Turning Rotors: a case study...

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With all of the debate about turning rotors I thought I'd do a bit of measuring and documenting while doing a brake job. Here is my results.
The car 1987(?) Honda accord; poor shape, 200,000K no brakes due to a
ruptured line. This soaked the pads and as per MAP standards required new pads. Now disregarding the brake line (and the springs off in the right read drum that he couldn't afford to repair) the car would appear as a prime candidate for a DIY brake job in the front driveway. Both rotors had no major grooves, very small ridge> I could picture this as a easy pad slap.
Now ASE standards have you measure several different angles to determine if a re-cut is needed. We concentrate on the warpage factor. I don't feel like buying a few hundred dollars more of measuring tools to tell me what one pass on a lathe will tell me.
The right front rotor measured at .747 to start I took one pass at .002 on each side Final measurement was .742
The left front rotor measured at .697 on the outside edge and .703 at the inside edge to start I took one pass at .002 on each side. the start of the cut was heavy, I thought I'd have to do a second cut. Often we will see the O/S edge a wee bit thinner than the inside edge, but when I returned to the lath one cut was all that was needed. Final measurement was .696
I forgot to write it down, but discard was in the .660 range.
Most people who do brakes at home won't measure the rotors and it was interesting to see the big difference in specs from the left to the right to start. We thought there would be at least one new rotor needed, but specs are specs.
You can see by the measurements that the lath action took off very little metal, not enough to make a difference on heat dissipation, but the little work guaranteed me that this car (the POS it was) had decent front brakes when he left. This is why I turn rotors every time. In the big picture, more metal is lost in the time between brake jobs by the pads than on a simple clean up on a lath.
--
Stephen W. Hansen
ASE Certified Master Automobile Technician
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Stephen H wrote:

what does measuring disk thickness tell you about how accurately the disk was kept axial with the bearing?
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If this were the only factor in the performance of brakes than I would be concerned. But by just removing the tire and rotor you can effect the bearing to rotor play if a small piece of dirt were to fall between them. In fact, how do you know your "new Honda replacement rotor is true to your old wheel bearing?. We do have a 5000$ on car lathe, that is suppose to be "the most accurate way to turn the brakes" but it broke again. (600$ was the last repair bill) and having a brother-in law that is a machinist, we can see no great advantage to turning it on the car of off, as long as the turn job is true.
Wear of the rotor and warpage (runout) are the two biggest factors of a brake job.
--
Stephen W. Hansen
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Stephen H wrote:

ok, so now let's /cut/ a disk with a piece of dirt under the rotor. now we have a disk that is perfectly planar with respect to the dirt, not the mounting. and that causes brake judder.

well, i've never had a problem with a new disk. i've had repeated problems with skimmed disks. you figure it out. labor and a $5k machine to futz about with a part that can be replaced with a flawless new disk for $50-$60 in about 10 minutes? forget it.

no kidding.
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You may have had problems, and in a perfect world everyone would replace parts with new when worn with high quality stuff, but many want it done cheap.
Anyway my point was to show how little metal comes off during a lathe job.
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Stephen W. Hansen
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Stephen H wrote:

i have two points:
1. it ain't cheap enough to be worth bothering with [unless it's a disk that requires complete disassembly of the hub] AND
2. the results frequently aren't good enough to be worth the trouble. in fact it often /causes/ problems.
i'd much rather live with a slightly scored disk than one that's been machined wrong and is now ruined.
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I don't think this issue can be resolved into a single best course of action. In another recent thread the subject of professional standards vs DIY standards came up, and I think that applies here. A DIYer who routinely leaves untouched disks that look good and have performed well is frugal and smart. A shop that routinely leaves untouched any disk is being sloppy.
Jim, I gather you and I are on the same page on the DIY way of doing it. If the disks are serviceable as-is there is no point in doing anything to them except a quick wash with brake cleaner when we get our mitts off them. If they need more, replacement is the way to go. If we had free access to a lathe we might do the same as Stephen does - clean the surface up. Or maybe not.
Mike
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Well said.
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Stephen W. Hansen
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Michael Pardee wrote:

i used to work in a shop and had access - that's how i know these things can be difficult to center, giving inconsistent results.
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Ahhh, New equipment.... Some are sweet!
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Stephen W. Hansen
ASE Certified Master Automobile Technician
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"jim>> Anyway my point was to show how little metal comes off during a lathe job.

My point is different; We do it for free with a brake job It takes about 10 minutes to do total
We have never had an issue with machining them wrong. Its to simple. But and idiot could play with the settings on the lath and screw it up... Perhaps the people who turned your rotors in the past were idiots?
I agree with pressed on rotors... What a wonderful idea someone had...
Take care,
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Stephen W. Hansen
ASE Certified Master Automobile Technician
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Personally I have had very poor results in dealing with warped rotors through turning them. On my '03 Accord the rotors warped enough to cause brake shudder. Turning them helped for about 50 miles, when they warped again. New Brembo rotors cured the problem completely. 20,000 miles later the brakes are still working perfectly.
John
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warped rotors

enough to

miles, when they

completely. 20,000

Where did you buy these? How much did they cost? They're not OEM, right?
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Elle wrote:

summitsportcompact.com has great prices on brembo disks. quality is very good.
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Elle wrote:

One of the online parts sources, though I forget which one. It was probably one of: rockauto.com, alleurasianautoparts.com or discountautoparts.com.
Personally I trust professional level aftermarket brands, but stay away from the no name and "value" brands of auto parts.
No need to go with OEM for these. I hated the rotors which came on the car, so why give Honda more of my money?
John
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one. It was

or
but stay away

which came on the

Sounds good; appreciate the info.
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John Horner wrote:

my experience exactly. thank you.

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A rotor that is "warped", not simply scored, is a very poor candidate for resurfacing. The metal's properties have been changed by the level of heat it has been exposed to, causing a permanent change in its structure. After machining, once heat is applied, it will revert to its prior condition.
I went through this issue with my wife's Malibu, arguing with the Chevrolet service manager every step of the way. They resurfaced the rotors 3 times to cure pedal shudder, but it never worked because the rotors were warped. In this case. it was a design flaw by GM - they made them too thin in the first place, and paired them up with lousy pads. When I finally gave up dealing with them and did the job myself, the rotors were ~0.003" thicker than the bare minimum, suggesting that they were able to shave off a minimal amount of metal each time they turned them. But so what? The rotors were junk the first time they warped.
OTOH, a rotor that is merely scored can be effectively resurfaced and returned to service, providing that not too much metal is removed. I've done this on cars when the pad wore too far and "kissed" ther rotor. A competent technician can do this.
Doug
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doug wrote:

sorry - unless this disk has been operated well into red heat, you're not going to get much change in microstructure. you can however have problems with a bad disk if it was not heat treated properly after casting. when operated hot, /that/ disk will distort. more likely is that there are simply different cooling rates on different parts of the disk leading to local temporary distortion. that shouldn't happen on a disk of sufficient thickness and whose internal vanes have not lost too much material through rust.
my experience is that the disk cutting process is far from perfect - those cutting machines see heavy usage, and you can mount the same disk up a dozen times and get 13 different centers.

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Your opinion and a dollar will get you a cup of coffee. That's all it's worth . Repeated heating and cooling cycles will definitely change the distribution of carbon atoms in an iron matrix - and it doesn't have to be "red" hot. The temperatures created by the crappy pads used by GM did the job in less than 15,000 miles.

If that's your experience, that's too bad. You need to find a better technician, or learn to do it yourself.
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