The myth of warped brake rotors? - PeachParts Mercedes-Benz Forum

Eric Eliel · #1 04-13-2004, 04:42 PM

Below is a link to a white paper published by Stop Tech, a high performance and racing brake parts supplier with loads of racing experience. I found the article very informative and wonder about YOUR experience with "warped rotors". Read carefully, the cavaet is:

"With one qualifier, presuming that the hub and wheel flange are flat and in good condition and that the wheel bolts or hat mounting hardware is in good condition, installed correctly and tightened uniformly and in the correct order to the recommended torque specification"

http://www.stoptech.com/whitepapers/warped_rotors_myth.htm

Other white papers on the subject of brakes are on this informative site.

dieseldiehard · #2 04-13-2004, 05:06 PM

Interesting paper, I have seen that pop up here on the 'forum b4.
I have had the bad experience of discs that were installed improperly. Darned mechanics that use impact wrenches on the max setting.
This is a good time for my suggestion for Ceramic brake pads. I am running a set of Akebono ACT ceramics on my '87 300D turbo. They did not require any break in - they are smooth - they make no noise - they show no fading under fairly heavy downhill braking - and they leave very little brake dust on my flat face wheels. I can't say enough great things about these pads.
http://www.akebonobrakes.com/aftermarket/proact/ceramic_disc_pads/index.html
I wonder if anyone else has "discovered" these?
DDH

psfred · #3 04-13-2004, 07:47 PM

Excessive runout isn't caused just by warped disks. Bad wheel bearings, or incorrectly installed races can cause the hub to run funny, badly mounted rotor, badly mounted wheel, etc.

Didn't read the article, but another cause of "warped rotors" is hard spots on the friction surface. This will cause thickness variations and intermittant "sticky" spots in rotation, causing thundering brakes, steering wheel vibration, etc. Cured by re-surfacing the rotor. Usually on requires a couple thousandths of an inch of material be removed, the effect seems to be very shallow. Never encountered this on a Benz, but GM cars in the 90's were absolutely plagued by it.

#4 04-13-2004, 08:03 PM

I have no scientific proof of what I am about to describe, only suspicion.

I never had trouble with "warped" rotors until I started using spray on tire foam. Since all the MB's I've had since that time have had alloy wheels, my theory is this; I think that stuff gets on the rotor and either causes what is described in the paper, or leaves a film of its own in spots.

I recently replaced rotors on my 300E. I dug in my scrap lumber and found a piece of OSB big enough to cut a 15" circle out of. I put a few finger holes in the middle and I use this to keep the foam from drifting onto the rotors through the wheel holes.

Time will tell, but I really suspect that this has been my problem.

BTW, while changing my front rotors, I stripped the bearings out of the hub and washed it in my parts washer, dried it and then spent some time with the hub face against the steel brush on my bench grinder. I made that sucker squeaky, shiny clean on the entire face.

I then washed the hub again, packed the bearings and put everything back together. For the first time in a long time, my 300E has smooth, great brakes.

Have a great day,

william rogers · #5 04-13-2004, 08:03 PM

psfred, Your explanation makes sense. I always questioned how a rotor could warp
but a machined casting which MB rotors are sure could have hard spots as any one knows that has worked with cast steel or cast iron............

William Rogers.........

TonyFromWestOz · #6 04-14-2004, 08:48 AM

I just had the lower ball joint replaced on the LHS front on my '84 300D.
I had been experiencing a brake "surging / pulsing" feel thru the pedal. It appears to have gone since the replacement, although I do not have many miles in it since the repair.
Could a badly worn ball joint cause this??

GregS · #7 04-14-2004, 12:35 PM

Larry,

When you install the bearings, how do you know how tight to tighten the hub-net. I know you can measure bearing play with a micrometer, but I don't own one and am wondering if there is a simpler way to reassemble. On my old Jetta you were supposed to tighten just to the point you couldn't feel play in the brake drum. Would the same practice apply to the front brake rotor on my w123? Somehow I doubt mechanics are pulling out micrometers when they reaassemble stuff like this.

GregS
'84 300D
'90 300CE

#8 04-14-2004, 01:48 PM

I have packed many, many wheel bearings so I do it by feel and would not be able to describe with words, but they always come out well, at least as far back as I remember. I probably screwed up a few when I was a kid.

If you don't have practice, it would probably be a good idea to borrow a dial indicator.

I will say this though. After repacking wheel bearings it is important to initially cinch down that nut really hard then back it off and retighten. This is to ensure that all the grease is squeezed out so you can feel or measure correctly.

Also, it is also a good idea to give the wheel a shake with both hands pulling/pushing in opposite directions after bolting it back on. If you feel any slack, it's loose. This is taking to account that the suspension is tight. If it is loose, inspect the suspension so that you know for sure it's the bearings.

Best of luck,

samiam4 · #9 04-14-2004, 10:25 PM

I haven't seen may warped rotors... but I agree with the GM cars in the 90's.

From engineering textbooks, it's common knowledge that green castings will (can) warp when they are heated the first few times. When rotors are cast, they cool. During the cooling process, there are residual stresses trapped until the rotor is brought back up to some temperature. So hot, the stresses want to relieve, which will change the shape slightly-bingo warping.

I've seen this in some replacement Jag rotors. Drove the cars with new rotors very hard... they warped, trued them on the brake lathe... reinstalled and everything was fine.

Chemical doesn't make any sense to me.
Bearing could cause heating by rubbing of the pads unevenly- I can see that. Seems like a basic check when inspecting brakes.

Michael

psfred · #10 04-14-2004, 10:54 PM

Micheal:

A bit of grit under the bearing race can make the rotor "orbit" rather than simply spin, giving it excessive runout. Ditto for grit between the rotor and hub. In both cases, it's not the rotor that's bad, it's just not in plane.

Rotors CAN warp -- the usual cause if a very hard, high speed stop followed by holding the brakes down. Rotor gets REAL hot, and stays hot under the pads while the rest of it cools off in the air. Differential heat cause the casting to warp. Worse on thin rotors, and some designs or suppliers will be worse than others. Non-uniform material makes this even worse, etc.

Anything that modifies the friction between the pads and rotor will do strange things to disk brakes 9or drums, for that matter). I'd NEVER use silicone spray or foam anywhere near my brakes -- the blasted stuff simply won't leave, and a dry lubricated spot on the rotor is certain to cause uneven wear and the good old "thumpa-thumpa-thumpa" thing.

Other things that will cause brake "pulstations" are loose calipers, worn pins and bushins on floating calipers (also causes noise and iffy brakes), stuck pads, and loos suspension parts. Sometimes hard to tell if the whole car is shaking or just hte steering wheel or just the brake pedal. Loose tie rods will sometimes allow the wheel to vibrate at some frequency, but usually braking stabliizes that.

Tires with lumps on them or loose belts (or just badly out of balance or out of round) will also give a sensation of warped rotors.

The only car I know of offhand in recent years to suffer from serious rotor warping problems was the "old" Ford Tarus/Sable. Rotors were too thin, warped pretty fast in service. Had crappy strut bearings too -- a friend of mine has a Sable he bought new in 94, has always groaned while turning and usually has minor vibration in the brakes.

Peter

TimFreeh · #11 04-15-2004, 11:04 AM

A couple of months ago I posted a question about shuddering brakes on my W124 sedan, the problem ended up being rear rotors but after I removed them I did check for runout and they were within factory specified limits. I was a bit puzzled by this until Larry Bible suggested that sometimes wheel cleaners can somehow cause friction differences on the surface that will mirror brake shudder during operation.

After reading his post I remembered that the last time I went to buy a new can of wheel wheel cleaner I switched brands from Armor-all quicksilver to a Meguirs (sp?) product. I did this because the McParts store I was at had run out of the armor-all but had plenty of other cleaners.

Well guess what? I detailed my girlfriends 400E a couple of weeks ago using the Meguirs wheel cleaner and now it has a shuddering brake problem.....

I'm not sure of the chemistry involved here but I think it's very coincidental that I've suffered this problem right after a good wheel cleaning with a new wheel cleaner product.

Tim

samiam4 · #12 04-15-2004, 12:35 PM

It's really hard for me to stomach that you are getting enough differential in friction due to wheel cleaner......

But, it's plausable. Yea thin rotors will warp.

Michael

#13 04-15-2004, 02:55 PM

My post was not about wheel cleaner, it was about tire foam. I believe that this is what caused problems for me. I NEVER had "warped rotors" before using that stuff. I have no evidence just suspicion. It does, after all, have an oily element to it, so you would think that it would not be healthy for the rotors and pads proper environment.

I now have a piece of OSB board cut to use as a shield when putting on the tire foam. I first tried using armor all and a rag, but the tire foam is just so fast and convenient.

Have a great day,

rickg · #14 04-15-2004, 03:35 PM

Personal case history of warped rotors(on front end):
We buy a '96 Chrysler Cirrus(new). At about 12,000 miles, the brakes start pulsing. By 15k, the steering wheel will about come right out of your hands if you hit the brakes hard at any speed, say, over 30mph. Have dealer check it out. Rotors and all are in spec they say, won't repair.
Ok. Fine. I'm a machinist, and have some proper equipment to check this out. Rotors show .003" TIR(total indicated runout) on one side, .004" on the other. Doesn't sound like much, but the book gives .002 TIR max allowable(dealer is now proven to be fulla you-know-what)
Take rotors in to a shop, have rotors turned. I make sure the locating surface on the hubs are clean, smooth and flat, as well as the inside of the rotors. Put rotors on, re-indicate. One side is down to .002, the other .0025.
Drive car, same thing. No improvement.
Screw it!

I buy new rotors. Problem goes away! Perfectly smooth brakes.
I now put new rotors on every 20k to 30k miles.
I found out later that Chrysler started putting thicker brake rotors on that model car starting in '98, due to all the customer complaints with front brake problems.
Yes Virginia, brake rotors do warp.

TimFreeh · #15 04-15-2004, 08:30 PM

samiam4

I'm not presenting my experiences as definitive. I have no idea how a wheel cleaner could cause changes to the rotor surface that would vary the resistance as a function of rotation angle, I'm not even remotely sure that the wheel cleaner caused the problem.

All I can say is that I have a used set of 300D rotors from my car that have around .002 of runout that will absolutely rattle your teeth in a high or medium speed stop. A new set of rotors had about .0005 or runout and are as smooth as butter.

When I replace the rotors on the 400E I'll get some more numbers.

Tim