum extension remains the same.Preload only tells the spring - Start working soaking up forces greater than the riders weight, which is x kilos.If the spring is told [ by virtue of preload ] the rider weighs less than he really does, the spring treats the weight difference as added force that must be damped. If the spring is told the rider weighs more than he really does, the spring will pass on forces equal to the weight difference up to the rider and starts damping only after it experiences forces greater then the preload it's set to.
Therefore, it is imperative to set preload correctly first and then move on to compression and rebound damping. Incorrect preload means incorrect rebound and compression damping as a rule.
3. Most lower priced bikes come with preload adjustment. It is possible the soften or stiffen the suspension to a certain extent by adding or removing preload. Going two up for longer distances mean setting the preload to allow for the extra weight.
4. The range of preload adjustment is determined by the spring rate, spring length and number of coils [ correct me if i have missed any others ] . If you weight a certain amount , get springs that work best for that weight range. If you get a spring that works best for someone 25 kilos heavier than you, it's kinda pointless.
Thanks to all who contributed and YD for starting the thread. Can we move on to a discussion on setting sag? I have never done this, perhaps those who have can share their experience with pictures?