Jim bailey - 928 International

Point is a V belt that is 13 mm wide at the top of the V has the surface area of BOTH sides of the V easily 18-20 mm total, if the V belt were an equilateral triangle the contact area would become some 26 MM but v-belts have the bottom of the v flattened out so it drops down into the groove.

pearlpower

I fully understand that , but again Jim, how does that apply here? You are not comparing two 13mm surface areas and in fact are reinforcing what i have been trying to state all along. Surface area is important and key. I have yet to see a brake rotor with a V shape groove which matches up with brake pads with similar V shaped grooves. Brake pads as you know use one surface of the lining against the rotor, not the sides as a V belt would.

Heck, I have even seen people that have completely worn away one complete side of a rotor and the pad (metal) was contacting the internal grooves/ribs of the rotor itself. Car still stopped. Just made a lot of sparks says the customer and a racket.

Jim bailey - 928 International

Any groove be it a U, V, W and the surface area of the pad and rotor is greater than two flat surfaces of the same width once the pad conforms to the grooves ...why do serpentine belts have ribs an irregular line between two points is always longer than a straight line... I had a pad delaminate and the steel backing plate got so hot that it was extruded and about 25 % larger than normal and bent 90 degrees up on the hat of the rotor...and yes it still stopped. Brakes are highly overated everytime I use them on the track I slow down

Jack Riffle

Oh man, now you did it Jim, might as well wear a "beat me" sign on your back in this thread.

Dennis Wilson

Pearlman is talking about new pads on worn rotors and Jim is talking about bedded pads where the rotor and pads have taken mirror image shapes. No misinformation. Porsche states that the break in period is up to 200km (125miles).

Dennis

69gaugeman

Even if there were poly V grooves running around the rotor, it would brake fine as long as LR (lateral run out) and TV (thickness variation) was small. it would be perfectly fine to use.

James-man

Other conventional wisdom is: don't put new pads on new or newly turned rotors.

Sounds strange when you know there are imperfect surfaces when resuing something old, but there you have it.

brutus

Brake pad material transfers to the rotor surface at a molecular level and then during braking they are pulling on each other with an electron attraction at the extreme heats. If it were simple abrasive friction the discs would wear very fast.

69gaugeman

If that is conventional wisdom EVERY car manufacturer has it wrong.

James-man

Post the question to the track forum. You will probably get over 90% concurrence rate. brutus provides insight for the why of it. Brake pads transfer material to the disk which significantly aids in braking.

None of this matters much for run of the mill street cars. The context here is on high performance.

To be fair, the other side of the coin is where people deliberately try to scrape old pad residue off the rotors as they switch between pad compounds. I would anticipate a much more complicated bedding in procedure after this is done, however, I would also anticipate that the molecular coating on the rotor would probably be more consistent when starting from scratch.

If you bed in new pads on new rotors, just make sure you are thorough. IMHO & YMMV

blown 87

And almost every pro wrench in the country.
I have never heard of this.

Jim bailey - 928 International

Street pads are a lot different than race pads. Some street pads have a break in material on the surface which helps the stopping power until the pads actually are bedded in. Brake pads manufacturers know that very few mechanics are going to go on a 30 minute drive with little traffic carefully heating then cooling the brakes. Porsche stated it was about 200 Km before brakes were ready to use and extreme braking was to be avoided except in an emergency...

Dennis Wilson

Jim,

I also noticed that Porsche says the wear minimum (18.6mm on ealier rotors) is smaller if the rotors are not turned when compared to turned rotors (19mm on earlier rotors). Would that be due to the turning process changing the hardness of the steel?

Dennis

Jim bailey - 928 International

This article makes everything clear ..."This work focuses on surface changes induced by repeated brake applications and tries to provide explanations, how such material modifications might affect friction and wear properties of automotive disc brakes. Surface films were investigated locally by transmission electron microscopy (TEM) after having prepared thin cross-sections with a focused ion beam instrument (FIB). Since the observed friction layers revealed a nanocrystalline structure, modelling with the method of movable cellular automata (MCA) was performed by assuming an array of linked nanometer-sized particles. In spite of complicated material combinations at the pad surface, two very characteristic features were always observed at both the pad and disc surface, namely a steel constituent—either ferritic (pad) or pearlitic (disc), partly covered with patches of nanocrystalline iron oxide, on a zone of severe plastic deformation with fragmented grain structure. When using an automata size of 10 nm, reasonable values for the mean coefficient of friction (COF) were obtained, namely 0.35 and 0.85 for oxide-on-oxide and metal-on-metal contacts, respectively. Immediately after brake application mass-mixing and bond-breaking was observed within a narrow zone at both surfaces. ...." That sure makes it understandable. !

Dennis Wilson

Jim,

Does that mean yes?

Dennis