Piston Slap: That Gritty Feeling in Your Brakes?
TTAC Commentator 1500cc writes:
Hi Sajeev,
I have an odd problem with the brakes on my 2014 Cadillac ATS 2.0 AWD. It has 35k miles on the clock and the Brembo brakes up front. It started happening last spring: when I’d apply the brakes I’d feel (more so than hear) a grinding or gritty feeling through the brake pedal. The odd part was that the brakes were nice and smooth when cold, and only started to act up after I drove a little bit and the brakes warmed up. Also, it only seemed to do this in the final few feet of a stop, say from 10 mph down to zero (or maybe I just couldn’t detect it at higher speeds). The grinding was proportionate to how hard I pressed on the pedal, but pulsed a bit with the rotation of the wheels as if the rotors were warped.
So the first thing I did was pull the front wheels (for some reason it felt like it was coming from the front) and while the rotors were good and rusty, there was lots of meat left on both the rotors and pads. But just to be sure, I installed new front rotors and pads (a shop said the rotors were too gnarly to turn). Didn’t help. Okay, let’s look at the rears, then. They actually were worn to the point of needing replacement, so on went new pads and rotors there, too. But the problem is unchanged, including the pulsing sensation.
Searching the internet, I’ve seen a few forum threads with pretty much the exact symptoms (fine when cold, grindy/gritty when warm) on everything from Explorers to pickups to Porsches. Unfortunately I couldn’t find one thread that actually had the answer. There were some left-field suggestions like an overfilled master cylinder (I took an ounce or so out of mine just to be sure), but nothing fixed it. What could it be?
Sajeev answers:
You weren’t kidding about the brand diversity of forum threads with this issue…and the lack of any conclusive repairs for a gritty brake feel when hot.
I was wondering if expansion on thinner metal bits — i.e. brake dust shields — as they heat could cause this, but that doesn’t make a lick of sense.
To actually feel the grittiness in the brake pedal, the only(?) logical reason is metal transfer/accumulation from brake rotor to brake pad. Or vice versa, depending on the pad material‘s content. Too many variables to armchair, but odds are The Internet doesn’t have an answer until someone digs deep into each rotor and pad. Or invents non-metallic rotors with ceramic/ organic brake pads that stop as efficiently and cheaply as metal stuff.
We know modern cars are heavier, sporting big-ass brakes to stop and/or keep them from skidding. What we don’t know is if the metallurgy within those oversized rotors and the brake pad material is the source of the gritty brake pedal feel when they heat up.
Perhaps we never will, as the secrets are locked in some Automotive R&D laboratory?
So with that in mind, off to you, Best and Brightest!
[Image: Shutterstock user Andrija Pajtic]
Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.
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OP here. Thanks for all the suggestions. I really don't think it's ABS related; you could feel the grind right down to the last revolution of the wheels, even on a gentle stop, and there were no typical ABS sounds or pulsing. It really sounds/feels like when you're down to the metal on the pads, but of course that's not it. My first troubleshooting attempt (which I didn't mention) was to ensure the sliders were moving freely and that the pistons weren't seized, and all was good there. When I had the rotors off I looked at both the calipers and rotors (especially the outside perimeter of the rotors) for some sign of improper contact but didn't see anything. When I replaced the brakes I didn't use OEM parts. The pads were MotoMaster (Canadian Tire brand) and I forget the name of the rotors, but they were a third party brand from Parts Source. So that would lead me to believe it isn't materials related. Having said all that, since the time I wrote into Sajeev I traded the ATS in on a Regal GS so unfortunately I'm not able to try out any suggestions.
My Volvo always feels like it's sliding a little on a gravelly surface when the ABS goes into effect (sudden stop). The only time I had to do that in another vehicle was a Chrysler minivan, also with ABS. It also felt like there was sand on the road at the end of the stop. The owner of the car in question may have a vehicle that engages ABS whenever the brakes are applied, instead of just when needed.