BTW, not trying to be a smart *** know it all, just happened to be someone that has worked on this stuff for a long time always had an interest in how things work.
All passenger cars and light trucks built since the late 50's/early 60's have what's known as self energizing drum brakes. You know how when you go to put new brake shoes on there's always a short shoe and a long shoe, the short shoe always going to the front? On these vehicles there is only one anchor pin, right at the top. When you apply the brakes the front(or primary) shoe comes in contact with the drum. When it does it tries to turn with the drum, pushing the secondary shoe against the drum through the adjuster wheel. Braking force is actually increased. This is not the case with big trucks though. They have two anchor pins and both shoes just pivot out towards the drum.
As to heat and surface area and brake fade, in my previous post I did say that disc brakes claim to fame was their ability to dissipate heat. I went on to say that they needed to because they stored more heat in a smaller area. A 10" by 2 1/2" drum (which is what you will find on the front of most GM A body cars of the 60's and 70's) has a surface area of 78.5 sq. in. (dia x pi x width) An 11" rotor has a braking band surface area about 2 1/2 inches wide. Doing the math this gives a surface area of 38.27 sq inches per side for a total of 76.54 sq inches. (pi x radius x radius of rotor minus pi x radius x radius of the hub portion) Almost exactly the same as the 10 inch drum. Now compare the size of the shoe/pad area. An 11' in rotor would have 2 pads about 2.5 x 5 or about 25 sq inches. Total shoe area would probably be at least 20" x 2.5 (I'm guestimating, but picturing a ten inch drum which has a 31 inch circumferance, 2/3's of it would have shoe contact x the shoe width).
So the disc system has the same amount of heat as the drum in a simaler surface area and a smaller amount of friction material. It can do this because it does lose heat quicker.
Brake fade from heat has a couple of factors. One is that, in organic linings, as the friction material heats up it's co-efficient of friction decreases, ultimately to the point that it can not stop the vehicle at all. The answer to that is metalic linings. The co-efficient of friction actually increases as they heat up. That's what's on those race cars with the glowing rotors. They've had them on drum brakes since the 60's. Factory GM race cars like the 63 Gran sport Corvettes had big drum brakes with metalic linings. The also had brake backing plates with air scoops built into them and finned aluminum drums with steel insert friction surfaces to help get the heat away. But they're not great on grocery getters because they don't work worth a darn when they're cold. But I still say for typical street use in passenger cars and light trucks there is little to no braking advantage to discs. The average driver would be hard pressed to heat the brakes in a small vehicle to the point they wouldn't work.
Race cars are a different animal. They make more heat and need to be able to get rid of it. Discs are the answer there. The other component of brake fade is the expansion of the drum when it's hot. Under extreme conditions the drum can expand so much that the shoes will not contact them when fully extended. Of course that can't happen with discs.
I live in the mountains in Kamloops BC. There's a steep hill, 8% grade at points that goes through town. Two weeks ago I watched a semi go through a red light at an intersection 1/4 mile from the bottom of the hill. He was barely moving, but didn't have enough brakes to stop him. He coasted to a stop a few hundred yards past the intersection, got out and emptied his drawers. Just this morning I walked out the door of the Tim Horton's at the bottom of the hill to the stench of brakes in the air. There's runaway lanes all over the place here and I see fresh tracks in them lots. They're not a thing of the past. If disc brakes are the answer, why do 90%or more of the semis out there have drum brakes when the technology has been around for 40 years. A poster in the thread that started this said they're starting to see discs on some tank trailers, but you sure don't see them much. That's a puzzle to me because heat clearly is their nemesis.
Again, sorry to ramble. Just thank you're lucky stars you're not in the stool beside me at the neighbourhood watering hole.
Steve g.
