Rick's Free Auto Repair Advice

Warped brake rotors

What causes warped brake rotors?

Let’s do a cold stop right here. You may have been told that you have warped brake rotors. That’s a myth because brake rotors DO NOT WARP. Yes, you read that correctly. To warp a brake rotor you’d need to generate heat in the 2,300°F range. There isn’t an automotive brake system in the world that’s capable of generating that kind of heat. In fact, your brake fluid would begin to boil at temps well below 2,300°F and your brakes would develop fade long before that.

Brake pads carbonize at around 1,200°F and ignite at temps above that. Period. I don’t care if you’re traveling straight down the side of a mountain with your brakes applied the entire way. Your brake pads will burn up completely before you even come close to 2,300°F.

Plus, driving with hot brakes and then hitting a puddle of water can’t possibly warp brake rotors either.

So what causes brake pedal pulsation?

Simple. It’s disc thickness variation caused by lateral runout. Disc thickness variation is either the uneven deposit of brake material onto the face of the rotor OR, the uneven wearing of the rotor face caused by abrasive brake pad material.

The brake pad bedding procedure is critical to ensure proper brake performance after new brake pads are install. The bedding procedure transfers a uniform film of brake friction material to the face of the rotor. There is no single bedding procedure for all brake pads.

What causes lateral runout?

Corrosion on the wheel hub is the #1 cause of lateral runout

The brake rotor must sit perfectly parallel with the wheel hub. If there’s lateral runout on the hub, either from the factory or due to an impact, the rotor won’t be sit in a vertical plane with the brake pads. It will wobble and that causes an uneven deposit of friction material on opposite portions of the rotor.

Corrosion on the hub can also cause the rotor to be out of parallel. Cleaning the corrosion off the wheel hub is a CRITICAL step in performing a brake job.

Failure to clean the hub will cause brake pedal pulsation withing 3,000 miles. If you replace the rotors with new ones but fail to clean the corrosion, the new rotors will develop brake pedal pulsation again in 3,000 miles.

How much brake pad deposit or wear causes disc thickness variation and brake pedal pulsation?

Not much. Most drivers become aware of brake pedal pulsation with as little as 0.0004″ buildup. When thickness variation reaches 0.001″ the pulsation is so bad that drivers begin to complain.

Improper brake pad bedding is the #2 cause of lateral runout

Racing pad bedding procedure

The bedding procedure for racing pads, for example requires a gradually increasing sequence of hard slow-downs, finally reaching top speeds of 60-80-MPH and then slowing to around 5-MPH (no complete stops) to heat up the pad sufficiently to transfer friction material film. Once the rotors are fully headted, don’t leave the brakes applied—that will cause friction material transfer in the shape fo the pad. This is called pad imprinting (shown below).

brake pad imprint

The shape of the brake pad friction material deposits onto the face of the rotor when the vehicle is stopped and the brake are left applied after performing a racing pad bedding procedure.

Street pad bedding procedure

Again, the procedure varies by brake pad manufacturer and the specific model of brake pads. But generally, street pad bedding usually requires 10 to 30 moderate stops from around 30-MPH to 0-mph with cooling periods in between.

What happens if pads aren’t “bedded” properly?

Improper bedding (or no bedding procedure at all) produces uneven friction material deposits on the face of the rotor. During braking, those high spots become hotter than the other portions. When the leading edge of the high spots contacts the brake pad, it generates temperatures in the 1,200 to 1,300°F range. That’s not hot enough to warp the rotor. But it IS hot enough to change the composition of the cast iron. Here’s how that happens:

Cast iron is an alloy of iron, silicone and carbon that’s heated to form a matrix of carbides. However, at after the initial casting, if temperatures reach around 1,200 to 1,300°F, the matrix changes to form cementite (3 iron atoms and 1 carbon atom).

Cementite is harder than the surrounding cast iron. It is also more abrasive. Worse yet, it doesn’t dissipate heat as easily as the surrounding cast iron, so it creates “hot spot” which grows hotter with every successive brake application, converting more cast iron into cementite.

What about installing new pads on a used rotor?

If the old rotors are within spec (above the discard thickness) and doesn’t have deep grooves, it can often be reused as long as you perform a bedding procedure with the new brake pads. Ideally, you should use the same friction material as the pads you’re removing. If you don’t know what pads were used, you can try bedding the new pads according to the manufacturer’s instructions. Just be aware that noise is the #1 complaint when installing new pads on used rotors.

How much deposit or wear causes disc thickness variation and brake pedal pulsation?

Not much. Most drivers become aware of brake pedal pulsation with as little as 0.0004″ buildup. When thickness variation reaches 0.001″ the pulsation is so bad that drivers begin to complain.

Improper lug nut torque is the #3 cause of lateral runout

In order to maintain a perfectly parallel fit of the brake rotor to the wheel hub, the clamping force from the lug nuts must be the same on all wheel studs. Never use an impact wrench to tighten lug nuts, even with a torque stick—unless you set your impact on the lowest setting and use the torque stick to seat the lug nuts just enough so you can perform the final tightening by hand with a torque wrench.

If you tighten lug nuts in a circlular pattern with an impact wrench, the first few lug nuts will pre-load the rotor, causing it to flex slightly. Then, even if you tighten the remaining lug nuts to the same torque, your torque will be off due to the flexed rotor absorbing the energy. You won’t feel brake pedal pulsation when the rotor cold. But as the rotor heats up, it will expand unevenly and cause lateral runout.

That’s why lug nuts must be tightened in a star pattern and by hand with a torque wrench.

Can heavy braking cause warped rotors?

Not really. When a vehicle develops brake pedal pulsation, so-called “experts” say the driver rode the brakes, got them too hot and the rotors warped like an old vinyl album left in a hot car.

It’s physically impossible for automotive brakes to generate enough heat to cause a cast iron rotor to change its shape. Here’s why:

Brake rotor melting point temperatures

If order for a brake rotor to warp or change shape, it must heat up to near it’s melting point. Most high quality auto rotors are made with G3000 or G4000 Gray Cast Iron. Here are the various melting points for cast iron:

G1800 Gray Cast Iron Melt temperature 1150°C 2102°F
G3000 Gray Cast Iron Melt temperature 1145°C 2093°F
G4000 Gray Cast Iron Melt temperature 1145°C 2093°F
1008 Plain Carbon Steel Melt temperature 1620°C 2964°F

During normal street use, brake rotors and pads generate around 200°F to 400°F. When racing or used heavily on a track, they can reach temperatures of up to 1,000°F — still not enough to distort a rotor.

As temperatures rise above 400°F the phenolic resins and other materials in the friction material change from a solid state into a gas. Gassing produce backpressure, literally pushing the brake pad away from the surface of the rotor. This is what causes a loss of braking called brake fade.

In other words, your street brake pads will fade long before then heat up enough to cause warped rotors. If the shop claims you have warped rotors due to heavy braking, ask them to show you the scorched brake pads. Again, they’ll burn up way before they reach cast iron melting temperatures.

See this post on how to prevent brake pedal pulsation.

©, 2019 Rick Muscoplat

Posted on by Rick Muscoplat

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