Bicycle Disc Brake Rotors by SRAM

In this article, we’ll be discussing Bicycle Disc Brake Rotors by SRAM, US publication 20200407009. The publication date is Dec. 31st, 2020 and the filing date is June 8th, 2020. This patent is a continuation of application No. 16/451,378 filed on June 25th, 2019. I believe this 2020 patent is just a broadened scope of the 2019 patent, since the 2019 patent has 20 claims and the 2020 patent has 44. Both the 2019 and 2020 patents are currently being prosecuted and have not been granted by the USPTO.

This particular patent is quite incredible to me. It’s less of a specific design and more of a brilliant combination of a new design and a very cool manufacturing process that leverages the new design.

Brief Summary (tl;dr)

SRAM are introducing a new brake rotor design that uses both aluminum and stainless steel, like the current Centerline X rotors available today. The difference between the currently available rotors, and these rotors, is that the rotors being patented here do not have any fasteners connecting the aluminum and stainless steel. The core (center part attached to the hub) is machined aluminum, and the stainless steel brake tracks (outer area where brake pads are applied) are sprayed onto the aluminum core using a thermal spray process, where the steel is melted and sprayed — kind of like spray paint. So, in the end, the rotor contains both lightweight/heat-dissipating aluminum everywhere except the part where the pads touch the rotor. SRAM don’t state whether these are mountain bike or road bike rotors, but I’d assume they can be used for both.

Background

Of all the components on my bikes, I prioritize brakes over everything else. As far as I’m concerned, they’re the most important part of a bike. Imagine scooting along at speed, grabbing that lever and nothing happens. I’m sure this has happened to more than one of you. I know this situation from personal experience, twice, both time racing karts. The first time sent me over turn 1 into a catch fence. Fortunately, the second time had enough runoff to stay on the track.  It’s easily the worst feeling while you’re going fast,; you feel it in your gut.

As far as the composition of a brake, brake rotors are metal discs solidly attached to a wheel hub. As the wheel spins, the rotor spins. The outer portion of the rotor is located inside a brake caliper, where the caliper contains two brake pads that squeeze against the outside of the rotor when the brake lever is applied. Squeeze the lever, pads squeeze the rotor, bike stops.

Intro

SRAM’s current high-end brake rotor is the Centerline X rotor (shown below). They’re two-piece rotors, where the core (black part) is aluminum, and the brake track (silver) is steel. The two pieces are attached via a set of fasteners. They do this for a few reasons: steel wears slower than aluminum, tends to warp less, and is more resilient at high temperatures. Additionally, aluminum dissipates heat significantly better than steel and weighs much less. So, the steel brake track transfers heat to the aluminum core for better heat control. With less heat in the steel brake track, the brakes operate in a better temperature range. The two images below show a current SRAM rotor configuration that use 6-bolts and a centerlock.

The brake track also includes some slots or holes. For many years, it was believed that these slots worked based on the escape of heated gases, causing a reduction in the heat of the rotor. While I worked in auto racing, some of the old heads did significant research on the effect of slots and holes in a brake rotor and determined that the slots do improve brake power and allow for a release in gases, but the improved effect on braking was less associated with heat dissipation than previously thought (though heat is still dissipated). Instead, they learned a microscopic layer of gases and dust that build up between the rotor and the pads. The slots and holes allow the gases and dust to escape, so the pad and rotor have a more pronounced and effective surface connection. These advancements have contributed to braking power, reliability, and longevity in a small package.

Intended novelty

The idea that SRAM are trying to protect is a two-material brake rotor without any fasteners. So, the intended novelty is a fastener-less junction between the brake core and the brake track with two different materials.

Why

Typically, this section is the hardest to fill. Patent documents are required to define what the inventor is doing and how they’re doing it. They’re not required to provide any why’s to what they’re doing. That being said, SRAM don’t mince words as to why they’ve developed these rotors, which is really great to read.

The reason they’re using aluminum and steel:

…the example brake rotor adds less weight to the bicycle than known stainless steel brake rotors… the aluminum is more thermally conductive than the stainless steel… the aluminum core acts as a heat sink that draws heat from the stainless steel brake track and dissipates the heat to the surrounding air…thereby reducing peak operating temperatures of the brake track.

The reason they’re slotting:

The recesses help clear or remove dirt and debris from the brake pads when the brake pads come into contact with the first and second tracks, thereby improving braking performance.

The reason they’re not using fasteners:

…the brake rotor does not include or require any mechanical fasteners (e.g., bolts, screws, etc.). Thus, the brake rotor is lighter than other known brake rotors that utilize fasteners to connect multiple parts or layers to form a core section.

The reason they’re using angled openings in the core:

…the openings act as aerodynamic features that improve airflow around and through the brake roto … this angled or slanted design increases the surface area of the intermediate portion of the core, which further improves heat dissipation.

Ultimately, the advantages are:

The example brake rotor has reduced braking surface temperatures for more consistent pad-rotor coefficient of friction, lower caliper temperatures for more consistent fluid pressure (when used with hydraulic systems), reduced component/seal damage (e.g., seals within the brake caliper that may be affected by heat), and resistance to brake fluid vapor fade (hydraulic fluid boil).

That all sounds good to me.

What

SRAM are introducing a new brake rotor that is composed of 2 different materials. In this case, the core is aluminum, and the brake track is stainless steel. The two materials are not connected via fasteners, bolts, screws, etc. The brake track is sprayed onto the core, then machined into a shape that we’re all familiar with. Fig 2. below shows the core in the center of the rotor and the brake track on the outer portion of the rotor.

SRAM state that this rotor also has a few other existing technologies that, when combined with the present novelty, provide (potentially) the most high-tech rotor available. The brake track can be both slotted and drilled for gas/debris dissipation (see components 228, 228, and 228b). The core also contains angled openings (component 220) to dissipate more heat away from the brake track. Notice the thoughtful location of the openings, just under the brake track. The location of the openings should provide maximum heat dissipation away from the brake track, through the aluminum.

How

SRAM provides an example manufacturing process for this new rotor.  We’ll start with the core.

…the core 202 may be stamped from a single piece of material, such as aluminum. The central opening 212, the fastener openings 214, the arms 216, the openings 220, and other any other edges or surfaces may be machined in the piece of aluminum.

So, the first process can be a stamping process of the core. A secondary machining process then cuts the openings, arms, etc. SRAM also state that this process can be performed in one shot by stamping, machining, forging, or casting.

This next part is where it gets interesting. The brake track, where the brake pad meets the rotor, and what provides the stopping power, is sprayed onto the aluminum core. This process removes the need for any necessary fasteners and permanently attaches the brake track to the core.

the brake track is formed using a thermal spray process (e.g., a flame spray process, an arc spray process). For example, a rod or powder of coating material (e.g., stainless steel) may be melted or sintered and sprayed at the brake surface core portion to form a coating on the first and second sides and the outer peripheral edge. The melted or sintered coating material bonds to the first and second sides and the outer peripheral edge. The coating material dries and hardens to form the brake track. The brake track is permanently coupled to the core. As such, no other fastening means (e.g., bolts, adhesives, etc.) are needed to couple the brake track to the core.

The brake track may be formed on the brake surface core portion via a thermal spray process…after the brake track is formed on the core, the brake track is machined, formed, or grinded… The first and second sets of recesses may then be machined, etched, stamped, forged, or coined in the first and second braking surfaces of the respective first and second tracks.

Figure 6 shows a cross section of the end portion of the brake rotor. Note how the stainless steel brake track molds perfectly around the aluminum core. This is an image after the spraying process is performed.

SRAM describes the thermal spray process as:

…a coating material (e.g., stainless steel) may be melted and sprayed at the outer surfaces of the brake surface core portion. The coating material covers the sides and the outer peripheral edge of the brake surface core portion. The coating material hardens on the brake surface core portion and forms the brake track.

In short, the stainless steel is melted into a liquid and sprayed through a spray gun, similar to an automotive paint applicator, and then hardens onto the core. This is an extremely high-tech manufacturing process with serious machinery. The image below (from open.edu) shows a diagram of the thermal spray process.

After the stainless steel brake track is sprayed onto the aluminum core, there is a secondary process to machine, etch, stamp, etc., the recesses (slots) into the brake track and clean up the track/edges. In the end, we have a two-piece rotor without the weight of fasteners and with better heat dissipation.

Conclusion

If this patent doesn’t blow your tits off, I don’t know what will. This is some high-level manufacturing that SRAM are implementing into the biking world. I don’t typically make future assumptions about the technologies I write about, but when/if this rotor becomes available to the public, it may be the best rotor ever available, up to now.

But, the burning question is… the price. How much is this thing going to cost? Currently, the MSRP for the 180mm Centerline X is $78 and the XR is as high as $105… for one rotor, and I can’t imagine this one being any less expensive. That’s insane to me. My car’s rotors cost less. That being said, if the released rotor is anything like the one presented in this patent, I might have to push back my car’s next brake job and grab a pair of these, because god damn these are looking good.

As always, thank you for reading, and I hope you enjoyed this one.

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