HYDRAULIC DISC BRAKES 02 Introduction 03 The Lever 03 The Brake Hose 04 The Caliper OVERVIEW 05 Closed and Open Systems 07 Braking Power 11 Four-Piston Calipers 14 Heat and Fade 16 Care INTRODUCTION HYDRAULIC FACTORS AT WORK IN A DISC BRAKE HYDRAULIC DISC BRAKE Actuation force Leverage A hydraulic disc brake incorporates a (lever squeeze) (mechanical and hydraulic) master piston in the lever, a hydraulic brake DISC BRAKE line, two or more opposing slave pistons in the caliper, and hydraulic fluid (DOT brake A disc brake is comprised of a handlebar- fluid or mineral oil). There are several factors at work when using mounted lever, a frame or fork-mounted Clamping force Brake pad a hydraulic disc brake. The amount of lever caliper, and a hub-mounted rotor. at the caliper material In a hydraulic brake system, braking is actuation combined with the leverage of the When the lever is actuated, a brake accomplished by actuating the lever, which brake lever and hydraulic system create a pad (friction material) clamps onto the advances the master piston inside the lever clamping force at the caliper. That clamping rotor creating a frictional force at the body and forces fluid into the brake hose. force combined with the brake pad material rotor/caliper interface. This frictional Rotor diameter Friction at The fluid then moves into the caliper and produces friction at the rotor. The amount force slows the wheel by converting the the rotor presses against the slave pistons. Brake of friction combined with the diameter of kinetic energy of the spinning wheel into pads are attached to the slave pistons so the rotor generates braking power. Braking heat. In other words, slowing down = that when the fluid presses against the power, along with the duration of braking, heating up. pistons, the brake pads clamp onto the rotor. determines how quickly speed is reduced as Braking power Braking duration Once the pads contact the rotor, additional well as the amount of heat generated. force at the lever increases pressure in the system and creates greater clamping force on the rotor. Speed reduction, heat buildup Hydraulic Disc Brakes Overview | Introduction SRAM | 02 THE LEVER RADIAL LEVER Brake lever designs generally arrange the master cylinder in one of two configurations, radial or inline. Master cylinder Radial designs place the master cylinder perpendicular to the handlebar, while inline designs place Master piston the master cylinder roughly parallel THE BRAKE with the handlebar. The primary difference between these two Lever pivot designs is the location of the lever pivot, which can have a dramatic HOSE effect on the ergonomics of the lever. INLINE LEVER The brake hose connects the lever and caliper, and is an integral aspect of overall brake design and performance. Brake hoses are specially designed to Master piston support internal pressures in excess of 2000 psi without significant expansion or stretching. The small amount of expansion Lever pivot that does occur can be controlled by design to manipulate lever feel at specific pressures. It is important to note that while this does not affect overall braking power, it does allow for greater control of Master cylinder the available braking power. Hydraulic Disc Brakes Overview | The Lever | The Brake Hose SRAM | 03 THE CALIPER The slave pistons in the brake caliper often the distance the pistons must travel Another function of these seals is to allow there is a point where the piston will slip use special seals that flex, or roll, slightly before the pads contact the rotor. This the pistons to self advance as the pads through the seal. This means that as brake when the pistons/pads are pushed toward distance reflects the amount of clearance and rotor wear. As pads and rotor wear, pads and rotor wear, the pistons will move the rotor during braking. When the brake between the pads and the rotor, as well the distance between them increases, further than the seals can roll. As a result, is released, the piston seals relax and pull as the amount of lever movement required which affects piston travel and deadband. the pistons constantly slip through the seal the pistons/pads away from the rotor. This before the pads contact the rotor, known Without the use of these special seals, until the pads contact the rotor. Once the is known as pad rollback. The amount of as deadband. The greater the rollback the pad and rotor wear would require constant brake is released, the pistons/pads return to rollback is an important factor in determining greater the clearance and deadband. adjustment of pad clearance to maintain a new resting position, closer to the rotor. consistent deadband. But because the This eliminates the need to adjust pad amount of roll in a piston seal is limited, clearance or deadband as components wear. LEVER ACTUATED – LEVER RELEASED – FLUID PRESSURIZED FLUID DEPRESSURIZED Hydraulic fluid Piston seal gland Slave piston Piston seal Fluid pressure advances Piston seals pull pistons, piston seals flex pistons back Hydraulic Disc Brakes Overview | The Caliper SRAM | 04 Some bicycle hydraulic disc brakes CLOSED AND incorporate a closed system, where the volume of fluid remains the same DIFFERENCES between the master and slave pistons. BETWEEN OPEN OPEN SYSTEMS Most bicycle hydraulic disc brakes AND CLOSED SYSTEMS incorporate an open system, where a reservoir contains an additional volume Heat compensation - As the brake heats up, of fluid and a small empty cavity. The the fluid expands. In a closed system, this fluid and empty cavity are separated expansion creates pressure which advances by a flexible bladder that expands and the pads towards the rotor, causing drag contracts as changes in fluid volume occur. and affecting lever feel. In an open system, the expanding fluid can move into the reservoir once the brakes are released. This allows the system to operate consistently across a wide range of temperatures. Pad/rotor wear compensation - As pads and rotors wear, the slave pistons must move Bladder further in order for the pads to contact the rotor. In a closed system, more slave cylinder travel means more lever travel. Therefore, as the pads continue to wear the lever must Reservoir travel further. In an open system, piston slippage/pad advancement manages the gap between the pads and the rotor. Surplus fluid in the reservoir moves into the system CLOSED SYSTEM OPEN SYSTEM ensuring that there is always an adequate volume of fluid between the master and slave pistons. This allows the system to operate consistently as parts wear. Hydraulic Disc Brakes Overview | Closed and Open Systems SRAM | 05 Timing port FLUID MANAGEMENT IN AN OPEN SYSTEM Timing ports - During braking, fluid access to the reservoir must be blocked so that the flexible bladder doesn’t affect brake feel and isn’t tasked with sealing at high pressure. Timing ports are small holes located in the master cylinder, just in front of the piston (when the brake is at rest). When the lever is actuated, the master piston advances past these holes and closes off access between the reservoir and the rest of the system. Hydraulic Disc Brakes Overview | Closed and Open Systems SRAM | 06 BRAKING L x 1 L x 1 LEVER POWER PIVOT 1/1 = 1:1 LEVERAGE RATIO Total braking power is dependent on three L x 1 different attributes of the braking system: L x 2 L x 2 • Leverage/mechanical advantage • Pad friction material • Rotor size 2:1 LEVERAGE RATIO L x 1 2/1 = 2:1 LEVERAGE RATIO LEVERAGE AND MODULATION Leverage, or mechanical advantage, amplifies On the brake lever, the location of the lever L x 7 the force input at the lever, through the pivot in relation to the input force and the system, and to the pads/rotor. This will also pushrod (the mechanism that drives the have an effect on the amount of lever travel master piston) is a key factor in determining required to move the pads a given amount. leverage. This relationship is expressed as a L x 1 The total amount of leverage in the system ratio, where the length of the lever on one can be controlled by the design of the lever, side of the pivot is divided by the length of master piston, and slave pistons. the lever on the other side of the pivot. 7:1 LEVERAGE RATIO Hydraulic Disc Brakes Overview | Braking Power SRAM | 07 Hard raking stopping Brake levers are designed with the Additionally, depending on how the push pivot positioned close to the pushrod. rod moves in relation to the lever pivot, This provides increased leverage, where leverage can change during the course movement on the force input side of of braking. This change in leverage is the lever produces less movement on referred to as modulation. There are three the pushrod side, but also multiplies types of modulation: Light raking the input force by a factor equal to speed control the leverage ratio to produce a greater • RISING RATE - leverage increases output force to the master piston. throughout the lever travel • FALLING RATE - leverage decreases BRAKING POWER throughout the lever travel • LINEAR RATE - leverage remains Braking power Braking consistent throughout the lever travel Rising rate Falling rate SRAM brakes utilize a specially tuned Linear rate rising rate. It generates lower braking power when the pads first touch the rotor - preventing accidental wheel lock-up and loss of control - and increased leverage LeverLEVER TRAVELtravel and power when pulling the lever further. The result is a wide range of usable braking power and greater control during different braking scenarios. Hydraulic Disc Brakes Overview | Braking Power SRAM | 08 The final factor in determining overall leverage in the system is the piston ratio.