disk rotor choice

I've seen the same, and I dont know the real answer, BUT if you compare this with brake pads price and quality, you will definitely see a quality difference with the more expensive pads.

And I would not put cheap rotors on my car, regardless.

Out of some sense of statistics, why did you find it necessary to replace the rotors?

Well, they're warped ... thump, thump, thump when I brake. As to why, and not to be blaming any one particular person, but it seems to have started after someone I'm married to went down a long hill. Which has happened in the past, depending on who's doing the remembering.

We bought the car used. Considering who we got it from, my guess is, if there is such a thing as cheap aftermarket rotors, that's probably what they had on there.

Take an old rotor to the store if possible. Sometimes they put unexpected rotors on cars at the factory. Had heavy duty rotors on my '88 Celebrity sedan. The auto store book said they should only be on wagons. Had to make a second trip. Think the only ones they had were 18 bucks each. Made in China, but they looked the same as the originals. Didn't have them cut true, as some people recommend. No problems with them. Unless you're a metallurgist and can test them, just compare them as best you can and pick what you think is best. The counterman can put both in front of you. Making sure the tires aren't over-torqued is one way to prevent warping. But I think some rotors will warp from overheating. My Celebrity rotors were thumping, but they were also deeply grooved. If they were warped that's probably what caused it. Had about 150k miles on them.

--Vic

Thanks, George. Warped rotors is a major cause of complaints nowadays. There is some disagreement about exactly why this happens, but I think that we would all agree that just using the brakes going down a long hill should not cause this.

If this link still works, there is a good bit of useful information on it:

In addition, never let any jerkwagon tire changer use an impact wrench to put your wheels on your car. Torque wrench all the way.

I think overheating will warp them too, especially if the torqueing is not properly done.

On things I consider critical, including brakes and tires, I dont bet my life on a couple of bucks. There are areas where one can go with the cheapo parts maybe.

You should be able to tell by looking at them. Have them get both kinds for you then check them when they come in. Some really cheap ones are thinner, don't have as many cooling fins, etc. Everything is made in China, but some are better than others.

I hear what you are saying, but when metallurgy is involved, I dont believe you can tell by looking. Certain good machine work looks better, usually, than crappy machine work. Metallurgy??? A different thing.

Go the the store and ask to see both rotors. Buy the one with more metal. In my case, I'd probably get the cheap rotors. My old Cavalier was a great car and served me well. For me, only the cheapest parts would go into that baby. :-)

I read a startling article that said that the pulsating rotors are actually caused by transference of pad material onto the surface of the rotor at a molecular level which causes uneven braking. Beats the heck out of me if this is true but if it is, then breaking-in the pads could be critical. My understanding of the article is that applying the brakes after heating them up will cause a visible imprint of the pad onto the rotor. Weird stuff!

I dont believe that. When I have chocked up the rotors in the brake lathe, there is definitely a significant warp. I have never seen much pad material transfer to the rotor surface, but then again I buy good pads.

There are several factors at work here.

My guess is that most car guys won't believe it.

Good point. Hopefully good machining and more metal is indicative of better metal. It would be a shame to use poor metal with good machining. There was an article on the subject in Pop. Mech. about a year ago. I will see if I can locate it.

Im sure you are right.

you may not have much choice with even "genuine" g.m. product since so much of their stuff is made in china, but here's the skinny on cheapo brake disks:

1. bad castings can crack. brakes go through significant thermal cycling during use. that means a lot of internal stress and if the casting molds are not made right, there can be stress risers between the disk faces like this:

vs.

it's not common because, frankly, not many people drive that hard, but with the unradiused casting, your worst case scenario could be disk separation. more commonly, you just get cracks migrate through to the braking face.

[cheapo disks are made unradiused because the molds are easier, and thus cheaper, to construct.]

1. metallurgy can be very variable. paradoxically, you're looking for a casting with less metal, not more as a visual on "good". less metal means the base is a stronger alloy and thus you don't need to make up with a bulk of cheap material to compensate for flaws and achieve minimum strength.

[the alloy is gray cast iron. it's called gray because under the microscope, you'll see it's full of graphite flakes. once machined and to the naked eye, some of this graphite smears on the surface and creates the gray appearance. if it's "too gray", carbon content is too high and you can get increased propensity to crack and lower friction. but it's also cheaper. if it's too white, carbon content is lower and it doesn't conduct heat as well. a good disk requires a fine balance between the two.]

1. casting quality can be revealed in machining, so look out for areas where there are surface holes [porosity], particularly tiny ones, and also uneven color.

2. examine the outside circumference of the disk for where it may have been machined for balance. balanced disks are always higher quality.

regarding brake shuddering during use, while brake disks /can/ warp subsequent to heat treatment and machining, it's not actually very common because the temperatures needed to cause it are very high and few disks are operated at that level. ["apparent" warping visible on disk lathes is more commonly a setting problem than actual material distortion.]

more likely, it's an issue with the wheel/hub interface that can be cured by scraping off surface rust from the hub wheel interface [and the bearing carrier/disk interface], applying a little antiseize, and torquing the lug nuts to spec in the correct sequence, and in a two or more stage process. this is particularly relevant on modern lightweight hubs where the hub can be elastically distorted by uneven torque because it's so thin. the procedure above allows the hub to set evenly minimizing elastic distortion, and thus run-out and will cure even "badly warped" disks on problem vehicles like hondas without machining or replacement.

this is also more of an issue on "modern" single piston calipers since the mass of the moving parts is different on each side of the disk. you can't even out the mass without using opposing piston calipers, but you can mitigate the issue by maintaining the caliper's sliders so they can float properly.

The foundry is not the party that designs the rotor and decides whether to put a radius or not. The foundry would prefer a radius but if the customer says no radius then that is the way its done. Usually rotor internal ribs do have a radius, but I don't see any evidence in your picture that the no radius fins on that rotor are causing a problem. In the rust belt where salt is used in the winter a big problem with rotors is that rust builds up in the space between the internal ribs and that rust blocks the airflow and cooling capability of the rotors.

Actually no. It is more difficult and expensive to eliminate the radius. You just make stuff up to support your fantasies.

You don't see many gray iron rotors nowadays. Back a few decades ago there were some gray iron rotors in use.

Anyway, your analysis of gray iron is a bunch of made up hooey. but since it irrelevant to modern rotors we won't go there.

Sure but with those sort of visible defects they will likely have been already scrapped before they get to parts store.

balancing would certainly cost more. Achieving balance thru good QA processes also costs money.

Rotor disks warp and wear unevenly and when they do that causes additional warping and wear due to uneven loads and heat. So once it starts it usually gets worse.

That is a fairly good description of one mechanism that will cause warped disks.

I don't know why you think single piston calipers are more modern, but you are correct that sticking calipers can quickly cause a tiny amount of warp to turn into a greater amount of uneven wear and warp.

-jim

that's because i haven't posted a picture of it!

which has nothing to do with casting process.

nope. to get the internal vents, the sand molds are made in two parts. to get radii on both disk halves, you have to make both molds with complex structure. if you don't radius, you only need one complex structure, and you eliminate any need to align. it's a significant cost saving.

no, i just have some experience with iron casting.

almost /all/ automotive rotors are gray iron. section, polish and put under a microscope. you can see it for yourself.

yes you'd think, but you'd be surprised.

most disk brakes do not get hot enough to physically permanently warp. to warp them, you need either out of plane loading while at a high temperature, or a residual stress left over from casting which is then raised to a high temperature. like cherry red. since almost all brake disks are adequately heat treated after casting in order for them to be machinable, the most common cause of brake problems is as described.

You are about 100 years behind the times. The process you are describing is how they used to manually mold sand cores. Even in China the foundries are automated. Rotors made in China are not machined manually either.

-jim

no, i'm describing current process for the design and deployment of the actual mold form. and the casting strategy.

whether a form is made by machine or by hand is a red herring. look up "lost wax casting".

red herring - machining is not casting.

Remembering a few decades ago, a lot of calipers had dual pistons and some even had four (Corvette and perhaps a few others). I would guess that this added complexity, weight, and expense, and flew in the face of the rule of thumb that the more working parts you have, the more failures you will have.

I agree that one should clean the hub and the mounting face of the disc to be sure that there is no runout due to dirt or rust. If you really wanted to go down that road, you can try the rotor disc at all five possible mounting positions and test the runout with a dial indicator. I've never done this, but may take the time, if I ever have to work on my own brakes again.

I've seen the ventilation channels get pretty dirty or rusty, but I never took the time to clean them. Have you? Agreed that if you are going to use cheap rotors, it might be easier just to buy new.

It is beginning to look like most brake mechanics these days are parts changers, and many have no idea nor really care what makes the rotors wobble. Not that parts changing is bad, mind you. It can sometimes be the cheapest, quickest, and surest cure.

Another thing you need to consider when purchasing cheap rotors (vented type) is the layout of the cooling fins. Usually cheap rotors have fewer fins and are layed out differently, and they also usually have thinner metal making them non turnable, but they are above the minimum thickness. Same with the solid rotors they are usually just thinner.

A lot of shops in my area usually purchase the cheap rotors, but put the good pads on. They usually don't have any trouble with them with this combination.