Mechanical or Hydraulic (Actuated)
Based on how the brake operates—or more precisely, the medium used to move the brake pads to clamp the disc—we categorize them into cable-actuated and hydraulic systems.
Mechanical disc brakes use the tension of the brake cable to make the pads inside the caliper rub against and clamp the disc, creating braking force. Hydraulic brakes use fluid as the medium: when you pull the brake lever, it actuates pistons that compress the fluid. This pressure travels through the hose directly to the caliper, moving its pistons to push the pads against the disc. Mechanical disc brakes offer convenience in compatibility—for instance, most V-brake levers can work with them. Hydraulic brakes, however, consist of an integrated set of lever, hose, and caliper designed to work together, and they share no compatibility with traditional cable-operated brakes. Some disc brake manufacturers have developed semi-hydraulic systems, which retain a hydraulic caliper but still rely on cable pull—however, the cable actuates a piston at the caliper. The benefit of hydraulic disc brakes lies in their ability to multiply the braking force from your hand many times over, resulting in highly sensitive and efficient braking. Their sealed nature also requires minimal maintenance. While hydraulic brakes are widely and maturely used in motor vehicles, applying the same technology to the unforgiving precision of bicycles involves numerous technical challenges—even something as seemingly simple as a bicycle can be tricky to perfect. However, hydraulic brake levers contain a fluid reservoir, making them larger in size. This must be considered when fitting them onto the often space-constrained handlebars, especially in terms of compatibility with STI-type shifters and GripShift rotary shifters. Traditional rim brakes also had hydraulic versions—can you guess which? Magura HS11, 33, and 66 are examples. On the other hand, mechanical disc brakes that rely on cable-pull mechanisms may lose braking power due to factors like cable length, elasticity, and housing friction. In wet or rainy conditions, brakes can often produce a screeching noise, which might make some riders cringe.
TIP: Some people habitually store their bikes upside down with both wheels in the air. If you have hydraulic disc brakes, air or gas in the reservoir might enter the hoses, reducing braking performance. Get into the habit of testing your brakes before riding.
Open or Closed System
There’s quite a bit of debate about the internal construction of hydraulic disc brakes. Starting with open and closed systems, the biggest point is this—oil expands and contracts with temperature changes.
In hydraulic disc brakes, if the oil heats up—especially during prolonged braking on hot days—it can expand too much and push the pads outward, possibly causing them to rub, drag, or even lock up. The so-called open hydraulic system addresses the thermal expansion of oil by featuring a special reservoir and extra space near the brake lever to accommodate the increased oil volume, as well as allowing fluid to flow back to compensate for pad wear. In contrast, a closed system isolates the pads, pistons, and oil, preventing the oil's expansion from pushing the pads outward excessively. As such, the oil used in these systems must be carefully selected (typically mineral oil). Since pads wear down, closed-system disc brakes come with sensitivity adjustment features on the reservoir.
TIP: If the expanding oil causes the caliper to lock the disc, simply releasing some oil yourself is definitely not the solution—in fact, it could risk brake failure. It’s safer to take the bike to a shop for a proper check-up.
Mineral Oil or DOT Standard Brake Fluid
MineraloilorDot-approved(departmentoftransportation)brakefluids
DOT-3 and DOT-4 are brake fluids specifically designed for automotive and motorcycle disc brakes. Their formulation is meant to address the issue of thermal expansion. When braking generates high temperatures, the fluid can boil and vaporize, producing gas. The problem is that gas is much more compressible than fluid—so when you pull the brake lever, you're actually just compressing gas bubbles, which won't move the pistons or pads effectively, leading to brake failure or severely reduced power. Additionally, DOT-3 and DOT-4 have lower boiling points than mineral oil, are toxic, corrosive, and can damage paint finishes—but they do absorb moisture, including water vapor from within the reservoir. Fluid containing water has a lower boiling point. On the other hand, Cannondale’s Coda disc brake fluid claims to be a specially formulated fluid developed by NASA, capable of withstanding the extreme heat of rocket launches—so bicycle braking is no problem at all. Magura was among the first to use mineral oil. Its composition is very pure; although non-toxic, it doesn’t absorb moisture or wash off paint. Its advantage is that braking power remains consistent even with prolonged use or high temperatures. Brake fluid should be replaced regularly; DOT fluids, because they absorb water, are generally recommended to be changed once a year. Mineral oil doesn’t need to be changed as frequently, but if you notice a drop in braking responsiveness, it’s worth checking whether it’s time for a change.
TIP: User manuals or reservoirs usually specify which type of fluid to use. DIYers, when changing the fluid, be sure not to mix or confuse these two types. DOT fluid can corrode seals in mineral oil systems, and vice versa. Hayes hydraulic disc brakes use DOT fluid. The Magura system uses mineral oil.
Floating or Fixed Caliper
There are two types of disc brake calipers: those fixed solidly in place, and those that can move slightly after being mounted to the frame or fork, typically sliding sideways. During the development of bicycle disc brakes, two schools of thought emerged. The design of floating calipers—which "can slide sideways"—is closely related to the drawbacks of single-sided braking. However, it offers benefits; for example, frames and forks may have manufacturing tolerances, and even variations in the thickness of paint on mounting surfaces can affect the precise alignment between the disc and pads. That’s why shims (washers/spacers) are often relied upon to correct and align everything. Some floating calipers specifically emphasize their ability to self-align the disc and pads, ensuring parallel contact without worrying about tolerances.
TIP: The paint on the disc brake mounts of frames and forks is usually scraped off during installation, as its thickness can cause assembly tolerances. But be careful not to damage the metal surface, which could backfire.
Single-Sided or Dual-Actuated
Single-sided and dual-actuated braking aren’t exclusive to either hydraulic or mechanical disc brakes, but mechanical disc brakes are mostly single-sided. Single-sided braking means only one piston pushes a pad against the disc, while the other pad waits passively for the disc to be pushed toward it, allowing both pads to engage. This has led to the two schools of thought regarding floating and fixed calipers. Dual-actuation, naturally, refers to calipers with opposing pistons that clamp the disc from both sides simultaneously. Currently, disc brake calipers can have 1 (single-sided), 2 (one pair), 4 (two pairs), or even up to 6 (three pairs) pistons. The number of pistons doesn’t necessarily equal the number of pads—some calipers have two pistons on one side but use a single elongated pad; having two pads doesn’t always mean dual-actuation, as it could still be single-sided. For example, the Magura Gustav M is single-sided—how many pistons and pads does it have? Most dual-actuated mechanical disc brakes are products of Taiwanese manufacturers, such as RST and Shimano. An example is the popular Shimano M1.
TIP: Simple test: After installing disc brakes or buying a bike equipped with them, park the bike, pull the brake lever by hand, and watch the disc and pads. Observe how they come together—especially with single-sided systems, check if the disc is being pushed too far to one side or wobbling.
Metallic or Organic Brake Pads
Brake pads made of different materials have different friction coefficients.
Organic brake pads are made from materials like Kevlar mixed with fiberglass, without any metal powder. They can deliver strong braking power, but their performance drops more dramatically in high heat and humidity compared to metallic pads. The advantage is that these pads don’t conduct heat, so there’s less worry about the effects of oil expansion—making them ideal for closed hydraulic systems. Metallic pads, on the other hand, usually contain copper or iron powder, making them more durable and resistant to the effects of dust and mud on braking power. However, they conduct heat too well, so they’re better suited for open hydraulic systems. Between the disc and the inner and outer pads, it’s generally emphasized that they should remain parallel with equal spacing, and careful alignment is done during assembly to achieve ideal synchronous and balanced braking. This approach leans toward the principle of dual-actuated calipers. Some floating, single-sided disc brakes have the disc positioned closer to the fixed, non-moving pad—sometimes even lightly touching—so the wheel doesn’t spin freely and may produce friction noise, which is also said to subtly reduce speed.
TIP: New disc brakes and pads require a break-in period. It’s risky to immediately test their full performance right after installation.
Disc Rotor or Disc
The disc’s main job is to endure punishment—it plays the role of the long-suffering component. Most are made of stainless steel, though some manufacturers produce aluminum discs, claiming better heat dissipation, as well as titanium or carbon models. Discs must be paired with compatible disc brake hubs. Converting a traditional rim-brake bike to disc brakes usually requires a complete replacement or special adapters. Hub-disc compatibility is another factor to consider. Standard discs have six bolt holes for mounting; early models had four, and some, like the older Cannondale Coda, still use this design, as does Italy’s Formula. The UK’s Hope uses five holes, but also offers the more common six-hole standard. Germany’s Bees uses a unique non-bolt mounting system, resembling a spline or ratchet design, and only works with their own hubs. Disc diameter isn’t standardized either—the most common is 160mm, though Hope offers smaller sizes like 130mm and 145mm. Generally, more aggressive riding styles call for larger discs, as greater braking power is needed, and front discs are usually larger than rear ones—or they may be the same size.
TIP: Normally, just wipe the dust off the disc; for a more thorough clean, use alcohol. During lubrication and maintenance, avoid getting oil on the disc—it’s the last thing you want for effective braking.