By Andrew Markel. There are some myths about brake pads, rotors and hydraulics that need to be busted. These myths can hurt and hinder a technician’s ability to diagnose and solve some brake problems and customer concerns.
On the surface, some of these myths make sense. The logic can seem sound and explain a problem, but they do not resolve the real issues with a brake system.
A Rotor’s Minimum Thickness Specifications are Based on Heat
False, the discard or minimum thickness specification is based on travel of the caliper piston if the pads were worn to the backing plates.
If you had worn pads and a rotor below specification, there is a possibility the piston could start leaking and possibly become dislodged from the bore causing a failure of the brake system. Heat, warping and fading have nothing to do with discard specifications.
Soft Pads and Hard Pads
False, with a pinch of engineering truth. The engineering term for measuring the hardness of brake pads is compressibility.
Engineers typically measure compressibility as a manufacturing quality control measurement and not as a performance measurement. Compressibility is an important characteristic and can influence pedal feel, but it has very little to do with noise, rotor wear and pulsation.
What the driver is experiencing is the type of friction (tribology) the friction material is using to stop a vehicle. A “hard pad” is a pad that is abrasive to the rotor. This could be classified as a semi-met pad. These types of pads can have very stable friction over a wide range of temperatures.
What a technician or customer think of as a “soft” pad is typically an organic or ceramic formulation. How these friction materials generate brake torque is by adhesion type of tribology. These friction materials leave or transfer a layer of friction material (transfer film or “seasoning”) on the rotor’s surface that some friction material companies claim can smooth out the rotor surface, thereby causing less excitation and noise at the friction coupling. Also, this transfer layer may not be as sensitive to heat induced brake torque variation. Some of these friction materials can be friendly to rotors.
Both of types of friction materials can be the same in terms of compressibility. Calling one material soft and one hard is not possible unless your shop has a $20,000 machine to measure the compressibility of the materials.
Compressibility can influence pedal feel, but only in extreme cases where the pad could be defective. What really influences pedal feel is the coefficient of friction of the brake pad.
Damaged Brake Hoses Can Cause Brakes to Drag
False, with a dash of truth. The myth usually takes place on a vehicle where the brakes are stuck on at just one wheel. The technician tries just about every thing and eventually theorizes it is a restriction in the brake hose.
Brake hoses can be damaged by road debris and some clamps. Stock brake hoses are typically two or three layers. All modern hoses have a stiff internal liner that is in contact with the fluid. The outer layers are typically a softer material designed to absorb impacts with road debris.
The myth typically states that the inner liner was damaged and created a flap or check valve in the line. This check valve prevents the pressure from releasing at the caliper. But, this has been know to happen older brake lines from the 1960s.
In theory, it does makes sense, but you have to ignore some facts. First, if this flap was created, it would not form a internalized flap. Chances are the entire liner would fail and the brake fluid would be up against the softer outer liner. This would make the hose bulge and eventually burst.
Chances are the restriction could be a stuck emergency brake, caliper slides or even a problem with the metering/combination valve. Also, many stuck brake problems are related to brake booster or meter/combination valve problems.
Wet Brake Rotors Increase Stopping Distances
False, Remember when you were first learning to drive and some adult told you to tap the brake pedal after you drove through a puddle? In the days of drum brakes, this was good advice, but with disc brakes this piece of advice does not hold water.
If a vehicle is moving, water is thrown off the face of the rotor by centrifugal force. Any water on the pads is inconsequential.
Replacement Brake Pads are Regulated by the Government
False, There are no government regulations concerning brake pad performance.
Brake Pads Need to Warm Up
False, Street brake pads are designed to produce even brake torque even at very low temperatures. This is even true for exotic carbon ceramic brake systems on street-driven vehicles.
The exception for this myth is high performance racing pads that require some heat in the friction material to generate its highest coefficient of friction. Manufacturers of these pads will say that these pads should only be used for off-highway purposes.
Brake Pads Are The Source of All Brake Noise
True & False, All brake pads do produce vibrations when they are applied. This happens on all brake systems. But, it is how the vibrations are transferred to the rest of the vehicle that will cause a driver to hear or not hear the noise. A brake pad is merely the a string on a guitar, it is up to the player or vehicle to decide how it sounds.
Humans have a limited range of hearing so the sound made by some brake pads might be unheard.
What can cause noise is a change of the friction material due to heat. A “consistent” friction material causes less vibrational excitation variation at the friction coupling by having consistent brake torque at environmental extremes of humidity and temperature (-40F to 500F).
Typically, high frequency noises come from the caliper, rotor or bracket. Low frequency noises, like growls, grunts and moans, can be caused by struts, knuckles or even the body structure.
The option for technicians are to isolate the pads with lubricants, shims and restoring the hardware to like-new condition.