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Road bike disc brakes: Everything you need to know

How disc brakes work, why your next bike should probably have them, and the different types explained

Technology typically improves in increments but the introduction of disc brakes to road bikes was a huge leap forward for the sport.

Disc brakes had first appeared on mountain bikes in the late seventies and they started to go mainstream in the nineties, since taking over completely. The first disc brakes were cable-operated but most modern brakes are hydraulic.

It wasn’t until 2013 that we saw SRAM become the first of the big three groupset makers 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 fitting both hydraulic and mechanical parts into a single unit.

However, since 2014, riders using Shimano have also been able to get their hands on the better braking provided by discs.

Photo: Lizzie Crabb

Campagnolo followed suit in 2017, and since then prices for this once aspirational technology have tumbled, and it’s now common to find disc brakes on road bikes costing less than £1,000.

In fact, things advanced so quickly that UCI regulations trailed behind for several years and it was only in 2018 that the organisation finally allowed the use of disc brakes in road racing competitions, leading to heated debates about the supposed risk of riders rear-ending each other or getting sliced by rotors.

Because of the delay, the adoption of disc brakes by the peloton was a rare occasion when amateur roadies were well ahead of the pros.

Given that the 2022 Tour de France saw no pro teams using rim brakes, disc brake advocates can now enjoy some sense of sanctimonious ‘I told you so’.

Though rim brakes still enjoy popular support, the power (both literal and metaphorical) of the disc brake cannot be denied.

For a deeper dive into this divisive topic, see our guide on everything you need to know about disc brakes vs. rim brakes.



What is a road bike disc brake?

Photo: Matthew Loveridge

Traditional rim brake calipers function by clamping a rubber-based brake pad against the rim of the wheel, typically operated by a brake cable running from the lever. The front and rearcalipers are mounted directly to the top of the fork, and a bridge between the rear seat stays respectively.

Disc brakes instead employ calipers that grab onto dedicated brake rotors (essentially steel discs, hence the name) mounted to the front and rear hubs. 

The brake pads will be pushed together either by cable-actuated or hydraulically actuated pistons. Comparatively new to road cycling, disc brakes have been around on cars and motorbikes since the early 1900s.

How do hydraulic disc brakes work? 

Photo: SRAM

Hydraulic disc brakes operate by moving fluid throughout the system. The brake lever will house a small reservoir of hydraulic fluid, called the master cylinder. This fluid runs through the brake hose to the brake caliper, creating a moveable column.

When the lever is pulled, it actuates a piston in the master cylinder which then applies pressure on the column of fluid. 

This fluid cannot be compressed and is being forced away from the lever, directing the pressure towards the caliper. With both sides of the brake caliper full of fluid, the pressure forces the only moveable parts – the caliper pistons – inwards, extending the piston seals as it does so.

These pistons, known as slave pistons, in turn push the brake pads inwards until they make contact with the disc rotor.

When the lever is released a spring returns it to its resting position, pulling the master cylinder piston back and drawing the brake fluid with it.

The negative pressure in the system and energy stored in the stretched piston seals retract the pistons and pads, releasing the brake rotor from their grip.

Better brakes on better bikes

Photo: Lizzie Crabb

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 SRAM’s J P 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 gravel bikes, wider rims and wider tyres, and more aerodynamic frame designs have all been accelerated by the switch to discs.

What are the different types of disc brakes?

Though most road bike disc brakes will now use a hydraulic system, this is not always the case. Let's run through the different types of disc brake on the market.

Cable or ‘mechanical’ disc brakes

Photo: Matthew Page

The most basic designs use a standard brake cable to manually actuate the caliper, and are known as ‘mechanical’ disc brakes. The caliper arm sits on a spring which applies the pressure needed to allow the pads to move back out again once brake lever has been released.

Some cable disc calipers have just one moving piston, whose pad pushes the rotor against a second fixed pad. Others, like the TRP Spyre caliper picture above have two moving pistons, mimicking the action of a hydraulic caliper. 

Cable-actuated hydraulic disc brakes

Photo: TRP

The next tier combines some elements of both cable and hydraulic performance. Whilst the lever remains unchanged, still using a cable to apply the initial pressure, the caliper itself contains its own hydraulic chamber.

The plus side to this is that you can still use mechanical levers, which tend to be cheaper to buy and maintain, but you still get the power and self-adjusting properties of a hydraulic caliper. 

The downside is that they are still susceptible to typical cable issues, such as rust, cable-routing-related kinks and compression, so will not be as good as their fully hydraulic counterparts.

Hydraulic disc brakes

Photo: Matthew Loveridge

Fully hydraulic is the king of disc braking. All of the punchiness, all of the modulation, fewer of the mechanical foibles. But they do come at a heftier price, in both initial starting costs and repairs.

Photo: SRAM

As for the disc rotors themselves, there are two types: 6-bolt and centre-lock. The clue is in the name in that 6-bolt rotors attach to the wheel via six T25 Torx bolts and centre-lock rotors slot directly onto the hub and are tightened down using a lockring.

What makes disc brakes worth having?

Feel

Hydraulic disc brakes provide a completely different feel to standard rim brakes. The levers will usually have a later biting point, which gives a wider range of braking modulation. Not only does this allow for more technical control, but also helps avoid locking up the wheels unless desired.

The hydraulic fluid and hoses eliminate the compression that standard cables and housing suffer from. This means they shouldn’t have any sponginess unless air has entered the system and it needs bleeding.

Hybrid mechanical-hydraulic brakes will provide some of this same feel, whereas almost none of this translates to the purely mechanical models.

Performance

Photo: Roo Fowler

Disc brakes will typically generate more stopping power than conventional calipers. But it’s not just about more power. Road bike disc brakes also excel in their greater consistency, wet-weather performance, and control.

As the braking surface is no longer the rim (which picks up water), braking remains consistently powerful regardless of the weather. The smaller contact point also helps prevent the brake pads becoming contaminated by water or debris.

Disc brakes take the heat build-up caused by braking away from dangerous areas like the wheel itself and, with specially designed rotors and pads, are tailor-made to dissipate that heat as quickly as possible.

Not only is this a safety enhancement, but also reduces ‘brake fade’ whereby braking efficiency is negatively impacted by heat build-up.

Maintenance

At the same time as being more powerful, disc brakes are designed to require less maintenance and hydraulic brakes will self-adjust to an extent to compensate for pad wear. 

Pad life is significantly longer but it is key to change them once they are at minimal thickness. If the pad material runs down completely, the metal backing can begin to wear into the rotor and will damage not only the disc but the brake caliper as well.

Cleanliness is also crucial. Disc brakes do not like any sort of lubricant or grease, and even oil picked up from the road can contaminate the pads/rotors. As such, not over-lubing your chain and covering the brakes when cleaning the bike are also part of disc brake preservation.

Regarding adjustments, disc brakes can prove tricky. The clearance between pad and rotor is minimal and any small wobbles in the disc can cause loud and irritating noises. With such fine margins, they can be difficult to adjust at home so are more likely to require a trip to the bike shop than rim brakes.

Servicing

Photo: SRAM

With more intricate componentry comes the need for higher mechanical skills. Many riders will book in for a bike service for big adjustments and full disc brake bleeds, as specialist tools are required and it can be a messy job. It's perfectly feasibly to DIY it however:

When hydraulic brakes become spongy, it can mean the pads and rotors are completely worn out or, more likely, that there is either water, air or dirty fluid somewhere in the system.

A brake burp can let out air and replace it with fluid, which is a relatively small job. Dirty fluid or water can require a full brake bleed which is a bigger and more expensive repair.

Being a sealed system, hydraulic disc brake fluid shouldn’t suffer from contamination by dirt and grit.

As far as pad contamination is concerned, rotors can sometimes be cleaned, depending on the extent of the damage, whereas brake pads will almost always need replacing once contaminated.

A sure sign of pad contamination is a howling sound when you apply your brakes, plus reduced stopping power. 

Cost

Photo: Matthew Loveridge

With greater performance unsurprisingly comes greater costs. Whereas mechanical disc brake calipers may not differ a huge amount from their rim brake alternatives, the hybrid and fully hydraulic calipers will be more expensive to purchase.

As previously mentioned, hydraulic disc brake servicing is also more time-consuming so will cost more than replacing brake cables or rim brake pads.

The flip side of this is that the components tend to have a longer lifespan so can be a more economical purchase long-term and is generally less wasteful.

After the initial start-up cost, the pads also typically last longer than rim brake pads and, when the rotor is worn down, it can simply be replaced at a much lower cost than replacing the entire rim of a rim brake wheel.

Glossary: Brake types and parts

Mechanical

Compatible with standard shifters. Somewhere between traditional calipers 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.

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.

Caliper

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

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.

Hose

The bit that connects levers to calipers. Not cable and outer, instead hose and fluid.

Hydraulic fluid

The medium through which braking force is transmitted – either DOT (car-style, as used by SRAM) or mineral oil (employed by brands such as Shimano, Magura etc.)

Fancy learning some more about road bikes components? Read our in-depth guide to road bike groupsets

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