"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 compare the OEM rear brake system components off the Stripper against the finalized rear section of the Custom Clubsport AP Racing kit critical to complete the braking bias shift to the rear." 
- Matt

A refresher. 

This is the longest running Special Series on EuroConnex for a very clear reason: brake systems are complex. My intention is to empower BMW M owners with the knowledge to make the right decisions for their objectives. 

For a refresher on the hardware fundamentals of this Bespoke AP Racing kit. Please review the front kit's AP Racing hardware here and the custom hardware here.

It's been an amazing learning experience for me and I feel better equipped to guide M owners in their selection. Reach out for a chat anytime.

As always, whether you buy the kit I had developed for my car doesn't matter - be aware of what you buy. 

Calipers

I address the caliper comparison first as the aesthetics, for better or for worse, are a primary driver in purchasing these Clubsport big brake kits. 

We selected a similar caliper as the front unit: AP Racing's 4 pot radical-2 CP9541 monoblock forged caliper with reduced piston area over the clamp. 

In short: it's light, it has a wide range of pad selections, it has dust boots for street use, sliding pads and has proper piston dimensions to achieve our target rearward bias with BMW M's ABS.

You can read the complete initial specs here or at the AP Racing catalogue here. 

One of the caliper's paint was unfortunately damaged during transport. It revealed a cost cutting short cut I wasn't pleased with: the painter didn't apply a sealant and primer prior to painted the calipers.

I touched on the critical importance to primers and their effect on paint longevity in this entry to my E46's restoration. The production calipers will have primer applied before paint. 

The measly OE calipers are floating single piston - not any different than the front calipers except for its juvenile dimensions. 

The AP Racing caliper is massive next to it, I'm sure they optimized for weight here, the originals come in at 2.8 kilos.

The AP racing calipers come in at 3.3 kilos - wait, we're adding weight? 

Least we forget about brackets.

Brackets play a vital role in connecting the calipers to the knuckle. A floating caliper requires a massive bracket as it's an structural component of the caliper. 

The Genuine BMW bracket on the right is steel. The bracket on the left is 7075 T6 aluminum. It's 30% of the weight while being smaller.

We compromise on added weight where it matters, and we optimize for it where it doesn't. 

Rotors. 

You won't find much weight savings here, but a massive increase in performance. The OEM rear rotors are 350mm diameter and 24mm thickness.

We selected the CP4542-106/107 from AP Racing's catalog. It's a larger and thicker 355x32mm slotted rotor. The rotor itself comes in a 7.3kg without the ring. BMW M did pretty good to achieve a lower weight, coming in at roughly 19.7 lbs / 8.93kg.

The thickness is the obvious difference you notice between the two rotors. 

Two 2 piece versus meh 2 piece. 

BMW M defines it as a 2 piece semi-floating design as the disc is separated from the hub - but it is purely by the casting process instead of using bolts. This has benefits, and downsides.

The benefit is lower production cost and lack of any noises related to thermal expansion. We can do better without compromising street-ability. . 

 

The custom bell and AP Racing rotors are two separate units, but still considered semi-floating as they use bolts instead of bobbins. The main hub retains the 202x T6 material used on the front hats for its properties: it's 30% stronger than the industry standard 606X semi-billet aluminum.

This is the ideal compromise for a Clubsport kit. Little to no noise, the reduced weight of the bell and the reduced cost when it comes to swapping rotors only. 

I touched on why this kit doesn't use bobbins at the front, nor the rear here. In short, bobbins on the street is a PITA. 

Slotted versus Drilled

The next obvious difference is the use of slotted design over drilled. There is a lot of science between which design is most suitable for your intended application.

Today, drilled rotors are usually paired with specific pad materials that require the holes to gas outflows to improve rotor longevity. If you've ever wondered why our pads and rotors can last over 100,000km - this is a big part of it. 

We picked a slotted, curved groove rotor from AP's catalog. It is the ideal compromised between performance, pad grip and longevity.

Why not J hook rotors? Similar to bobbins, they're track products: they eat through pads faster as they provide better bite - and are also much more expensive. 

Vanes.

If your brakes get too hot, they fade. Heat dissipation is key. Vanes are a major component to the longevity of performance. Years of motorsport experiences have made AP the reference of brake disc technology.

The vanes are directional, the OEM brakes are as well. There are design subtleties that play a major impact on airflow. The dimensions of the vanes, the degrees of curvatures, etc.  

The OEM rotors' vanes are wide, and less numerous than AP. Science says say this is worse for heat dissipation. 

The AP's vanes massive, yet tighter. It enables AP to cast over 48 of them in this specific rotor. This is where the potential heat dissipation gains are mostly made over stock. 

A similar sized J Hook rotor would have 72 vanes. Do you need this for a dual purpose brake kit? I don't think so. 

E-Brake Compatibility. 

I highlight this section as this is an often misunderstood, and straight up unexplained limitations of many aftermarket rear big brake kits for the M3 V8. 

We used a good ol' drum-disc based emergency brake system. What's new? Well, nothing - but it requires a very specific rear hat design that some companies don't bother with. Instead, they will upsell an electrical e-brake system or just straight tell you the e-brake doesn't work anymore [...] in the small print. 

We once again opted for a compromise: the target owners for this kit are Clubsport guys. We like to think we're track stars but 95% of our usage is on the street. 

The rear hat features the necessary hat adaptations for the e-brake drum brakes to reliably make contact with. 

We still wanted to optimize for performance and weight reduction here. We designed and manufactured a unique 2 piece hub. The top piece is a zinc-steel that is extremely light weight. 

There's one downside: you can't drift with this setup. Pulling e-brakes induced slides is game over - not that I personally ever did those anyways. 

Pads

The pad sweep and actual contact surface & compound are variables in bias calculations critical to the rear bias of this complete kit. Choosing the right pad size was critical to achieving our objectives. 

The AP selection.

This AP Racing caliper is ideal for the Clubsport kit due to its widespread adoption. All major pad manufacturers have various compounds available for this caliper's pad shape. The caliper uses the pad shape CP6600D55. You can see the options I listed in a table of the previous article here. 

As with the front kit, I opted APF404 pad material. They are an AP Racing European catalogue product and retails for around 200$ for a set of 4. 

AP describes it as their most track suitable pads that can be used on the street - and I can confirm they feel absolutely amazing with no squeaking. 

The pad shape is perfect for this rotor's dimensions. 

+/- 2x the surface area

This is another area where improvements are massive. The AP Racing pad shape is 64.6 square centimeters. It has nearly twice the surface area over OEM. 

The pads above are Akebono ceramic replacement pads for the OEM calipers and were used for 25,000km. 

Longer service life*

I put an asterisk here as pad thickness is more than likely not the driving factor to pad life. Compounds used, driving style, heat will all play a part. Essex, the official USA importer of AP Racing uses a 2.5cm thick pad: it adds weight with increased pad durability and a slight cost benefit. 

They chose this to optimize for track time - we didn't optimize for frequency of pad change considering my original development mindset. 

The pads are 1.675cm thickness, thicker than OEM pads as well. 

Brake lines

Last but not least, upgrading the rubber brake lines to braided is a must in any performance oriented kit. 

As with front, we used custom made high temperature braided brake lines from Woodridge with rubber adapters. They are designed for the OEM brake line holder giving you a more consistent pedal pressure under hard use.

The production units will be made by HEL instead of Woodridge. 

There is a small twist to fit the lines during installation that I will touch on in the next entry. 

 

The rear kit was a more technically challenging product to develop, and as such, took more time to receive. 

Nonetheless, we live in the present, and it's time to install it. 

Up next: we torch things up, and get to feel the full effects of a purposely engineered rearward bias brake kit.