Root cause of warped rotors? - PeachParts Mercedes-Benz Forum

bobs · #1 12-29-2009, 09:01 AM

So, for the second time in 5000 miles, my '97 S420 has warped its right front rotor. I know that the two most likely causes would be a sticking caliper or a collapsed rubber brake line, so my question is how do you make the diagnosis as to which one it is?

I know I already have to spring for another rotor but I'd rather not have to buy a caliper too if I can at all help it.

Thanks,
Bob.

mbdoc · #2 12-29-2009, 09:10 AM

Excessive heat is the root cause of brake disc warpage.

ON cars with traction systems like yours...mis-matches tires can cause that type of problem. Even a 1/4" difference in circumference can cause that.

Have the disc re-trued at machine shop.

truckinik · #3 12-29-2009, 09:38 AM

Well, if I were you I'd just purchase another new rotor.
You asked how to diagnose bad caliper or rubber line, here's how.

Remove rubber line from the caliper end. Compress caliper. If the caliper fails to compress, than that's your problem. If the caliper compresses without a problem, than your brake line is your problem. You won't be able to tell anything, if you don't remove the line off of the caliper end first.

lkchris · #4 12-29-2009, 11:35 AM

Make sure mouting surface on hub is clean and flat.

Use high quality (from dealer) rotors.

Go to dealer or qualified independent and tell them your problem and ask them to fix it and guarantee it.

long-gone · #5 12-29-2009, 03:47 PM

Another cause of warped rotors is a failure to torque all the lug nuts equally and to spec (usually about 90-110 ftlbs). And yes, do make sure the mating surfaces are clean.

truckinik · #6 12-29-2009, 06:12 PM

And for gods sakes, please stop riding the brakes...

dhjenkins · #7 12-29-2009, 06:47 PM

I've seen more than my fair share of rotors that were bad right out of the box. Make sure the shop tests run-out on the new/machined rotor.

Of course, crappy rotors will warp quick as well, due to the inferior metallurgy.

bobs · #8 12-30-2009, 04:39 AM

Thanks for all the advice, guys. FYI, the rotor was an M-B OE part that I installed. The other three rotors I installed at the same time are still working fine so I can't really blame the mechanic.

I called the shop I got the parts from and they told me that the rotor was still under warranty. They told me to bring the car by and they will diagnose the problem. If it is a bad rotor, they will replace it for me free (incl. labor!). If it's something else, I'll owe them for an hour of shop time plus any parts they replace. That doesn't sound like too bad a deal to me, so off to the shop I go!

POS · #9 12-30-2009, 08:34 AM

Using hard ceramic pads on soft rotors will warp them too.

Kestas · #10 12-30-2009, 01:55 PM

Explain to me how inferior metallurgy can cause rotors to warp? What can a foundry do to cause rotors to warp? I just don't see it.

dhjenkins · #11 12-30-2009, 03:09 PM

Quote:

Originally Posted by Kestas

Explain to me how inferior metallurgy can cause rotors to warp? What can a foundry do to cause rotors to warp? I just don't see it.

Inferior metal will have a lower melting/softening point than higher grade metal. If it gets "too hot" while braking, then it will lose the surface hardness it gained through the hardening process, and degradation of the structure will continue at an even faster rate.

Metallurgy, along with thickness & design determine the maximum braking temperature a rotor can undergo before failure.

Kestas · #12 12-30-2009, 03:34 PM

Not so. Brake rotors are not hardened. They are usually made of ferritic gray iron.

The melting point of a material is only important for the foundry, not for rotor performance. Pure iron has a much higher melting point than carbon steel or cast iron, yet it is soft and has little strength, so melting point has nothing to do with metal performance.

It's rather hard to screw up cast iron metallurgy as it relates to rotors. The chemistry control for gray iron industry is not tightly controlled, nor should it be tightly controlled. Gray iron is produced to specified grades of minimum strength and ductility limits. Rotors are the least demanding of gray iron material properties. The only stringent requirement I remember is that max carbon flake size should be 0.01" MIN so that brake rotors won't be noisy. Most applications have a MAX flake size requirement for strength. It's usually poor foundry practices that result in a bad product, i.e., porosity, improper shakeout, and bad sand control. I've worked in the foundry business and qualified product for the automotive industry.

long-gone · #13 12-30-2009, 05:38 PM

Not meaning to disparage or step on your obvious experience,

Quote:

Originally Posted by Kestas

It's rather hard to screw up cast iron metallurgy as it relates to rotors.......It's usually poor foundry practices that result in a bad product, i.e., porosity, improper shakeout, and bad sand control.

So, it's hard to screw it up, or not??

Quote:

Originally Posted by Kestas

The only stringent requirement I remember is that max carbon flake size should be 0.01" MIN so that brake rotors won't be noisy. Most applications have a MAX flake size requirement for strength.

I've always believed that noisy brakes are usually caused by the metal backing on the brake pad chattering against the metal brake piston under pressure and friction, not by resonant rotors. Perhaps if the pads and rotors get very glazed, that might cause some squealing from the pad/rotor surface.

Regarding what a layman may generally refer to as "inferior metallurgy":
I've not worked in a foundry, so I surely would have to bow to whatever experience you have there, but regardless of standards for certain grades of iron, I would still have to think that the quality of the overall manufacturing process, including any alloys, fluxes, retention of impurities, annealing/tempering if any, stringent quality controls like batch testing and control of the mentioned flake size/structure and stress, and the quality of the final machining of the finished product would all factor in to some extent for the propensity of any particular manufacturers product to warp under various conditions. While the melting point may not really have an affect, many other things likely do that would vary from maker to maker.
There are most certainly great differences in quality and (quality control) between brands and manufacturers. If an iron rotor was merely an infallibly produced hunk of metal, all the rotors we buy would come from the cheapest sources available.

Kestas · #14 12-30-2009, 07:20 PM

You're correct on many points. But I had to chime in when I read statements like "Inferior metal will have a lower melting/softening point than higher grade metal", "... lose the surface hardness...", and "... degradation of the structure...". I couldn't let this misinformation go unanswered, which I am uniquely qualified to catch.

What bothers me is that, way too often, I read discussions on rotors and warping, and invariably someone will chime in and blame the "metallurgy" of a rotor and/or its chinese origin in the same breath. Metallurgy is a mystery to most people, including many engineers, and it's all too easy to make such a blanket statement blaming the "metallurgy", because it's easy to do and few people are familiar with metallurgy. Nor have any posters done any forensic metallurgical study to develop any experience on this matter. I'm all to familiar with metallurgy. It's no mystery to me, especially rotor material, which is the most forgiving to produce. Today, one poster on another site even blamed rotor corrosion on "bad metallurgy".

If rotors are warped, it is more likely that they aren't true, too thin, overtorqued, or overheated, rather than the "metallurgy" or simply being of chinese origin. The first five answers to the OP were the best.

dhjenkins · #15 12-30-2009, 08:45 PM

So a rotor made from an iron with lots of impurities, a non-homogeneous/incomplete blend will be just as good as a rotor made from a properly created "good" iron?

Yes, if there is a mistake in the metallic make-up of something it is more than likely the foundry's fault, however the result of that screw-up will show up in the molecular structure/composition of the end product. Blaming the foundry vs. blaming the metallurgy is semantics when it comes to the end user.

For example, I have a pasta sauce that is too salty to eat. It is too salty because it has too much salt in it. The fact that I'm the one that put too much salt in it doesn't take anything away from the fact that it has too much salt in it - the end result is that it has too much salt in it. Who put too much salt in it is irrelevant when it comes to the saltiness.

Further more, iron rotors are hardened during the casting process by controlling the rate of cooling. If you cool them too quickly, they'll be harder and more brittle. If you cool them too slowly, they'll be softer.