Braking System

brakes
Main

Timeline:


  • 1899 - Wilhelm Maybach invents an early version of a drum brake
  • 1902 - Louis Renault invents a more modern drum brake William Lanchester patents the disk brake
  • 1918 - first hydraulic brakes used by Duesenberg brothers in their A Model car
  • 1929 - Hydraulic brakes as standard
  • 1950s - Chrysler introduces disk brakes in many of their cars
  • 1976 - All safety tests for brakes must have stopping distance as a component
  • 1995 - All cars must meet standard FMVSS No. 105
  • 2000 - All cars must meet safety standard FMVSS No. 135 (An international safety standard)
  • 2005 - FMVSS applies to vehicles over 10,000 lbs too.

Rules and Regulations


FMVSS No. 105
This standard applies mainly to vehicles over 7,716 lbs because they use hydraulic or electric brake systems. This sets the standards for those two types of systems.

FMVSS N0. 135
This applies to passenger car systems.

All vehicles under 7716 gross pounds must have a stopping distance of less than 230 ft at a starting speed of 62 mph with no more petal effort than 368 ft. lbs. Also, if power brakes fail, the vehicle must be able to stop in less than 551 ft at the same starting speed with the same amount of effort. These requirements apply to cold brakes only.

Hot brakes have a different set of standards. Brakes are considered "hot" when there are two or more emergency or "panic" stops back to back. Hot brakes must be able to stop within 292 ft.

Physics:

Braking System
Brakes are probably the most important safety feature ever invented for cars. While the engine creates a positive acceleration relative to a forward direction of motion brakes provide the negative acceleration in this direction. Brakes use a force of friction provided by brake pads to accelerate the car. Most modern day brakes are electronic however all brakes work with hydraulics.

Basic Hydraulic Brakes
All cars use some for of hydraulic brake to stop. The purpose of brakes is to provide a retarding force to stop the car. According to Newton’s second law of motion it would be impossible for a person to supply a force large enough to cause the needed acceleration to stop a car with only their foot. This is where hydraulics come in. A more detailed explanation of hydraulics will follow later but the overall concept is that hydraulics allow a small force to apply a much larger force. This allows the driver to easily provide the force needed to accelerate a car.

Electronic Brakes
Electronic brakes differ just slightly from traditional hydraulic brakes. They are activated via an onboard computer. These type of brakes are advantageous because they eliminate brake cables, the weakest link of traditional brakes. With electronic brakes, upon hitting the brake pedal instead of a force being mechanically delivered to the hydraulic system a certain potential difference is created that is proportional to the force applied on the pedal. This voltage creates a current that flow to the onboard computer and is interpreted. The computer will then activate the brakes.


How Hydraulics Work
Hydraulics_with_Brakes.png
Hydraulics use the idea of pressurized fluid to mechanically increase force. Hydraulics use the idea of pressure. The pressure of the fluid at the point F1 is equal to the pressure at F2 as long as the system is closed. Pressure is defined as Force over Area (F/A). So if the initial area is minimized a small initial force can provide a much larger final force due to a larger final area. This idea is used very frequently in modern day mechanical devices and is the key to automobile brakes.


Important Equations for Brakes

Braking_NsL.png

Newton’s second law makes it impossible for a person to apply a sufficient force to accelerate a car because the mass of the car is far too high and the force is directly proportional to the mass.

Braking_Fµn.png
Brakes use friction to stop cars. In this equation the force of friction and the actual sopping force is represented by F. The force applied by the hydraulics is represented by N. Brake pads have a very high coefficient of friction this farther increases the stopping force that the brakes can provide.
Braking_inbrd_com.png
The force the driver provides on the pedal creates a voltage directly proportional to it and that voltage then provides a current proportional to that original force.
Braking_Hdrlcs.png
This equation models hydraulic force. The equation states that the Pressure at point one is equal to the pressure at point two, in a closed system. This can be used to create a large force from a much smaller one.

Connection:

Manufacturers need to create brakes that can stop vehicles within the distance required by the government. The brakes must vary due to the weight of cars, which makes things more complicated for manufacturers. It takes more force to stop a vehicle weighing 10,000 lbs than one that weighs 5,000 lbs, yet the requirements for these two vehicles are the same.

Sources:

http://www.autoevolution.com/news/braking-systems-history-6933.html
http://www.carhistory4u.com/the-last-100-years/inventions
http://www.conceptcarz.com/vehicle/z15406/Duesenberg-Model-A.aspx
http://www.articledashboard.com/Article/A-Show-Stopping-Invention---The-History-of-Brakes/153896
http://www.nhtsa.gov/cars/rules/import/fmvss/index.html#SN105
http://www.nhtsa.gov/cars/rules/import/fmvss/index.html#SN135
http://www.tomorrowstechnician.com/Article/687/i_want_you__to_understand_brake_performance.aspx
http://deepblue.lib.umich.edu/handle/2027.42/55414
http://www.unece.org/trans/main/wp29/wp29wgs/wp29gen/wp29glob_candidate.html