Re: Rubbers on traction bars
By tightening the bar's clamps to draw it up against the spring you are flatening out the spring which puts a load on the spring (hence preloading). Springs are rated at pounds per inch of deflection, so it doesn't take much of a deflection of the spring to generate 300 pounds to 600 pounds of preload.
How much load you put on the spring is determined by how your car launches. You can tune the bars change tire pressure and if you have a posi add clutch discs or increase the spring rate of the coil springs inside your posi unit to drive the car in a straight line upon launch.