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The Science of Stopping: It's All About Friction - Bicycle Brakes Convert Kinetic Energy (Motion) Into Thermal Energy (Heat).

Braking Distance

The approximate braking distance can be found by determining the work required to dissipate the bike’s kinetic energy:

work = ( μ × mass × gravity ) × distance kinetic   Energy = 1 2 × mass × velocity 2

Through the Work-Energy Principle it can then be said that:

( μ × mass × gravity ) × distance = 1 2 × mass × velocity 2

Finally, by rearranging the equation and cancelling like terms we can form an equation for braking distance:

distance = velocity 2 ( 2 × μ × gravity )

μ = coefficient of friction

Rim Brake

  • How's it Work? Rubber pads are pressed against the rim of the wheel.
  • Advantages: inexpensive, lightweight, easy to maintain, mechanically simple
  • Disadvantages: easily contaminated, less braking power

Disc Brake

  • How's it Work? Metallic or ceramic pads are pressed against a metal rotor that's attached to the wheel.
  • Advantages: powerful, protected from contaminates, better heat dissipation
  • Disadvantages: expensive, heavy, difficult to maintain
Figure 32: This diagram corresponds to the terms used for calculating braking distance in the equations above. Furthermore, it depicts the components and locations of a rear rim brake and front disc brake. *Note - bicycles generally have either a set of rim or disc brakes, not a mismatch of both.  
Friction moves against the wheel. As the bike pushes forward, velocity increases. The bike frame is where mass meets gravity.