Brake System

Last updated: November 30, 2023

Overview

The Cybertruck brake system is a hydraulic-based brake system, consisting of 4 brake calipers actuated by an electromechanical brake booster and an Electronic Stability Program (ESP) system. The rear brake calipers have an integrated Electronic Park Brake (EPB) motor mounted to each caliper for the electronically actuated parking brake.

Brake System Components

Front Brake Calipers

The front brake system uses a mono-block aluminum four-piston design fixed caliper with dual bleed nipples.

Rear Brake Calipers

The rear brake system uses a single piston sliding caliper with an integrated EPB and a single bleed nipple.

Brake Pads

The brake pads consist of semi-metallic copper-free friction material bound to a steel backing plate located between the caliper pistons and the rotor. The brake pads are equipped with wear indicators, which are thin metal strips attached to the brake pads that squeal as they rub against the rotors when the pads wear down. This squealing sound indicates that the brake pads have reached the end of their service life and require replacement.

Brake Rotors

The brake rotors are of a single-piece internally-ventilated cast iron structure with a corrosion prevention coating. Rotors are not serviceable by machining. Brake rotors should be replaced if the thickness of the rotors falls below the minimum specification in the Service Manual.

Electronic Park Brake (EPB)

The Electronic Park Brake (EPB) system is integrated into the rear foundation brake calipers, which includes key components such as the EPB drive unit, brake caliper, piston assembly, brake pad assembly, and two electronic control units (ECUs). These ECUs have distinct roles, with one being integrated into the rear vehicle control unit (VCREAR) and the other into the left vehicle control unit (VCLEFT).

There are different ways to engage the electronic parking brake:

• On the left side of the user interface (UI), there is a vehicle direction selection slider bar, with a 'P' icon located at the top.
  • Touching this icon will activate the parking brake.
• On the touchscreen, go to Controls > Safety & Security > Parking Brake menu and follow the on-screen instructions.
• The parking brake will automatically shift the vehicle into Park if the driver exits while the vehicle is in Neutral or Drive when it is stationary.

Where applicable, the EPB motor may be serviced separately from the hydraulic components to avoid requiring bleeding.

When the parking brake is applied, the RED parking brake indicator lights up on the touchscreen.

If a fault has been detected with the EPB, the YELLOW parking brake lights up on the touchscreen with a fault message.

Electromechanical Brake Booster (iBooster)

The iBooster is a vacuum-free electromechanical actuated system which is an integral part of the brake system.

If the iBooster suffers an irrecoverable fault, such as disruption of supply power, the driver can still brake unhindered. The driver's brake energy is transmitted mechanically through the booster to the Tandem Master Cylinder (TMC). The ESP system detects booster faults and generates additional brake force to aid the driver by using its own hydraulic pump (normally used for stability and traction control events). The brake force is metered proportional to the measured upstream hydraulic pressure from the TMC, to allow reasonable modulation of brake force by the driver. During this time, pumping noise and pedal pulsations will be felt through the brake pedal by the driver. This is the intended backup operating mode, and it allows a much greater vehicle deceleration to be achieved than by the driver’s muscular energy alone. This function is called Hydraulic Brake Boost (HBB).

Brake Fluid Reservoir

The brake fluid reservoir, mounted on top of the Tandem Master Cylinder (TMC), is a specially formed, clear plastic fluid container designed to fill to the necessary brake fluid volume. The brake fluid reservoir houses a fluid level indicator which will set a RED warning light on the touchscreen and will log an alert in the log data if the brake fluid level is too low.

Warning

Only Tesla-approved brake fluid is permitted in the brake system. The use of any other unapproved fluid may result in performance degradation or component damage. Do not use fluid from unsealed containers, as it might have absorbed moisture from the atmosphere.

Booster Assembly

Warning

No third-party modification of this is permissible. Foreign magnetic fields must always be kept away from the ECU. All parts are non-serviceable.

The booster assembly can be broken down to several key functional areas to aid service and troubleshooting discussions:

• Powerpack: Consists of the ECU and motor housing. All the electronic controls are located within the black plastic ECU housing. The housing is permanently attached to the booster motor housing and is non-serviceable. The 12V-brushless DC motor is located in the cylindrical steel housing fastened to the booster assembly. Motor rotation is measured using a rotational hall-effect sensor. There is one electrical interface on the ECU housing to connect to the vehicle.

• Gear housing: The primary booster mechanical components are located inside the steel housing. The internal components are physically separate from the master cylinder and brake fluid volume, as well as the external environment. This is a clean zone. No foreign contaminants, such as dirt, liquid, or any solvents or lubricants are permitted to enter the housing. Translation of the motor shaft rotation to linear motion is done by a gearset inside this housing. A mechanical device to enable the driver to modulate brake force is packaged with the transmission. The Differential Travel Sensor (DTS) is located in the gear housing. The steel gear housing helps shield the DTS from magnetic interference.

• Input Rod: The input tie-rod is the mechanical link between the brake pedal arm and the booster internal mechanism. It is connected to the pedal arm by a clevis and pin, and it enters the booster housing through a rubber bellows. This is extended through the entire booster via another tie rod to rigidly couple with the TMC. The input tie rod is very sensitive to all mechanical loading while not mounted. Extreme care should be taken when handling the booster while not fully assembled in the vehicle, to avoid permanently damaging the booster mechanism.

Electronic Stability Program (ESP) Unit

The electronic stability program unit is an electromechanically actuated system which is an integral part of the brake system. The ESP is specifically designed for vehicles with diagonal brake circuit distribution. Because the ESP is combined with the electromechanical brake booster iBooster, vacuum generating components are not required, and a separate pedal-travel sensor is not necessary because it is already integrated in the iBooster.

The ESP hydraulic control unit is separate from the iBooster and the calipers in the hydraulic circuit. This allows the control unit to serve as a redundant brake actuation system in case of iBooster faults. One of the key capabilities of the ESP unit is the ability to independently build brake pressure at any brake caliper. This feature enhances stability control during dynamic driving when the vehicle detects oversteering or understeering events.

The ESP unit can be broken down to several key functional areas to aid service and troubleshooting discussions:

• ECU housing: The electronic controls are located within the black plastic ECU housing. The housing is permanently attached to the valve block and is non-serviceable.
• Valve block: The hydraulic valves and accumulators are located within the aluminum valve block. This part is also non-serviceable.
• ESP pump motor: The DC motor is located in the cylindrical steel housing fastened to the valve block. This part is also non-serviceable.

Wheel Speed Sensor

The wheel speed sensors provide the digital wheel speed signals to the ESP unit. A wheel speed sensor module is installed in each knuckle assembly. The sensing element is situated to align with the magnetic tone ring, which is integrated with the inboard bearing seal of the hub bearing unit. Each seal contains magnetic elements arranged in pole pairs that make up the tone ring. Only one side of the bearing contains the integrated tone ring. As the wheels rotates, the pole pairs in the seals induce sinusoidal voltage fluctuations in the wheel speed sensors (Giant Magnetoresistance or GMR sensors) that are converted into square wave signals. The signal frequency is proportional to the speed of each wheel.

The wheel speed sensors are active sensors. They output a current-based signal that has been converted from analogue form and processed in the sensor unit before being sent to the ESP unit. This ensures that disturbances and errors in the raw GMR signals are not passed to the ESP unit. The pulse width carries further direction and diagnostic information and is not proportional to vehicle speed. Also, the sensor monitors several internal diagnostics and can enter a safe state to prevent passing erroneous wheel speed information to the ESP unit.

Each wheel speed sensor module includes two sensing elements that are electrically separate from one another. One is connected to the ESP unit on the vehicle left power domain, while the other is connected to a vehicle controller on the right power domain. The second sensing element allows backup ABS functionality via iBooster in the event of an ESP unit failure or power domain loss. Each sensing element operates on the same principle as described above.

Each wheel speed sensor module also includes a 3-axis accelerometer for other uses on the vehicle.

Inertial Measuring Unit (IMU)

The Inertial Measurement Unit is an electronic device that measures yaw, roll, and pitch of the vehicle. This is integrated into the Restraints Control Module (RCM) and communicates with the ESP and iBooster over the chassis controller area network (CAN). The sensor is used for ESP operation and measures the vehicle’s rotation around its vertical axis (yaw rate), while at the same time measuring the acceleration perpendicular to the driving direction. The ESP function uses the sensor inputs to detect the onset of side slip during cornering.

Pedal Assembly

The brake pedal and accelerator pedal form a single assembly. The brake pedal arm connects to the booster input rod using a clevis. The accelerator pedal assembly has a housing at its pivot containing an electromagnetic induction device to sense rotation angle. There are two circuits for redundancy.

Brake system operation modes

Normal Braking

The iBooster detects the driver’s brake request by an integrated pedal travel sensor, and this information is sent to the integrated control unit. The control unit operates the electric motor, while a two-stage gear unit converts the motor torque into the necessary longitudinal boost force. The force supplied by the booster and the driver is converted into hydraulic pressure in a conventional tandem master brake cylinder. This is the farthest upstream element of the “wet” hydraulic system. This is then distributed into the rest of the hydraulic system via the ESP unit.

The tandem master cylinder consists of two floating hydraulic pistons in series. There are two hydraulic inlets from the brake fluid reservoir (one for each piston) and two hydraulic outputs routed to the ESP unit (one for each piston).

When the brake pedal is released, the springs in the master cylinder return the master cylinder pistons to the rest position. Brake system pressure is relieved, the caliper pistons retract, and the brake pads no longer apply a braking force to the rotors.

Electronic Stability Program

The ESP function assists the driver in maintaining control of the vehicle while cornering. ESP differs from conventional Anti-lock Braking System (ABS) / Traction Control (TC) systems by using yaw rate and acceleration sensor information in addition to the four-wheel speed sensors (hall-effect sensors), located on the four-wheel hubs, which provide the wheel speed signal for each wheel. ESP calculates the intended path of the vehicle based on the driver’s inputs (from a steering angle sensor) and compares it to the measured rate of turn of the vehicle from the yaw rate sensor.

ESP monitors for understeer or oversteer events during cornering. In the case of understeering, braking the rear inside wheel results in a positive yaw torque that helps the vehicle turn into the corner. During oversteering, braking the front outside wheel results in a negative yaw torque that steers the vehicle out of the corner and helps the rear axle regain traction. If the vehicle path deviates from the driver’s path, ESP briefly applies the brakes at individual wheels to help steer the vehicle back to the intended path. ESP also controls and limits motor power to the extent necessary to support lateral tire grip during cornering.

ESP works on all road surfaces and weather conditions to help utilize all available road force to keep the vehicle stable. However, ESP cannot evade the laws of physics, and it does not prevent loss of control if a driver carries excessive speed into a corner. Furthermore, ESP only assists the vehicle to follow the path the driver is steering. ESP does not prevent the driver from steering a vehicle off the road.

The ESP also operates a function commonly known as Anti-lock Brake System (ABS). This adjusts the maximum brake pressure transmitted to the wheels, preventing loss of roadholding under all tire and road conditions caused by wheel locking. The system has been designed to integrate, but not replace, the normal mechanical braking system.

The ESP controller constantly monitors and compares signals received from the 4 speed sensors. When the brake is applied, a wheel decreases speed in relation to the other wheels, and is about to lock, the ESP modulator reduces the brake pressure at the wheel to prevent wheel locking. This alteration of brake pressure can be felt as a pulsing sensation through the brake pedal. This demonstrates that the ESP is operating and is not a cause for concern. When the wheel speed stabilizes, the ESP controller promptly adjusts brake pressure to maintain optimal braking force. The ESP control system performs these adjustments several hundred times per second on each wheel independently. This enables the driver to achieve the highest brake force physically possible while maintaining vehicle stability and steering over any road surface.

The ABS indicator briefly flashes yellow on the touchscreen when the vehicle enters drive. If this indicator lights up at any other time during the drive cycle, it indicates an ESP fault, showing that the ESP is not operating. The braking system remains fully operational and is not affected by an ESP fault. However, braking distances may increase.

Traction Control

The Traction Control (TC) system constantly monitors the speed of the front and rear wheels. If a loss of traction is detected by analysis of the wheel speed sensors, the system minimizes wheel spin by controlling brake pressure and motor power. By default, the TC system is on. Under normal conditions, it should remain on to ensure maximum control of the vehicle.

The yellow indicator flashes on the touchscreen whenever the TC system is actively controlling brake pressure and motor power to minimize wheel spin. If the indicator stays on, this indicates that a fault is detected with the TC system.

Hydraulic Fade Compensation

Hydraulic Fade Compensation (HFC) is a function that increases the braking pressure when brake fade is detected by the ESP system. It looks at the pressure in the brake master cylinder, compares this to what is happening at the wheel, and uses the ESP pump to actively build pressure in the system up to ABS initiation levels. HFC will normally trigger when there is not enough friction between brake pad and rotor due to overheated or wet/iced brake pads.

Automatic Emergency Braking

Warning

Automatic Emergency Braking is designed to reduce the severity of an impact. It is not designed to avoid a collision.

Warning

The brake pedal moves downward abruptly during automatic braking events. Always ensure that the brake pedal can move freely.

When Automatic Emergency Braking (AEB) applies the brakes, the touchscreen displays a visual warning and sounds a chime. An abrupt downward movement of the brake pedal also occurs. The brake lights turn on to alert other road users that the vehicle is slowing down.

Automatic Emergency Braking logic is performed by the Driver Assistance System. For more details on how this feature operates, see the Automatic Emergency Braking section of the HW 4 – Vehicles built after May 2023 in Theory of Operation.

Automatic Emergency Braking does not apply the brakes or stops applying the brakes, when:

• The steering wheel is turned sharply.
• The brake pedal is pressed and released while Automatic Emergency Braking is applying the brakes.
• The accelerator is pressed hard while Automatic Emergency Braking is applying the brakes.
• The obstacle is no longer detected ahead.

Automatic Emergency Braking is automatically enabled. To disable it for a drive session, use the touchscreen: Controls > Autopilot > Automatic Emergency Braking

Vehicle Hold

Vehicle Hold can continue to apply the brakes even after the foot brake has been released. After coming to a complete stop, press the brake pedal again until the touchscreen displays the Vehicle Hold indicator light to enable Vehicle Hold. The driver can then release the brake pedal and remain stopped, even on a hill. The indicator below displays on the touchscreen whenever Vehicle Hold is actively braking. To disengage Vehicle Hold, press the accelerator pedal, or press and release the brake pedal. Shifting into Neutral also disengages the Vehicle Hold Function.

Note

Vehicle Hold automatically shifts into Park after approximately 10 minutes or less of use or when the driver exits the vehicle. If the vehicle remains stationary after an automatic shift into another gear, it can shift back into 'Park' earlier for safety and convenience.

Slip Start

To allow the wheels to spin at a limited speed, Slip Start can be enabled. Slip Start can be enabled at any speed. However, it is less effective at higher speeds.

Under normal conditions, Slip Start is off by default. Only activate it when intentional wheel spin is necessary:

• Starting on a loose surface, i.e., gravel / snow.
• Driving in deep snow, sand, or mud.
• Rocking out of a hole or deep rut.

To activate Slip Start, use the touchscreen menu: Controls > Driving > Traction Control > Slip Start

Slip Start is not available while using Traffic Aware Cruise Control (TACC). If TACC is activated while in Slip Start, the system will revert to normal traction control conditions.

Note

Slip Start is automatically disabled the next start cycle.

System Faults

If the ESP system faults, normal braking is maintained. However, braking distances may increase and wheels may lock under heavy braking. The RED brake and/or YELLOW ABS fault indicator lights display in the instrument cluster along with a fault message.

If the vehicle power is interrupted, and the Hydraulic Boost Compensation (HBC) is also not available, the brake system is still able to provide a deceleration through a mechanical push-through through the iBooster, which provides a direct connection between the pedal and the master cylinder.

×