"These entries are part of a Special Series on the development of the M3 V8's Bespoke AP Racing Clubsport Brake Kit.

In this entry, I explore the inspiration, research, I dig out archives of historical testing, and provide insights on how we improved upon the concept of increasing rear bias on the E9X M3."
- Matt

The inspiration to shifting bias rearward [...]

A blast from the past: the PFC 355mm kit.

The OG owners of the E9x M3 platform will remember the PFC brake kit for the E8x/E9x M cars. 

They were arguably the first company to bring to market a kit that shifted bias to the rear while focusing on a much lighter 4 piston. The front kit was 10 kg / 22.2 lbs lighter than stock. 

When you cut through the marketing charabia on the brochure, you would find ingenious designs. It had snap-in rings to fit the rotors to the bells. 

It still had some engineering shortcomings: the caliper design was quite crude: it traded machining costs and overall caliper weight to achieve stiffness. 

This is their very similar Z4M kit. The PFC kit is unfortunately discontinued. 

When magazines were cool: the SportAuto test. 

Better performance and lighter weight? Bold claims. It was put to the test. The PFC kit was famously tested by SportAuto, a legendary German magazine known for their intensive Nurburgring reviews and unfiltered lap time leaderboard. 

The lineup. 

They lined up the OEM brakes versus off-the-shelf big brake kits from major brands such as the AP Racing, Brembo, KSport, StopTech and PFC. 

The tests were conducted using an E92 M3 GT4 at the DEKRA test centre on the same pavement in similar weather conditions.

They standardized tires by fitting new 245/35 and 265/35 Michelin Cup2 tires every time they changed BBK brand. Lastly, an independent specialist oversaw the comparo. 

They tested 100-0, 200-0 and 250-0. They did not perform repeated braking tests to evaluate heat dissipation. The 250-0 gives somewhat a measure of heat dissipation considering the kinetic energy that needs to be transferred to heat. 

The pads used. 

The pads used were the specific compounds supplied off the shelves by the manufacturers and as approved by the TÜV for road legal use in Germany. 

As such, wasn't a true 1:1 test of the hardware, but rather a comparaison in the development philosophy of each brand. 

The surprising results.

Scroll down for the transcript. 

The OEM brakes. 

The stock brakes are single piston floating calipers with 360mm rotors. The front brakes are boat anchors coming in 35.8 kg. 

• 200-0: 127.3 meters
• 250-0: 206.2 meters

Brake distances were good, showing the stock brakes are fine for daily use and occasional tracking driving. 

It gets problematic once you increase temperatures. They reach their critical thresholds fast, and consumables become expensive as you burn through rotors and pads that can't dissipate heat appropriately. 

The K-Sport is non-starter.

The KSport kit is what happens when you try to undercut expensive engineering with brute force. 

KSsport selected a ridiculously sized 8 piston caliper with 380mm rotors clamping on Pagid RS29 pad coming in at 37 kg.

• 200-0: 122 meters
• 250-0: 190.4 meters

There were critical issues. The Ksport suffered from severe vibrations after repeated use while weighting more than stock. 

Additionally, the KSport used the same pads as the AP Racing kit and barely outperformed it on stopping distances from 250 kmh to 0. 

The Brembo GT results were atrocious.

The Brembo GT was fitted with 6 piston front calipers and 380mm rotors weighting in at 29kg. 

• 200-0: 129.2 meters
• 250-0: 214.2 meters

It objectively had the least braking power of the bunch. 

The reviewers commented on the initial bite being inconsistent. They further noted how it felt very low. After re-bleeding the pressure point was still inconsistent and often required a double braking action to restore the pressure point.

It was claimed retroactively that Brembo delivered the wrong kit - mmm, sure Brembo. 

The AP Racing kit was proper. 

The AP kit at the time was fitted with the 6 piston CP5555-49 and 378mm rotors with a Pagid RS29 clubsport pad. The kit weighted in at 30kg.

• 200-0: 121.8 meters
• 250-0: 195.0 meters

Reviewers commented on a very consistent brake pressure point. APRacing hadn't yet released their Radi-Cal2 calipers. They proved their ability to refine and improve upon this kit in recent years. 

The StopTech is cheap, effective braking power.

SportAuto used the ST60 front kit with 380mm rotors. It's heavy, coming in at 34kg. 

• 200-0: 122.6 meters
• 250-0: 191.2 meters

Stoptech had short, consistent pedal travel and the best modulation. Highest deceleration and no fade.

The StopTech was however one of the heaviest kits in the test, and the caliper looks crude. 

The PFC kit was surprising. 

PFC fitted its kit with a 4 piston caliper grabbing a 372mm rotor. It came in at 24kg. That's 10kg lighter than the stock brakes and lighter than any other BBK in this test.

• 200-0: 124.5 meters
• 250-0: 188.9 meters

The 250-0 test result were intriguing. It was the best result in this group test. This was hinting that shifting bias rearward could potentially have positive effects on heat dissipation, and repeated stopping power while being much lighter than it competitors.

The commenters noticed a consistent and shorter pedal travel as well as being easy to modulate: all good things.  

Building upon these experiments [...] using AP Racing hardware.

In a previous entry to this series, I took a deep dive into the hardware selection and its +/- 

The section belows covers how the hardware selection fundamentally shifts bias rearward while improving feel & performance. 

Increased Pad & Clamping Area.

The caliper uses the pad shape CP6600D55. The 64.60cm2 surface area is a 15% increase in pad area, it equates to which 15% increase in clamping area. 

Furthermore, increased clamping area provides better heat and force distribution onto the rotor resulting in better brake feel, brake bite and modulation.

Pad Area - cm² (in²)

64.6 (10.01)

 

Multi-Pistons Calipers.

The AP Racing 4-piston caliper has many benefits over the OE calipers. Relevant to shifting bias rearward, the multi-piston design has equal force distribution over the rotor. 

Equal forces eliminates pad tapering, technical wordage to indicate uneven pad wear between the caliper and the rotor. This improves modulation, which has a direct impact on bias when considering the next point. 

Change in center of pressure.

The positioning of the 4 pistons is partly dictated by the bracket. When comparing to the single piston OEM calipers, the AP Racing caliper sits slightly higher. The caliper pistons are staggered diameters at Ø38.1mm and Ø41.3 mm. 

This creates a very small difference in brake bias compared to stock. HaimusRacing was confident this small change would be well within the limitations of the smart ABS adaptation capabilities. 

Up next: I review the objective performance results and I review how this translates to my subjective experience with the kit after a few months of tests.