warped rotors
 

I have read many accounts, both here and on the Volvo 'brickboard' site, that attribute warped brake rotors to improper tightening of the lug bolts/nuts. Initially I bought this, but the more thought I give to it the harder it is for me to accept. I took a degree in mechanical engineering (ok, a long time ago) but never practiced it, and have worked in construction management since graduation. So granted, I have lots of years away from theory and lots of time spent in another area, but here I go. The wheel's mounting surface is flat against the 'hat' section of the rotor, which is flat against the axle's flange, and the 3 flat surfaces are bolted together in 4 or 5 locations. If the torque on each of these bolts varies, then I assume only the immediate areas of wheel, the 'hat' section, and the flange adjacent to the bolt hole experience different stress. At this point if I'm wrong, then I should stop. However, assuming I'm correct, then how do these extremely localized different stresses contribute to warping the rotor? Also, if the force is insufficient to stress the material beyond it's elastic limit, and I assume this is so since visual inspection shows no deformation, then the material will return to it's original shape when the force is removed and the vibration caused by 'warped rotors' during braking will disappear. However, this does not appear to be the case in my experience with warped rotors.

So, I'm at the end of my reasoning, and looking someone(s) to explain how this works.
regards,
Mark

I do know that European cars tend to have less meaty, thinner rotors that are not turnable (less unsprung weight).

I was impressed at a tire shop in Atlanta where there were no pneumatic wrenches in the tire installation area. The manager told me that they had to buy a Jaguar owner four new rotors ($$$$ at the time) because of warping caused by overtorquing of the lug bolts. This is why I remove and mount my tires myself here.

Thanks, but this is more of the same that perpetuates the 'theory' that overtightening the lugs warps the rotors. I am asking for engineering facts, not the factual story of a business owner deciding to spend a few ($$$$) dollars to satisfy a customer. By the way I am easily convinced by facts, and if my reasoning is faulty then I will welcome the truth.
regards,
Mark

Here's a credible source that argues that there is no such thing as a warped brake rotor:

http://www.stoptech.com/tech_info/wp_warped_brakedisk.shtml

so it's all a lie?

Could it be that on these extremely localized areas of increased stress, lateral forces due to streering, uneven road surfaces, and constant heat expansion and contraction multiply the amount of stress on the given area?

Of course, I don't know what I'm talking about.

also the sept !05 issue of STARTUNED magazine has an article on brakes and warpage, do a google search for STARTUNED, it's a daimler chrysler publication for independent mechanics.

It mentions the localized area of pad on rotor at a stop after hard usage, and the cooling rate differences between that and the remaining surface of the rotor not contacted.

There is also a recommendation for racing drivers ( cannot remember if this is in the startuned article ) that after any track time slow down but don't keep the pads in contact with the rotor for extended periods or when the car is stationary and cooling down.

Should have said Sept 2005 issue of STARTUNED, they have an archive

i dont know the answer to any of this, but i offer the following observation about mercedes benz cars i have owned:

most have had solid front rotors. this has only been a problem on one, my 82 123 wagon, which would warp a brand new rotor in short time (or deposit brake material on it perhaps). i attributed this to the fact that it was heavier than my other 123s and had an automatic tranny. i assumed they were warping from heat.

all of my mb cars with vented rotors have not warped that i can remember.

my wifes pt cruiser has lumpy rotors at the present with 45k miles. it has vented rotors.

if the article posted is true, why cant i see the brake material that is deposited on the rotors? and why can they be turned and trued if they are not warped?

i have decided that i dont think you can warp them by torquing them wrong based on nothing except my thoughts. i could be convinced otherwise, but i am thinking that at present.

tom w

OK...this is a fun discussion!! First, there are three things that happen to discs that people all call 'warped'- one is runout- truely warpage- not true with the flange surface, one is thickness variation- different thicknesses at different parts of the rotor, and parrellism(sp?)- the two faces of the rotor not running true to each other.
Third one is easiest- occurs when the ribs corrode and either A- swell and push the faces away from each other or B-collapse and allow the faces to come closer together. Can also be caused by heat.
Second one is what tom was talking about- actually, its not pad deposits that usually cause this- it's the pad ripping material from the disc. Usually occurs when the car sits for a while- with the metallic materials in pads now, it is a given that they will corrode to the rotor with very little time/moisture involved. You jump in the car, put in drive and RIPPP..... away goes some rotor surface(and pad surface)- this can be driven out of the car, at least on an MB, by braking hard, or using the sanding pads, if it's not too bad.
The first one is the real question- does it happen...yes- seen it, as too why- the best I can figure is this- (assuming things like rust on the hub have been cleaned,etc) when the lug is tightened, the cone of the bolt actually deforms the wheel cone to help hold it- you can't see it with alloys, but watch a steel wheel when tightening, if using an impact, when the cone distorts a little- the wheel is tight (and actually close to torque). So, it stands to reason that if the wheel is distorting, the rotor could too, actually I guess the hub could too, but you would have to be a freak to do that! As to the science of it- the pulling of one side tight while allowing the other side to lift pushs the lifting side against the hub with the center hole- hub holds still (obviously), and the disc's mating surface gets pushed outward at the edge of the hole-driving the rotor face away from the outside of the car- remember, we are talking about .0001ths of an inch here. Then that bolt gets tightened..and TAADAA, we have runout. What mark was saying was right, without the other force (the sideways push of the hub against the center hole).
Having said all that- the biggest problem is sloppy techs (usually young), not being taught right, having the tool guy sell them a new 'Thundergun' (because they are the best, which they are, but dangerous in the wrong hands), and you have a recipe for disaster!! I could probably do wheels up with my gun and not have a problem(and have), but I don't usually, just because it's ingrained to torque them now, and it also sets a real bad example for the young techs.

OK.. after I posted- I went and read the article- you notice he says 'hub clean and true and torqued correctly'

There is some good info there, some left out, and some salesmanship. To say he has never seen a warped disc (what is he defining?), only thickness deviation, suggests he hasn't seen too many northern cars with parrellism (sp)problems or dealt with too many of the $12 disc that come from off shore that came warped out of the box, etc, etc...

Thanks to MBtech21 for a very good explaination.

However, I have a few questions...

MBtech21 wrote:
< Second one is what tom was talking about- actually, its not pad deposits that usually cause this- it's the pad ripping material from the disc. Usually occurs when the car sits for a while- with the metallic materials in pads now, it is a given that they will corrode to the rotor with very little time/moisture involved. You jump in the car, put in drive and RIPPP..... away goes some rotor surface(and pad surface)- this can be driven out of the car, at least on an MB, by braking hard, or using the sanding pads, >

Note: 11,000 miles on 4 new rotors & pads. After my 94 e320 sat for about a month, I discovered an accumulation of material on both sides of both rear rotors from the leading and trailing edges of the pads. I hand sanded with 400 grit until the disk surface feels smooth. After about 500 miles of city driving, a slight thumping still is present.

Is it more likely the rotors or the pads?
Why just the back and not also the front?
About how may miles would it take to drive/brake it smooth?

Until 1964, my father had a 2 bay "Gas Station" - the kind that fixed most anything and have been converted into mini-marts. He taught me; mount the wheel, attach each lug nut in a star pattern with a speed wrench (didn't have an air gun in those days) and then again in the star pattern, use a 1/2" breaker bar to tighten each to a squeek.

Other than using a torque wrench instead of a breaker bar, isn't this still the prescribed method?

Bob

According to the Carroll Smith article sanding must be with garnet paper, not aluminum oxide sandpaper. Most sandpaper is aluminuim oxide these days but garnet paper is available at most good hardware stores - should say so on the back or on the package. Garnet apparently doesn't last as long as the other but apparently still has some prescribed uses, like this one.

in my experience, use will get rid of most problems related to sitting.

i cant remember ever sanding a disc.

tom w

So far it doesn't seem like anyone supports the theory that incorrect torque of the lugs will warp the brake rotors.
regards,
Mark

I don't know how other MBs are, but on a 108/109/111/112, if improper torque caused warping of rotors, the hub would also become warped. They are bolted together quite well.

Also, if improper torque caused rotor warping, wouldn't it also cause the WHEEL ITSELF to warp as well? They're mated flat and tight against each other with a LOT of force. If one warps, the other will have to warp as well.

Tom, you make a good point; If the wheel or hub were distorted by improper torque, then perhaps you would experience the same 'shudder' without application of the brakes. Of course if the runout is minor, then the elasticity of the tire may mask it. Regardless I am yet to be convinced that improper torque will deform a rotor. Anyone?

Mark

The rears take longer to 'fix' as they don't do much braking.. hence why the rear discs on MBs corrode first.
As far as the sandpaper goes- MB has made special tools for years that are basically brake pads with sandpaper on them to 'surface' rotors- far as I can tell, it is regular sandpaper like you would get at Home Depot.
I guess every one missed my point- tightening them to the wrong torque wouldn't warp the discs, but tightening them wrong WOULD!! Your father was right, if you tightened the wheel like that and had wrong torque on each bolt- no problem with warpage- however, very few do it like that any more, so you get the scenario I described.

if you look, when tightening a steel wheel you will see that the parts around the bolts indeed does flex a bit. because of the shape of the wheel, with the rim being such a large mass, if the center is warped a bit here and there the flexibility of the overall wheel will absorb the differences and no effect will noticable.

now with an aluminum wheel because of the mass of metal around the hub area, the flexing is a lot less noticalbe, but it will be there too.

you know, "if you drop a feather on an anvil, it will deflect....it will be hard to measure though".

tom w

I have a 95 E320 and there is an issue with my 7k old rotors (call it "warped" or not) they make a pulsating sound. My MB mechanic says my calipers are fine, but the lug nuts were too tight and he said it was the lug nuts being too tight.

My MB mechanic has over 30 years of experience so he appears to be an authority to me on the subject.

What else am I to think? Calipers are operating fine so I need to chagne teh rotors again and torque the lugs equally to the required spec.

As far as the tire shops using torque wrenches versus impact wrenches, that is mostly due to protecting the alloy rims, which CAN warp if improperly torqued.

Using torque wrenches to eliminate warped rotor complaints may only serve to placate customers.

I don't buy the warped rotor theory either...I'm more inclined to believe that a bad caliper or pad can create issues that would appear that the rotor is warped (i.e., high vs low spots measurable with a caliper), but I believe visible damage such as scoring would be present.

Well, I have no degrees in any field but I do know that uneven torque on the lug nuts where some are TOO tight and some are TOO loose will cause the rotors to warp. It is not immigrant but takes 3 or 4 weeks of normal driving to happen. A trip to the mountains and getting the rotors extremal hot will cause it to happen sooner. We replaced the pads and turned the rotors for a gentleman and all was good. He later had a new set of tires installed at a tire shop. About a month latter, he came back to us with a pulsating brake peddle. We took care of the problem. About 2 months later he returned with the pulsations again. Again we took care of the problem. Another 2 months later, same problem. This time we ask a few questions. Have you had your tires rotated or anything? Well, he got a lifetime rotation with the new tires. We ask that the next time he had his tires rotated if he would come by ASAP and let us torque the lug nuts correctly. He did, and had no problems for the next 6 months. We lost so much money so we could keep a customer. He then believed what we were saying about the torque being very important that he took all the receipts we have given him (we make a receipt for our files and for the customers files even if it was free) to the tire place and demanded that they pay us a fair amount for all the work we had done that they had caused. Believe it or not, they paid us the labor for 5 brake jobs and one set of rotors. What we learned from all of this is that lug nut torque is very critical and needs to be the same on each lug nut and applied in a star pattern. This is only one of many stories about this problem we have had.
science and education may disagree, but real life tell a different story. Did you know that science says it is impossible for a bumble bee to fly but the bumble bee proves them wrong every day.
Paul