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.

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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?

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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

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

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

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

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