2.5 Braking Systems Flashcards
(27 cards)
What are the key hypotheses for analyzing braking scenarios?
The key hypotheses include:
- 1 DOF (motion along a straight line)
- Neglect vehicle resistances different from inertial forces
- Neglect suspension deformations and sprung mass motions
- Flat road (no transversal inclination)
What are the main tasks of a braking system?
The main tasks are:
- To stop the vehicle completely
- To control speed when natural deceleration is insufficient
- To keep the vehicle stopped on a slope
What are the required braking systems according to regulations?
Three required systems:
- Service braking system for normal speed reduction or stopping
- Emergency/secondary braking system for the same function in case of service brake failure
- Parking braking system for keeping the vehicle stationary, especially on slopes
How is static load distributed in a vehicle at rest on a flat road?
When V=0 and aG=0, the static load is distributed between front and rear axles according to the position of the center of gravity relative to the wheelbase.
What happens to vertical load distribution during braking?
During braking, longitudinal load transfer occurs, increasing the front axle vertical load and decreasing the rear axle vertical load by a factor proportional to m|aG|h/L.
What are the formulas for dynamic vertical forces during braking?
The formulas are:
- FZF = FZF,st + (m|aG|h)/L (front axle load increases)
- FZR = FZR,st - (m|aG|h)/L (rear axle load decreases)
What is ideal braking?
Ideal braking means the same exploitation level of the available friction on both axles according to their actual vertical load. This means:
- FXF = μFZF
- FXR = μFZR
What is the maximum deceleration possible during ideal braking?
The maximum deceleration is: aG = -μg, where μ is the coefficient of friction between tires and road.
What is the ideal braking parabola?
The ideal braking parabola is a curve representing the optimal distribution of braking forces between front and rear axles to achieve maximum deceleration without wheel lock. It’s described by parametric equations:
- FXF = (mg/L)μ(b+μh)
- FXR = (mg/L)μ(a-μh)
How is the optimal front/rear braking force ratio calculated?
The optimal front/rear braking force ratio F/R is calculated as:
F/R = (b+μh)/(a-μh)
where b is the distance from CG to rear axle, a is the distance from CG to front axle, h is the height of CG, and μ is the friction coefficient.
How can the optimal braking force distribution be expressed in percentages?
The optimal force distribution in percentages is:
- %F = [F/R/(1+F/R)]×100
- %R = 100-%F
What is the condition for front wheels locking?
Front wheels lock when:
FXF = FXFlim = μFZF = μ(mgb/L - maGh/L)
What is the condition for rear wheels locking?
Rear wheels lock when:
FXR = FXRlim = μFZR = μ(mga/L + maGh/L)
What is braking efficiency?
Braking efficiency is defined as the ratio between actual vehicle deceleration and maximum possible deceleration: ε = aG/aG,max = aG/μg
Which wheels typically lock first on wet asphalt?
Rear wheels typically lock first on low-friction surfaces like wet asphalt.
What braking circuits layouts are allowed by EU regulations?
EU regulations allow several circuit layouts:
- TT: Front axle / Rear axle
- K: Front right and rear left wheels / Front left and rear right wheels
- HT: All wheels / Front axle
- LL: Front axle and rear right wheel / Front axle and rear left wheel
- HH: All wheels / All wheels
What are the main components of an ABS system?
Main components include:
- Wheel speed sensors
- Hydraulic modulator (with inlet/outlet valves)
- Control unit
- Warning lamp
- Return pump
- Brake-fluid accumulator
How does ABS function?
ABS (Anti-lock Braking System) functions by monitoring wheel speeds and modulating brake pressure to prevent wheel lock. When a wheel approaches lock-up, the inlet valve closes and the outlet valve opens to reduce pressure to that wheel’s brake, allowing the wheel to rotate again.
What are the main types of disk brakes?
The main types are:
- Fixed-caliper brake
- Floating-caliper brake
- Sliding-caliper brake
What is the basic formula for calculating braking force at the wheels?
The braking force at the wheels can be calculated as:
|FBRAKING,F| = |FXF| = 2(pF-p0,F)AWC,F BFF RDISC,F/RW
|FBRAKING,R| = |FXR| = 2(pR-p0,R)AWC,R BFR RDISC,R/RW
How do electro-hydraulic actuators (EHAs) function?
Electro-hydraulic actuators (EHAs) apply braking pressure through an electric motor coupled to a mechanical system that translates rotational movement into linear displacement of a piston, which displaces fluid through hydraulic calipers. Pressure is monitored by a sensor providing feedback to the Brake Control Unit.
How do electro-mechanical brakes function?
In electro-mechanical brakes, braking force is applied through an electric motor that transmits torque to a floating caliper, which applies force to the discs. A force sensor monitors the applied force and feeds information back to the Brake Control Unit.
What is regenerative braking?
Regenerative braking is a system that captures energy that would otherwise be dissipated as heat during braking and converts it to electrical energy to be stored in a battery or mechanical energy stored in a flywheel.
What are the categories of regenerative braking systems according to EU legislation?
EU legislation divides regenerative braking systems into three categories:
- Category A: Regenerative braking system is not part of the braking system (only during throttle-off)
- Category B (non-phased): Regenerative braking is part of the braking system, delivered together with friction braking (parallel strategy)
- Category B (phased): Regenerative braking is part of the brake system and can be generated before friction braking (serial strategy)
