Dan540

Dan540
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Joined: Oct 2003
Location: Seal Beach, CA
My Car: 2003 540iA Sport

On turning or not turning rotors: Brake rotors are manfactured on CNC machines that cost 100s of 1000s of dollars. Specs are held to about .003 inch. The process includes straddle cutting the rotor, double disc grinding and balancing. I worked with a company in California that manufactured about 100,000 rotors per month, so I am familiar with the process (I also spent about a year with Brembo). Bottom line: out of the box, a new rotor is ready to install (after washing) and needs no machining, nor should it be machined (many mechanics still machine new rotors before installing).

Turning a rotor on a $3000 Aimco lathe in the back of a dirty machine shop or installation shop is NOT going to produce the same parallelism, finish and accuracy! For sensative brake systems (Ford calls these 'Tuning Fork brake systems') the runout that would be introduced by these inferior and often worn machines will manifest as vibration and/or pedal pulsation. Most lathes have at least .001 runout on the shaft, with no weight. Add the various cones and adapters (which are usually worn) along with the rotor and that runout can easily be .010. I call this the train wreck, with all the various adapters and the rotors cinched together on the lathe.

This is OK for an older vehicle, something less sensative perhaps, but it will drive you crazy on a BMW (or any German car, Volvo, Saab, for that matter). In Europe, brake rotors are sold in pairs, packaged 2 to a box. Not only do they replace the rotors, but they do it in pairs

DZeckhausen
www.zeckhausen.com

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Joined: Mar 2003
Location: Maplewood, NJ
My Car: 2001 540i 6-Speed

Quote:

Originally Posted by Malachi
Holy smokes! Are you saying our E39 OEM rotors can be turned? I have heard it more than once that these rotors cannot be turned.

So I have two options, have them turned or use garnet paper.

edit, 3rd option is to replace the front rotors.

Sure they can be turned! You've got 1.6mm of allowable wear. If you take a couple thousandths of an inch off both sides, you can turn the rotors 16 times before you've removed enough material to have to replace them.

That being said, the BMW rotors are on the thin side when compared to other manufacturers. For example, Mercedes allows 2.4mm of wear before the rotors are replaced.

In general, I am against turning rotors because it's a procedure that is way overdone and often done poorly. Some people are of the mistaken belief that you NEED to turn rotors when swapping pads. This is not the case, unless you have a serious deposition issue (like yours!) or if your rotors have deep grooves in them. When turning rotors for deposition, you want to remove the least possible amount of material, since the deposition is on the order of several ten-thousandths of an inch thick.

A major problem with turning rotors is that some shops are so careless with the maintenence and set-up of their equipment that they can easily introduce several thousandths of runout simply by being careless. Don't take your rotors to Pep Boys where some kid is going to throw them on the brake lathe without checking that they are properly fastened to the fixture and then set the machine to rip off a huge hunk of material and just walk away. There's a reason Pep Boys charges only $5/rotor to do this.

In general, replacement rotors from Brembo, Balo, ATE, or Zimmerman are so inexpensive that it makes more sense to just slap new ones on.

DZeckhausen
www.zeckhausen.com

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Joined: Mar 2003
Location: Maplewood, NJ
My Car: 2001 540i 6-Speed

Quote:

Originally Posted by Malachi
When I apply the brakes I get a pulsating sensation and some vibration through the brake pedal.

Background. Changed from OEM brake pads to Axxis Ultimates about 2,000 miles ago, now have 30K. I embedded them per Dave Z. instructions then 2 days later took the car to a road coarse. So I am wondering if the new brakes pads were too much for the old rotors at the track.

Time for new rotors>

The Axxis Ultimate pads are NOT track pads. What happened is that you were good enough at the track to drive those pads above 1200 degrees F and they began to leave uneven deposits on your rotors.

You don't have warped rotors, you have a thickness variation across the face of them. You should be able to see dark gray, uneven streaks across the friction surfaces. If you were to take them off the car and measure in about 15 - 20 places around the circumference, using a micrometer, you would find thickness differences on the order of 0.0005". That's an order of magnitude less than the runout ("warping") required to notice a pedal puslastion under braking of about 0.006".

There are a few ways you could fix this:

Get some "garnet" paper from a hardware store and attempt to remove the deposition layer from the rotor surface. Sandpaper has aluminum oxide and will react with the iron under heat, ruining the rotors. You MUST use garnet paper.
Take the rotors to a shop and have them turned, only removing as much as required to get the deposition off. We're talking about less than half a thousandth.
Replace the front rotors.

Next time you go to the track, use a set of Hawk HT10 race pads in front. The Axxis Ultimate are fine in the back. Install the HT10 pads, then drive around on the street long enough for the abrasive race pads to chew away the transfer layer left behind by the street pads. Then, bed the HT10 pads in and do your track event. After the event, drive around for a while until the HT10 pads chew away their own transfer layer. Then install and re-bed the street pads. This technique will ensure that you will be vibration free, both during and after your track events.

You don't want to install the track pads at the track, then bed them in on top of the transfer layer from the street pads! Otherwise you will have vibration problems when you get the brakes heated up past 1200 degrees.

The track pads run in an abrasive mode when cold. Once heated, they are more adherent mode and will lay down a transfer layer, just like a street pad. But if you drive around with them on the street, they will quickly chew away that transfer layer and then begin to grind away at your rotors. Leave them on your car for a couple of weeks and you'll be getting new rotors!

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Dave Zeckhausen
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DZeckhausen
www.zeckhausen.com

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Joined: Mar 2003
Location: Maplewood, NJ
My Car: 2001 540i 6-Speed

Quote:

Originally Posted by Malachi
Its my understanding that you don't reface or "turn" these rotor because they are mad of aluminum and not steel. So, no....

They aren't aluminum. They are iron. There is very little metallurgical difference between rotors from BMW, Mercedes, Brembo, Zimmerman, StopTech, etc. The manufacturers play around a little bit with impurities (e.g., phosphorus) to trade off characteristics such as hardness for susceptibility to pad deposition. But all mainstream rotors are made from what is called gray iron.

There have been repeated attempts to use aluminum alloys in rotors to reduce weight, the latest being something called metal matrix rotors by a company called CoolTech. But at high temperatures, even temperatures that could be experienced on the street, the coefficient of friction dropped dramatically, making them unsafe.