By on June 7, 2019

 

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 [email protected]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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26 Comments on “Piston Slap: That Gritty Feeling in Your Brakes?...”


  • avatar

    Would there be any connection to ABS? Seems like, if taking ABS out of the equation, all the clues would point to exactly what 1500cc looked at. The fact that none of the “old school usual” things made any difference when addressed, does it make sense to look to the relatively newer systems that are now a part of a vehicles brake system?

    • 0 avatar
      Vulpine

      Was the exact same thing I was thinking about. When you have some cars now capable of doing motorcycle-style ‘stoppies’, anti-lock may be operating so quickly that it causes that pulsation at a noticeable frequency and feels, more than sounds, like a gritty, grinding sensation.

    • 0 avatar
      NormSV650

      Probably flushing the overdue fluid would exacerbate the problem?

  • avatar
    sirwired

    The ABS systems on some cars could be described as “gritty” and “pulsing” in the pedal when activated. I wonder what would happen if they tried driving without ABS for a little while (by pulling the fuse for the controller.) One wheel’s ABS sensor acting up could do the trick.

    • 0 avatar
      JimZ

      in any of my recent cars, when ABS kicks in you not only feel the pedal pulsating, the pedal also goes rock hard and you can’t press it any further (the ABS module has taken over hydraulic pressure.) ABS activation is not subtle.

      • 0 avatar
        sirwired

        I’ve had different cars react differently. My ’98 Corolla sounded like the braking system was going to explode while the pedal kicked at me… bang! bang! bang!. My ’04 Passat felt exactly like what this letter was describing; a gritty feeling on the pedal and a faint pulsing.

        This would especially be the case if the ABS was only activated on a single wheel; that could keep the pedal feel just fine for the remaining three.

  • avatar
    indi500fan

    Spray your rotors every morning with WD-40? /jk

    In a more serious approach, the ABS investigation is a good one to try first.

  • avatar
    theBrandler

    Silly as this might sound, the brakes might actually be doing the same thing your wipers occasionally do, stick and and unstick rapidly. Think of it like this: wet your fingertip and then press it on glass with some force, and then, while still applying that force, quickly drag your finger across the glass. You’ll move smoothly until your finger is no longer wet.

    Now try the same trick, but instead of dragging your finger quickly, drag it slowly. Instead of a smooth motion, you get a series of jittery short movements, and likely some squeaky noises, as your finger sticks/unsticks rapidly.

    The modern ultra effective braking systems may be exhibiting a similar behavior. When cold, they would be far less sticky. Once warm though, the pad and rotor are at optimum effective temperature, and when the rotors stop spinning through the caliper fast enough to have a smooth motion, you get the stick/unstick oscillation that feels like grit because it’s happening so fast. It would happen faster with stiff materials like metal/ceramic because they don’t flex very much under compression.

    Now, I’m still arm-chairing this one, obviously, so I could be totally wrong.

  • avatar
    ajla

    ’18 and ’19 Stingers are having a very similar sounding issue, sometimes starting as low as 100 miles. We might actually have the same front calipers.
    Forum consensus is that the pads and rotors aren’t compatible with each other and it’s causing deposits on the surfaces of the brake hardware. Kia offers a semi-metallic pad for the European market and a handful of people have swapped to that. They report it solved the “grinding”, although at the expense of increased brake dust.
    I know you already changed the pads, but if you used OEM-spec then you might not have solved the issue (if that is the problem). Another possibility is that the new pads weren’t bedded properly.
    So my suggestion would be to try out a bedding procedure and if that doesn’t work try a semi-metallic or more agressive ceramic pad.

  • avatar
    Flipper35

    Have you bedded the brakes properly? A lot of performance pads and rotors do not work well together if they have not been properly bedded.

  • avatar
    smartascii

    Armchair quarterbacking and all that, but the part about the rear brakes being almost completely worn out while the fronts were lightly worn makes me wonder if this car has EBD and if it’s apportioning excessive force to the rears and overheating them. Hot rear brakes + ABS + malfunctioning EBD could trigger all sorts of weirdness. Alternatively, there could be air in the front lines and the computer is going bonkers trying to compensate, though it seems like it’d be weird when the brakes were cold if that were the case.

    • 0 avatar
      Robotdawn

      I know nothing about cars, but I do know having your back brakes worn at 35k miles and your front brakes still OK flashes giant warning signs to me that the fronts aren’t working well at all and the backs are compensating.

      • 0 avatar
        indi500fan

        It’s not uncommon these days to have rears wear faster than fronts.
        They proportion more force to the rear (under non-ABS operating conditions) to reduce front end dive on stops.

  • avatar

    I retract my initial guess. I think everyone discussing ABS and electronic brake force distribution are on to something here.

  • avatar
    cdotson

    A note about “bedding” and metal/pad transfer:

    Bedding brakes is the process of heating up the pads and discs for the purpose of embedding a fine layer, or “glaze” of friction material from the pad into the surface of the disc.

    The best stopping is achieved when the friction is pad/pad, with the disc surfaces carrying pad material diffused into the metal. This can only happen when the brakes are relatively hot, necessitating the bedding procedure. I know Wilwood has a good bedding procedure you can find via Google, although it is somewhat race-oriented.

    Properly-bedded brake discs only ever get a light surface rust, mostly spotting. If your disc ever looks as if it’s sat submerged in saltwater for a week and is fully rust-coated you need to re-bed your brakes as the surface layer of diffused pad material has been removed. This can happen with extensive highway driving with only occasional light brake usage as the brakes never get warm enough to maintain the pad material on the disc. Also over heating the brakes to the point of noticing fade can burn off the pad material from the disc.

    Continuing to drive without the bedding layer on the discs is what causes excessive brake wear.

    I’m not an automotive brake engineer, but I am an engineer and in a former life had to solve an off-highway vehicle brake wear issue in consultation with Wilwood engineers. I also drove a 1/2-ton truck 150k miles on factory brake pads and rotors and they had life when I replaced them.

  • avatar
    JMII

    Normally I’d point to the rotors but you said you just replaced them. So I’d check the wheel bearings. Also check the caliper pistons, seals and any pins/slides to ensure nothing is hanging up. Next would be a switch to a different pad material followed by re-bedding. Since you mentioned rust I’d also check the mounting surface between the wheel and the rotor hubs. A full brake fluid flush might be advisable if it hasn’t been done in the last two years.

    Since the problem is related to temperature I suspect something is out of tolerance. Oddly most brake issue I’ve encountered happen when they are cold. Once warm they work great until the point where they get too hot. So maybe the pistons are not retracting properly? Especially in the back if the rears wore sooner then the fronts. A sticky caliper would have similar symptoms to what you have described.

    • 0 avatar
      Vulpine

      What you describe reminds me of an issue I had myself on a used F-150 I purchased some years back. A quick way to discover if the brakes are dragging would be to just lay your hand on the wheel hubs after driving for a bit, both front and rear, both sides. Dragging brakes will have the affected wheels very warm while no braking would be relatively cold. If the temperature is the same all around, they’re operating properly. An infrared cooking thermometer could give you specific readings.

      In the case of my truck, I had to replace all the brake lines and pistons. In the case of another vehicle, I discovered the handbrake/parking brake had broken/skipped a tooth and never fully released. The touch method told me very quickly which brake/brakes were dragging.

  • avatar
    jalop1991

    I just replaced wheel bearings for a “gritty” noise.

    However, the diagnosis for that is so elementary, I can’t imagine anyone screwing that up.

    But as they say, the universe will make bigger idiots…

  • avatar
    1500cc

    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.

    • 0 avatar
      28-Cars-Later

      Why did you go with the Regal GS?

      • 0 avatar
        1500cc

        I don’t like SUVs/CUVs or pickups, but I liked the utility of the hatch. Also I wasn’t a fan of the 2.0 turbo in my ATS (not that I had any problems with it; I just didn’t like the way it delivered power), so I wanted a V6, something that enjoyed revving and didn’t sound like a cow.

        I also looked at a Stinger, but in the end I was able to get a far better deal on a loaded GS than even a mid-level Stinger GT. And TBH I was a little leery of the Kia’s long term reliability. So far the Regal’s been great and I’m very happy with the decision.

  • avatar
    RHD

    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.

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