Time to Apply the Brakes! As a tire of a bicycle or car is rolling down the road, the main friction sources working on the wheel is the friction from within the hub, and the friction created between the rubber tire and the road surface. When a tire is spinning normally, there is no sliding action between the tire and the road, maintaining traction. While the vehicle and wheel hub are in motion relative to the road, the bottom surface of the tire in contact with the road is stationary.     Simple brakes work by pushing a non-spinning high-friction surface (brake pads) against a spinning surface of the wheel (usually brake drums or disk rotors). This generates a frictional force that stops the vehicle and heats up the brake system. Essentially, brakes transform the rolling energy of the vehicle into heat, thus stopping the vehicle.                    (Kind 2000)   On surfaces with high coefficient of friction such as dry pavement, this system works well. It is on surfaces with low coefficient of friction such as wet roads or ice where problems can occur. The problem is that when the brakes are applied, the friction between the brake pads and the brake drum is higher than the friction between the tire and the road. This causes the tire to stop spinning completely, (i.e. the hubs lock) and the non-spinning tire slides across the surface of the road.  This causes the driver to loose control of the vehicle, and often, the rear tires move forward beyond the front tires in a ‘spin out’.   Ever ran and slid on the ice? Simple brakes react in a similar way!   Anti-lock brake systems, or ABS, work differently than simple brakes. Through a series of hydraulics and computer monitoring systems, the hubs will not lock in a car equipped with ABS. Instead, when the brakes are applied and the tire locks up, the computer will open a series of hydraulic valves, which temporarily (on the order of 0.1 second) disengage the brakes from the tire, allowing it to freely spin with the momentum of the vehicle before reapplying the brake pad. The driver will feel a pulsing in the brake pedal, which is the valves rapidly opening and closing. Simply put, ABS creates a tire that rolls and stops rapidly (around 15 times per second). By allowing the tire to roll for short period of time, an area of tire that would otherwise be skidding across the road is allowed to roll with it. As discussed before, a stationary object has higher traction than a moving one, and a larger force is required to move that object from stationary to moving than the force necessary to maintain a velocity. ABS break the traction between the road and the tire again and again, which will absorb more energy than sliding on the road, thus stopping the vehicle faster.

Winter Driving Scenario Friction & Traction Applying the Brakes  Bibliography