Hit The Brakes
Here is a little write up that can save some headaches. Big wheels, wheel spacers, giant brakes, you are ready to hit the track with all those Big-By-Bigger parts all the cool kids on the internet use. You hit the autoX or road course and BOOM!!!! brake pedal goes soft. You gotta pump the brakes, the dreaded piston knock back washes away all that time, money, Johnny-racer right out of you. Why? well all the cool kids on the net don't tell you that behind all the cool pics and excitement they portray on FB, it's all for show. Those cool looking 14" weight penalizing rotors you put on move a lot more at the outer diameter than smaller rotors AND move at a higher speed than say a 11 or 12" would, the amount of deflection of the rotor is also greater. Wheels with high amounts of negative offset will also apply greater bending force on the hub further exacerbating the problem. Wider wheels and tires also will generate more force on the hub.
On most of our F-Bodies 1-3rd gens the knuckle has a spindle pin with serviceable bearings, effective low cost. However, they were designed for passenger cars using 5-8" wide wheels, centrally mounted (zero offset). Now we are putting on 9-11" wide wheels with sticky tires. The spindle pins flex, binds up the bearings, causes the rotor to move in and out in relationship to the caliper. The bigger the rotor the greater the amount of movement at the caliper piston radius. This pushes the pistons back in their bores (piston knock back). This is especially aggravated with fixed, opposed piston calipers such as 4-8 piston calipers. How to minimize this problem of piston knock back? -Floater calipers. (this may not be possible on high end braking systems) -Keep the rotor 13" or less. -Use wheels with near zero offset or some positive offset. -Use knuckles with the largest pin available. -Use bearing spacers.