Disc brakes for bicycles is not new. Mountain bikes have been using mechanical and hydraulic disc brakes for over 20 years, and it is already well accepted, almost compulsory in fact. Even entry level mountain bikes use hydraulic disc brakes, not just high end trail or downhill bikes.
Typical hardtail mountain bike with hydraulic disc brakes
Road bikes have not transitioned to using disc brakes, because of two main reasons. The first is because disc brake systems are currently heavier than caliper brake systems. This is true, but with recent developments in component technologies, the weight difference is getting smaller. As most road cyclists would understand, getting the road bike as lightweight as practically possible is important. Whether or not it really improves your speed is another matter altogether.
The second reason is because there is usually no need for the higher braking power of disc brakes. For many years, caliper brakes have provided sufficient braking power in most situations. However, when the weather turns wet, or when the terrain gets steep, caliper brakes are no longer able to provide sufficient braking power, especially when it is a wet downhill route. Disc brakes are able to cover this deficiency by providing good braking power in all weather conditions.
For a more detailed comparison of the pros and cons of different brake systems, refer to this link for more information.
Now, for road bikes, it is not as easy to change over from a caliper brake system to a disc brake system. First of all, the frame needs to be different, as disc brake mounts are required for mounting the disc brake calipers. Then, the brake levers might need to be different, especially when hydraulic disc brakes are to be used. Lastly, the wheel hubs also need to have disc brake rotor mounting.
Hydraulic disc brake on a road bike. Note the disc brake mounts required on the fork, and the brake rotor mounting required on the wheel hub.
Road bikes have been using brake/shifter lever combos for many years. These are also called road shifters, where the brake levers and shifters are integrated into one unit. The braking system has always been using steel brake inner cables, which can be connected to caliper brakes or in certain cases, mechanical disc brakes.
However, mechanical disc brakes have long suffered from poor braking power and a mushy lever feel, and is vastly underpowered compared to hydraulic disc brakes. In my opinion, if a disc brake system is used, it should be a hydraulic disc brake system.
The tricky part for implementing hydraulic brake levers on road bikes has been the challenge of integrating the hydraulic master cylinder into the road shifters. There is simply no space to fit both the master cylinder and the shifting mechanism into the road shifter, without making it too big and bulky.
This problem was solved with the introduction of Di2 electronic shifting systems, where the shifting mechanism is no longer required in the shifters. The Di2 road shifters have been slimmed down, and the complex shifting mechanism has been replaced by electronic shift buttons and a simple circuit board. With this breakthrough, there is now space for the hydraulic master cylinder to be placed in a Di2 road shifter!
As the first generation of road Di2 shifter with integrated hydraulic brake lever, the Shimano ST-R785 is a non series road shifter designed for Di2 equipped road bikes with a hydraulic disc brake system. It is being marketed as a Ultegra grade Di2 road shifter with hydraulic brakes. Let's take a closer look at this road shifter and see how the integration is done!
Shimano ST-R785, a Di2 road shifter with hydraulic brakes. Slightly taller at the front of the hood as compared to mechanical road shifters.
Being a Di2 shifter, what you get are two buttons instead of shifting levers. The buttons are textured differently for easy identification.
The bracket cover removed from the main bracket. Note the multiple protrusions from the rubber, designed to hold the cover securely to the bracket.
Side view of the bracket, which is cast out of aluminium, unlike the engineering composite used for mechanical road shifters. The hydraulic brake hose will connect to the rear of the bracket, above the clamp band.
Clamp band is still tightened to the handlebar in the same way, accessible from the side.
The Di2 electronics, which is just a little box with a wire that connects to the buttons. The other end of the box has one port for plugging in a Di2 wire.
The pivots for the hydraulic brake lever is located higher up than on mechanical road shifters. This gives a different braking feel due to the different movement arc of the brake levers. The free stroke adjustment screw is also shown here.
To bleed the hydraulic master cylinder, the chrome cover needs to be removed first. The master cylinder is integrated within the aluminium bracket.
Bleeding port is located way up on the bracket. The brake lever reach adjustment screw is also shown here.
Hydraulic piston located inside the bracket, which pushes the hydraulic fluid out of the bracket and towards the brake caliper.
Similar weight to the Shimano 105 ST-5800 mechanical road shifters, despite having no shifting mechanism. This is mainly due to the large aluminium bracket which is heavier than engineering composite material.
As shown above, that was the non series ST-R785 Di2 road shifter for hydraulic disc brakes. The master cylinder is able to fit into the bracket without making the entire shifter too big.
The thing is, not every rider is able to afford Di2 shifting components, and so there needs to be a way to integrate mechanical shifting with hydraulic brake levers. This is where it gets really challenging, which is to fit the shifting mechanism plus the hydraulic master cylinder into a road shifter, without making the road shifter too bulky. Amazingly, it has been done, and the result is the non series Shimano ST-RS685 road shifter, which has mechanical shifting and hydraulic brake levers.
Shimano ST-RS685 road shifter, with mechanical shifting and hydraulic brake levers.
Still looks similar to a normal mechanical road shifter, just that it has a longer bracket and a taller hood. Quite amazing that everything fits into the bracket!
Front view of this non series road shifter
Taller hood, but not as large as the size found on SRAM road shifters with hydraulic brakes
Once the rubber cover is removed, the full layout can be seen! There is a metal pipe that links the master cylinder at the front of the bracket to the hose connector at the rear.
The hydraulic hose will be plugged into the connector located at the rear of the bracket.
Triangular shaped master cylinder located at the top of the hoods
This hydraulic brake lever design also incorporates Servo-Wave technology, as shown here with the use of a cam mechanism.
Shifting mechanism is located well within the bracket. Shown here is the inner cable insertion point, similar to Shimano 11 speed mechanical road shifters.
The shifter inner cable passes through the bracket and leads here, then turns 90 degrees to exit from the back of the bracket.
With the bottom cover removed, the complex shifting mechanism can be seen, ingeniously squeezed into the bracket.
Weight of one side of the road shifter is 322 grams, which is quite heavy. Once again, this is due to the combined weight of the aluminium bracket, shifting mechanism and the master cylinder assembly.
With that, a brief introduction to Shimano road hydraulic brakes + shifters is complete! It is a feat of engineering to be able to fit all the mechanisms into the road shifter, without making it look too bulky or affecting the gripping ergonomics.
If you are buying a new road bike, it is worth considering hydraulic disc brake options, as that is the trend that road bikes are moving towards. With a disc brake compatible frame, you can be sure that you will be able to install the latest innovations in road disc brake technologies, and have a future proof bike.
Road bikes with hydraulic disc brakes are the next wave of new technology, and it is already here, as can be seen from major bike brands all offering disc brake road bike options. Professional cyclists are still using caliper brakes, as the UCI has not approved disc brakes for road cycling competitions yet. However, that is just a matter of time, as seen by the on-going disc brake trials going on in some road cycling competitions. Once disc brakes are approved for top tier road cycling competitions, you can be sure that things will move and change very fast. That said, traditional road bikes with caliper brakes are here to stay, and will not be phased out anytime soon, as they still have their own charm and advantages.