What is the "preload" setting on the front fork suspension?
I've made a photo of the actual "thing" as I find it very difficult to explain otherwise:
I'm wondering what this "thing" sets and how it actually works. So, what is "preload" and what happens when I turn it clockwise (plus) and anticlockwise (minus). I tried rotating it both ways to its maximum limit and I didn't notice any change in anything.
The bike the photo has been taken from is Merida Matts 40-D.
I always turn it to max(-) it fitted for me but I weigh 55kg.
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This fork setting exists so that the fork can be customized to your weight (major reason) and riding style (minor reason). It's simply the initial compression of the internal spring in the fork. The more it's compressed, the stiffer the fork will feel.
Bigger preload compresses the spring more, and so it's best for heavier riders and/or people who need/prefer the stiffer ride (racers etc.) Lighter riders should use less(-) preload. Less(-) preload will also give you plusher ride, but the bike will e.g. dive more during braking.
So rotating it to the maximum of anticlockwise (maximum minus), it will be suitable for a lighter rider, is that right?
Typically so, but just to be sure I would consult the manual for your fork to see if it adheres to the convention.
I have a similar model, from RST, and @ttarchala explanation is perfectly exact and complete. BUT, in some models (like these RSTs), the difference is not so big, and you'll probably feel it only on the worst terrains (like roads with rocks or stones, or even slowly going down a stairway).
For some riders the effect of the preload setting can be completely void specially if they are very low weighted, as their weight won't be enough to compress the suspension coil anyway, so tightening it won't make things much different. In a similar way a rider that is "too heavy" for the suspension will compress the spring to the bottom, so preload will simply do nothing.