Car Question - Rotors

[Gary_T]

*Originally posted by Xema *
Our disagreements concerns the consequences of doing a casual job of cleaning the thin preservative coating from new rotors.

Well, as it turns out, I misread your first post and was speaking to the issue of not cleaning the rotors at all, or perhaps giving them the most cursory wipe. Apparently you are speaking of cleaning them, but not necessarily thoroughly.

The 5% figure was in the vein of “how do you know the effectiveness won’t be as little as 5%?” rather than a contention that it would definitely be that low. I perceived (erroneously) a suggestion that one needn’t bother cleaning the rotors, the brakes will still work 97% as well, and was quite fearful they wouldn’t. I envisioned a pad getting significantly contaminated, which is certainly possible if the rotor is not cleaned at all. Knowing how squirrelly brakes can be with contaminated linings, I was, well, alarmed. Hence my alarmist tone.

Since you apparently have done just what you speak of, I will bow to your experience in the matter. I have never installed rotors that were not thoroughly clean and dry.

I will caution, however, that degree of contamination and the specific contaminant can vary with each situation, and it would be unwise to assume that the results will be the same in each case.

[Balle_M]

*Originally posted by Rick *
**Now as far as your statements about instructions go, auto parts are sold for professionals to repair cars. **

False.

I don’t have 35 years auto-repair experience, but I DO have 20+ years behind a parts counter. Just going over this months sales, over 65% of the parts I sold have gone to do-it-yourselfers, not professional mechanics. I would imagine that our store is not alone in this respect.

FWIW I recommend using a brake cleaner followed by soap and water to anyone buying new rotors.

[Rick]

OK Balle_M I was not trying to say that auto parts were not sold to non professionals. Of course they are, but they are designed and packaged (for the most part) to be sold to professionals. This means that they do not come with instructions.
for example take a front wheel bearing. Good old tapered roller bearing.
I can off the top of my head name three different cars that use tapered roller bearings and each have installation instructions that are light years apart. That is my point, proper installation of a part is the responsibility of the installer.

[Rick]

OH, one more quick thing. A good parts man (or woman) is worth their weight in GOLD
When I am fixing cars, a good parts person makes me look good

[Xema]

*Originally posted by Boo Boo Foo *
It’s very common knowledge that pads and rotors have to be contaminant free - at all times. It’s part of the reason why it’s good practice to machine your discs and replace your pads every three years - even if your pads have lots of life left in them. Pads pick up gunk and they diminish in their effectiveness as a result.

I think you’re both disagreeing and agreeing with me here. It’s obviously desirable that pads and rotors be clean, and the cleaner the better. But, as you note, gunk and contamination are a part of normal life. “Contaminant-free” is quite rare.

And yet, despite a tolerably contaminated life, brakes are really very reliable - they can obviously handle normal contamination and still give reasonable performance. Which is certainly not the same thing as saying that clean rotors are a bad idea.

It would be great if all mechanics were as fastidious as Gary T. obviously is. It would be a disaster if we had brake designs that required this level of cleanliness for safe performance.

[Doc_Nickel]

** And yet, despite a tolerably contaminated life, brakes are really very reliable - they can obviously handle normal contamination and still give reasonable performance. Which is certainly not the same thing as saying that clean rotors are a bad idea.
**

-Actually, the usual in-use environmental contamination is water and dirt/dust, or in combination as mud.

Disc brakes, besides being better at actual braking, are better than drum brakes at “self cleaning” due to their very design- the rotor slides through two pads in tight contact with the rotor, and any solid contaminants- dirt and dust- tends to be squeegied off rather well.

Water doesn’t brush off as easily, but the heat of the rotor caused by braking friction (that’s what a brake does, is convert the car’s kinetic energy into heat) boils it off and evaporates it quite easily.

Oil, grease and cosmoline, however, don’t just “brush off” as easily. The main problem here is that they get thin with heat, but typically don’t boil away like water. The thin, hot oil soaks into the porous fiber resin of the brake pads, significantly reducing it’s frictional qualities.

Once so contaminated, it’s difficult, if not at times impossible, to get all the traces of grease and oil out of the shoes, so they must be replaced.

Similarly, if the surface of the rotor or pad has “glazed”- formed a smooth, burnished surface- again, braking power is reduced. Both rotors and pads need to be turned or replaced regularly to reduce glazing, to keep braking power the maximum it can be.

When replacing rotors, I tend to wear latex surgical-type gloves, both to keep my hand oils off the rotor, and to keep the car’s oils (brake fluid, bearing grease) off my hands. Once the rotors are installed but before the calipers are mounted, I’ll liberally scrub the rotor with carb cleaner and a clean rag, then rinse it with brake cleaner, then air-blow it dry.

A rotor and pad simply cannot be too clean, and not taking a few minutes to clean the parts carefully simply means the resulting brake job will not be as good as it can be. It’s like painting a house with an airless sprayer but not masking the windows. Why bother?