X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from [65.173.216.67] (account marv@lancaironline.net) by logan.com (CommuniGate Pro WebUser 5.0c2) with HTTP id 734570 for lml@lancaironline.net; Tue, 27 Sep 2005 09:58:13 -0400 From: "Marvin Kaye" Subject: Re: Drilled Brake Rotor To: lml X-Mailer: CommuniGate Pro WebUser v5.0c2 Date: Tue, 27 Sep 2005 09:58:13 -0400 Message-ID: In-Reply-To: References: MIME-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1"; format="flowed" Content-Transfer-Encoding: 8bit Posted for Gary Casey : Colyn, Craig, Rob, I guess I shouldn't have sounded so positive in my posting (people actually read these things!) - yes there are a number of combinations that work for race cars. My experience was only with TransAm cars which are relatively heavy (2700 pounds, 750 hp) and have to stop for a couple of hours. The hot setup (literally) back in the 80's was to use massive rotors and organic pads. The primary failure mode was from boiling fluid and the organic pad provided better insulation. We had to add ballast anyway, so the heavy rotors didn't increase vehicle weight. They also had a very high and consistent coefficient as long as the rotor stayed reasonably cool. The wear rate was very fast, usually completely using up a set of pads in a race, and these pads were about twice the thickness of street car pads. The slots or holes were there to sweep away the pad debris, liquid binder and gases from the face of the pad. Rotor temperatures stayed fairly cool because of the massive rotors and good cooling and the high coefficient allowed the system to be run without a booster. Another combination that could have been run is a high metallic- content pad, but we couldn't figure out a way to effectively insulate the fluid, even with ceramic insulators and ceramic pistons - and a booster was required. HIgh metallic content, depending on the binder chosen, might be expected to slough off less, reducing the advantage of drilling the rotors. And, yes, drilling the rotors reduces the mass of the rotor, a bad thing. We (Bendix) found that for a single stop a good assumption to use is that ALL the energy goes into heating the rotor. Some goes into heating the pad, but not nearly as much. Some goes into the air, but in the time it takes to make a stop virtually none has been dissipated. Most large-aircraft brakes appear to be designed (and they are) with no air cooling capability at all. The heat is absorbed into the brake and then dissipated while the next batch of passengers is being loaded. Carbon brakes are interesting in that the materials themselves are relatively poor at absorbing heat (the figure of merit - I forgot what it is called - is the density times the heat transfer coefficient and that sort of times the melting point). Copper is the "best" material and aluminum would be except for the low density and low melting point. The advantage of carbon is that it essentially has no melting point and its strength is retained or even increases as the temperature goes up. Red-hot iron brakes are pretty hot, but hard-working white-hot carbon brakes could light up the sky. Carbon burns, so why doesn't the brake just burn up? It does, although slowly enough to last a race. Aircraft brakes are intentionally designed to keep air out to increase the life of the brakes. That being said, I don't see any advantage of using carbon pads with steel rotors, as the rotors can't be run any hotter with the "carbon" pads than without. I suspect that people are selling "carbon" pads that are simply conventional pads with enough carbon added to turn them black. Real "carbon" brakes are designed to use a carbon/carbon friction pair. Another incidental thing we found was that very little heat was transferred to the air within the slots of a ventilated rotor - the big advantage of the "ventilated" design is that the rotor is structurally more rigid and tends to warp less. Iron that is very far away from the friction surface isn't useful because the low thermal transfer coefficient of iron prevents heat from traveling that far, so ventilating the center portion reduces weight and increases rigidity without degrading the thermal capacity. Almost all the heat is dissipated directly from the friction surface as the rotor turns - not usually recommended to come to a complete stop with the rotors very hot (the rotor inside the caliper cools at a different rate than the rest of the rotor), but that's not usually a problem with aircraft as we usually taxi at low speed before coming to a complete stop. What's all this have to do with our aircraft? I'm not sure, but the "best" compromise might be to use a ceramet (metallic with ceramic binder) pad with lots of mechanical advantage to overcome the low coefficient and then expect to change the rotors often. A disadvantage of this combination is that the coefficient is very low when cold - expect erratic braking when first touching the brakes after landing - that would make me very nervous. We don't have power brakes like the airliners, and increasing the pedal ratio isn't all that practical, although I'm thinking about drilling new holes in my pedals. A problem with that is that any rotor warpage could push back the piston enough so that the brakes couldn't be actuated with available pedal stroke. Too many questions, not enough answers... Gary Casey