[Phil325i]

Is it just me or is there a fundamental design flaw with single piston rear calipers? Just stripped down my pads and discs. As you can see from the pics below there was plenty of meat left on the discs (no rind).

The front of the discs (the face you see) was fine, but the reverse was very badly corroded and the swept area of the disc was reduced, with consequent gouging into the pads. It seems to me the caliper design does not transmit enough grip to the 'back' pads to bite into the disc and also the passage of air (and water) through the disc venting causes build-up of rust. Not impressed...

[Tambohamilton]

Not really, unless your sliders are seriously stuck. The clamping force is massive compared with the friction on the sliders, so the difference in force across the disc should be negligible

[Foghorn]

Looks to me like you have ceramic brake pads and don’t use the car much or are gentle with the brakes.

[Phil325i]

Originally Posted by Foghorn

Looks to me like you have ceramic brake pads and don’t use the car much or are gentle with the brakes.

Wrong on all three counts.

[Phil325i]

Originally Posted by Tambohamilton

Not really, unless your sliders are seriously stuck. The clamping force is massive compared with the friction on the sliders, so the difference in force across the disc should be negligible

Thanks Tambo. Think I will have to dial 'clean and lubricate sliders' into regular maintenance...

[edycol]

Originally Posted by Phil325i

Thanks Tambo. Think I will have to dial 'clean and lubricate sliders' into regular maintenance...

ATE sliders are dry! I would clean them or buy new ones.

[Welcome to NBA Jam]

Originally Posted by Phil325i

Thanks Tambo. Think I will have to dial 'clean and lubricate sliders' into regular maintenance...

For some reason BMW does not want you to lube them. Better to take them to the wire wheel reinstall them clean, shiny and dry, otherwise replace if they have any grooves. It would be a great time to replace your caliper bushings as well. Real cheap and easy.

The odd thing is this style of calipers with slide pins like this are used on other brands like VW, Volvo and I think one particular model of a Chrysler but I can't remember. Some recommend lube, some don't.

[Tambohamilton]

Also go to town on the runners where the pads locate on the carrier. Make sure everything can move smoothly and freely.

[edycol]

Originally Posted by Welcome to NBA Jam

For some reason BMW does not want you to lube them. Better to take them to the wire wheel reinstall them clean, shiny and dry, otherwise replace if they have any grooves. It would be a great time to replace your caliper bushings as well. Real cheap and easy.

The odd thing is this style of calipers with slide pins like this are used on other brands like VW, Volvo and I think one particular model of a Chrysler but I can't remember. Some recommend lube, some don't.

It is ATE brand caliper. Second largest supplier of brake systems in the world, and by far the largest on German vehicles.
Also, German vehicles in general use this style regardless of who makes caliper. Usually, issue is when boots are compromised and start leaking moisture inside. Boots are like $2 in Auto Zone.

[Phil325i]

Originally Posted by Tambohamilton

Also go to town on the runners where the pads locate on the carrier. Make sure everything can move smoothly and freely.

Thanks. Yeah got that. All thoroughly cleaned and lubed. In fact the sliders were in good shape with no corrosion and still some lube in place.

I still feel there is something weak in the caliper design that causes this. It happened recently to my son's Sirocco at the front, would you believe. Outer face of the discs perfect, inner face massively corroded...

[Tambohamilton]

It might just be as simple as the outer face drying faster than the inner when the car is standing, therefore it rusts less. Once uneven wear starts, it tends to accelerate too; bit of a vicious circle.

[Jaronbwall]

The rear brakes don't get nearly as hot as the front. About 70-80% of braking force comes from the front. Now I'm curious what front rotors look like... I agree with Tambohamilton and edycol

[Phil325i]

Originally Posted by Jaronbwall

The rear brakes don't get nearly as hot as the front. About 70-80% of braking force comes from the front. Now I'm curious what front rotors look like... I agree with Tambohamilton and edycol

They look fine...on the outer face. I will only know the condition of the inner face when I change them (not due just yet).

While you're right about front/rear braking force, my car has cruise control which means on downhill sections of road where speed would otherwise increase, the cruise control applies the rear brakes. So in my case they wear out first!

[Tambohamilton]

Didn't know cruise only applied the rear brakes... You'll have e-diff too, which will apply a rear brake occasionally

[Brian86]

If you want to prevent that, you need to drive the car considerably harder.

Race cars don't suffer from rusty rings on the rotors.

There isn't a design flaw. Fixed caliper brakes suffer the same symptoms when the rotor is run for tens of thousands of miles of grocery getting. It's just the way the pad and rotor and corrosive elements all interact.

[edycol]

Originally Posted by Phil325i

They look fine...on the outer face. I will only know the condition of the inner face when I change them (not due just yet).

While you're right about front/rear braking force, my car has cruise control which means on downhill sections of road where speed would otherwise increase, the cruise control applies the rear brakes. So in my case they wear out first!

There is your problem. Cruise control.
Drive harder. Drive it the way it supposed to be driven. I track my 328, and trust me, no issues with calipers. I sometimes go through set of rears in two track days bcs. ediff. Etc.

[Tambohamilton]

Originally Posted by edycol

There is your problem. Cruise control.
Drive harder. Drive it the way it supposed to be driven. I track my 328, and trust me, no issues with calipers. I sometimes go through set of rears in two track days bcs. ediff. Etc.

Eh? You have to track your e9x in order for the brake discs to not get corroded? Sorry, that's BS.

I have never tracked my car and have no issues with the brakes. I'm not hard on the brakes in daily driving either, because MPG.

[edycol]

Originally Posted by Tambohamilton

Eh? You have to track your e9x in order for the brake discs to not get corroded? Sorry, that's BS.

I have never tracked my car and have no issues with the brakes. I'm not hard on the brakes in daily driving either, because MPG.

No, that is not what I said. I drive it aggressively, and among other things I track it.
Also, what pads and rotors are you using?
There is no design flaw with calipers. But there is one in maintenance or driving.

[Tambohamilton]

It's still BS that you have to drive aggressively to prevent rust buildup or uneven wear on your discs. Simply not true!

Front; not sure - on the car when I got it. 99% sure they're just cheap parts though.
Rear; febi discs and ate pads. Nothing weird or special, just run of the mill OEM stuff.

Yes, I do agree that maybe a mistake in maintenance has been made on the OP's car. Maybe replacing the discs without also replacing the pads or something.

[edycol]

Originally Posted by Tambohamilton

It's still BS that you have to drive aggressively to prevent rust buildup or uneven wear on your discs. Simply not true!

Front; not sure - on the car when I got it. 99% sure they're just cheap parts though.
Rear; febi discs and ate pads. Nothing weird or special, just run of the mill OEM stuff.

Yes, I do agree that maybe a mistake in maintenance has been made on the OP's car. Maybe replacing the discs without also replacing the pads or something.

Ok, several things:
1. Inner pad will always wear out faster than outer.
2. FEBI is budget rotor! Get ATE, Zimmerman, Pagid.
3. Again, slider boots could be compromised, how is your piston boot? There is a reason why there are repair kits.
4. It is not BS. BMW designs cars based on their culture which revolves around autobahn. There is always Lexus ES or Toyota Camry as an option. But I have two Toyota's and you should see shit show with brakes there.

[Tambohamilton]

1. Sure.
2. Why? I don't have any problems.
3. See 2.
4. Have you driven in Germany, or on unrestricted autobahn? There is nothing about it which mandates heavy braking on a regular basis. This brake design is used on heaps of cars across a broad spectrum of performance levels, and has been for years, with no significant ill effects; there's nothing about it which requires heavy braking to operate or wear properly.

[edycol]

Originally Posted by Tambohamilton

1. Sure.
2. Why? I don't have any problems.
3. See 2.
4. Have you driven in Germany, or on unrestricted autobahn? There is nothing about it which mandates heavy braking on a regular basis. This brake design is used on heaps of cars across a broad spectrum of performance levels, and has been for years, with no significant ill effects; there's nothing about it which requires heavy braking to operate or wear properly.

I confused you with OP.
Yes, I have driven on the autobahn. I was the test driver for an oil company and later car magazine in Europe, and I am from there.
My point was larger, just did not know I have to draw a picture. It is a cultural thing. Does that mean bcs. no heavy braking on autobahn BMW should put smaller brakes? Why 370X30mm on F30 335? Going 300X24mm would save A LOT of rotational mass and improve acceleration, consumption etc.
My point is that German cars or Italian cars or whatever are developed bcs. local inherent culture from the beginning of times. As much as the autobahn is nice to get to top speed, the southern alps are even better to have fun.
Brakes are fine on E90, and the design of calipers is fine. For various reasons that are not the topic of this discussion BMW's like speed and rpm as other European vehicles. Brakes including. With such small braking force in the back, coupled with poor maintenance things like this will happen.
I personally never had this issue, and I had numerous BMW's and other Euro vehicles with same caliper design, from Alfa to VW.