Answer: Because preload isn’t stiffness. It’s ride height. People think they’re making the bike ‘harder’ for the track, but all they’re doing is messing up the geometry and the sag. If the bike is mushy, your spring rate is wrong for your weight or your valving is shot. Cranking a knob isn’t a magic fix for a bike that needs a real setup.
Sport bikes come with a multitude of suspension adjusters, and most riders don’t take advantage of this adjustability. Everything needs to work together here, spring preload, compression & rebound damping front and rear. For the most part the adjustability from the factory will allow some basic set up but for the track you need to take things up a notch.
Start with getting the correct Race sag front & rear, approximately 30-35mm front and 25-30mm rear. You may or may not be able to achieve these sag numbers with the stock spring rates and may need to replace springs. You have the correct spring rate when you hit your sag numbers, and the spring is at the recommended preload. Spring free length minus its installed (compressed) height is the preload. Once the bike has the correct springs and sag then it’s time to dial in the compression & rebound damping, you want the suspension to absorb bumps and pavement irregularities and return in a controlled manner. Too stiff on compression and the tire will deflect when it hits a bump, too soft, and the fork will blow thru the stroke too easily. Too much rebound and the fork will not return in time for the next bump, too little rebound and the fork extends too quickly causing the bike to run wide at the apex when you begin to open the throttle.