Tech / Tech Articles

6 Brake Rotor Myths Debunked

ACDelco brake rotor

(Image/automotivespaces.com)

It’s time to debunk some brake rotor myths.

Myths that, on the surface, seem to make sense in many cases. But ultimately, believing these myths won’t resolve actual brake system problems. These myths can hurt an automotive technician’s ability to diagnose common brake problems and solve them effectively. This is why people complain about taking their car into the shop, spend a bunch of money, and tell all their friends the following week how the car is still doing the thing they took it in for.

1. A Rotor’s Minimum Thickness Specifications are Based on Heat

FALSE.

The minimum thickness specification—or discard—is based on the travel of the caliper piston if the pads are worn to the back plates.

If your pads are particularly worn and the brake rotor is below specification, there’s a chance the piston will start to leak and dislodge from the bore, causing total brake system failure. Which is bad.

Heat, warping, and fading have nothing to do with discard, or minimum thickness, specifications.

2. Wet Brake Rotors Increase Stopping Distances

TRUE and FALSE.

Remember when you were first learning to drive and were taught to tap the brake pedal after driving through puddles? Well, back in the days of drum brakes, this was solid advice. But now we’re mostly dealing with disc brakes, and this brake pedal-tapping advice no longer holds water—literally.

When vehicles move, the disc brake rotors are turning, causing the water to be thrown off the face of the rotor face from centrifugal force. Any water on the brake pads is inconsequential. However, it should be noted that some vehicles, a Mercedes-Benz for example, will auto-pulse brakes to remove water if the rain-sensing wiper system detects water on the windshield.

3. Brake Rotors Warp

FALSE.

Rotors don’t warp.

Back in the 1970s, in what was probably a scene right out of the movie Dazed and Confused, someone came up with a theory about brake-rotor “warping,” and it totally stuck. Why? Probably because, in layman’s terms, it makes sense.

But then real life happens and this faulty explanation doesn’t hold up to scientific scrutiny.

What is often described as “warping” is actually the presence of two separate phenomena that can happen in isolation or in combination with each other, and neither has anything to do with warping.

These phenomena are:

  • Brake torque variation (BTV), and
  • Disc thickness variation (DTV)

BTV is a variation of torque across the rotor’s face which causes the rotor to slip and catch as the brake caliper is engaged. The differences in torque across the rotor may be caused by inconsistencies in the rotor’s finish or metallurgy.

BTV can be caused by uneven deposits of friction material. This might not cause a pulsation in pedal feel, but it will cause vehicle judder or vibration.

DTV, on the other hand, is the result of measuring the thickness of the rotor surface in multiple spots around the rotor. DTV measurement can be found simply by finding the difference between the thickest part of the rotor from the thinnest.

As the rotor’s variable thicknesses pass through the restricted caliper, the piston moves in and out, causing pulsations in the brake pedal while stopping.

4. All Rotors are the Same

FALSE.

Even if a rotor fits a vehicle, it may not be the most appropriate rotor for that particular vehicle or driver. Low-quality rotors may have compromises in structure and metallurgy that might feel good on the wallet, but won’t seem so awesome when it comes time to stop safely or have your brake system perfoming up to standard during any type of competition. Do some shopping around to find the right fit.

5. New Rotors Need to be Machined

FALSE.

New rotors should be finished to specifications and ready to install out of the box.

There shouldn’t be a reason to give them a “clean-up” cut. If they do need a cut, then you should probably find a new rotor supplier. Machining new rotors will shorten their life. It may also leave a rougher finish on the rotor surface.

The manufacturing tolerances for rotor runout on most new rotors average about 0.001 inches or less, with a maximum upper limit of 0.004 inches. Some vehicles are particularly sensitive to rotor runout, so much so that as little as 0.0015 inches of runout can produce noticeable pedal pulsations.

6. Micrometers Aren’t Necessary

FALSE.

Some shops measure rotors with the naked eye. Some brake techs suggest that quality measurement tools either don’t exist or aren’t affordable. Not true.

If you want to perform high-quality brake pad or rotor replacement jobs, you should definitely have a quality micrometer in your toolbox to measure rotor thickness.

SOURCE: Raybestos

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21 Comments

  1. Stephen O says:

    An exception to the wet brake rotor myth is immediately after going through a car wash. My repeated experience is with the wheels barely turning thru the wash, the water doesn’t get thrown off til your up to speed in traffic. The first 1 or 2 stops exiting the gas station area are pretty wishy washy (LOL!).

  2. Brake rotor warp – otherwise known as “Run Out” or the deviation of the braking surface to he center line of the hub. there is even a specification for the maximum allowable run out by the vehicle manufacturer. or in layman’s terms, “Warped”. verb (used with object)
    1. to bend or twist out of shape, especially from a straight or flat form. this is what is being referred to and can be felt as the steering wheel moving from side to side as the brakes are applied. So contrary to the myth buster explanation offered here and elsewhere, rotors do in fact “warp” as can be measured with a dial indicator with the absence of Disc thickness variation (DTV). not saying that Brake torque variation (BTV) is not real but does not explain run out.

    • I 100% agree with you.
      I’ve proved they will warp a number of times by putting a dial indicator on them on the vehicle (removed tire/wheel and reinstall lug nuts to seat rotor) and on the brake lathe before turning.
      Little to no variations in thickness but as much as .150 in run out.

    • Joseph Masciantonio says:

      Everyone is smarter than the person who wrote the article that they searched for…

  3. Anyone who thinks that brake rotors don’t warp has never put one on a brake lathe.

  4. T. Lumpkin says:

    I agree with both replies on the brake rotor warping topic. Why then is there a minimum run out spec on rotors. It doesn’t take much run out in my 81 corvette to have total brake failure and the brake pedal goes to the floor. If you do not believe me I will re-install all 4 rotors and let you drive the car. Don’t worry I will have an ambulance on stand by for ya.

  5. MaintainYourRide says:

    If you get your rotors hot enough, they’ll warp…

  6. Pingback: 5 Myths You Probably Believe About Brake Rotors

  7. Dennis Workman says:

    I agree with the article. Rotors don’t warp. The measured “runout” is due to material deposites on the rotor – usually the result of keeping your foot on the brake pedal after hard stops.

  8. KEITH M. GOOD says:

    So those of you who say they don’t warp, and to put then on a lathe….I’m assuming they would wobble when in motion….so wouldnt they wobble as well being that thickness and weight were not reasonably even causing it to appear warped? Why would people claim to machine rotors in order to fix them when warped? How does a lathe untwist metal back to its intended shape? If it were warped then that means not just the area where the brakes engage was twisted, but also the part that attaches to the hub and wheel….so wouldnt the car shake at all times without the brake engaged?? Kinda like when the wheels are improperly balanced…I’m just a laymen trying to understand different theories….

    • A lathe does not ‘untwist’ the metal it removes it with the goal of removing just enough material to return both surfaces to flat and parallel. Rather than trying to describe the process may I suggest you head over to youbtube to look for brake rotor turning.

  9. I ran my Porsche 924 one night so hard that when I looked at the car the front rotors were bright orange and I mean BRIGHT molten metal orange. I never knew that was even possible. To my amazement they worked the same after that night.

  10. I had a 91 TAURUS and used two sets of Chinese rotors that warped. I got more expensive Canadian rotors that did not warp.

  11. Robert Vedell says:

    So what causes front brake shudder above 65mph. Do the rotors need replacement or surface cleaned.

  12. Amanda Sutton says:

    I had my rotors warp 2times in the last 3 months of buying new. Only thing I can think of that is warped them is delivering food for hours then going into car washes 2times a day

  13. Centrifugal force is a myth. Ask any freshman physics student.

    • Yes, but linear momentum still causes water to sling off a rotating disk. Ask any freshman physics student. The layman’s term is centrifugal force. The point made in the article is still valid.

  14. Most shudder complaints are caused by by hard spots on the rotor! This is a metallurgical change in the rotor. Machining the rotor will make the brakes feel better for a short time but the shudder WILL return!

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