Is it possible to clean rotor discs

[WobblyHand]

If it had not been for the US government (Not politics, just fact) back in 2008 and 2009 GM would have had more in common with AMC and Studebaker than with Ford and Dodge. Even with that said, they had to close several of their brads and a huge number of their dealers. Say what you will but GM isn't the most awesome thing on the road, Ford weathered the downturn without any government assistance and Fiat say promise in Chrysler and bought them paying back Their Government loans.

Might be true, but not a helpful post in this context...

 

[verbotenwhisky]

Might be true, but not a helpful post in this context...

Fair enough....

 

[tim81]

I’m a General Motors technician and that’s what the manufacturer of these vehicle are saying is the root cause to comebacks for warped rotors. They have gone as far as to send out accuturn on vehicle lathes partially due to this. Also instructed us to clean the hub flanges with abrasives to ensure that there’s No contamination between the hat of the rotor and the hub flange. Makes a ton of sense if you think about it. If a rotor begins with over two thou runout it’s not gonna get any better now is it? Theees tons of information supporting this if you search for it. Not to mention the support of one of the largest automakers in the world. Let’s hear your take, since you obviously have more training and hands on experience than a General Motors Master and world class technician

Lol I am not contradicting you. Just will do some measurements. It’s a good exercise for me as I am new and will try to get the hang on how to measure and make fixtures etc. at least with this one I will learn some useful stuff.

 

[tim81]

That I would agree with. The mounting surfaces need to be clean. Can't get good fit and low run out with crud between the surfaces. Thought that was common knowledge, at least for anyone doing mechanical work.

Over torquing, might cause trouble too, especially with high grade bolts or studs. I don't use air guns for this kind of work. I torque to spec whenever possible. Guys in a shop may take short cuts and torque the daylights out of things. We have all seen that.

No disrespect meant to you or your training. It seemed odd to me that coatings would really matter. What matters are clean, smooth surfaces for the mating assemblies. If GM uses coatings, that's good. Coatings would help parts stay rust free. But coatings don't last forever, at least in the northeast. And not all manufacturers may employ the same strategies.

I was wondering I have see mechanics that have never cleaned the metal matings surface for the rotors. Then if they do so, do they alter the flatness. What about the shoulder on the stud for small cars? Does it change the geometry when it is tighten and it keeps that spot a bit higher. Tightening is very important I do use the torque wrench whenever I can. Got the whole set around 2017. “Needed it at that time for the ram1500”

 

[WobblyHand]

I was wondering I have see mechanics that have never cleaned the metal matings surface for the rotors. Then if they do so, do they alter the flatness. What about the shoulder on the stud for small cars? Does it change the geometry when it is tighten and it keeps that spot a bit higher. Tightening is very important I do use the torque wrench whenever I can. Got the whole set around 2017. “Needed it at that time for the ram1500”

I am sure we have all experienced stuff going together in a less than perfect way. If the surfaces are not clean, then there's obviously no guarantee that the result will be square or true. The geometry will be off. Now whether that matters or not, that depends on the application.

As for the stud shoulders, the wheels should have a small countersink or counter-bore on the back side to prevent geometry changes due to the stud shoulder. I hope I'm not misunderstanding your point on wheel studs.

 

[tim81]

I am sure we have all experienced stuff going together in a less than perfect way. If the surfaces are not clean, then there's obviously no guarantee that the result will be square or true. The geometry will be off. Now whether that matters or not, that depends on the application.

As for the stud shoulders, the wheels should have a small countersink or counter-bore on the back side to prevent geometry changes due to the stud shoulder. I hope I'm not misunderstanding your point on wheel studs.

Just thinking of what the runout would be in a real world scenarios. Then again they have the sliders, I guess they are there for this situation. Back in Eastern Europe nobody cleaned the mating surface when changing rotors or resurfaced the rotors . They told the customers to go slow the first 10s of kilometers and use the brakes like 10 or more times at first to have the pads form to the surface of the rotors. It’s in no way perfect or recommended but it is like that. And the pads uhhh. The pads were china quality of perfection. the rotors where consumed faster then the pads.

 

[WobblyHand]

Just thinking of what the runout would be in a real world scenarios. Then again they have the sliders, I guess they are there for this situation. Back in Eastern Europe nobody cleaned the mating surface when changing rotors or resurfaced the rotors . They told the customers to go slow the first 10s of kilometers and use the brakes like 10 or more times at first to have the pads form to the surface of the rotors. It’s in no way perfect or recommended but it is like that. And the pads uhhh. The pads were china quality of perfection. the rotors where consumed faster then the pads.

All I ever did was wipe off any rust or debris with a rag. If it was really rusty, I'd use a wire brush to knock things down to metal and just wipe it clean. These are cars or trucks, not spacecraft. There's some margin built in. No rotor or any other part is perfect. Trying to achieve ultimate perfection in a consumer grade vehicle is nonsensical.

I bed the pads, more or less like you describe. Works for me.

 

[tim81]

All I ever did was wipe off any rust or debris with a rag. If it was really rusty, I'd use a wire brush to knock things down to metal and just wipe it clean. These are cars or trucks, not spacecraft. There's some margin built in. No rotor or any other part is perfect. Trying to achieve ultimate perfection in a consumer grade vehicle is nonsensical.

I bed the pads, more or less like you describe. Works for me.

Me too.

 

[Jake M]

I was wondering I have see mechanics that have never cleaned the metal matings surface for the rotors. Then if they do so, do they alter the flatness. What about the shoulder on the stud for small cars? Does it change the geometry when it is tighten and it keeps that spot a bit higher. Tightening is very important I do use the torque wrench whenever I can. Got the whole set around 2017. “Needed it at that time for the ram1500”

You'd better believe that a mechanic can change the geometry of a mating surface, and without even breaking a sweat. The scotch brite disks that "everyone" says are safe, the bristle disks that "everyone" says is safe... They take metal, they change geometry, but they're fast and easy, and laying a straight edge across a clearly no longer flat surface.... It gets violent reactions, because the tools are fast and easy. Those of us that give a (crap) learned long ago to eyeball a surface. Brake rotors are a good one. Rust on a surface is fine. Just not on the bearing surface. Wheels, rotors, and lots of other car parts don't really want to be cleaned up too much. Rust takes away metal. By the time you've cleaned the rust, the mating/bearing surface then MUST be reduced to match... There IS metal being removed. 95 percent of the abrasive "cleaning" you see a mechanic do, is detrimental.

I am sure we have all experienced stuff going together in a less than perfect way. If the surfaces are not clean, then there's obviously no guarantee that the result will be square or true. The geometry will be off. Now whether that matters or not, that depends on the application.

You are very correct on that. Car's (like most of the world) are not as "perfect" as we'd like to think they are.

As for the stud shoulders, the wheels should have a small countersink or counter-bore on the back side to prevent geometry changes due to the stud shoulder. I hope I'm not misunderstanding your point on wheel studs.

Indeed. And there's a perfect example of rust that SHOULDN'T be removed. Not abrasively anyhow. That countersink on the back side of the rim at each stud, that'll rust under there. Removing metal. But the bearing surface where the wheel sits, that's gonna be clean. (Typically. There's ALWAYS an exception when you generalize....). So if you take the rust down to the layer of the clean bearing surface where the wheel sat, you still have rust. When you dig out the rust (which is irrelevent, because it's not a bearing surface), you also lower the actual bearing surface.

So in the end, rust removal/cleanup is not a simple thing. One must (if they give a crap) pay attention and use smarts and judgement. Or on the other hand, a lot of cars have survived a lot of butch repairs.... How perfect must it be?

Just thinking of what the runout would be in a real world scenarios. Then again they have the sliders, I guess they are there for this situation. Back in Eastern Europe nobody cleaned the mating surface when changing rotors or resurfaced the rotors . They told the customers to go slow the first 10s of kilometers and use the brakes like 10 or more times at first to have the pads form to the surface of the rotors. It’s in no way perfect or recommended but it is like that. And the pads uhhh. The pads were china quality of perfection. the rotors where consumed faster then the pads.

Runout on a brake rotor has a target of two or three thousandths of an inch. If the rotor ran "perfect", the pads would never retract properly.

When you buy discount or DIY parts store rotors, if you ever check them, you'd be blown away. Most of that is from poor storage/stacking. And just as it was "introduced" into a finished part, if you bolt it up and heat cycle it a bit, it'll relax back to pretty near as good as it ever was. (Which probably was more than the target range). You might see as much as 5 or 6 thousandths of runout that lives a happy life, depending on the rotor and the pads. Of course like a lot of things, there's no hard or fast rule about what's works and what will give issues a little too soon or a lot too soon.

Folks talk about warped rotors all the time. Warps in a rotor DO NOT give problems. Noobdy cares. You'd (within reason) never know there was a problem with the rotor. What folks who say "warped rotor" really mean is that the brake pedal pulsates. That's not a warped rotor, that's a thickness variation what does that. Some of the worst one's I've seen out of the DIY parts stores are, on initial installation, have been at or over 60 thousandths of runout. (They're not ALL that bad...) And while they do settle in and typically run way truer than that, there is ZERO issues with them in the meanwhile.

The pulsation that one might feel, that's from a localized spot. It can be poor inconsistant metallurgy that wears unevenly, but tyically not. Most typically that is what I'd call a "fold" more so than a warp. The uneven lug nut tightening, overtightening even if it is equal, spirited driving that keeps the brakes plenty hot, so the rotor takes on some shape as it grows. Almost a single point "bend" in the rotor surface. As that makes a very local deformation, it is not a flat surface, the caliper can't follow it because it's no longer planar as a flat/parallel rotor with normal runout would bel, and that very local high spot will wear down even in the non-braking driving time. It makes a thin spot. This changes the travel needed to apply the braking force, and this is where pulsatations come from. Thickness variation pushing the pistons back in right after the "thin" spot. That particular measurements is in tenths. If you've got a thousandth of thickness variation, you can feel it. You may not care at that point depending on your own maintenance plans, your own sense of adventure, or your own sense of humor, but by then it's real.

Somebody (Sorry, forgot who) mentioned GM and their on the car brake lathes. I was with a GM dealer when the first of those got delivered.... They told us ALL DAY LONG about ALL the things that us techs were doing wrong, all the rust damage bollixing things up, and just every possible excuse in the book. We had two options as we were "introduced" to the lathe. We could use the lathe, or continue using the shims. Wedges essentially, that were installed between the hub and the rotor to true up the rotor. Here's the thing. On a Brand new hub (Their wheel bearings were not reliable at that time), on a BRAND NEW hub, with a BRAND NEW rotor, you could use their method, and true stufff up.... BRAND NEW. NOT RUSTED. NEVER TIGHTENED. NEVER TORQUED. Required shims and/or cutting the rotor to get stuff in spec. Here's the coolest part of that. If some tech wanted to pick a fight with them, one could take the new rotor, on the new hub, UNCORRECTED, and clock it five ways (or however many lug nuts it has). The runout that they were trying to fix, it followed the HUB, not the rotor. Repeatedly. Car after car. GM rotors sucked, at least back then anyhow. We solved the problem that GM was trying to solve. We started offering Centric Rotors as a more long lived option than the originals, some of which were eating pads in 8000 milles. Or we'd cut or shim the GM ones, Tech's option at that time. No pulsations out of the whole thing though. Just unacceptable pad wear.

 

[tim81]

You'd better believe that a mechanic can change the geometry of a mating surface, and without even breaking a sweat. The scotch brite disks that "everyone" says are safe, the bristle disks that "everyone" says is safe... They take metal, they change geometry, but they're fast and easy, and laying a straight edge across a clearly no longer flat surface.... It gets violent reactions, because the tools are fast and easy. Those of us that give a (crap) learned long ago to eyeball a surface. Brake rotors are a good one. Rust on a surface is fine. Just not on the bearing surface. Wheels, rotors, and lots of other car parts don't really want to be cleaned up too much. Rust takes away metal. By the time you've cleaned the rust, the mating/bearing surface then MUST be reduced to match... There IS metal being removed. 95 percent of the abrasive "cleaning" you see a mechanic do, is detrimental.



You are very correct on that. Car's (like most of the world) are not as "perfect" as we'd like to think they are.



Indeed. And there's a perfect example of rust that SHOULDN'T be removed. Not abrasively anyhow. That countersink on the back side of the rim at each stud, that'll rust under there. Removing metal. But the bearing surface where the wheel sits, that's gonna be clean. (Typically. There's ALWAYS an exception when you generalize....). So if you take the rust down to the layer of the clean bearing surface where the wheel sat, you still have rust. When you dig out the rust (which is irrelevent, because it's not a bearing surface), you also lower the actual bearing surface.

So in the end, rust removal/cleanup is not a simple thing. One must (if they give a crap) pay attention and use smarts and judgement. Or on the other hand, a lot of cars have survived a lot of butch repairs.... How perfect must it be?



Runout on a brake rotor has a target of two or three thousandths of an inch. If the rotor ran "perfect", the pads would never retract properly.

When you buy discount or DIY parts store rotors, if you ever check them, you'd be blown away. Most of that is from poor storage/stacking. And just as it was "introduced" into a finished part, if you bolt it up and heat cycle it a bit, it'll relax back to pretty near as good as it ever was. (Which probably was more than the target range). You might see as much as 5 or 6 thousandths of runout that lives a happy life, depending on the rotor and the pads. Of course like a lot of things, there's no hard or fast rule about what's works and what will give issues a little too soon or a lot too soon.

Folks talk about warped rotors all the time. Warps in a rotor DO NOT give problems. Noobdy cares. You'd (within reason) never know there was a problem with the rotor. What folks who say "warped rotor" really mean is that the brake pedal pulsates. That's not a warped rotor, that's a thickness variation what does that. Some of the worst one's I've seen out of the DIY parts stores are, on initial installation, have been at or over 60 thousandths of runout. (They're not ALL that bad...) And while they do settle in and typically run way truer than that, there is ZERO issues with them in the meanwhile.

The pulsation that one might feel, that's from a localized spot. It can be poor inconsistant metallurgy that wears unevenly, but tyically not. Most typically that is what I'd call a "fold" more so than a warp. The uneven lug nut tightening, overtightening even if it is equal, spirited driving that keeps the brakes plenty hot, so the rotor takes on some shape as it grows. Almost a single point "bend" in the rotor surface. As that makes a very local deformation, it is not a flat surface, the caliper can't follow it because it's no longer planar as a flat/parallel rotor with normal runout would bel, and that very local high spot will wear down even in the non-braking driving time. It makes a thin spot. This changes the travel needed to apply the braking force, and this is where pulsatations come from. Thickness variation pushing the pistons back in right after the "thin" spot. That particular measurements is in tenths. If you've got a thousandth of thickness variation, you can feel it. You may not care at that point depending on your own maintenance plans, your own sense of adventure, or your own sense of humor, but by then it's real.

Somebody (Sorry, forgot who) mentioned GM and their on the car brake lathes. I was with a GM dealer when the first of those got delivered.... They told us ALL DAY LONG about ALL the things that us techs were doing wrong, all the rust damage bollixing things up, and just every possible excuse in the book. We had two options as we were "introduced" to the lathe. We could use the lathe, or continue using the shims. Wedges essentially, that were installed between the hub and the rotor to true up the rotor. Here's the thing. On a Brand new hub (Their wheel bearings were not reliable at that time), on a BRAND NEW hub, with a BRAND NEW rotor, you could use their method, and true stufff up.... BRAND NEW. NOT RUSTED. NEVER TIGHTENED. NEVER TORQUED. Required shims and/or cutting the rotor to get stuff in spec. Here's the coolest part of that. If some tech wanted to pick a fight with them, one could take the new rotor, on the new hub, UNCORRECTED, and clock it five ways (or however many lug nuts it has). The runout that they were trying to fix, it followed the HUB, not the rotor. Repeatedly. Car after car. GM rotors sucked, at least back then anyhow. We solved the problem that GM was trying to solve. We started offering Centric Rotors as a more long lived option than the originals, some of which were eating pads in 8000 milles. Or we'd cut or shim the GM ones, Tech's option at that time. No pulsations out of the whole thing though. Just unacceptable pad wear.

Awesome explanation.