Nice data point from Bimmerpost today.

Customer G81 Touring (~4300+ lbs) ran the kit hard enough on track to discolor the anodized bell. That typically implies sustained bell temps north of ~250C, which generally corresponds to rotor surface peak temps in the ~700-800C range. Customer reported zero loss of bite. The visible gas noted is consistent with expected street pad offgassing in the ~400-500C range when pushed into track temps.

He also reported no noise on the street and no dust. That is exactly the operating window I am targeting: stable friction under high thermal load without compromising daily drivability.



https://g80.bimmerpost.com/forums/sh...&postcount=430

Nice to see BSPK wheels and brakes both getting properly exercised.

 

Hi John, I'm wondering if you can explain that top ceramic coat where it makes the top surface look like a dry desert. Is that layer a good thing or a bad thing? I have no idea. A competitor obviously claims it's a bad thing that masks the quality of the whole rotor.

 

It is a good thing as he has explained in the past.

 

I have a more detailed write up on the SiC rich outer layer I elected to pursue. I had intended to get it out this weekend but various other commitments got in the way. I’ll prioritize getting it out tomorrow so please be on the lookout for that. Apologies for the delay.

 

Question about the gt4rs, does your kit require spacers for the calipers considering the rotors have a different diameter ?

what about the gt4rs with OEM PCCB. Are the calipers the same and positioned the same as steel ?

 

On the rear if you have steels you need a 5mm spacer and longer bolts to fit 390mm, on the front is just from 408 to 410 so not a problem

 

@Hinz Motorsport will sell you a hardware kit, you want one-stop shop.

 

Is that one for 390mm or for ST 400mm?

 

That’s a 400mm kit pictured, but they can supply for any of the configurations, with an email.
For steel to 390mm. They can provide the 5mm spacer that normally would go in the PCCB -> ST conversion. With a PCCB length bolt.

 

I supply the necessary brackets and hardware. Here's the bracket for reference.



Instead of using individual shim/washer spacers, BSPK uses a single-piece, CNC-machined caliper spacer bracket. This approach increases stiffness, distributes clamp load across a larger, purpose-made contact surface, and helps maintain caliper alignment during high thermal and braking loads. The result is a more repeatable installation and a solution better suited to demanding street + track use.

Note that no bracket is needed for the front to go from 408 to 410. The 1mm difference is a function of how the disc sits within the caliper bridge, not a geometry difference from the factory.

 

This part is nice.

Would you sell that standalone?

 

Sorry for the delay. I shouldn't have said yesterday as I failed to account for CNY and everything that comes with that in my household (I won’t bore people). Happy Year of the Horse / 恭喜发财!

This topic can spider web quickly into durability, friction modality, and failure modes. I’ll hit the key points below, and I'm happy to expand as follow-up questions come in.

One disclaimer: I have zero interest in debating what other vendors chose to do. I can only speak to my engineering choices and the rationale behind them. I'll focus on the underlying physics and manufacturing realities. I think this audience can separate signal from noise.

Without further ado.

1) What a “coating” is, and what it is not

The term “coating” gets used loosely in the carbon ceramic world, and I have used it at times because it is common consumer language. The problem is that “coating” can mean three totally different things.

A) A true post-applied coating
Something added after the disc is already manufactured. Think “paint on a wall.”

B) An in-situ engineered surface zone
A near-surface region created as part of the manufacturing sequence. It is chemically and structurally continuous with the disc.

C) A pad transfer film
A third-body layer of pad material deposited onto the rotor surface during bedding and use.

For BSPK discs, the SiC-dense friction surface is category B. It is formed in-situ during liquid silicon infiltration (LSI). It is part of the composite itself, not something applied afterward to “cover” anything. A better mental model is “crust on a loaf,” not “paint on a wall.”

2) Two styles of CLF disc (same foundation, different surface philosophy)

Surface Transforms helped bring continuous long-fiber reinforcement into high-profile automotive applications (often associated with early hypercar programs). Moving from more discontinuous architectures to continuous fiber reinforcement improved toughness and damage tolerance.

Their commonly understood surface philosophy is more carbon-rich and carbon-participating at the friction face. That naturally supports a refurbishment narrative in some use cases. If the friction surface wears and enough thickness remains, refurbishment can sometimes restore a usable friction face within spec.

That message resonated, and it became the reference architecture for a long time. From a product development standpoint, it also explains why many newer entrants gravitate toward a carbon-rich surface philosophy. The market understands the refurbishment story, and it avoids having to solve the harder problem of engineering an SiC-dominant surface zone that behaves predictably with the right pad chemistry. There is nothing wrong with that. It is simply a different surface philosophy with different lifecycle tradeoffs.

3) Modern direction: engineered surface zones, and why porosity is the control ****

The next evolution is learning how to engineer the near-surface region during manufacturing so the friction face is more wear-resistant and more oxidation-resistant, without relying on periodic resurfacing. That is where SiC-dominant integrated surface zones come in.

If you want to understand why an in-situ SiC-dominant surface zone can exist, you have to understand porosity. A carbon ceramic disc is not a solid chunk of material. It is a composite with a carbon fiber skeleton and a matrix. Before silicon infiltration, the structure contains an engineered pore network.

Porosity matters for three reasons.

A) Porosity controls silicon access
In LSI, molten silicon infiltrates the carbon structure through the connected pore network. The infiltration is capillary-driven. If the pore network is too closed, silicon cannot reach where you want reaction to occur. If it is too open, you can end up with a different near-surface phase balance than intended.

B) Porosity controls reaction kinetics and phase balance
When silicon reaches carbon, it reacts to form SiC. How much SiC forms in a region depends on how much silicon reaches that region, how long silicon is present, local carbon availability, and the pore geometry. If you bias silicon availability and reaction near the surface, you can create a surface region that is more SiC-dominant, while the bulk remains the intended C/C-SiC composite. This is the real distinction. It is not a top coat. It is a deliberately engineered near-surface phase balance created during infiltration and reaction.

C) Porosity gradient is how you create an in-situ surface zone
Instead of having a sharp boundary like “layer stuck on top,” you can design a gradient. Near the surface, the pore network and carbon availability are tuned to promote more silicon infiltration and more SiC formation. As you go deeper, the structure is tuned for the desired bulk properties. That is why I call it a surface zone. It is continuous with the substrate. There is no clean adhesive boundary that can simply peel off.

On my discs, the nominal SiC-dominant zone is about 0.8 mm. That is far greater than surface crazing depth, and it is formed as part of the composite during LSI.

4) Two surface philosophies and two friction modalities

At a high level, we are comparing two surface philosophies that can both exist on continuous long-fiber LSI C/C-SiC discs.

Carbon-rich, carbon-participating friction face
The interface includes more rotor participation and tends to be more pad-forgiving. A practical benefit is that it can support a refurbishment narrative in some cases, assuming sufficient thickness and the disc is otherwise within spec. This became the reference architecture for a long time, which is why many newer entrants gravitated to it. It is proven, market-understood, and avoids solving the harder problem below.

SiC-dominant integrated surface zone
Instead of leaning on refurbishability, the goal is to engineer a more wear-resistant, oxidation-resistant friction face so the disc itself wears extremely slowly. This is not a post-applied layer. It is a surface zone created as part of the manufacturing sequence. It has tremendous benefits to rotor longevity and μ stability.



5) Abrasive vs adherent friction, and what it does to μ

Assuming correct pad selection and proper bedding, both surface philosophies can work extremely well. The difference is what dominates at the interface, where the wear lives over time, and how μ behaves across a session.

First, the interface is a third body. Brakes are not pad rubbing directly on rotor. The working interface is a thin third-body layer made of compacted wear debris and reaction products. The pad and rotor continuously build, shear, and rebuild this layer. That third-body dynamics is what drives bite consistency, modulation, judder, and μ stability.

μ is not a fixed number
μ is not a single constant for a brake system. It is a response curve that depends on temperature, pressure, sliding speed, and the state of the transfer film. When drivers say a setup feels “consistent,” they usually mean two things.

A) μ is stable lap to lap once the system is at temperature
B) μ changes predictably with pedal pressure and trail braking inputs

That is what builds driver confidence and repeatable threshold and trail braking.

Abrasive dominated friction (more carbon-participating surfaces)
Abrasive friction means a meaningful portion of braking work comes from micro-cutting, micro-plowing, and microfracture of the surface and third body. On a carbon-rich or carbon-participating friction face, the rotor can contribute more materially to the third body.

What that tends to do to μ:

• μ can be strong early and feels progressive, especially once hot.
• Over long sessions, the friction surface is evolving because you are mechanically consuming rotor material as part of the friction system. That can introduce more μ drift across a weekend.
• The system can be more tolerant of different pad chemistries because something will always generate friction mechanically.

Lifecycle trade: more of the wear tends to be carried by the rotor.

Adherent or film controlled friction (more SiC-dominant surfaces)
Adherent or film controlled friction means braking work is dominated by shearing within a stable pad-derived transfer film on a wear-resistant counterface. The rotor behaves more like a stable platform for the film rather than a consumable contributor.

What that tends to do to μ:

• Once established, μ tends to be very stable lap to lap because the counterface does not change much. That translates into consistent bite, consistent release, and easier muscle memory for threshold and trail braking.
• The primary variable becomes the transfer film and pad condition, rather than the rotor surface progressively wearing or changing texture.
• The system tends to resist classic fade. When you do see issues, they usually present as deposit behavior or judder rather than a dramatic loss of braking.

Lifecycle trade: more of the wear tends to be carried by the pad.

Bottom line
Both approaches can deliver excellent performance. The design choice is where you want wear to live and what you want to be most stable over time. Carbon-participating systems can be more pad-forgiving and often feel strong early. SiC-dominant systems are engineered to keep the rotor surface and μ behavior more stable over repeated high-energy cycles, with the pad becoming the sacrificial element.



6) Why I chose SiC-dominant for BSPK (track priorities: μ stability and confidence)

From the beginning, my priority was not just peak bite. It was repeatability. On track, what separates a fast brake setup from a good one is whether it delivers the same response at the end of a 20 minute session as it did on lap 2.

For a driver, μ stability shows up as three practical things.

A) Consistent threshold point
You can reliably find the limit without hunting for it as temps rise.

B) Predictable trail braking support
As you bleed off pedal pressure, the car stays balanced and the braking force tapers linearly instead of stepping up or dropping away unexpectedly.

C) Repeatable release
Release characteristics matter as much as initial bite. Unpredictable release is what destabilizes the chassis on corner entry, especially on fast cars with high aero sensitivity or rear weight transfer.

A key reason I chose an SiC-dominant surface zone is that it makes the friction face far more wear-resistant and more oxidation-resistant. That matters directly to μ stability because it reduces one major variable in the system: the rotor surface itself evolving over time.

In a carbon-participating surface philosophy, the rotor is more involved in generating the third-body layer. That can work extremely well, but it also means the rotor surface is progressively changing and the system is more likely to drift over a weekend as the surface and film evolve.

With an SiC-dominant surface zone, the rotor behaves more like a stable counterface. Once the correct transfer film is established, μ tends to stay consistent because the counterface does not materially change from lap to lap. The wear burden shifts more to the pad, which is cheaper and easier to manage than consuming rotor life.

This choice also aligns with real track failure modes. At the extreme end, the limiting factor is rarely “does SiC survive.” The limiting factor is heat management of the whole system, pad condition, and keeping the interface stable over long sessions. If μ shifts unexpectedly at high temperature, the driver feels it immediately as a change in threshold point or a step change in trail braking behavior. Anyone who tracks regularly knows how unsettling that is.

One additional point on intent and audience. I fully expect most of these systems to live a street life. That said, I design and validate around track duty because it is the harshest environment for thermal cycling, interface stability, and repeatability. To borrow the old motorsports line, win on Sunday, sell on Monday. If a brake system is genuinely stable and predictable at track temperatures, it earns the credibility to be boring and confidence-inspiring on the street.

So the design intent is simple. Prioritize a stable, durable friction face that supports consistent μ, consistent modulation, and a repeatable threshold and release, while pushing wear toward consumable pads instead of expensive rotors.

 

Nice post John. Very informative and transparent on the difference between the ST type rotors and BSPK's.

This type of knowledge and the ability to present it to the community in such a manner is a major part of why I decided to work with John on the introduction of his carbon rotors to our community. Not many vendors have this level of understanding about the engineering of their products.

On a side note here, I would also put Scott, the owner of Antigravity Battery, in that same category as well.

 

C8 RS6 crowd might be all over this.

 

Yes fantastic post, thank you.

There are no problems with running pads like the Endless W007 or Pagid RSC2 (not RSC1) with these rotors on track right?