Hydraulic Disc Brakes: A Closer Look
Hydraulic disc brakes, vital for safe cycling, rely on the use of specific fluid to function optimally. When you engage the brake lever, the fluid within the system triggers the pads to make contact with the disc rotor, effectively bringing you to a secure stop.
There are two primary types of brake fluid utilized in bicycle brakes – mineral oil and DOT fluid.
It is of utmost importance that you adhere to the fluid specified by your brake manufacturer. Neglecting to do so will inevitably result in premature brake failure.
In the following sections, we will delve deeper into the distinct types of brake fluid, the systems in which they are utilized, and provide answers to common questions regarding these fluids.
The Significance of Brake Fluid
The correct brake fluid plays a pivotal role in ensuring optimal brake performance. When you activate the brake lever, a piston within the lever is depressed, causing the fluid to flow through the hydraulic hose and reach the brake caliper. This action then compels the pistons to extend outward from their housing within the caliper, commonly referred to as the bores. The brake pads are then pressed against the pistons, resulting in direct contact with the disc rotor.
Once you release the lever, the fluid returns to the reservoir.
Brake fluid is specifically designed to withstand high temperatures and possesses a boiling point considerably higher than what a brake will typically be exposed to under normal conditions.
Furthermore, brake fluid is non-compressible, effectively transferring all the power from the lever to the pads.
Compared to mechanical cable-operated disc brakes or rim brakes, hydraulic braking systems offer superior modulation. Due to the sealed hydraulic setup with no cables to stretch, they offer more reliable braking. As the brake pads gradually wear down, the fluid within the lever reservoir continuously replenishes the system, ensuring consistent braking performance.
This enhanced modulation and control over the amount of brake force exerted, depending on how far the lever is pulled, also minimizes the risk of wheel lock-up when applying the brakes.
What is mineral oil?
Mineral oil, a vague and unregulated term, is utilized by notable brands such as Shimano, Campagnolo, and Magura. Each manufacturer employs their own distinct variety of fluid to meet their specific needs.
Advantages and disadvantages of mineral oil
The benefits of mineral oil include its durability, as it tends to last longer, and its non-corrosive nature towards your bike’s paintwork. Nonetheless, if any residue manages to come into contact with a component or the paintwork, it should still be promptly wiped off using isopropyl alcohol and a rag.
In the event that any accidental contact occurs with your hands, mineral oil typically does not cause irritation. However, as a safety precaution, we recommend wearing eye protection and gloves when bleeding your brakes.
On the downside, mineral oil lacks regulation, leading to variations in quality amongst different mineral oils. Consequently, it is advisable to avoid mixing mineral oils and to purchase it from a local bike shop or online bicycle retailer. In contrast, DOT fluid, commonly used in the automotive industry, is more widely accessible.
Nonetheless, the absence of regulation can be seen as advantageous for manufacturers, as it grants them complete control over the brake fluid manufacturing process.
Furthermore, although mineral oil does not absorb moisture from the air (known as being hygroscopic), if water does enter the system, it will tend to accumulate at the caliper, resulting in decreased braking performance.
Which brands use mineral oil?
Shimano, with their unique formulation, is a prominent user of mineral oil. Other major manufacturers, including Magura and Campagnolo, also rely on mineral oil. Formula brakes employ it as well, while SRAM has recently introduced their own mineral oil brake, the DB8, although they primarily utilize DOT fluid.
Clarks, Promax and Tektro / TRP also use mineral oil.
Shimano
Shimano utilizes its own variety of mineral oil that is compatible with all of its brakes and comes in a distinctive purple or red hue.
According to Shimano, it is crucial to only use their designated mineral oil as using any other alternative will not only void the warranty but also cause damage to the system’s seals.
Magura
Magura employs its own unique mineral oil called ‘Royal Blood’, which is characterized by its blue color.
Campagnolo
In Campagnolo’s original road groupsets, the brand utilized Magura’s Royal Blood mineral oil (rebranded as Campagnolo).
In their Ekar brakes, the Italian brand introduced their own red-colored mineral oil. Campagnolo plans to use this fluid for all their hydraulic disc brakes moving forward.
While it remains compatible with Magura Royal Blood, it is important to thoroughly purge the entire system of any red fluid before using it.
SRAM
Prior to the introduction of the DB8, SRAM’s brakes traditionally utilized DOT fluid. However, the brand recently released the DB8, which makes use of mineral oil instead. This brake is positioned as a mineral oil alternative to the Code (SRAM’s downhill brake), though SRAM notes that it may not provide as much power due to the fluid change.
SRAM collaborated with Maxima to develop its own mineral oil, which is claimed to be water-resistant and requires less frequent changing or bleeding compared to DOT 5.1.
What is DOT fluid?
With the exception of the DB8, all of SRAM’s brakes rely on DOT 5.1 fluid. Warren Rossiter / Immediate MediaDOT fluid is the other main type of brake fluid. Unlike mineral oil, DOT fluid is regulated and primarily used in the automotive industry. Its name is derived from the ‘Department of Transport’. There are typically four types of DOT fluid categorized by their boiling points – DOT 3, DOT 4, DOT 5, and DOT 5.1.
The Reason DOT 5 is Not Used on Bicycle Brakes
DOT 5 is not utilized in bicycle brakes due to its silicon-based composition, unlike the other fluids that are glycol-based. Implementing DOT 5 would result in the swelling of the seals.
Advantages and Disadvantages of DOT Fluid
Advantages of DOT Fluid
To begin with, it is crucial to cover any areas that DOT fluid could potentially leak onto with a rag as it possesses corrosive properties. DOT fluid has the benefit of being widely accessible, not limited to bike shops or specialized online retailers, as it is also employed in the automotive industry. Additionally, it is generally more economical compared to mineral oil.
Disadvantages of DOT Fluid
The primary downside of DOT fluid is its hygroscopic nature, which means it has the tendency to absorb water, unlike mineral oil. As more moisture infiltrates the system over time, the boiling point decreases, consequently affecting brake performance. This leads to reduced stopping power and the possibility of brake fade. Therefore, it is crucial to bleed your brakes at least once a year to prevent these issues from occurring.
The minimum boiling point of DOT 5.1 fluid is 270 degrees Celsius when dry and 190 degrees Celsius when wet (with some water present in the system). While it is highly unlikely that your brakes will reach these extreme temperatures, there is an elevated risk of brake fade when the fluid contains water over time and is subjected to high heat, such as during a long descent in the Alps as opposed to when it has been freshly bled.
DOT fluid is known to allow approximately two to three percent of its volume in water per year, although this percentage may increase in particularly humid conditions. However, it is important to note that while DOT fluid permits moisture to enter, the water does not accumulate in one place but rather disperses evenly throughout the entirety of the fluid, thereby maintaining optimal performance.
Moreover, DOT fluid possesses corrosive properties and has the potential to damage paintwork. When bleeding a brake with this fluid, caution should be exercised. It is recommended to wear suitable protective eyewear and gloves, and any spills should be promptly wiped away using isopropyl alcohol.
Brands That Utilize DOT Fluid
Brands That Use DOT Fluid
SRAM’s Bleeding Edge connectors facilitate a much easier and less messy process of bleeding hydraulic disc brakes. As a result, SRAM and Hope are the primary brands that employ DOT fluid. Additionally, after-market brake manufacturer Hayes and some Formula brakes incorporate this type of fluid.
SRAM Brakes: What Brake Fluid to Use
Introduction
SRAM brakes utilize DOT 5.1 fluid for optimal performance. However, it is worth noting that DOT 4 fluid can also be used as an alternative, although it has a lower boiling point.
Hope Brakes: Dot 5.1 Fluid
Hope brakes, like SRAM brakes, also rely on DOT 5.1 fluid for efficient operation.
Can Brake Fluids be Mixed?
The Importance of Avoiding Fluid Mixing
Mixing different types of brake fluids is strongly discouraged. This practice can lead to the degradation of crucial seals, ultimately resulting in premature brake failure.
If you are unsure which type of fluid your brakes require, check for any indications on the lever or caliper. Alternatively, seeking advice from your manufacturer or local bike shop is always a reliable option.
The Shelf Life of Brake Fluid
Maximizing Fluid Shelf Life
Mineral oil, once opened, can be stored indefinitely without compromising its braking performance.
On the other hand, DOT fluid tends to attract moisture upon opening. To ensure optimal results, consider purchasing smaller quantities, around 100ml, which should suffice for two to three bleeding sessions. According to SRAM, the typical shelf life of DOT fluid is approximately one year.
Brake Bleeding Frequency
The Importance of Regular Brake Maintenance
Regularly bleeding or servicing your brakes can have a significant impact on their overall performance. In general, it is recommended to bleed your brakes once a year or whenever the brake lever feels spongy.
Over time, water, moisture, and contaminants can infiltrate the brake system. Without bleeding, it is impossible to determine if the fluid has discolored, which indicates the need for fresh fluid.
If you observe discolored fluid during the bleeding process, it is essential to replace it and continue bleeding until the syringe or bleed bucket no longer contains any discolored fluid.
Remember to use dedicated bleed kits for each specific type of fluid and avoid mixing different fluids within the same kit.
