[Zheez]

Hi all,

Just wondering if anyone has any ideas on what to look at on my rear brakes - it appears that the pad isn't making contact with the outer part of the rotor/disc. It's doing it on both sides although one side has a larger area than the other.

It was perfectly fine up until December and there were no visible problems at all at that point. I didn't drive my car much over the Xmas period and I live near the coast so with the weather as it was, I expected some surface corrosion - especially as it had been stood idle for nearly 3 weeks.

When I did next drive it, I went on a 900 mile trip. I've done another 800 or so miles on top of that too so after 1700 miles I would've expected it to all have sorted itself out. There is no issue with braking performance, no noticeable juddering or any other signs of any problems but just part will not clear up:



There's obviously signs of light surface corrosion appearing again since I haven't driven it for a little bit but I'm not worried about that, it's just the outer ring that's concerning. I'm going to take the calipers off and check everything out this week some time but just wondering if anyone has any thoughts. Discs/rotors and pads were all changed out in August, believe it or not and the car has probably only done about 8k miles since then.

[Billfitz]

Normal. Pads never reach the rotor edge. The rust doesn't bother anything, but I still sand it off mine as part of my yearly maintenance routine, which includes tire rotation and touching up the paint on the calipers and rotor hubs. As a result mine looked like this at 7 years old:

[HighlandPete]

Originally Posted by Zheez

....but just wondering if anyone has any thoughts. Discs/rotors and pads were all changed out in August, believe it or not and the car has probably only done about 8k miles since then.

What I see in your picture, is typical of what we have to deal with up here every winter. Any sitting in humid and particularly a salty atmosphere, (worse up here when roads have been salted), rust can be so dominant it will rub the pads off, just as your disc, rather than 'clean' the whole of the disc friction face.

I end up cleaning the rust off with a flat oscillating sander, when I change over wheel sets, spring and autumn. Allows me to get a bit more life out of discs.

I've even got a tool I've made, a swivel abrasive pad on a modified screwdriver, to rub off rust through the spokes.

My experience and other users up here, once the rust abrades part of the pad, driving alone, doesn't easily get rid of it.

[Skyhigh]

Originally Posted by Billfitz

Normal. Pads never reach the rotor edge. The rust doesn't bother anything, but I still sand it off mine as part of my yearly maintenance routine, which includes tire rotation and touching up the paint on the calipers and rotor hubs. As a result mine looked like this at 7 years old:

What is visible in his photo is (way) beyond the normal unreachable outer edge of the rotor.
This is very typical for cars in humid environments which are not driven much/often. Unfortunately, once it starts and progresses, there is very little one can do (except to remove and sand down the rotors flat on both sides.

Here is what my rear Audi rotors progressed to with time...:


@Zheez
At the early phase, what would help is doing some runs with slowly pulling the hand-brake from speed like 50-80 km/h. Basically to clean the rust off using the pads. It will only work if the rust is still at the very surface. Yours don't look too bad yet! I think you have a good chance. You need to do it quite a few times to see results though. Make sure you are not overheating your brakes in the process... leave some time in-between every few hand-brakes.

Other tips I can give you from my experience:
- NEVER wash your car (or wheels/brakes specifically) and put it in the garage straight away for a prolonged period of time. You will find the pads stuck on the rotors and rotors corroded. I've done that mistake. Now, when I know I will not be driving the car for a while, I only wash the car but don't spray water on the wheels to protect the brakes.
- Every time you see the rear rotors slightly corroded - use your hand-brake to slow your vehicle down a couple of times (don't do it on slippery surface or in a turn!)
- Inspect both sides of the rotors every time you switch wheels (summer-winter). Sometimes rust develops only on the inside. At least in Europe, the regulation says that if the contact surface area on the front and back of a rotor deviates with more than 20%, you are not roadworthy and would not pass inspection.

[johnung]

Originally Posted by Zheez

Hi all,

Just wondering if anyone has any ideas on what to look at on my rear brakes - it appears that the pad isn't making contact with the outer part of the rotor/disc. It's doing it on both sides although one side has a larger area than the other.

It was perfectly fine up until December and there were no visible problems at all at that point. I didn't drive my car much over the Xmas period and I live near the coast so with the weather as it was, I expected some surface corrosion - especially as it had been stood idle for nearly 3 weeks.

When I did next drive it, I went on a 900 mile trip. I've done another 800 or so miles on top of that too so after 1700 miles I would've expected it to all have sorted itself out. There is no issue with braking performance, no noticeable juddering or any other signs of any problems but just part will not clear up:



There's obviously signs of light surface corrosion appearing again since I haven't driven it for a little bit but I'm not worried about that, it's just the outer ring that's concerning. I'm going to take the calipers off and check everything out this week some time but just wondering if anyone has any thoughts. Discs/rotors and pads were all changed out in August, believe it or not and the car has probably only done about 8k miles since then.

Your photo looks pretty typical for BMW rotors. They don’t treat them with a zinc coating to prevent rust so the areas unswept by the pads tend to rust quickly. You can see it on brand new BMWs just sitting on the dealer lot. Another BMW mentality thing that I don’t understand.

If rotor rust really bothers you the best method is to soak them for up to 24 hours, no more, in regular white household vinegar. I buy gallons of vinegar cheap at Sam’s Club.

Get a low, long plastic container that will hold two rotors at a time. Soak them upside down. I fill in empty spaces with filled water bottles to bring up the vinegar level. Do this outside. Cover and comeback the next day.

The white vinegar will have turned disgusting. This will stain pavement so pick a good spot, pull out the rotors, spray lots of Formula 409 (it works better than anything else) and wire brush the rotors. Then use brake cleaner to spray out the vanes and outer surfaces. Huge difference! Easier and better result than sanding.

I got tired of Genuine BMW rotor hats rusting so I painted then with Silver G2USA Caliper Paint. Epoxy paint that lasts forever, never needs repainting. See photos.

Hope this helps!

[Billfitz]

Originally Posted by Skyhigh

What is visible in his photo is (way) beyond the normal unreachable outer edge of the rotor.

IME it's not out of the ordinary. It is in the pictures that you posted. Rust happens. If your car is left outside in the rain within hours rust will form on the rotors. It's no big deal, driving a hundred yards will remove it. It only gets nasty if you don't drive often enough to keep the rust at bay.

[Skyhigh]

I believe you are not looking where you should be - he is talking about the outer rusty board! It is too wide, wider than what’s normal! Normal is a a couple of mm, maybe half width of what he has there. Problem is - it will get wider and wider unless addressed (or prevented, if at all possible).

[casualDIYer]

Hi Zheez which rear brakes do you have on your car? A single-piston type in which one brake pad has a spring clip that plugs into the brake piston? Or do you have 2 piston brakes, in which the pads are pressed on by one piston on either side of the caliper, and no spring clip is used to hold a pad to the piston?

With the one piston type it is common for rust to form the 'ears' of the pad and on the brake caliper, fusing the two together in a matrimony of Rust Hell. As the world isn't perfect, sometimes it will happen to just one ear so that the pad is prone to skewing when it is pressed into the disk.

How blunderful.

You will need a wire brush to clean up the the pad carrier (a cast iron part that straddles the disk and where the brake pads hang on to). Once cleaned up, apply a little bit of anti-seize to the ears of the brake pads. More isn't better here. Anti-seize is wonderful stuff but it's sticky. It helps keeps parts from seizing but also will trap particles of dust. A 'smidge' is all that is required. You will see this mentioned in BMW's own brake pad replacement instructions. Of course, being translated from German, I doubt BMW will say 'smidge' but probably will say 'apply the correct amount'. The 'correct amount'. So intuitive. Once you've applied the anti-seize, place the front pad into the carrier and attempt to move it back and forth, as best you can, perpendicular to the disk. If it gets jammed up, get out the wire brush and give the carrier a scrub. Eventually you will observe the pad will move more freely within the carrier. Repeat for the rear brake pad.

[Billfitz]

Originally Posted by Skyhigh

I believe you are not looking where you should be - he is talking about the outer rusty board! It is too wide, wider than what’s normal! Normal is a a couple of mm.

IME normal is from 3 to 5mm. That might be 6mm, indicating the pad is undersized, but it's not something I'd lose sleep over, especially on the rear.

[HighlandPete]

It's the speed rust and corrosion can build up, even when used regularly in humid environments. So hard to clear through braking, once the pad is compromised.

The attached image is from my E91 330d front disc, before it even went through its first winter from new. About 8mm of rust on the outer rim of the friction face.

[Markyboyt]

Originally Posted by Skyhigh

What is visible in his photo is (way) beyond the normal unreachable outer edge of the rotor.
This is very typical for cars in humid environments which are not driven much/often. Unfortunately, once it starts and progresses, there is very little one can do (except to remove and sand down the rotors flat on both sides.

Here is what my rear Audi rotors progressed to with time...:

@Zheez
At the early phase, what would help is doing some runs with slowly pulling the hand-brake from speed like 50-80 km/h. Basically to clean the rust off using the pads. It will only work if the rust is still at the very surface. Yours don't look too bad yet! I think you have a good chance. You need to do it quite a few times to see results though. Make sure you are not overheating your brakes in the process... leave some time in-between every few hand-brakes.

Other tips I can give you from my experience:
- NEVER wash your car (or wheels/brakes specifically) and put it in the garage straight away for a prolonged period of time. You will find the pads stuck on the rotors and rotors corroded. I've done that mistake. Now, when I know I will not be driving the car for a while, I only wash the car but don't spray water on the wheels to protect the brakes.
- Every time you see the rear rotors slightly corroded - use your hand-brake to slow your vehicle down a couple of times (don't do it on slippery surface or in a turn!)
- Inspect both sides of the rotors every time you switch wheels (summer-winter). Sometimes rust develops only on the inside. At least in Europe, the regulation says that if the contact surface area on the front and back of a rotor deviates with more than 20%, you are not roadworthy and would not pass inspection.

I agree this is within the pads friction area as you can see where they have polished some of the rust up, it’s just too not enough so possibly sticking in the carrier a little as someone else suggested.
Unfortunately the handbrake won’t have any influence because it doesn’t operate the brake pads, it has shoes inside the disc.

[Skyhigh]

I completely forgot BMW have the hand brake differently (for reasons I don’t get)!! Good point - hand braking will not help here unfortunately.

And yes, the slightly polished area indicates it is well within the coverage of the pad and well outside of the normal edge with no-friction per design.

[Billfitz]

Originally Posted by Skyhigh

I completely forgot BMW have the hand brake differently (for reasons I don’t get).

Hand braking with the rear pads is the exception, not the rule. Only one car that I've owned over the last forty years had that. The rest all used combination drum handbrake/disc main brake rears.

[Zheez]

Thanks for the replies everyone, some things to think about there. Earlier on today I took the rear brake calipers off to take a look at everything and clean it all up. Here's what I found - nothing looks particularly bad, except you can see the parts of the pad that aren't effective against the rotor for whatever reason because they're a slightly different colour.




This is after driving another 350 miles on Thursday. I made sure to do some harsher braking than usual - not quite emergency stops but some fairly abrupt braking from 60mph, but it didn't seem to make much difference to the rotor surface.

So I took it all apart and I cleaned everything up - pads, calipers and carriers and I lightly sanded the areas of the brake carriers where the pads sit to remove any corrosion there. I also lightly sanded the surface of the pads before cleaning it all up again then I put it all back together with fresh anti-squeal paste. Haven't driven the car anywhere yet but I'll soon see if it makes any difference. Hopefully now it might have a hope of clearing itself up with some heavy braking.

I did also spot that one of my front rotors has a small patch on the outer edge that's very similar, but it's not as much as the rears. The other front rotor is absolutely fine though. If this makes any difference at all to my rears then I'll sort the front out in the same way.

One thing's for sure though, I'm not going to let my car sit idle for that long again. I did give it a clean in December and then I left it to sit for weeks after that so no doubt that has contributed to it and I also need to remember to drive the car for a bit after cleaning it in winter Normally it doesn't sit idle for that long because I drive all over the country for work but that was an unusually quiet period for me.

[Skyhigh]

Originally Posted by Billfitz

Hand braking with the rear pads is the exception, not the rule. Only one car that I've owned over the last forty years had that. The rest all used combination drum handbrake/disc main brake rears.

Not sure what cars you’ve owned, but none of those I’ve had or driven (rentals aside) had drum handbrake in addition to disc brakes. (European cars only - Audi, VW, Skoda amongst others)

I don’t get the advantage - more components, more weight, more complexity, dormant failures, potential for rusting due to rare usage, inability to visually inspect...

[Billfitz]

Nissan, Toyota, Mitsubishi, even a Renault. The advantage to a drum parking brake is that because of the larger contact area it does a better job of locking the rotor than a pad, if it's properly adjusted.