Help diagnosing sportwagen rear disk brake issue

Maintenance/Repairs
, ,
1

Guys, I need help determining an issue with a rear disk brake.

Pulled the wheel because I was hearing whisssh , whissh sound there, twice per rotation. I suspected a brake pad dragging twice per rotation (rotor warped?).

But when I looked at the rotor, I saw that the outboard pad was only touching the rotor in two places.

And the inboard side of the rotor has strange marrings on the surface that almost look like it’s been welded and ground. These marrings on the surface are higher friction when I run my finger over them, so they could possibly be the source of the noise.

Oh. And strangely, the noise doesn’t go away when braking.

Any help diagnosing would be appreciated. I’ve attached pictures, although I don’t know how useful they’ll be.

Welded looking area is a tiny bit clockwise form 12 oclock.

IMG_20200629_205737969
IMG_20200629_2057379693264×2448 860 KB

2

Thats typical of a worned rotor.Replace both with new ones along with the pads.You also need to lubricate the slider pins with brake grease.Who is doing the brake job?

3

Only 15k on the pads and rotors, and I drive like your Grandma.

Rotor is still at initial thickness of 10mm. Pads have plenty left too, although outboard one is wearing slightly faster at the bottom.

Weird.

I know the default answer for any brake problem like this is new pads and rotors, but is there anything else it could be?

Forgot to say:. I’m doing the brake job.

4

Bent axle ?

5

Hope not!. The wife would be in trouble :slight_smile:

I’m going to try and sand the rotor and see is it’s a simple as some rust.

6

Does this have a small drum brake that fits inside the hat section of the rotor for the parking brake? If it does, you are probably hearing the drum brake shoe rub the rather large amount of rust on the INside of that hat section.

1 Like
7

I think we’re looking at the inside, pretty rusty if there’s a drum parking brake.

8

@Mustangman @texases

No drum brake. Parking brake is actuating the caliper piston.

I guess the rotor is probably just warped. Should I try to get away with just replacing that rotor? It’s a PITA to replace everything else at the same time, since it only has 15K on it.

9

Fitted new rotor (but using old pads - wanted to be able to return things if rotor / pads weren’t the issue). Issue still there - rotor spins 3/4 around with clearance, then rubs for the last 1/4.

The next turtle down is the hub not spinning true. Going to buy a dial indicator today to see if I can confirm that.

10

Maybe the pads are worn uneven or perhaps a wheel bearing or axle problem

11

When you did the brakes 15,000 miles ago, did you lube the slider pins? Are the calipers moving freely?

12

I didn’t do them last time. They were done by the dealer where I bought the car used.

That should ring alarm bells, eh?!

But I didn’t have any trouble for 15K. This started suddenly.

Did lube the slide pins. One returns more freely than the other, but surprisingly the pins themselves have different designs (one has a little rubber spline on it) so I suppose it’s on purpose.

I hadn’t reattached the caliper when doing this test. Just the caliper carrier bracket with the pads in.

13

Certainly beginning to look that way. Don’t really want to do a wheel bearing or axle myself. Don’t trust myself enough.

What on earth would have caused this to happen suddenly?

This started right after I replaced front shocks/springs. Don’t see how that could affect the rear axle / wheel bearing though.

-g

blocked.gif

14

Don’t just start replacing parts that are suggested here. Do a diagnosis first. There are lots of youtube videos that can help you.

15

There’s another possibility . . . and your wife won’t like it, since she’s apparently the one driving the car

Any chance she did a curb check on that corner . . . the one with the problem?

If so, perhaps the caliper bracket isn’t situated properly, because things are bent . . .

the more I think about it, the way you describe it . . . something may very well be bent

16

Curb check? Her hub caps are a wear item :slight_smile:

I can’t think how one of the static parts getting bent would cause an “intermittent” (only 1/4 of the rotation) dragging though. Can you?

But I can see how it could be the hub or bearing that’s somehow bent or mis-seated though.

Gonna hit it with the dial indicator and see what we see.

-g

blocked.gif

17

I think we’re onto something

1 Like
18

So, procured a dial indicator. 0.002" runout at the hub. 0.007" runout on the disc.

I have the shop manual, but for the life of me I can’t find the manufacturer specs. Do those numbers sound reasonable to you guys?

19

Update: The hub runout is more like 0.001"

20

Ok. I always like to circle back to provide a conclusion for any future readers. But this seems to be some sort of quantum effect. By measuring the system, I have fixed it.

After measuring things with the dial indicator, I put everything back together with the old parts, assuming that I had reached the end of the line, and would take it to my mechanic. Lo and behold putting it back together this time solved the problem, although it never did before.

To be honest, there is still some small sound, but I have to really listen for it, with the windows open. Before, it was a clear loud noise in the cabin.

Lesson for future readers: get a dial indicator :slight_smile:

1 Like