brake rotor turning - PeachParts Mercedes-Benz Forum

whitcre · #1 12-01-2003, 10:47 PM

the brakes on my 2000 e320 (21000 miles) vibrate when i am stopping. it feels like rotors need to be turned or the anti lock feature is kicking in. can the rotors be turned?

#2 12-02-2003, 12:01 AM

Generally, MB doesn't recommend turning rotors. It's usually justs as easy and cheap to replace MB rotors if they are warped or worn below minimum thickness.

Sometimes rotors can be warped by over tightening the wheel mounting bolts. That's why I always torque the bolts after someone has used an impact wrench to mount wheels on my car.

It's less likely with a Mercedes than some other brands but I suppose the rotors could be warped. A Cadillac dealer did it to my brother-in-law's DeVille.

Mike Murrell · #3 12-02-2003, 10:33 AM

Regarding the turning of MB rotors....

Look at what MBDoc had to say in this thread:

http://www.peachparts.com/shopforum/showthread.php?threadid=12589

I went thru about 4 bad sets of OEM rotors a few yrs. ago. One right after another. Got sick of spending my weekends replacing rotors. Had the last new set I put on turned. Problem solved.

#4 12-02-2003, 11:34 AM

Mercedes is saying to resurface rotors for warranty work. You have to question their motives. I assume it's cheaper for Mercedes to have the dealer resurface than to change the rotors. I don't think that's true for a D.I.Y.'er.

I've always bought my rotors from a dealer and have never had a problem. With the old blue anti-rust coating, it used to take more time taking that off than it did changing the rotors. That's no longer a problem.

In any case, I'll bow to the experts.

euronatura · #5 12-02-2003, 11:56 AM

Sounds to me like the rotors are warped. But when you say that the brakes vibrate, do you mean that the brake pedal vibrates? If its the pedal - its the rotors. If it is the steering wheel most likely need alignment.

Iggy

RKreutzer · #6 12-02-2003, 06:48 PM

When I bought my '87 420 SEL, I also felt a pulse in the brake pedal. Not knowing much about MB's and before I found this site, I went to a tire dealer and had them look at the rotors. They did measure the runout of the rotors and the right rear was the worse. So they turned the rear rotors for I think $ 15 per rotor.
A test drive showed it was much better, but still a bit of pulse left.
So I had him do the fronts, too.

Brakes were ok for over a year or about 5,000 miles.

Then I measured the rotor thickness and found the front to be slightly below the service level (25.8 mm vs 26 mm), the rears are still way above service level.
So I got new front rotors and brake pads for front and rear and put them on (Thank you, thomaspin, for your website).

My experience would suggest that it can work, but you should ask for the thickness of the rotors before and after the turning and know the service level and the minimum level for your car.

Overall, the front rotors lasted for 50,000 miles

psfred · #7 12-02-2003, 07:01 PM

Normally, a Benz rotor is thin enough (to save weight and improve handling) that turning it will take it below minimum thickness. Certainly attempting to turn a rotor after a couple of sets of brake pads will do it in. They are expected to be replaced every third pad set or so.

However, it is possible to get rotors with some irregular surface hardening from the factory grinding, giving you pulsating brakes as soon as they start to wear. This isn't limited to Benz by any means -- I've heard the same problem on a number of American cars. Don't know about Japanese. The cure in this case is a VERY light turning, just enough to remove a couple thousandths of an inch of material to remove the surface hard spots. Those spots wear more slowly than the rest of the rotor, leaving them high. NOT warped, just too much runout on one side.

If you have a distinct lip at the outer edge of the rotor, replace it. Rears are something like $20 on Fastlane, fronts are about $60 -- pretty cheap, and not that much more than it costs to get them turned.

Peter

JamesStein · #8 02-22-2005, 04:11 PM

Man I love digging up old threads but from what I've read this one is the most appropriate for my question..

My wifes 300D has sat stationary for around 2 months while I was waiting to have time to do the timing chain. I live in Florida, its wet.. things rust. Heck the CV axle I put in a while back is covered in it. Rust that is.

A couple months ago I put new pads, calipers, rotors and hoses all around. Anyhow the new rotors are now pretty rusty, I can actually see an outline of the pad. I'm assuming this rust is causing brake pulsation. Will this wear off on its own? As a corolary if I let it wear off on its own with the pulsating ever really stop?
Is it safe to get them resurfaced to get rid of the surface irregularities that are there now?

I don't mind replacing them if thats the wisest choice. I just hate doing it when I just did it.

Ferdman · #9 02-22-2005, 04:43 PM

James, the rust should wear off on its own; however, if it were my car and the rust appeared fairly heavy I'd use some fine steel wool to remove as much of the rust film as possible rather than foul up the new brake pads. If the rust is causing the brakes to pulse it must be quite heavy. Rusted rotors are common on pre-owned cars on the MB dealers' lots. You may even notice a light rust film on the rotors after washing your car.

spartonboat1 · #10 02-22-2005, 04:52 PM

Disk brake rotors are mfg'd to some of the tightest specs on any brand of vehicle, esp. MB, down to 10 thousandths of an inch, both in thickness, as well as whether the braking surface is square to the centerline of the bearing.

Regarding surface machining of rotors, American rotors are machined from cast iron castings in three steps: roughing (1st 20-50 thouths), finishing (down to 3-10 thsds) and a final grinding by a 3-4' dia very precise grinding wheel that brings the specs to sub-one thousandths.

When you get your rotors "turned", whether at MB or American dealer or independent, you are barely getting the precision of the second step, since field level machines and setup are by nature imprecise. More importantly you are entirely skipping the final very precise grinding operation that comes in at 10's of thosandths of an inch precision.

The pulsation is usually due to a condition that occurs as rotors wear, usually termed "tri-lobe", because the bad wear occurs as three high and low points. On a 360 degree measurement graphic, it appears as three lumps or lobes. This condition can actually occur at the factory during machining, if hard castings are machined or if the machining tooling wears in a number of ways.

However, what separates the MB or BMW from "normal" rotors, is their much higher hardness, such as in Rockwell Hardness. Soft cast iron might have RH of 80-90, where the MB, BMW (Lexus, Jag, new Cads?) have 110-120 Rockwell Hardness, resulting in a rotor much more resistant to wear and able to absorb more abuse, while returning high braking levels.

MB rotors are designed for running at 155mph/300kph and so the factory specs are going to disallow field level turning to a much poorer quality of configuration.

But, I always go by the feel of the brakes- pulsing pedal? Then they are warped/worn. Get them turned asap, as long as not below min thickness.

Do not use non-OEM pads that are actually a little soft compared to a Midas or other after market. They are designed to fit the system, match the rotor hardness, and wear-in smoothly.

My brother in law simply installs brand new pads yearly, regardless of condition, which is as close as you will get to the factory grinding operation, while you are running and using the brakes.