The Stovebolt Site Forums The Parking Lot Big Bolts Drum vs Disc

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.

I wonder if it isn't the problem of replacing rotors on the big rigs, seems everyone is always replacing their rotors when they have disc brakes on cars and pickups.
Heat and water are the only dis-advantage that I see to drum brakes, and both can somewhat be avoided if one pays attention.

You can lock up the brakes disc or drum if adjusted right

and thoes big trucks with smokiing brakes coming down the hill are completly out of controll, and it is driver error unless something broke

All big trucks that I seen have drum brakes.

Disc brakes are much lighter, better for lighter vehicles such as compact or sports cars. They are perhaps easier to manufacture and adjust.

When drums heat up they expand away from the brake shoes. This requires more pedal travel on hot drums to reach the same braking force (brake fade). However, when disc's heat up they expand into the shoes, requiring less pedal travel to reach the same braking force.

I think the big trucks haven't switched to disk brakes because of the weight. Imagine a 25" diameter, 3" thick rotor and a 200 lb caliper......plus I'm not sure that air disk brakes would be all that safe. I think you need the power of hydraulics to keep the pads against the rotors. So, you'd have to have a completely separate system just for the front brakes.

IOW, in that application, it doesn't make a great deal of sense.

I chuckle at my pal who turns a lot of rotors at the local machine shop. When it rains for a day or two he says he stays busy for a week turning rotors that are "warped" due to hot metal mixed with cold water!
After retiring I delivered parts for a local parts store and find that more and more shops are putting on the CERAMIC pads. They say less dust, quieter and even stop better. Has anyone tried them?? I plan to put them on my daughters car soon because even the best pads seem to make to much "metal to metal" noise when she stops. I tried a couple different ones but they do the same. Maybe its the rotor metal? I've turned them a time or two since 2000 so maybe I'll its time for new rotors and pads.
Any comments on the Ceramic pads?

In the late eighties Volvo was building quit a few class 8 trucks with disc brakes. I was working at my Dads' tire shop and seen quite a few come though then, all the drivers and owners I talked to loved them. Since I started driving truck in the early nineties I have heard vary little about disc on big rigs and after putting my butt in the seat of hundreds of different trucks have never driven one.

jhaa-lives,
It was my understanding that disc brakes are heavier than drum brakes? I'm adding cross-drilled drums to my 58 NAPCO, that should take care of water and help release heat and gasses.

one reason you aren't seeing air disc on semi's yet may be no one wants to be the lab rat. they would rather deal with technology (drum Brakes) they are familar wih and can predict cost maintenence and downtime.oh boy do i need spellcheck today