[alexwhittemore]

So I just brought my car in for the recall service and had them look at a couple other things - one remote didn't work, etc, etc. My SA took a look at the rear brakes and added a line to have them checked, out of which I got new pads and rotors. I think all pads were last changed together around 25k, car's currently at 41k, and I'm pretty sure rotors have never been replaced (until now).

So realizing that the rear rotors were warn enough to warrant replacement, and knowing that the wear lip on my front rotors was even worse, I just went out with the calipers to measure the thickness on the fronts to find that it's 20.96mm. The rotor is stamped with "Minimum thickness: 22.4mm"

Of course, somehow both the SA and the tech who did my back brakes missed that, it never got mentioned.

So what I'm thinking is this: On the one hand, I'd love new rotors on the front also, since I plan to paint the back ones black now that they're nice and fresh (well, hubs anyway, obviously not the contact surface). On the other hand, my free maintenance is up October 29th and the closer I get to that before replacing them the longer before I have to actually spend money on brakes. Granted, I'll replace them myself out of maintenance and warranty, so it's not the most expensive thing in the world, but you know. I'm cheap.

For reference, the front pads had 3mm more meat on them (10mm vs 7) than the back when I got the car around 37k, despite all having been replaced together at 25k. Point being I'm not sure if they're close to dead or not.

The more I think about it the more I think I'm just going to make an appointment tomorrow to get the fronts done too, but my question remains: what's the safety factor on E90 328 front rotors? What's the realistic risk I run of having my rotors shatter on me within the next 4-5k miles that I'll put on before October?

[Efthreeoh]

First, you must just totally drive in traffic all the time to have the rotors wear to under the minimum thickness in 41,000 miles. But if you did measure correctly then I'd say BMW should replace the rotors and new pads too. This also proves my point I always make when the question of new rotors come up. It has been my experience that BMW rotors never last all the way through the second set of pads, which is why you should always replace the rotors when replacing the pads.

To answer your question though, the rotors will not "shatter". They may warp a slight bit if you get on the brakes real hard one time and heat the rotors up real good, but the rotor in street use will never shatter, so don't worry about it.

[alexwhittemore]

Originally Posted by Efthreeoh

First, you must just totally drive in traffic all the time to have the rotors wear to under the minimum thickness in 41,000 miles. But if you did measure correctly then I'd say BMW should replace the rotors and new pads too. This also proves my point I always make when the question of new rotors come up. It has been my experience that BMW rotors never last all the way through the second set of pads, which is why you should always replace the rotors when replacing the pads.

To answer your question though, the rotors will not "shatter". They may warp a slight bit if you get on the brakes real hard one time and heat the rotors up real good, but the rotor in street use will never shatter, so don't worry about it.

I personally don't brake hard (if you brake a lot, it means you're wasting gas) although I am in city traffic fairly often. What I can't speak to is the first 37k, which was as a lease so anything is possible.

As for warping and shattering: hard braking doesn't warp rotors, it variably deposits pad material, baking it in. That's unrelated to rotor thickness (although if they're so thin you don't have room to machine it off, I guess they'd need replacement.)

And the rotors for SURE WILL shatter at some point. Cast iron is brittle. That's why there's a minimum wear spec at all - there comes a point when the wall thickness isn't enough to hold the torque exerted on it under maximum braking force and it snaps. The point is, there's some safety factor built into that wear spec to be sure it never gets that bad. 1.5 wouldn't be incredibly surprising. Even 2 (i.e. the rotor is rated for 2mm of wear but can handle 4 before its close to dangerous). But I wonder if anyone had actual experience with deeply worn rotors to say what it is.

Anyway, ask me how I know shattering is a real possibility: my grandfather is super cheap. For a while he was using our old (92?) Nissan maxima as an island car. Wanting it to finish out the year, he refused to replace the brakes well after the pads were dead. Like, stopping distance: multiple feet from 5mph. Anyway one day we're coming down the hill into their driveway, him gingerly using the pedal, engine braking, and using the hand brake, and sure enough this ENORMOUS snapping noise comes from one of the front wheel wells. About an 8-10 square inch section of only one side of the rotor had just snapped clear off. Although I suppose "snap oddly" is a better description than "shatter"

[Efthreeoh]

Originally Posted by alexwhittemore

I personally don't brake hard (if you brake a lot, it means you're wasting gas) although I am in city traffic fairly often. What I can't speak to is the first 37k, which was as a lease so anything is possible.

As for warping and shattering: hard braking doesn't warp rotors, it variably deposits pad material, baking it in. That's unrelated to rotor thickness (although if they're so thin you don't have room to machine it off, I guess they'd need replacement.)

And the rotors for SURE WILL shatter at some point. Cast iron is brittle. That's why there's a minimum wear spec at all - there comes a point when the wall thickness isn't enough to hold the torque exerted on it under maximum braking force and it snaps. The point is, there's some safety factor built into that wear spec to be sure it never gets that bad. 1.5 wouldn't be incredibly surprising. Even 2 (i.e. the rotor is rated for 2mm of wear but can handle 4 before its close to dangerous). But I wonder if anyone had actual experience with deeply worn rotors to say what it is.

Anyway, ask me how I know shattering is a real possibility: my grandfather is super cheap. For a while he was using our old (92?) Nissan maxima as an island car. Wanting it to finish out the year, he refused to replace the brakes well after the pads were dead. Like, stopping distance: multiple feet from 5mph. Anyway one day we're coming down the hill into their driveway, him gingerly using the pedal, engine braking, and using the hand brake, and sure enough this ENORMOUS snapping noise comes from one of the front wheel wells. About an 8-10 square inch section of only one side of the rotor had just snapped clear off. Although I suppose "snap oddly" is a better description than "shatter"

So apparently you like to ask questions that you think you already know the answer to, then try to prove wrong someone else who properly answers the question.

I can't speak to your cheap-ass grandfather's situation with his Nissan since I wasn't there, but I'd guess that what really happened (if you're not making it up) is the brake had no pad left and was on the backing plate against the rotor, which would rapidly wear the rotor down to the point where I suspect the face finally wore through and the backing plate caught the cooling fins inside the rotor, and the rotor broke. Hopefully your grandfather hasn't yet killed anyone.

If you are so educated on the subject of automotive brakes and in metallurgy, you would understand that from your measurement of the rotor the rotor was only 1.44 millimeters under spec, which puts the rotor nowhere near being in danger of failure. Also, hard braking does exactly that; warp brake rotors. Apparently you must have been off your game when you wrote this since you know that hard braking adds large amounts of heat to the brake system and heat the rotors up, which in turn can lead to warping of the rotor disk by changing the tension of the metal. And I'm sure you also understand that constant moderate braking where the rotor does not have enough time or not enough proper air flow to properly dissipate the heat allows the heat constantly build to the point that the rotor overheats and can warp.

I'm almost positive that you understand the physics behind the brake system where the little tidbit about energy not being created nor destroyed but just changing state comes into play and the kinetic energy of the moving car is changed to heat energy through the process of energy (frictional) transference by heating up the brake pads, brake rotors, brake caliper, brake fluid, wheel, wheel hub, and the physical action of loss of pad material and loss of rotor material, noise (and light energy - like when you see red-hot rotors on race cars, say at the 24 hours of Seabring), etc.

[alexwhittemore]

Originally Posted by Efthreeoh

So apparently you like to ask questions that you think you already know the answer to, then try to prove wrong someone else who properly answers the question.

The question was "how much wiggle room" and the answer "it'll never be a problem" is not really correct and, anyway, unhelpful.

Originally Posted by Efthreeoh

I'd guess that what really happened (if you're not making it up) is the brake had no pad left and was on the backing plate against the rotor, which would rapidly wear the rotor down to the point where I suspect the face finally wore through and the backing plate caught the cooling fins inside the rotor, and the rotor broke. Hopefully your grandfather hasn't yet killed anyone.

That was my reaction too. Luckily his cars have to pass inspection now that he's in mainland Florida. The accelerated wear due to no pad is for sure true. But the piece that snapped off had quite in tact face. A good question WOULD be why did it wear asymmetrically, and it could be that the plate was wider on the non-piston side vs the piston side or something, I'm not really sure. I do seem to recall that it was the wheel-facing side that snapped, but I don't remember.

Originally Posted by Efthreeoh

If you are so educated on the subject of automotive brakes and in metallurgy, you would understand that from your measurement of the rotor the rotor was only 1.44 millimeters under spec, which puts the rotor nowhere near being in danger of failure.

For Christ sake, that's my question. Quantify "nowhere near". I generally think you're right, but then hell, there's a minimum wear spec. How conservative is it?

Originally Posted by Efthreeoh

Also, hard braking does exactly that; warp brake rotors. Apparently you must have been off your game when you wrote this since you know that hard braking adds large amounts of heat to the brake system and heat the rotors up, which in turn can lead to warping of the rotor disk by changing the tension of the metal. And I'm sure you also understand that constant moderate braking where the rotor does not have enough time or not enough proper air flow to properly dissipate the heat allows the heat constantly build to the point that the rotor overheats and can warp.

http://www.stoptech.com/technical-su...nd-other-myths for that matter, this

Originally Posted by Efthreeoh

I'm almost positive that you understand the physics behind the brake system where the little tidbit about energy not being created nor destroyed but just changing state comes into play and the kinetic energy of the moving car is changed to heat energy through the process of energy (frictional) transference by heating up the brake pads, brake rotors, brake caliper, brake fluid, wheel, wheel hub, and the physical action of loss of pad material and loss of rotor material, noise (and light energy - like when you see red-hot rotors on race cars, say at the 24 hours of Seabring), etc.

In fact, light energy even when you don't see red-hot rotors. Blackbody radiation. You're putting off IR light RIGHT NOW! (And theoretically, an immeasurably low amount of visible light as well.) Anyway, I'm not arguing your point that brakes generate heat, obviously that'd be silly. My point is that said heat doesn't get to the point of yielding the iron rotors.

[alexwhittemore]

Actually, knowing the cooling channel thickness should, to some extent, make the answer to my question calculable. It's not exactly that straightforward, but it'd be a good ballpark.

[Efthreeoh]

Well I'll try and answer your question with some recent personal experience then. I have a 2009 Hummer H3T. It has about 35,000 on it. In February, after an extended trip up North, I noticed a prominent brake noise coming from the left rear. I could see how I'd have gone through rear brakes on the H3T in 35,000 even because I had inspected them at the last tire rotation at 30,000 and I never heard the tuning fork wear indicator. When I inspected the problem, for some reason the pad had disintegrated and the backing plate was rubbing on the rotor outer face (the wear indicator is on the piston side of the caliber. The pad that went bad was the outer floating pad. The backing plate had cut into the rotor face so bad that it was half the thickness as the inside rotor face (measuring from the cooling fins out to the face of the rotors). No issues other than grinding noise and some reduced braking performance. It was safe to say the rotor was way below the minimum thickness.

[Squark]

FWIW, my SA told me the minimum thickness is for when you change the pads. As long as it's thicker than the min when the pads are installed it will be fine until the next pad change. I was close to the min when my pads were changed and they wouldn't give me new rotors.

[alexwhittemore]

Originally Posted by Efthreeoh

The backing plate had cut into the rotor face so bad that it was half the thickness as the inside rotor face (measuring from the cooling fins out to the face of the rotors). No issues other than grinding noise and some reduced braking performance. It was safe to say the rotor was way below the minimum thickness.

Interesting. And the footprint of the backing plate that had worn through that side of the rotor was about the same as the pad? I mean, basically that entire face was worn down?

Originally Posted by Squark

FWIW, my SA told me the minimum thickness is for when you change the pads. As long as it's thicker than the min when the pads are installed it will be fine until the next pad change. I was close to the min when my pads were changed and they wouldn't give me new rotors.

That is very interesting and makes lots of sense. That pretty much settles my fears.