Essex Designed AP Racing Radi-CAL Competition Brake Kit (Front 9660/355)
Complete front Essex Designed AP Racing Radi-CAL Competition Brake Kit for the Subaru WRX STI
What's in the box?
- One pair AP Racing CP9660 Radi-CAL Pro5000R racing calipers
- One pair 355x32mm AP Racing CP5773, 72 vane, J Hook racing discs, with fully floating 6061 hard anodized aluminum hats and attachment hardware
- One pair caliper mounting brackets and hardware
- One pair Spiegler Stainless Steel Brake Lines with abrasion resistant coating
- Detailed installation instructions
- Brake pads are NOT included with the system.
“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. For more details on the design concept and what these calipers have achieved in professional racing.
- Massive Stiffness Increase- A 30+ % increase in both static and dynamic stiffness vs. conventional calipers allows for far less deflection under load, which means superior pedal feel & modulation, more even pad wear, and longer caliper service life.
- Considerable Mass Reduction- Removing all extraneous caliper mass lowers the caliper weight, despite the huge stiffness increases.
- Optimized Airflow- Air moves around and through the caliper more efficiently, providing superior heat evacuation and cooling.
- Efficient Packaging- The asymmetric caliper profile and internal fluid porting allows the caliper to fit into tighter spaces.
CP9660 PRO5000R, The Everyman radi-CAL
It has taken eight years, numerous generations of the design concept, and advancements in manufacturing techniques, but the Radi-CAL has finally evolved into a viable solution for racers and enthusiasts of all levels and budgets. While many products are supposedly born in racing, there can be no doubt about the Pro5000R pedigree. These calipers are direct descendants of the current crop of F1 and Sprint Cup calipers. They don't look ordinary, because they're not ordinary. They're the epitome of pure racing design: elegant, sparse, and brutally effective. And while they embody and employ the Radi-CAL design philosophy of the past, they also add some outstanding new features that enhance their practicality, convenience, and appeal. Quite simply, they are the lightest, stiffest, and most technologically advanced brake calipers that have ever been within the average enthusiast's grasp.
These intricate calipers have traditionally been machined from proprietary aluminum alloy billets. As one can imagine, machining away all of that material to achieve the final form is both time-consuming and costly. As a result, the price of these calipers has historically been prohibitive for the average club racer, time-trialer, or HPDE participant. Fortunately, the Pro5000R calipers use a new drop-forging technique that has dramatically lowered the manufacturing cost and the resulting price of entry, bringing this incredible technology to the masses.
Radi-CAL = Massive Dynamic Stiffness Increase
Over the years, many different strategies have been employed to shrink the brake caliper footprint while maintaining adequate stiffness: More robust caliper bridge designs, two-piece calipers with steel through-bolts, exotic materials (i.e. Beryllium, MMC), forging rather than casting, monobloc design, etc. All of these innovations pushed calipers towards becoming lighter, smaller, and stiffer. However, until the advent of the Radi-CAL, the same basic rectangular, boxy shape remained the accepted standard.
Is a rectangular box really the most appropriate and efficient shape for a brake caliper? AP Racing has now proven that it is not. As a brake caliper clamps brake pads against a spinning disc, a tremendous tangential friction force occurs between the pads and the spinning disc. In other words, the caliper is pulled strongly in the direction of the spinning disc, as represented by the downward pointing arrow on the right side in Figure 1 below. The caliper resists that force via the spindle hard mounting points, depicted by the upward arrow on the left. The dynamic load placed on the caliper twists or pulls the caliper into a parallelogram shape.
The dynamic force exerted on the calipers during a braking event is the key to the magic of the Radi-CAL design philosophy. Brake calipers of the past were designed to some extent in relative isolation from the forces that they were attempting to combat. They were designed to be the stiffest box possible while resting on a table, with much of the caliper mass residing at either end. The Radi-CAL is dramatically different because it was deliberately designed around the braking event forces that will be acting upon it. This is achieved by optimally distributing the body mass of the caliper on a diagonal, rather than at the two caliper ends. As such, the forces dynamically acting on the caliper are supported by the caliper mass, rather than what historically has been an empty box (see Figure 2 below). At the same time, this also allows for a significant reduction in overall mass, because all non-essential, non-load-bearing caliper body material is removed. So in the case of the Radi-CAL, it's not just about the material that is put into the caliper, it's about the material that is taken away!
What does the above mean for you, the driver? It means a significantly firmer brake pedal with superior modulation under all conditions. It also means more even pad wear with less tapering, and a longer caliper service life.
Discs-AP Racing Competition J Hook
The AP Racing J Hook discs in our system are the epitome of endurance racing components. They will hold up extremely well to any abuse you plan to throw at them. These discs have been proven time and again in professional racing, winning many races and even championships (ALMS, Rolex, Grand Am, etc.)
Two-piece Floating Design with Aluminum Hat
As is the case with most metal, iron brake discs grow substantially when heated. As it is heated, a disc expands radially, increasing in diameter and circumference. One-piece disc designs run into problems when this occurs. Look at the picture below and imagine the disc is being heated on the track. As the disc expands, the outer edges of the disc are pulling away from the center of the disc, but there are no built-in provisions to allow for that expansion. The edges of the disc therefore pull, lift, and distort, which is called coning. Now imagine that disc vertical on the car, running in your caliper. Coning directly impacts the brake pads’ contact with the disc, leading to uneven wear and tapering, and even a long brake pedal.
Two-piece discs on the other hand, compensate for the expansion of the disc as it heats. This is accomplished by building ‘float’ into either the disc itself, or the disc hat/bell. In this case, the float is in the disc. If you look closely at the picture below, you can see that the mounting holes for the hat attachment bobbins are not round. They are an oblong shape. These channels allow the hat mounting hardware to slide as the disc is heated and expands, allowing the disc to run true in the caliper without distortion. That means less distortion, stress cracks, and pad taper.
This Part Fits
- 2015-2017 Subaru STI Essex Designed AP Racing Radi-CAL Competition Brake Kit
- 2015-2017 Subaru WRX Essex Designed AP Racing Radi-CAL Competition Brake Kit
- 2018+ Subaru STI Essex Designed AP Racing Radi-CAL Competition Brake Kit
- 2018+ Subaru WRX Essex Designed AP Racing Radi-CAL Competition Brake Kit
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