The main point of difference is their extremely compact reservoir and master cylinder and a ton of adjustability and an elliptical cam system to drive the pushrod into the master cylinder with a very straight path and linear response.
Starting with the master cylinder, the shape allows them to orient the brakes in any way they need to. For now, that means a svelte package that places the gloriously straight lever blade close to the bar. Now, let’s take a look inside…
Another really nice feature is their “quick draw” activation. The port timing holes are placed very close to the primary seal, so as soon as you start pulling the lever, the brakes start working.
As a quick primer on how hydro brakes work, the timing port holes (E) let brake fluid flow freely into the reservoir (B – which holds the expansion bladder) through a port (G) when not engaged. As the lever moves the push rod into the master cylinder, the holes are moved past the primary seal (F) to close the system between the seal and the caliper, which forces fluid to push the pads toward the rotor.
Other items on the FSA brakes are the bleed port (A), stroke adjust bezel (D) and reach adjust bezel (C).
The tool-free stroke adjust lets you dial the timing ports back a bit to add some free stroke, but that simply means you’ll be pulling the lever further before the brakes start doing anything.
Once that’s set, you can adjust reach with another tool-free knob, and there’s a wide range of adjustment. Left to right shows it dialed all the way in and out. It’s worth noting that if you have them dialed in really close and add some free stroke, they’ll be hitting the grips before you have full braking power.
The reservoir is offset slightly from the master cylinder and has an angled interior wall so the highest point is at the bleed port when it’s level. The lever bodies are the same, so you can flip flop between normal and moto-style braking, but that means you’ll always see one bleed port facing up and the other facing down when they’re on the bike.
The caliper is a one-piece mono block piece with a very stiff bridge between each side. Pistons are 22mm diameter. Lower bleed port is located in the center of the banjo.
Conveniently, their brake pads are the same size and shape as Shimano pads.
The pads are a custom semi-metallic that’s designed to break in quickly and has a very high coefficient of friction, so it’s strong. Because the lever pushes in a linear fashion, it can have that strength without being grabby.
Lastly, they’re using front and rear specific brake hoses. Each is made with different expansion rates (front has more expansion, rear is stiffer) so they feel exactly the same in both hands. The reason, they say, is that many modern bikes require very long hose runs for the rear brakes, sometimes twice as long. Since all of the opportunity for expansion comes from the hose, the solution was to tune them so they feel the same. Brake guy Joel Richardson says unless you’re running a hard line to the brakes, you’ll always get some expansion.
They wanted to offer something will all the bells and whistles most riders want and keep it light. With system weights just under 300g. That’s good on its own, but even though the top model is currently pegged K-Force, they could go much lighter in the future by losing the stroke adjust and making a tooled reach adjust.
Differences in the brakes are materials, not function. The K-Force gets a magnesium body, alloy caliper and carbon lever with titanium hardware. They said the reason magnesium can’t be used at the caliper is because it’s highly affected by heat, which would cause a lot of problems there. The Afterburner is all alloy with steel hardware.
The K-Force gets a 2-piece rotor with alloy carrier arm for aftermarket. OEM customers will have the option of this 1-piece K-Force rotor, too, but the 2-piece is stiffer since the oversized alloy spider runs pretty far out toward the braking surface. Weight between the two are very similar.
The Afterburner rotor (shown below on the scale) gets much more mass and twice the surface area for better heat management. Interestingly, FSA engineer Herb Tai says friction force is not a function of the surface contact area, it’s just that great surface area means better heat dissipation.
Graphics for each model will match those on the other K-Force and Afterburner parts.
The K-Force brakeset comes in at 195g with 185cm of hose (it ships with 160cm). Bolt kit is 9g.
Rotors are 98g for K-Force two-piece and 95g for one-piece. Afterburner rotor is 142g. All are 160mm.