The Audi S5 (B9) is a brilliant grand tourer, blending Quattro traction with everyday usability. However, like many Audis, it carries significant weight over the front axle. This nose-heavy distribution can make turn-in feel reluctant, especially when the brakes heat up on a twisty road. This case study details the ultimate remedy: a Custom Big Brake Kit featuring the legendary AP Racing Radi-CAL™ calipers and StopFlex Carbon-Ceramic rotors.
The Struggle
While the factory 6-piston Akebono brakes are competent for the street, they reveal limitations when pushed:
- Understeer: Heavy iron rotors add unsprung mass to the front corners, exacerbating the car's natural tendency to understeer.
- Thermal Fade: The S5 is a heavy car (approx. 1,700 kg). Repeated high-speed stops can overwhelm the thermal capacity of standard iron rotors, leading to a soft pedal.
- Brake Dust: The factory pads generate excessive dust, requiring constant cleaning to keep the intricate wheels looking their best.
Technical Specification
This is a complete Custom Big Brake Kit (BBK) overhaul. It replaces the factory front calipers with motorsport-derived AP Racing Radi-CAL™ units and pairs them with lightweight StopFlex Carbon-Ceramic rotors.
| Vehicle | Audi S5 (B9 / B9.5) |
|---|---|
| Front Hardware | AP Racing CP8521 6-Piston + 393mm CCB |
| Rear Hardware | Factory Single Piston (Retained) |
| Weight Saving | ≈ 16 kg (35 lbs) Reduction Front Axle |
| Pad Material | StopFlex Low-Dust Ceramic Compound |
Compatibility Redline
This is a Custom Big Brake Kit (BBK) application. It upgrades the front axle to AP Racing World Radi-CAL II (CP8521) 6-piston fixed calipers. This system requires 20-inch wheels for clearance and is not a direct replacement for standard calipers. If you wish to retain your factory calipers, please see our OE-Replacement Carbon-Ceramic Rotor upgrades.
Engineering Validation
1. Neutralizing Understeer
StopFlex carbon-ceramic discs are over 45% lighter than factory iron discs. Combined with the lightweight AP Racing calipers, this setup removes approx. 16 kg of unsprung mass from the front axle. This reduction in rotating mass significantly sharpens turn-in response, making the front end feel lighter and more eager to change direction.
2. Motorsport Pedal Feel
The AP Racing Radi-CAL™ design is famous for its asymmetrical stiffness. It resists flex under heavy load far better than standard calipers. When paired with the immense friction of the StopFlex rotor, the result is a pedal that feels incredibly firm and linear, allowing for precise trail-braking.
3. Fade-Proof Endurance
The 393mm (15.5 in) StopFlex rotor provides immense thermal capacity. It can absorb and dissipate the heat generated by repeated high-speed stops without fading, giving you the confidence to push harder on track days or spirited drives.
4. Clean Aesthetics
The AP Racing caliper is a work of art, and its open design showcases the carbon-ceramic rotor perfectly. Our low-dust ceramic pads keep the entire assembly clean, ensuring your wheels always look their best.
Installation Gallery
In-Depth S5 Bespoke FAQ
Q: Why choose AP Racing calipers over the factory Akebono ones?
The factory Akebono calipers are good, but the AP Racing Radi-CAL™ system is a dedicated motorsport-derived upgrade. It offers superior stiffness (less flex), better cooling airflow, and lighter weight. It is the preferred choice for drivers who demand the absolute best pedal feel and modulation.
Q: Will these fit under my factory 19-inch wheels?
It depends. The 393mm rotor combined with the large 6-piston caliper typically requires 20-inch wheels for comfortable clearance. However, some high-clearance 19-inch motorsport wheels may fit. We always recommend verifying clearance with a template before ordering.
Q: How do I maintain these rotors?
Carbon-ceramic rotors are incredibly low-maintenance. They do not rust, and they wear extremely slowly. Simply keep them clean during regular car washes and avoid using acidic wheel cleaners. With proper care, they can often outlast the vehicle ownership period on the street.
Q: Is this system noisy?
No. Despite its race-bred DNA, the system is tuned for street refinement. The StopFlex ceramic pads and anti-rattle hardware ensure silent operation, making it perfectly livable for a daily driver.
Q: Can I upgrade the rear brakes too?
Yes. While this kit focuses on the front axle (which does 80% of the work), we offer matching rear rotors and pads to complete the look and performance balance. However, replacing the rear *caliper* is complex due to the electronic parking brake, so most owners stick with a rotor/pad upgrade for the rear.
