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The Cyclist guide to road bike disc brakes

Joseph Delves
6 Apr 2021

A Cyclist guide to road bike disc brakes - including how they work and why they should be on your next bike

Technology improves in increments. Still, there’s always a moment when something shifts and what you previously accepted suddenly looks so old-fashioned you wonder why you put up with it for so long.

Disc brakes first appeared on mountain bikes in the late ’90s and have been standard equipment almost ever since. Yet it wasn’t until 2013 that Sram became the first of the big three groupset producers to make it to market with a complete hydraulic disc brake and shifter system for road bikes.

Shimano took a little longer with its first offering, which was restricted to the electronic Di2 system due to the difficulties of cramming both hydraulic and mechanical parts into a single unit. But since 2014, riders using Shimano have also been able to get their hands on the better braking provided by discs.

Campagnolo followed in 2017, and since then prices for this once aspirational technology have tumbled, and it’s now common to find disc brakes on bikes costing just a few hundred pounds.

In fact, things have come on so fast, it took a quick Google to confirm that it was indeed only in 2018 that the UCI finally allowed the use of disc brakes in road racing competition. Worried by concerns about riders tail-ending each other, or getting sliced by rotors, the adoption of disc brakes by the peloton was a rare occasion when amateur roadies were well ahead of the pros.

It’s now funny to think that using disc brakes on a road bike was once contentious stuff. The argument that better braking makes for anything other than safer riding has since pretty much disappeared. As in every comparable sport, better braking is now equated with faster and safer racing.

Shop Sram disc brakes on Chain Reaction Cycles

What is a road bike disc brake?

Replacing the traditional system whereby a calliper was used to directly slow the bicycle by gripping its rim, disc brakes instead employ callipers mounted to the frame and fork. Actuated either via a cable or hydraulic fluid, this then acts on rotors bolted to the hubs. Although comparatively new to cycling, it’ll be a system familiar to any car or motorbike mechanic.

On a bicycle, disc brakes generate more stopping power than conventional callipers. But it’s not just about more power. Road bike disc brakes also excel in their greater consistency, wet-weather performance, and control.

Skinny road tyres have a small contact area with the road, so road bike disc brakes are deliberately made less powerful to avoid locking up the wheels.

However, the extent to which you can modify the power is much greater than with a rim brake. Not only is this crucial in the wet, but as the braking surface is no longer the rim (which picks up water), braking remains consistently powerful regardless of the weather.

‘Not only are disc brakes more powerful, but they also retain 92% of that power in the wet,’ explains Sram’s J P McCarthy.

At the same time as being more powerful, disc brakes require less maintenance. Pad life is significantly longer, and because the rotors and not the rims wear out, they’ll extend the life of your wheels, particularly if you ride in foul conditions. Being a sealed system, hydraulic versions don’t suffer from contamination by dirt and grit either.

Check out the Cyclist guide to the best road bike groupsets

Better brakes on better bikes

Historically, part of the reason for the switch to discs has been the improvement of other areas of bicycle design, such as the introduction of carbon frames, stiffer wheels, and better tyres.

‘Frames are now more rigid and stable thanks to better materials and bigger tubes,’ says McCarthy. ‘They can descend faster and so need better brakes. The popularity of cyclocross and the emergence of gravel bikes has also pushed the boundaries of what drop bar bikes are capable of. Brakes started becoming a limiting factor’.

Once adopted, switching to disc brakes quickly changed the entire process of bicycle design. As early pioneers found, it’s not enough to just slap some mounts on an existing frame design. Instead, the greater torque exerted means frames have needed to be redesigned from the ground up.

This shift presented lots of opportunities – and is partly responsible for many of the varied styles of bikes we all now ride. The trend for adventure bikes, wider rims and tyres, and more aerodynamic forms have all been accelerated by the switch to discs.

Shop disc brakes from Shimano and Sram on Tredz

For and against

Arguments for disc brakes on road bikes include improved power and control, more consistent braking in the wet, lower maintenance, better clearance, and decreased wheel wear. Cons remain increased weight, the elevated cost of hydraulic models, and the fact they can be trickier to adjust.

Of these, the most commonly cited downside is the still slightly increased weight of disc versus calliper brakes. However, this is consistently coming down. And with the possibility of integrated hydraulic lines being built into the next generation of frames, it might fall even quicker than it has been.

Strangely, the cost has also become almost a non-issue. Particularly at the lower end of the market, there’s now almost no difference between disc and non-disc models.

Of course, racers will continue to have their preferences often based more on tradition than science, but I defy anyone to go around a corner in the rain and be 100% glad they chose not to opt for the better braking provided by discs.

If you haven’t made the switch yet, perhaps our list of the best road bikes can change your mind.

The best road bikes of 2020 & 2021

Road bike disc brake guide

Cable versus hydraulic

Cable-operated discs adapt readily to road use as they can work with conventional shifters. However, they’re heavy and clunky compared to hydraulic systems, while also lacking their power.

Using hydraulic fluid rather than traditional Bowden cables, hydraulic brakes require less servicing. At the same time, integrating a hydraulic system into the combined brake/shift levers found on modern road bikes is a difficult proposition.

This means that hydraulic brake/shifter units are both larger and more expensive than mechanical alternatives.

Shop disc brakes from Shimano and Sram on Evans Cycles 

Glossary: Brake types and parts

Mechanical

Compatible with standard shifters. Somewhere between traditional callipers and hydraulic systems in terms of power. Good wet weather consistency. Operated using cables, they can require frequent maintenance and adjustment, plus they’re a bit heavier.

Mechanical-Hydraulic

Cable-actuated hydraulic systems include TRP’s Hy/Rd. Providing similar power to a fully hydraulic system without the need to swap shifters, downsides include graceless aesthetics and indirect feel due to the use of cables rather than hydraulic hoses. Two systems in one means increased maintenance.

Mechanical-hydraulic converter

Early attempts to combine mechanical shifters spawned boxy converters that fitted underneath the stem. Effective but fiddly to set up. Still found in a more refined form on some Giant bikes, they’re an evolutionary dead end.

Fully hydraulic

The way to go if the cost is no object. Most powerful, yet lowest maintenance.

Rotor

A metal disc attached to the hub that provides the braking surface. These come in either six-bolt or centre-lock varieties. The larger the diameter, the greater the braking power.

Shop disc brake rotors on Wiggle

Calliper

The unit that houses the brake pads that in turn squeeze the disc.

Shop disc brake callipers on Wiggle

Pad

Resembling those on a car. Metal backing plate with either long-lasting sintered or quieter organic material that does the job of stopping the rotor.

Shop disc brake pads on Wiggle

Hose

Not cable and outer, instead hose and fluid.

Hydraulic fluid

The medium through which braking force is transmitted – either DOT (as used by Sram) or mineral oil (favoured by Shimano).

What does it mean to bleed a disc brake?

Bleeding SRAM Disc Brakes Tutorial Step 2

While requiring less maintenance than conventional brakes, hydraulic systems need specific servicing. Ace mechanic Greg Conti, explains what it means to ‘bleed’ a brake.

‘Bleeding is something you do for one of two reasons: the fluid in the system may have absorbed water, or there may be air bubbles trapped inside.

‘Either way, you need to purge the system and flood it with fresh fluid. Systems using DOT fluid need this done around once a year, while with mineral oil it’s considerably longer.

‘A disc brake comprises a piston at one end that pushes the fluid and a piston at the other end that receives it and squeezes the brake pads. You need to ensure that the space in between is full of fresh fluid.

‘Bleed ports at either end allow you to connect a syringe with which you push fluid from one end to the other. When the old fluid exits the system, you may see air bubbles or the oil change colour.

‘At the end of the process, you should see clear, bubble-free fluid at which point you reseal the system. It’s not a difficult process but it can be messy.

‘Squirting hydraulic fluid across the kitchen is unlikely to endear you to anyone you share accommodation with. Yet with a degree of mechanical aptitude, it’s really a case of taking your time and having the right equipment. Most brands provide kits that make life much easier.

‘If you take your bike to a mechanic, expect to pay around half an hour’s labour per brake.’

Cyclist guide: how to bleed Shimano hydraulic disc brakes  
Cyclist guide: how to bleed Sram hydraulic disc brakes

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