Quick look: Shimano RS505 road bike disc brake system

In 2013, Shimano finally introduced hydraulic disc brakes to road bikes, but it wasn’t until 2015 that the technology trickled down to a more accessible level. Recently I chanced upon a bike that had this new hardware.


Hyro, my TCX, uses post-mount frame tabs to mount a TRP Spyre disc brake caliper on the non-drive chainstay.

Disc brakes are not new technology; they’ve seen service in automobiles and motorcycles for decades. In the realm of bicycles, disc brakes were an innovation first popularized by bicycle tourers.

They then found widespread adoption in mountain biking, where they quickly superseded rim brakes and their susceptibility to mud and water by the end of the 20th century. The disc brake calipers were mounted to MTB frames and forks by either the perpendicular IS (International Standard) mount, or the more ubiquitous Post-Mount system.

In 2010, the UCI (Union Cycliste Internationale), the governing body of bicycle competition, allowed the use of disc brakes in cyclocross races. Prior to this, cyclocross bikes were essentially fully rigid mountain bikes with drop handlebars, road bike drivetrains, and cantilever rim brakes. These brakes were chosen for their mud-clearing ability, not exactly for their stopping power. With disc brakes, cyclocross racers could now enter corners faster and brake later, while remaining consistent regardless of conditions. This meant that the adoption of these brakes on road bikes was only a matter of time.

For bicycle component juggernaut Shimano, it was relatively straightforward to adapt its existing mountain bike hydraulic disc brake hardware to road use. Indeed, it took its Deore XT BR-M785 disc brake caliper and re-sold it as the road-use BR-R785 and BR-RS785, retaining Post-Mount as well. Later on however, Shimano introduced a new mounting standard called “Flat Mount” for disc-braked road bikes, citing better aerodynamics and integration. Indeed, after the introduction of Flat Mount, the only hydraulic road disc brake calipers that retain Post-Mount compatibility are the two R(S)785 units.


Yep, it costs a pretty penny. You can see the front of the ST-RS505 levers here.

On one of my visits to LifeCycle’s former branch in Makati, while having Hyro maintained, I noticed a flashy red bike on display. It was a brand-new Eddy Merckx Mourenx69 endurance road bike, made out of carbon fiber…and sporting Shimano’s RS505 disc brake hardware. I decided to take a better look since this was the first time I saw these parts out in the wild.

This is the BR-RS505 brake caliper. Note how they’re mounted to the bike’s chainstay; Flat Mount bolts them directly on. The bolts actually go through the chainstay from the underside and thread directly into the caliper. Merckx fitted this bike with a 140 mm rear rotor; the caliper will require a thin adapter plate to fit a 160 mm rotor.

Note the fins on the brake pad and the brake rotor. The finned center portion of the brake rotor is also made up of aluminum, sandwiched between the steel layers of the rotor’s main brake surface. All these features are supposed to mitigate heat buildup from braking, and let it escape into the atmosphere – a bigger concern for the higher speeds and braking loads of road bikes.

The ST-RS505 control lever as seen from the inboard side.

Unlike cable-actuated road bike brakes (either rim or disc), Shimano’s hydraulic road disc brakes tend to come as a kit of the brake calipers themselves and a matching set of STI levers. This is because the STI levers contain the master cylinders that push the hydraulic fluid toward the calipers. The first R785 brake systems paired the hydraulics with electronic Di2 shifting, currently available only on the top-end Dura-Ace and Ultegra groupsets. Later, Shimano reintroduced 2×11 mechanical shifting with the Ultegra-level ST-RS685 and these, the 105-level ST-RS505 STI levers.

The ST-RS505 control levers as seen from the outboard side. Note the Fulcrum Racing 5 DB wheels the Mourenx69 has – that explains some of the price.

You will have noticed by now the shape of these ST-RS505 levers. The earlier ST-R785 and ST-RS685 levers are very similar in shape to an Ultegra 6800 or Dura-Ace 9000 STI lever. These RS505s on the Mourenx69 look larger and rather bulbous by comparison. The look certainly divides opinion.

The brake hoods of the ST-RS505 control levers as seen from above. You can also see how flush the Flat Mount front brake caliper is with the non-drive side fork leg.

I tried holding the RS505s by the brake hoods. As per glove sizing convention, I have medium-sized hands, and I had no problems with comfort. If anything, the front bulb makes for a nice extra handhold for an aero tuck while riding on the hoods. When held the normal way, it does feel like you’re holding a microphone.

The square-edged bleed port at the very rear of the brake hood tends to rub some people the wrong way, I’ve heard.

On some other cycling websites that have reviewed these STI levers, I read comments about a square-edged nub on the brake hood that rubbed some cyclists the wrong way and detracted from comfort. Apparently this problem area is the bleed port. Personally I never really noticed it, but it will depend on the individual rider.

If you’ve followed this site for any amount of time, you know I’m totally sold on the merits of disc brakes. The addition of hydraulics increases power to the modulation and consistency of the disc brake design. My only real reservations are the price and the backward compatibility of these calipers to road and cross bike frames with Post-Mount fittings, especially as I have no plans of getting rid of my TCX any time soon.

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