Complete front Essex Designed AP Racing Radi-CAL Competition Brake Kit for all C5 Corvettes.
What do I get?
“Radi-CAL” is a blanket term used to describe AP Racing’s patented asymmetrical brake caliper design. Radi-Cal technology is grounded in Computer Aided Design (CAD) and Finite Element Analysis (FEA), and allows for organic, alien-looking designs that are a radical departure from conventional caliper designs of the past. Radi-CAL's are widely considered the pinnacle of current brake caliper technology. Since their inception in 2007, these revolutionary calipers have amassed a lengthy string of race victories at all levels of professional motorsport, while redefining brake performance expectations.
The key benefits of the Radi-CAL design:
As mentioned above, the Radi-CAL design philosophy is just as much about what isn't there, as it is about what is there. The Radi-CAL design relocates caliper mass, creating voids that would traditionally not be located where they are. As such, considerable mass is removed from the body, and the entire envelope of the caliper is optimized. The result is an extraordinarily lightweight and compact footprint. Whereas many competing six piston calipers weigh in the 9-12 lb. range, the CP9660 weighs an astounding 6.1 lbs.!
The Essex mantra when designing brake kits has always been, "Anything larger than necessary to get the job done is simply dead weight to drag around," and the Radi-CAL aligns perfectly with those values. One of the major problems with many of the brake packages currently on the market is wheel fitment. You’re offered gigantic discs and 12 piston calipers, with a pat on the back and a, “Good luck finding wheels to clear those things (insert sinister chuckle here).” The reality is that many casual racers want to use their OEM wheels on the track, or the smallest, lightest wheel they can find. Not only is saving unsprung weight critical, R compound tires are much more plentiful and cheaper for smaller wheels. If the components are designed properly with heavy use in mind, you don’t need to cram boat anchors under your wheels. If you’re worried about the loss of stiffness due to mass reduction, don’t. Some manufacturers use a heavier six piston caliper, but that's because the caliper wasn't designed or optimized for racing. In those cases, the same caliper may have been designed for use on much heavier road cars, and even trucks! AP Racing’s Pro5000R calipers are incredibly stiff, and designed from scratch with only the racetrack in mind.
Pad Thickness, CP9660 caliper (18mm) vs. CP9668 caliper (25mm)
For many of the platforms we are servicing, we have both the CP9660 and CP9668 calipers available. The biggest difference between these two calipers is the thickness of the pads that they can accommodate. The CP9660 caliper uses an 18mm thick pad in the above shape, while the CP9668 caliper uses a 25mm thick pad. Which one is right for you? If you're running multi-hour endurance races, or if you want to reduce the frequency of pad changes, the CP9668 is likely your proper choice. If you're running standard 20-40 minute HPDE/Time Trial sessions, or sprint races, the 18mm thick pads will be more than ample. There are two primary tradeoffs when going with the CP9668 caliper: It is about a pound heavier (including the difference in pad weight), and roughly 14mm wider than the CP9660. You will lose that 14mm on wheel spoke clearance vs. the CP9660 kit, so please make sure to check both fitment templates if you're debating on caliper choice.
There are people who will tell you that aluminum pistons are great for track calipers. They will tell you that the expansion rates of the pistons and caliper body need to be the same when heated. This argument is completely invalid and unproven. Those same people tend to get upset when you point out the fact that every serious race caliper, from every serious race caliper manufacturer on the planet uses either stainless steel or titanium pistons, period. There is a reason for this: they're better!
Stainless steel pistons are far superior to aluminum pistons in creating a thermal barrier. They are much better at keeping heat out of your brake fluid and preventing a soft pedal from fluid fade on the track. This has been proven over and over again at all levels of motorsport. While most aftermarket calipers use a pressed aluminum piston, the Pro5000R's use an expensive machined stainless steel piston.
To add stiffness to the pistons, AP designed the back of the piston with a domed back. At first glance this seems like a trivial design element. It is not. When domed back pistons were introduced in professional racing, driver feedback was immediate and resoundingly positive. The domed back adds considerable stiffness that can be felt through the pedal, and they have now become the standard vs. which all designs are judged.
For even greater heat resistance, there is ventilation on each piston. The air gaps around the piston edge allow for even more cooling air circulation around the pistons. All of these features slow and repel the influx of heat into the brake fluid, preventing brake fluid boiling and fade
Not only are the pistons stainless steel, they are fitted with anti-knockback springs. Springs in pistons you ask? Yes, springs. If you’ve ever gone through a series of S turns and then had your pedal drop when going into the following brake zone, you have experienced knockback. To say it is disconcerting is an understatement. You’ll often see pro drivers ‘pre-tap’ their brakes lightly when approaching a brake zone. They are fighting knockback.
Knockback is a phenomenon that is common with fixed calipers. Knockback occurs when your car’s wheel, hub, and bearings deflect during cornering, allowing your brake disc to move out of sync with your caliper and brake pads. The amount of knockback varies by vehicle, and depends on the amount of deflection seen in the parts listed above. As the brake disc deflects, it actually pushes the pads away from each other, forcing the caliper pistons back into their bores. The piston seals don’t have enough tension in them to completely return the pistons to their original location. That means there is slack in the system that needs to be taken up. When you press the brake pedal, it will continue to drop until that slack is taken up.
Anti-knockback springs help alleviate this situation by putting some tension on the back side of the pistons. When the disc deflects and makes contact with the pistons, the springs push the pistons back into their proper location, reducing slack in the system. That means less pedal drop and far fewer pucker-factor moments when going into heavy brake zones.
There are no major downsides to lightweight AKB spring as long as the caliper is designed to accommodate them. More specifically, AKB springs do not create any increased drag or wear on the pads and discs as long as the shape and material of the piston seals takes them into account.
As you're driving the suspension is constantly compressing, the disc is moving around laterally, and the pads are being pushed slightly away from the disc. Think of the seals in the caliper as a spring or hinge attached to the side of the piston, rather than just a ring through which the piston slides. In an AP Racing competition caliper, the groove in which the seal resides isn't a square cut groove.It has angles. When the pistons slide in or out there is friction between the outer piston wall and the seal, and the seal distorts a bit as shown in the illustration below.
A caliper piston sliding out to the left would distort the seal in this manner (the slashes are the seals on either side of the piston):
As the piston slides back in to the right, the seal does this:
There is a certain amount of tension or friction that needs to be overcome before the piston actually starts moving through the seal ring. That tension/friction keeps the piston from dragging on the disc once the pistons are pushed back into the bores by the disc/suspension movement.
When AKB springs are added, a little more force is required to push the pistons back into their bores than would be required without them.After the spring is compressed, it unloads and pushes the piston back to 'neutral.'
With the proper seal and spring the goal is to keep the piston in the 'neutral' position, not pressed against the disc.The piston is still able to slide freely in either direction, but a bit of friction or tension needs to be overcome initially to get it moving in either direction. The seal offers that first bit of friction to limit movement, and then the spring provides additional resistance. The end result is that the properly designed AP Racing calipers won't drag or create additional or unnecessary