6 Symptoms of a Warped Brake Disc
Let’s clarify the terminology from the beginning. The term “warped brake disc” is somewhat misleading. Deformation is a change in the geometric shape of the disc. The shape of the brake disc remains the same. It is not geometrically distorted. We will explain why later. First, let’s look at what a warped brake disc is and the common symptoms associated with this condition.
What are warped discs?When you brake your car, the brake pads continuously wear down. The discs also wear down, but not as quickly as the pads.
During this process, the brake pad material transfers an extremely durable coating to the disc, made up of complex resins that hold the pad’s friction material together. During normal driving and braking, these resin deposits are applied evenly and smoothly to the rotor surface. They are microscopic and cannot be seen by the naked eye.
If the rotor is functioning properly, it will always look shiny and smooth with a uniform resin coating, and may even have a very fine groove pattern around the perimeter.
Insufficient coating of the rotor with lining resin can cause an effect called rotor distortion. The coating may be uneven and vary in height, which leads to thickness variations on the rotor surface. This can be assessed using a depth gauge attached to a nearby solid structure.
The rotor surface can be measured while rotating the rotor by hand. A height deviation of more than 0.002 inches (0.051 mm) is enough to create the sensation of a warped rotor when braking.
Some cars also alert the driver that the rotor is warped if the rotor thickness deviation is less than this limit.
Signs of a Warped Brake Rotor
The condition often incorrectly referred to as a warped rotor is indicated by one of these unpleasant braking actions:
1) Brake squeal
When coming to a slow stop, your brakes may squeal intermittently.
2) Car vibrates when braking
When you apply the brakes lightly, your car will not stop smoothly and will pulsate (vibrate). You can feel this slight vibration through the seat.
3) Steering wheel vibrates
If your front brake discs are warped, your steering wheel may pulsate or vibrate when coming to a stop. The frequency of this vibration will decrease as your car slows down.
4) Vehicle pulls to one side
The car may pull to the right or left when stopped, which can be very dangerous in heavy traffic.
5) The brake discs have an uneven surface
Upon visual inspection, one or more of the glazings may not appear smooth and shiny. Instead, each disc may have an uneven pattern of dark and light areas, or a pattern that ranges from bluish to black, indicating localized overheating.
6) Brake performance deteriorates over time
Minor impacted discs will function normally when a quick stop is required. But over time, the condition worsens, and at some point such rotors will no longer be able to brake properly when needed.
What causes rotors to warp?This harmful coating on the rotor surface can occur during any of the following events:
1) Hard braking
such as very hard braking B. If you brake suddenly from highway speed and then suddenly stop your vehicle on the shoulder of the highway. When a moving but hot rotor comes into contact with hot brake pads, resin stains can be transferred to each surface of the rotor.
As the brakes cool, this resin hardens, making it less likely to wear down during subsequent driving. This is an extreme case and probably a rare occurrence for most drivers. One thing to keep in mind when driving on the highway is that if you need to slow down significantly (such as to avoid a collision), don’t stop immediately. Continue driving for at least another 5 miles until your brakes cool.
When taking rest breaks or scenic spots on the highway, slow down as much as possible. Your car is going to sit there for a while at stops like this, and you don’t want to sit there with very hot brakes.
2) Stop-and-go traffic
Frequent stop-and-go traffic at speeds of 35 mph or more and the inability to maintain a consistent speed to allow time for brake cooling may be characteristic of driving home from work.
This type of daily driving can lead to uneven buildup of hard resins on the brake discs, eventually causing brake disc deformation. This condition is limited to the front brakes, which are responsible for most of the work in stopping the vehicle.
#3) Long, steep downhill
Long downhill slopes on steep mountain roads can lead to brake disc deformation. For example, consider the exit for the eastbound Interstate 70 from El Rancho, Colorado, into the Denver suburbs, or the freeway exit from the summit of Mount Washington, New Hampshire. Think for a moment about your worst downhill stretch. Use the brakes to control your speed. If equipped, always use the brake cooling switch. But only pause, and resume driving as soon as conditions permit.
Continuing driving improves even cooling of the rotor and minimizes hardening of deposits between the brake pads and rotor.
In summary, a warped rotor is one whose surface has been damaged by one or more of the phenomena listed above. The resulting deposits lead to uneven rotor thickness.
This condition worsens over time as the thickly coated areas of the rotor get much hotter during braking than the lower areas, resulting in further pad resin buildup.
How Brake Rotors Are Created
The manufacturing procedure for most rotors is a casting process which starts with a molten steel or iron alloy being poured into a mold. Once it gradually cools and hardens, the rough rotor casting is removed from the mold and finish-machined to the appearance you will see when the rotor is brand new.
This process trues up the rotor surfaces to the mounting face on the rotor hub. Mounting holes or alternate means of attachment are added as well. The new rotor will be spun and dynamically balanced.
The new rotor may then be plated with an anti-rust coating. It will also be rust-proofed with a tacky preservative oil and placed in a sturdy box. Then it gets shipped off to the car manufacturer, to a car dealer or to your local parts store. Once the rust proofing is removed the rotor is ready for installation on your car.
Because the basic structure of a brake rotor derives from the initial casting process accomplished at a very high temperature, a brake rotor will never warp. It simply will never reach the temperature necessary to weaken the material and permit warping. So when we have brake problems we may think we have a warped rotor, but what is this problem, really?
Note that throughout this article we will repeatedly use the term “warped rotor” to refer to rotors that have the degraded condition and qualities we will describe below.
How To Tell Whether the Front or Rear Rotors Are Warped
If the front rotors are warped, you will sense the pulsation or vibration through the steering wheel. If you feel this pulsation through the car seat but the steering wheel is not vibrating, the rotor problem is likely at the rear wheels.
However, warping of the rear rotors is less likely than at the front because the fronts become much hotter than the rears while stopping. So the risk of pad resin transfer is higher for the front rotors.
Are Warped Rotors Dangerous?
Any condition that adversely affects your brakes can be dangerous. Warped rotors can degrade the stopping efficiency of your brakes resulting in longer stopping distances. This is an adverse outcome which could end with a collision.
Any time you sense pulsating during deceleration when applying the brakes should be cause for concern. A trip to your dealer or mechanic for evaluation and corrective action should soon follow.
This pulsating condition during stopping can also cause increased wear on the suspension and steering systems. Such wear will be cumulative over time and will add cost to needed repairs if the warped rotor problem is not soon corrected.
Can Bedding-in the Pads Correct the Warped Rotor Behavior?
When first noticed, a mild amount of brake pulsations can sometimes be corrected by repeating the new brake pad bedding procedure. This procedure is explained in Step 16 of our “How to Change Brake Pads” article.
If this procedure fails to correct a brake pulsation problem, having the rotors resurfaced or replaced will be required.
Can Warped Rotors Be Resurfaced? (Or Should They be Replaced?)
If there is sufficient material thickness on the rotors, they can be resurfaced. And this will correct the warped rotor problem.