Friction

Overview

Friction is a physical property that resists the relative motion of solid surfaces sliding against each other and is found throughout an automobile. Without friction a car’s tire could not grip the road, or a brake pad could not slow down a car. Conversely friction would quickly destroy an engine that is run without oil. So, understanding friction and some of its basic properties can help a technician diagnose friction-related systems such as tires, brakes, and clutches.

The actual amount of friction generated when two objects are sliding against each other is based on many factors such as:

  • The force pressing the objects together
  • The material each object is made of
  • The surface finish of each object
  • The temperature of the two objects
  • And many lesser variables

Scientists developed a rather simple way to measure and compare the level of friction between two objects at rest (static friction) and in motion (kinetic friction).

Static Coefficient of Friction (COF)

Static friction is between two solid objects that are not moving relative to each other. Some examples of static friction on an automobile are:

  • The brakes are applied on a car that is stopped and there is no movement between the brake pad and rotor.
  • On a car equipped with a manual transmission if the clutch is released, the clutch mechanism may be rotating as fast as several thousand RPM; however, there is no movement between the friction components of the clutch assembly.
  • Even when a wheel is in motion, the patch of the tire in contact with the ground is stationary relative to the ground, so it is still static friction.

The unit of measure for friction is the coefficient of friction (COF) which is simply the amount of force it takes to move an object horizontally divided by its downward force (weight). In the illustration below, it takes 75 pounds of force to initially move a 100-pound block of steel horizontally along a steel surface. The static COF is .75 (75/100).

75 pounds / 100 pounds = Static COF of .75

Kinetic Coefficient of Friction (COF)

Kinetic friction is friction between two solid objects that are moving relative to each other. Some examples of kinetic friction on an automobile are:

  • The brakes are applied on a car that is slowing down and there is still movement between the brake pad and rotor.
  • On a car equipped with a manual transmission when the clutch is initially released and the car is stopped, the clutch mechanism is rotating as fast as the engine; however, the input shaft of the transmission is not initially rotating. During the time when the transmission’s input shaft is catching up with the flywheel’s speed, kinetic COF is at play.
  • When a car is skidding, the friction between the tires and the road is kinetic friction.

In the illustration below, it takes 52 pounds of force to keep a 100-pound block of steel moving horizontally along a steel surface. The kinetic COF is .52 (52/100).

52 pounds / 100 pounds = Kinetic COF of .52

COF Characteristics

  • The COF between two objects is primarily determined by the
    • materials of each object
    • surface smoothness of each object
  • COF can’t be calculated
    • it is determined by testing.
  • COF is a ratio of two forces
    • it is not affected by dimensions (the actual surface area of the two objects does not contribute to the COF)

Table: Static and Kinetic COF of Common Materials

MaterialMaterialStatic COFKinetic COF
AluminumSteel.61.47
BrassSteel.35 – .51.44
SteelSteel.74 – .80.42 – .62
ConcreteRubber1.0.6 – .85

COF and Automotive Applications

Brakes

The braking system of a car is heavily related to COF from both the friction material side (brake pad to disc rotor surface) and the force side (brake hydraulics).

Brake Pad / Rotor Surface

One of the most serviced systems on a car, the disc brake system, functions by friction. The brake pads make up one friction surface and the disk rotor makes up the other surface as shown in the image below.

To duplicate the original brake stopping power and feel, it is important that the original pad to rotor COF is maintained. This means the brake pad composition and rotor finish are the same as the original. Some points to consider are:

Use OEM Pads

The best way to match the OEM brake feel is to simply use OEM brake pads. The actual composition of the OEM brake pad material is somewhat of a trade secret and some aftermarket brake pad manufacturers aren’t willing to spend the money on premium ingredients even if they knew the “secret” recipe.

Most of the better brake pad manufacturers have an OEM style pad that should perform similar to the original. They are not cheap, but brake parts aren’t the place to go low budget.

Match Edge Codes

The last two letters “FF” are the edge code

If you are going to use aftermarket brake pads you could also try and match the COF edge codes of the pads coming off the car. A brake pad edge code is mandated by the Department of Transportation (DOT) and painted on all street legal brake pads. The 7th and 8th letters (“FF” in the image) represent the COF values taken from a standardized Society of Automotive Engineers (SAE) recommended practice. To preserve the original brake feel, try to match the edge code to the OEM edge code.

This approach is not foolproof since the edge code is sometimes unreadable due to years of dirt, grime, and heat cycles. Also, the wide COF ranges associated with an edge code (as shown in the table below) ultimately just provides a rough indication of a pad’s thermal performance.

DOT Code1COF @ 2500 FCOF @ 6000 FComments
EE.25 – .35.25 – .350-25% fade at 600 F possible
FE.25 – .35.35 – .452% to 44% fade at 600 F possible
FF.35 – .45.35 – .450-22% fade at 600 F possible
GG.45 – .55.45 – .55Very Rare
HH.55 – .65.55 – .65Carbon/Carbon only.
O.K. up to 3000 F where it glows

Side to Side Pulls on Braking

To assure there is no left/right pull on braking, the COF must match on each axle. Some points to consider are:

  • Always replace brake pads as a set (from the same box) to assure the COF is the same from side-to-side
  • Rotor finish from side-to-side must be the same
    • If replacing both rotors make sure they are the same brand and the finish looks the same. If any question, turn both on a brake lathe to assure the same finish on both sides.
    • If replacing one rotor, turn both on a brake lathe to assure the same finish on both sides.

Hydraulics

The force part of the COF equation on a brake system is coming from the hydraulic system. To prevent any changes in the braking COF from the hydraulic system follow these practices.

  • When changing the master cylinder if the original bore is not maintained, the brakes will perform differently. If the parts book lists more than one master cylinder the difference is often in the bore size. If the bore size is not cast into the master cylinder housing, pull it apart and measure the bore.
    • If a master cylinder with a larger bore is installed, the line pressure will be lower than the original and it will take much more brake pedal force to stop the car.
    • If a mater cylinder with a smaller bore is installed, the line pressure will be higher than the original and it will cause the brakes to be too sensitive.
  • When changing a caliper or wheel cylinder also make sure te bore size is the same.
    • If you install a caliper or wheel cylinder with a smaller than stock bore the force applied to the brake components will be less than the original force.
    • If you install a caliper or wheel cylinder with a larger than stock bore the force applied to the brake components will be more than the original force.
    • If you end up with a mismatched wheel cylinder or caliper bore from side to side, the car will have a pull on hard braking.

Tires

Tires are actually the most important friction-related component on a car. The COF of the tire with the road is what keeps the car on the road during accelerating, braking, and cornering.

Tire Traction Grade

Traction grades are an indication of a tire’s ability to stop on wet pavement. A higher graded tire should allow a car to stop on wet roads in a shorter distance than a tire with a lower grade. Traction is graded from highest to lowest as “AA”, “A”, “B”, and “C”.

Traction grade percentages of all tires currently being sold:

  • 15% are rated “AA”
  • 77% are rated “A”
  • 7% are rated “B”
  • Only 4 lines of tires are rated “C”

Tires in the Kinetic COF Zone (Not Good!)

As we saw earlier in this lesson, once the static COF of two objects is exceeded, the kinetic COF drops to a much lower value. The COF between tires and the road are the same way. At any speed, as long as the tires are not slipping, the tire has a static COF relationship with the road through the tire patch.

If you exceed the static COF by excessive acceleration, deceleration, or cornering forces the tire will start sliding and the static COF drops to the kinetic COF level almost instantly. This happens in racing when a car almost instantly darts up and hits the outside wall. The driver pushed the car beyond the “hump” as shown in the graph shown below.

Ther slip angle is the difference between where the tire is pointed and the car is actually going, a low tech definition is “tire squish”.

Video Resources (YouTube)

What Are The Best Brake Pads? Cheap vs Expensive Tested!

Race Friction Dyno Testing by Raybestos Brakes

Raybestos Racing Friction Manufacturing