Introduction to Mountain Bike Disc Brakes
I. Disc Brake History and Overview
A. Basic Brake Design
Disc brake systems generate braking force by clamping brake pads onto a rotor that is mounted to the hub. The high mechanical advantage of hydraulic and mechanical disc brakes allows a small lever input force at the handlebar to be converted into a large clamp force at the wheel. This large clamp force pinches the rotor with friction material pads and generates brake power.
Hydraulic disc brakes utilize a master cylinder mounted on the handlebar to produce the input force at the lever and push brake fluid to a hydraulic caliper at the wheel which generates the clamp force.
Mechanical disc brakes utilize normal bicycle cable brake levers to pull a cable and actuate a mechanical caliper. Mechanical Calipers have internal components that can convert the cable force into a clamp force.
Brake Power is generated when the caliper brake pads clamp the rotor. As a general rule, three factors will determine how much brake force is generated:
- Clamp Force generated by the caliper: The tighter the rotor is pinched, the more brake force will be generated. This can be changed through the design of the hydraulic pistons or mechanical internals.
- Friction force generated by the friction material: The higher the coefficient of friction for the pad, the more brake power will be generated. Various friction materials will provide more power than others. See Coefficient of Friction below for further explanation.
- Rotor Diameter or effective radius of the rotor: Larger diameter rotors have longer torque arms and can generate more brake power with the same amount of clamp force than a smaller diameter rotor.
Brake power is only useful if it can be controlled and the tire does not lock up.
II. Brake System Terms and Characteristics
A. Lever Stroke – Lever stroke can be divided into three categories:
- Dead-stroke – The initial lever stroke when the primary seal pushes fluid into the reservoir instead of toward the caliper. Once the primary seal has stroked past the porting to the reservoir, all fluid is then pushed toward the caliper. This only occurs with open hydraulic master cylinders
- Pad Gap stroke – Stroke required to move the caliper pads out to contact the disc. The amount of stroke required for this stage will depend of the amount of clearance between the pads and disc as well as the brake design. Pad gap stroke begins immediately with mechanical or closed hydraulic brake systems.
- Modulation – The brake pads are now clamping the rotor and by stroking the lever further, additional brake power will be generated. See below for additional information about modulation.
B. Retraction – The action of the brake pads being physically pulled away from the rotor. Typically hydraulic calipers use a rubber seal (square seal) that deforms when the brake is applied, and then returns to its normal shape and pulls the pads away from the rotor when pressure is released. A few hydraulic and almost all mechanical systems use springs to pull the pads away from the rotor.
C. Burnish – Brake power is generated by the friction material on the pads embedding into the surface of the rotor, re-bonding to the friction material still on the pads and then breaking apart or shearing. In order for this bonding/shearing to occur, the friction material must first be displaced onto the surface of the rotor. This typically happens during the first 10 – 50 stops of a brake system and is referred to as “burnishing” the rotor and pads. When a rotor is cleaned, it will need to be re-burnished again to re-deposit the friction material onto the surface.
D. Coefficient of Friction (µ – pronounced “mew”) – A number measuring the “grip” of a material used in brake pads. Coefficient of friction can vary depending on the type of material used for the brake rotor. Typically service brakes are concerned with dynamic coefficient of friction, or the coefficient of friction measured while the vehicle is moving. The coefficient of friction may change as the brake system is required to perform through different applications. Below are a few of the main characteristics. Depending on the desired performance, the characteristics can be minimized or maximized.
- Speed Sensitive – Coefficient of friction typically drops as the speed of the vehicle increases.
- Pressure Sensitive – Coefficient of friction typically drops as more clamp force is generated.
- Temperature Sensitive – Coefficient of friction typically drops as the temperature of the brake system increases.
E. Modulation – Usually incorrectly referred to as a characteristic of a brake system. Modulation actually refers to the process of a rider accurately controlling the amount of brake power required without locking the wheel. Typically modulation is best with a brake system that has a “firm” or “hard” lever. The amount of lever stroke required to increase the amount of brake power generated is minimal. Soft levers require stroke to go towards caliper and hose expansion instead of brake power. This type of soft lever is inherently more difficult to control. Levers can feel soft due to mechanical/hydraulic advantage or hose and caliper stiffness properties.
F. Fade – Fade is generically defined as a decrease or loss of brake power and typically occurs in two ways:
- Friction Material Fade – When pads reach high temperatures they can sometime “outgas” chemicals that redeposit themselves on the surface of the brake pad. This decreases the coefficient of friction and results in a decrease of brake power. The lever will remain firm however the brake will not generate the normal amount of power. This is also described as when pads “glaze” over.
- Brake Fluid Fade – This type of fade occurs when the brake fluid inside a hydraulic caliper boils. An important characteristic of brake fluid is that it is incompressible. When a brake fluid boils, gas is formed within the system that is compressible and any lever stroke available goes toward compressing the gas instead of generating brake power. Interestingly enough, when a fluid is under pressure, it is very difficult for the fluid to boil. If a brake system is under pressure, the fluid temperature can rise above the boiling temperature without the fluid actually boiling. Once the pressure is released, the fluid will instantly boil and fade will occur.
G. Thermal Characteristics – Designing brake systems to handle high temperatures is just as important as designing them to be powerful. Below are three key elements for a system to properly handle high temperatures
- Thermal Mass – A brake system must be sized appropriately to not only be able to provide enough power for a vehicle, but have enough material mass to properly handle the temperatures during braking applications. Removing material from a system to reduce size and weight also removes material that would otherwise have helped a system absorb and diffuse heat generated by braking.
- Cooling Air – Keeping a brake system in the flow of air for cooling can also help reduce operating temperatures. Many calipers have open bridges that allow for air to flow through the caliper and remove heat from the braking surface.
- Surface Area – The more surface area available on a brake system, the better heat dissipation will be via convection. Cooling fins are often used in systems that are attempting to reduce operating temperatures because they greatly increase the surface area.
- Material Selection – Material selection is important in trying to control where the heat goes once generated. Insulators can be used to prevent heat from being conducted to the brake fluid. In the same manor, good heat conductors can be utilized to draw heat away from critical components.
H. Bleeding Brakes – Bleeding brakes is the process of “bleeding” the air out of a hydraulic system and filling it with brake fluid. The Hayes Brake Bleed Process forces fluid into the caliper up through the master cylinder allowing the air within the system to flow to the top and out of the system.
III. Hayes Disc Brake History
1972 – Schwinn 200E Series bicycle disc brake
1993 – Production of DiaCompe Speed Check Disk Brake
1997 – HFX Mag
1999 – Cable Actuated Hydraulic
2000 – Redesigned flip-flop Mag MC 2 piece clamp
G1 Caliper 74mm post mount
2001 – HMX-1 Mechanical
2002 – HFX-Comp
HML Mechanical Levers
2003 – HFX Mag Plus
HFX Nine MC
2004 – HFX Nine Carbon
2005 – El Camino
2 Piece Nine MC
V-Series 6 & 8" Rotors
2006 – MX 3
2007 – UNDER DEVELOPMENT
IV. Hayes Pioneering – Bike Industry Firsts and Standards:
- 203 mm Rotor Size
- 10 mm Quick Release Hub Rotor Offset
- 15 mm 20mm Thru and Rear Hub Rotor Offset
- 0.070” Thick Rotors
- Forward arcing rotor splines for thermal capabilities and strength.
- 74 mm Post Mount Calipers w/ Slotted mount feet.
- Flip/Flop Universal Lever Design with 2-piece clamp master cylinder body.
- T25 Low Profile Disc Screws
- Three Layer Hose Construction
- Zero spill brake bleed process.
- Tool-free brake pad change.
- Magnesium Master Cylinder Bodies
- Bladder/Cartridge Master Cylinder Design
- Hydraulic Master Cylinder Power/Modulation Adjustment
- Ball socket caliper pistons.
V. Future of Disc Brakes
Numerous opportunities exist with the potential for smaller, lighter, more powerful designs. There are also possibilities for further component integration with other parts on the bicycle. Disc Brake technology can improve performance through responsiveness, durability and control features such as brake by wire or wireless braking systems.
The Disc Brake Advantage
Disc brakes have a “power” advantage over rim brakes in two ways: First, disc brakes (mechanical or hydraulic) are able to generate much higher clamp forces than rim brakes. Second, the interface between the brake pads and the rotor can be customized for maximum brake system performance.
II. Environmental Consistency
Disc brakes are made to thrive in harsh environments. Disc brakes are located at the center of the wheel, and are thereby much more protected from the environment. Water, Mud, etc need to be splashed up on to the rotor or caliper to affect the braking surface. Also, friction materials can be designed to specifically remove water from the braking surface such that performance is not affected. These types of materials are not realistic for rims and rim brakes. Disc brake rotor materials can withstand the aggressive nature of the pads.
In addition to the protection from the environment, the location of the brake systems at the center of the wheel prevents out of true wheels from affecting the setup or performance of disc brakes.
Mountain Bike Disc Brake Industry Standards
I. Post Mount
II. Front IS
III. Front 20mm IS
IV. Rear IS
Mountain Bike Disc Brake System Materials and Coatings
Lightweight and strong. Good manufacturability. Capable of being anodized. Stiff material. Does not have an infinite fatigue life.
Strong and heavy. Depending on heat treat can offer a variety of properties.
A good material option stronger than aluminum but lighter than steel. Material is usually soft (ductile).
Extremely light weight material. Is not ductile and can crack under loading. Limited options for coating.
E. Stainless Steel
Corrosion resistant material offering various properties depending on alloy. Rotors are typically stainless steel offering corrosion protection, good wear resistance and excellent strength under extreme temperatures
F. Carbon Fiber
Lightweight and strong. Must be designed for custom applications to guarantee strength in the correct areas. Susceptible to cracks and has poor impact resistance
Unlimited composition offerings covering all types of physical properties.
H. Thermoset Plastics
Typically used thermal insulators with good compressive strength properties.
I. Aramid (Kevlar)
Known for its strength and toughness, Aramid is also extremely lightweight. Bullet proof vests are manufactured with this same material. Brake systems use this to strengthen hydraulic brake hoses.
EPDM (Glycol compatible) or BUNA-N (other) rubber compositions are used mostly for seals and covers inside brake assemblies.
Hayes Mountain Bike Disc Brake Operating Specs.
I. Torque Range: 0-230 ft-lbs
II. Clamping Force: 0-1200 lbs
III. Operating Temperatures:
El Camino, Mag, MX: -20ºF – 120ºF
HFX Nine & Sole: 0ºF – 120ºF
IV. Hydraulic Pressures
Mag, Nine, Sole: 0-1700 psi
El Camino: 0-2300 psi
V. Cable Tensions
MX 1, 2, 3, 4: 0-100 lbs
VI. Burnish Times
0-10 stops for 80% of full burnish. 30 stops for 100% burnish