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
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
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
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
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
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.