Brake Pad Properties
Published: February 14, 2018
Brakes work by way of pistons clamping brake pads against the rotors, with the resulting friction decelerating the car. As a result, a crucial property of brake pad material is its friction coefficient. A higher friction coefficient means higher deceleration rate, assuming otherwise identical braking systems.
When changing brake pads on the same car, a higher friction brake pad will produce higher deceleration rate given the same brake pedal pressure.
Heavier cars generally use brake pads with a higher friction coefficient, as the kinetic energy dissipated by the brake system as heat during a stop is proportional to the mass of the vehicle. In turn a heavier car would often use wider tires to obtain surface traction to effect this larger energy delta.
Friction Coefficient Curve
Unlike a high school physics class which assumes the coefficient of friction between two materials is always the same, when it comes to brake pads, as the temperature of brake pad material changes, so does its coefficient of friction. Brake pads are coolest at the end of a straight at the beginning of a braking zone, and are hottest at the end of a braking zone. The longer the straight before a braking zone and the larger speed drop in the braking zone are, the higher the temperature rise in the braking system, and of the brake pads in particular, ends up being.
Brake pad manufacturers sometimes talk about "friction coefficient rise with temperature" which is referring to the same concept and specifies that at higher temperatures the brake pad produces a higher friction coefficient.
A flat friction coefficient curve is usually desirable in a track environment, as this makes the car easier to drive at the limit. A pad with a rising friction coefficient requires the driver to be on the brakes harder early in the brake zone and back off a bit toward the end of the braking zone to maintain the same effective deceleration rate, whereas a pad with a flat curve requires steady brake pedal pressure throughout.
Brake pads with a high friction coeffecient rise with temperatures are prone to locking up tires under late and hard braking, where the driver applies more than usual braking force thus causing the brake system to heat quicker.
Initial bite refers to the rate at which the brake pad goes from zero applied force to maximum applied force for a given brake pedal depression. Pads with a smooth initial bite have a longer ramp up time, while pads with an aggressive initial bite have a shorter ramp up time.
Sometimes brake pad manufacturers talk about simply "bite" - this can refer to either initial bite or the ultimate friction coefficient, and therefore can be a confusing term to use. When I refer to "bite" in this essay, I mean both the initial bite and the steady state friction coefficient.
Higher initial bite means the brakes are applied quicker, or more abruptly.
A pad with too much initial bite is hard to modulate, especially in non-ABS cars. Such brake pads are prone to causing tire lock ups under braking, as the car goes from no braking force to full braking force very quickly. The driver might refer to brakes as an "on/off switch".
A pad with insufficient initial bite will not decelerate the car as the driver intends early in the brake zones. The driver may think of the car as "not having enough brakes". Alternatively the driver may consider the brake system "soft" and "spongy" if they compensate for lack of initial bite by pressing harder on the brake pedal. In extreme cases the driver may end up applying the brakes too abruptly thus causing tire lockup in non-ABS cars and unsettling the car during the forward weight transfer that happens during braking.
A heavier car generally will need a pad with higher initial bite as there is more inertia in the wheel assemblies (wheels, tires, brake rotors and hubs are normally heavier on heavier cars). As well, since the car overall is heavier, there is more weight being moved from the back to the front during braking.
Sometimes called "optimal temperature range", this is a range of temperatures within which the brake pad attains its stated performance characteristics which are all of the properties we are presently discussing: friction coefficient, initial bite and wear rate.
The temperature is that of the brake pad itself, and brake pads are in their optimal temperature range only in the braking zones under braking, not on the straights, in the corners or in the pit lane.
The temperature that a brake pad attains in operation depends on: - ambient temperature; - precipitation and water on track surface; - weight of the vehicle; - percentage of time that brakes are used over a lap; - speed differential in a particular braking zone, or an average over all braking zones; - how aggressively the brakes are applied, or how quickly the car decelerates; - brake pad compound.
Brake pads increase their temperature in each braking zone. They cool down between the braking zones.
Pad Wear Rate
Pad wear rate is, as one would expect, how quickly the brake pads are wearing when used.
There can be a huge difference in wear rate between brake pads manufactured by different vendors on the same vehicle even when the different brake pads provide similar driving characteristics like pedal effort and braking distances.
The same brake pad can also experience widely varying wear rates depending on the temperature it is being operated at. Some brake pad compounds, for example, advertise having the same friction coefficient at any temperature. The catch may be that above a certain temperature, the pad continues to generate the same friction force but wears much more rapidly. Other brake pad compounds are not designed to work when cold and experience elevated wear when they are below their optimal temperature range.
Rotor Wear Rate
Brake rotors are the other consumable in a brake system. Just as different brake pads have themselves different wear rates, different brake pad compounds wear the rotors at different rates. Pads with low rotor wear rates are called "gentle on rotors"; pads with high rotor wear rates are called "abrasive".
As with pad wear rate, rotor wear rate depends on operating temperature.
Brake rotors used on track cars generally experience cracking before they go below their acceptable minimum thickness. As a rotor wears down it becomes lighter and thinner which further accelerates the process of cracking. Brake pad compounds with more gradual temperature ramp up during braking tend to make the rotors last longer before cracking than brake pads that shock the rotors with thermal loads.
The two opposites to help illustrate rotor conditioning are typical Hawk and PFC brake pad compounds. Hawk compounds (HPS, HP Plus and DTC all have this property in my experience) groove the rotors over time. Grooves in the rotors reduce the contact area between rotors and pads as the pads are not contacting the rotors evenly. When grooved rotors are used with new pads, the contact area is reduced further and the pads are being worn unevenly. If the rotor has ridges they dig into the pad and make the pad run hotter than it should. Drivers using Hawk brake pads thus experience strong pressure to replace rotors every time brake pads are replaced, which can be a nuisance or a significant expenditure depending on the car, or live with a less than great brake system.
PFC brake pads, on the other hand, if installed on grooved rotors, flatten and polish the rotors. If starting with brand new rotors, PFC pads tend to maintain the flat contact surface of the rotors throughout the rotor life. PFC calls this "superior disc conditioning".
As best as I can tell this refers to the smoothness of brake release by the brake pads, which in turn affects smoothness of weight transfer from the front to the rear of the car. A pad with good release characteristics should be easy to trailbrake with, and specifically control how much rotation the car is undergoing via brake pedal pressure during brake release.