As nitros was saying brakes crack when they exceed the heat capacity of the metal- if you want a technical explination read further if not kewl.
ill try and make it short- its about a 50 page segment in the book Engineer to Win by Carrol Smith, MBI Publishing Co. Osceola, WI 1984. You sdhould purchase this book if you have any of these sort of questions- Carrol was the race engineer to Shelby aMerican during most of his life and if considered a guenius (sp) in the industry- his books are very well written and can be understood by anyone.- they are all about $25 and the ones to read are:
Engineer to Win
Prepare to Win
Nuts, Bolts and Fasteners
Tune to Win
If you read these you will be VASTLY better at all things automotive.
You must first understand the properties of metals and the basic checmical compounds and behaviors of crystalin structures (thats the atomic structure that all metals have). Metals like to arraing themselves in to c=rystaline latices and form stable "blocks"- but when rotors are made they are made of cast iron - more specifically "Gray Iron"-
(Quoting from the book)
"Gray IRon is the most common tyoe of cast iron used in the foundry. Gray iron is basically a matrix of iron and silicon in a solid solution intersperced with pparticles of carbon in the form of very thin interconnected FLAKES of graphite. Because more carbon can be dissolved in molten iron than can be dissolved in to iron in a solid state, the graphite is precipitated out of solution with the iron during the cooling process. The mechanical properties of gry iron are affected be how much carbon is present (ie how much space within the lattice do the flakes use), the size and distribution of the flakes and by the hardness of the metallic matrix. Simply put, the higher the carbon and/or silicon content and the slower the cooling process, the more and larger will be the graphite flakes and softer and weaker will be the matrix. Like most materials in flake form (corn flakes for instance) the graphite flakes themselves are both weak and brittle and the volume that they occupy within the matrix has little mechanical strength. Thr presence of the flakes however does give gray iron its excellent castability and machinability so that complex parts may be cast and machined economically. The material has modest tensle strength values (up to about 55,000 psi, ultimte), good wear resitence, excellent internal vibration damping ability and thermal stability and a good resitance to galling." Engineer to Win: page 50
that is the makeup of your rotors.
now why they crack:
Plastic and Elastic deformation of metals
(this is along and complex chapter so ill summerize)
When you place a metal under a tension or compression load you are trying to change its matrix shape. The matel by nature and its atomic bonds will deform a certain degree and still have the ability to "spring" back into its original shape- you all have done this with metal - bend it but then it goes back to normal. if you exceed the bonds ability to stay together the metal the goes from elastic to plastic deformation. This is when you stress the atomic matrix to such and extent that it will actually shift its self to accomodate the pressure. when you add heat to this process everything becomes easier to do- ie try to bend a cold bar as opposed to a white hot one- the hot one is easier. This is why if you are going to puchase metal - be sure you get the cold rolled type- it is much stronger and often not much more money.
So back to heat distorion or plastic deformation.
then your gray iron rotors are clamped by the calipers it is in a compressive type of stress- Gray iron is very good in that application of stress and that is why it used in brake rotors- but then these pads create friction and now the rotor is transormed into a giant heatsink. Iron is not as good as say aluminum at dispersing heat- it tends to retain much of the applied heat- so when it gets HOT the rotor now under extream heat and compressive force begins to exceed its capabilities at the atomic level and a shift in the matrix occures, manytimes this is felt as brake whobble. As often as the rotors warp they also get "baked" or exceed their heat abosorbtion capabilities- this is when crackes occure- they are called stress fractures. Caused by a combination of excess heat and stress they form along Grain boundries within the atomic structure of the metal- (these can be seen easily on a piece of galvinized sheet metal- all the different colored spots are grain boundries) these crackes are also prevelant along these flakes and also any casting and or machining flwas within the rotor. once your rotors havew begun to crack they are destined for the scrap heap- get rid of them as soon as you can afford to- and get a better uality rotor- and these arent always the nicest or most expensive rotors on the market-
any brake rotor company worth your time should be able to tell you the exact metalic compund and structure and give you a data sheet (paper showing the batch resistance to fatigue heat sheer tensile ect) of your batch ect.
hope this helps- if not my wrists ache and ill answer any other questions later.
scot
