How Hydraulic Brakes Work
Hydraulic brakes have transformed mountain bike disciplines ever since their arrival. They enable us to go faster and stop harder. So what is it about hydraulic brakes which make them the stoppers of choice for professionals and trail riders alike? First a little bit about hydraulics..
The principle behind any hydraulic system is simple: forces that are applied at one point are transmitted to another point by means of an incompressible fluid. In brakes we call this brake fluid of which there are a few different varieties, but more on that later.
As is common in hydraulics the initial force which is applied to operate the system is multiplied in the process. The amount of multiplication can be found by comparing the sizes of the pistons at either end. In braking systems for example, the piston driving the fluid is smaller than the pistons operating the brake pads therefore the force is multiplied helping you to brake easily and more efficiently.
Another convenient characteristic of hydraulics is that the pipes containing the fluid can be any size, length or shape allowing the lines to be fed almost anywhere. They can also be split to enable one master cylinder to operate two or more slave cylinders if needed.
Now that we understand hydraulics let's take a look at the different parts which make up the hydraulic brake. The entire braking system can be broken down into the following main parts:
- Master cylinder (Lever)
- Slave cylinder (Calliper)
Next we will explain these components in more detail.
The master cylinder, mounted to the handlebar, houses the brake lever and together they produce the input force needed to push hydraulic brake fluid to the slave cylinder (or calliper) and cause the brake pads to clamp the rotor.
The lever stroke can be divided into 3 categories:
1. Dead-stroke - This is the initial part of the lever stroke when the primary seal pushes fluid toward the reservoir before it goes on to push fluid on to the calliper via the brake lines.
2. Pad Gap Stroke - This is the part between the calliper beginning to push the pistons out of their housings and the pads contacting the disc (as the dead space between the pads and rotor is taken up).
3. Contact & Modulation - The pads are now clamping the rotor and by stroking the lever further, additional brake power will be generated. Modulation is rider controlled and not necessarily a characteristic of the braking system, however some brakes may allow the rider to better modulate or control the braking forces than others.
Open or Closed?
Master cylinder systems can be categorised into two groups - open and closed.
An open system includes a reservoir and bladder which allow for fluid to be added or removed from the braking system automatically during use. Reservoirs are the overflow for fluid which has expanded due to heat produced by braking. The bladder has the ability to expand and contract therefore as the fluid expands the bladder will compensate without any adverse effects on the 'feel' of the brake. Reservoirs also provide the additional fluid needed as the pads begin to wear resulting in the need for the pistons to protrude further to compensate for the reduced pad material.
A closed system also utilises a reservoir of brake fluid, however the lack of an internal bladder to compensate for the expansion in brake fluid and also to compensate for pad wear means that any adjustments to the levels of brake fluid within the working system need to be made manually.
Hydraulic brake lines or hoses play the important role of connecting the two main working parts of the brake, i.e. the master cylinder and slave cylinder. We've already mentioned that hydraulic systems can be very versatile in that their lines or hoses can be routed almost anywhere so let's take a closer look.
Hydraulic hoses are multi-layered in their construction and usually consist of 3 layers:
1. Inner Tube - this layer of tubing is designed to hold the fluid. Teflon is usually the material of choice here as it does not react or corrode with brake fluid.
2. Aramid (Kevlar) Layer - provides the strength and structure of the hose. This woven layer is flexible and handles the high pressures of the hydraulic system efficiently in that it should not expand. Kevlar is also very light, which is a desirable attribute for any cycle component, and also it can be cut easily and re-assembled using standard hose fittings.
3. Outer Casing - Serves as a protection layer for both the Kevlar layer and the bike frame to reduce abrasions.
The layers that make up an average hydraulic brake line.
Steel Braided Brake Lines
Steel braided hoses can provide some advantages over standard hydraulic hoses. Steel braided hoses are also usually a 3-layer construction, the inner most layer contains the brake fluid and there is an outer most layer which provides protection against abrasions. The key difference is in the middle layer which is made up of a stainless steel braid.
This stainless steel layer is designed to be more resistant against expansion than that of standard lines. This can be an advantage because when the brake lever is applied we want all of the force we put in to be transferred to the calliper to cause braking. Any expansion in the hydraulic line due to the pressures within will mean that some of that pressure will not be transferred to the calliper. This will be wasted effort and will require additional lever input by the rider to compensate.
Steel braided lines may also be more appealing aesthetically. Many riders believe that they look better than the standard, boring black hoses that are supplied with the vast majority of brakes on the market.
Formula R1 brake with braided brake lines.
Hydraulic braking systems typically use one of two types of brake fluid - DOT fluid or mineral oil. An important thing to note before we get into the properties of each is that the two fluids should never be mixed. They are made up of very different chemicals and the seals within the braking system are suited to either fluid and not both; therefor mixing or replacing one fluid with the other is likely to corrode the internals of your brake.
On the other hand, mixing fluid from the same family is allowed but not generally advised. For example you may mix DOT 4 fluid with DOT 5.1 without harming your braking system.
DOT Brake Fluid
DOT brake fluid is approved and controlled by the Department of Transportation. It has to meet certain performance criteria to be used within braking systems and is classified by its performance properties - mainly its boiling points.
DOT 3, 4 and 5.1 brake fluid are glycol-ether based and are made up of various solvents and chemicals. Glycol-ether brake fluids are hygroscopic, which means they absorb water from the environment even at normal atmospheric pressure levels. The typical absorption rate is quoted to be around 3% per year. This water content within the brake fluid will affect the performance by reducing its boiling point. Which is why it is recommended to change brake fluid every 1-2 years at most.
The table below shows DOT brake fluid in its various derivatives with its corresponding boiling temperatures. Wet boiling point refers to fluid with water content after 1 years' service.
DRY BOILING POINT
WET BOILING POINT
205 °C (401 °F)
140 °C (284 °F)
230 °C (446 °F)
155 °C (311 °F)
260 °C (500 °F)
180 °C (356 °F)
270 °C (518 °F)
190 °C (374 °F)
DOT brake fluid is commonly used in Avid, Formula, Hayes and Hope brakes.
DOT 5 Brake Fluid
DOT 5 brake fluid (not to be mistaken for DOT 5.1) is very different from other DOT fluids as it is silicone based and not glycol-ether based. This silicone based brake fluid is hydrophobic (non water absorbing) and must never be mixed with any other DOT brake fluid.
DOT 5 can maintain an acceptable boiling point throughout its service life although the way in which it repels water can cause any water content to pool and freeze/boil in the system over time - the main reason that hygroscopic fluids are more commonly used.
Mineral oil is less controlled as a brake fluid, unlike DOT fluid which is required to meet a specific criteria, therefore less is known regarding its performance and boiling points from brand to brand.
Manufacturers such as Shimano and Magura design their brakes around their own brand of mineral oil and should never be introduced to DOT brake fluid as this will likely have an adverse effect on the brake's seals.
An advantage of mineral oil is that, unlike most DOT fluids, it does not absorb water. This means that the brake will not need to be serviced as often, but any water content within the braking system could pool and freeze/boil adversely affecting the performance of the brake.
Mineral oil is also non-corrosive meaning handling of the fluid and spillages are less of a concern.
The brake callipers reside at each wheel and respond to the lever input generated by the user. This lever input is converted to clamping force as the pistons move the brake pads to contact the rotor. Callipers can be fixed by a rigid mount to the frame or floating. Fixed callipers are combined with a fixed rotor which offers the only way of achieving zero free running drag, one drawback of this design is that it is much less tolerant of rotor imperfections. Floating callipers slide axially and self-centre with each braking application.
Calliper construction can fall into two categories - mono-block and two piece. The difference here is the 'bridge' design, the bridge is the part of the calliper above the pistons which connects the two halves together and provides the strength to endure the clamping forces generated by the pistons.
1. Mono-block - A mono-block calliper is actually a one piece design formed from one piece of material. This can offer a unique design and usually a lighter calliper as there is no need for steel bolts joining both halves as in a two piece design. Also the lack of a transfer port seal means there is one less opportunity for fluid leaks at the half way seam. Servicing a mono-block calliper can be tricky however and manufacturing and assembly are usually more difficult.
2. Two piece - These two piece callipers are constructed as two separate halves and are then held together with steel bolts which can provide additional strength over a mono-block design. Servicing, manufacturing and assembly are simplified. Steel bolts and additional seals are a means of additional weight and can be problematic during servicing.
Exploded view of an Avid two-piece caliper design.
The pistons are the cylindrical components housed within the calliper body. Upon lever input they protrude to push the brake pads which contact the rotor. The number of pistons within a calliper or brake can differ. Many hydraulic mountain bike brakes have 2 piston callipers, some may have 4 pistons. Whereas some automobile brake callipers have 6 or even 8 pistons. It is an important note that brake power is not determined by piston quantity. A more reliable indicator would be total piston contact area, e.g. 4 smaller pistons can be just as powerful as 2 larger pistons.
Pistons can be either opposed or single sided. Opposed pistons both protrude with lever input to push the brake pads equal amounts to meet the rotor at both sides. Whereas single sided calliper pistons stroke on one side and float the rotor to the opposite pad.
Choosing the right brake pads can mean the difference between a great and a poor performing brake. With the sheer diversity of brake pad materials out there it is quite easy to get it wrong when the time comes to replace the pads.
Let's jump right in and take a look at the different pad materials available and their properties.
Organic brake pads contain no metal content. They are made up of a variation of materials which used to include asbestos until its use was banned. These days you will commonly find materials such as rubber, Kevlar and even glass. These various materials are then bonded with a high-heat-withstanding resin. An advantage of organic pads is that they're made up of materials that don't pollute as they wear. They are also softer than other brake pads and as a result quieter. Also they inflict much less wear upon the brakes' rotor. However organic pads wear down faster and they perform especially poorly in wet gritty conditions (UK readers take note :).
Organic pads then are probably more suited to less aggressive riding in mostly dry conditions.
The metallic content of semi-metallic pads can vary from anything between 30% and 65%. The introduction of metal content into the friction material changes things slightly. It can improve the lifespan of the pad quite significantly as metal wears slower than organic materials. Also heat dissipation is improved as it is transferred between the pad material and the backing plate. Some disadvantages can include increased noise during use and the harder compound means increased wear on the rotor.
Sintered brake pads are made up of hardened metallic ingredients which are bound together with pressure and high temperature. The advantages of this compound are better heat dissipation, a longer lasting pad, better resistance to fading and superior performance in wet conditions. The trade-offs are more noise, longer bed-in time and a poor initial bite until the friction material has chance to warm.
Ceramic brake pads are now seen more and more as an alternative/upgrade mountain bike brake pad. Traditionally ceramic brake pads would only be seen on high performance racing cars with brakes which need to perform under intense heat. Heat like that is not usually a problem for the average mountain bike brake and therefor for most people ceramic pads would be overkill, however they might have other desirable properties. The advantages of a ceramic material then is one which can cope with extreme heat and keep performing strongly; this is in part down to its great dissipating abilities. They also last longer than other pads and noise is less of an issue. They're also easier on brake rotors and produce a lot less dust that other brake pad compounds.
Rotor size has a direct effect on braking power. The larger the brake rotor the more power will be produced for any given input. This can be a concern with larger rotors as they tend to have more of a 'grabby' feel making the brake more difficult to modulate.
Mountain bike rotors tend to range in size from 160mm to 203mm, with smaller rotors geared toward XC type riding and larger rotors designed for downhill riding.
Important specifications of rotor design include hardness, thickness and rub area.
The material used to manufacture rotors must be hard and durable due to the aggressive forces inflicted upon them from the pad friction material. This has a direct impact on rotor wear.
Rotors must also have no thickness variations. Differences in thickness around the circumference of the rotor can have undesired effects on the braking system including pulsing as thicker and thinner sections pass between the pads. Rotors also need to run true. Any lateral wobble in the rotor during use can cause the brake to contact the pads intermittently during riding.
Left to right: Formula Lightweight, Avid G3 Clean Sweep, Ashima AiRotor.
A rotor's rub area can take the form of many different designs. The three rotors above show this in detail. Rub area design can affect the weight and strength of the rotor. It also has a direct effect on pad lifetime.
Six Bolt or CenterLock?
The two types of rotor on the market today are ISO standard 6-bolt rotors and CenterLock rotors. Both have their pros and cons.
6 Bolt - Readily available and interchangeable between many brake models, this is the most common rotor fixing system in use today and was adopted by all manufacturers in the late 1990's. With no shortage of hub options, cross-compatibility with other products is rarely a problem. However installation of six fixing bolts can be cumbersome and there is always the risk of stripping a thread on fixing bolts and hub mounting points.
CenterLock - The Shimano CenterLock system eliminates the risk of stripping threads as there are no bolts to worry about, just one centre locking ring. Installation and removal is also simplified, although you will need a CenterLock tool. Lack of mass-market adoption means that hub choices are limited and brake choice may also be limited due to odd sized rotors. CenterLock rotors are also generally slightly heavier and can come at a price premium.
Left to right: ISO standard 6-bolt, Shimano CenterLock.
2-Piece rotors are supplied as standard with some higher priced brake sets and can also be bought separately as a brake upgrade.
In contrast to standard stainless steel rotors, 2-piece rotors combine a stainless steel rub area with an aluminium carrier (or spider). The advantage of the alloy carrier are a cooler running disc as aluminium has superior heat dissipation qualities to that of stainless steel. This will also help to keep your pads, calliper and fluid cooler. Aluminium is also lighter than stainless steel so a reduction in weight can be expected.
Formula 2-Piece Stainless Steel / Aluminium Rotor.
Why Brakes Fail
Hydraulic brakes can fail or temporarily stop working for numerous reasons such as a simple (but potentially catastrophic) fluid leak or eventual brake fade after prolonged use. Knowing the causes of brake failure can be valuable knowledge in curing the problem and preventing future episodes.
As we know there are a couple of important principles behind hydraulic brakes. Hydraulics rely on pressure within the system and brakes rely on friction. Absence of either will result in failure of the system. For example, a loss of brake fluid will decrease the pressure within the system as the lever has nothing to transfer the input forces to resulting in a need to bleed the system of air. On the other hand if brake fluid contacts the brake pads or rotor, a loss of friction will occur due to the lubricating nature of brake fluid.
The above examples should be obvious to most but what about the less obvious causes of brake failure? Earlier we mentioned brake fade, a term which I bet many of you have heard, however did you know that there are multiple types of brake fade? Below is an overview of the three different types.
All friction material (the stuff your pads are made of) has a coefficient of friction curve over temperature. Friction materials have an optimal working temperature where the coefficient of friction is at its highest. Further hard use of the brake will send the friction material over the optimal working temperature causing the coefficient of friction curve to decline.
This high temperature can cause certain elements within the friction material to melt or smear causing a lubrication effect, this is the classic glazed pad. Usually the binding resin starts to fail first, then even the metallic particles of the friction material can melt. At very high temperatures the friction material can start to vaporise causing the pad to slide on a layer of vaporised material which acts as a lubricant.
The characteristics of pad fade are a firm, non-spongy lever feel in a brake that won't stop, even if you are squeezing as hard as you can. Usually the onset is slow giving you time to compensate but some friction materials have a sudden drop off of friction under high temperatures resulting in sudden fade.
Green fade is perhaps the most dangerous type of fade which manifests itself on brand new brake pads. Brake pads are made of different types of heat resistant materials bound together with a resin binder. On a new brake pad these resins will cure when used hard on their first few heat cycles and the new pad can hydroplane on this layer of excreted gas.
Green fade is considered the most dangerous as it can catch users unaware given its quick onset. Many people would consider new brake pads to be perfect and may be used hard from the word 'go'.
Correct bedding-in of the brake pads can prevent green fade. This process removes the top layer of the friction material and keys the new pad and rotor together under controlled conditions.
Fluid fade is caused by heat induced boiling of the brake fluid in the callipers and brake lines. When used under extreme conditions heat from the pads can transfer to the calliper and brake fluid causing it to boil, producing bubbles in the braking system. Since bubbles are compressible this results in a spongy lever feel and prevents the lever input from being sent to the calliper.
The major cause of fluid fade is absorbed water from the air under normal atmospheric conditions which reduces the boiling temperature of the brake fluid. DOT brake fluid has an affinity for absorbing water from the air around it, especially in hot humid conditions. This is the main reason why we replace brake fluid on an annual basis.
Fortunately fluid fade has a gradual onset giving the user time to compensate for potential loss of braking.