drums vs disks

[tnord]

i know disc brakes are better than drum brakes........but why? i have a basic understanding of how drum brakes work, and a pretty good one of disc brakes. so what is it about the design that makes drum brakes inferior? what are the properties that lead to the downfall? and such and such........

[lbcklik1486]

Drum brakes don't decipate heat as good as disk brakes.

[DSC]

*hijack*

Not only do they have inferior braking but drum brakes have more parts, more parts = more $$$
So why are they still used today?

My mom's 2002 *censored* even has drum brakes on the back

[tnord]

Originally posted by lbcklik1486
Drum brakes don't decipate heat as good as disk brakes.

i figured as much, but i was looking for a little more in-depth explination than that

[misnomer]

Bah, speak informed dammit!!

Drums usually have greater breaking power due to a servo action when you hit them. When you press on the break, the shoe is pushed into the drum, where it grips, and the force of that helps push it around it's pivot into the drum with even greater force. Most drums will give you greater stopping power than disks due to this servo action. This is why they are still used in many vehicles today. Hell dood, my dad's Sentra has at least rear drums (dunrecall the front) and it has noticeably more responsive braking (as far as knocking you forward when you tap the brake goes :P ).

HOWEVER!! Due to their design, drums cannot dissipate heat very well. They fade much faster and will remain faded for longer than disks. This is why you have disks on performance cars, where it's common to brake frequently. The disks heat up, but cool down quickly because they have more surface area open to the air. Particularly the important, quick heating contact path of the pads, versus drums where they are "inside" the drum. Hense that same Sentra, if I'm braking for a while, will fade to mush going down a long hill whilst my 240 is still good to go.

[kandyflip445]

misnomer pretty much got it. But I'd like to add that drum brakes, due to their design hold the brake dust IN the drum. Which can cause problems if you drive hard a lot. Also, self-adjusters aren't very reliable if you romp on your brakes frequently. Especially in older systems that have had a lot of dust, heat cycling, etc. The piston seal in a caliper is much more reliable compared to the self-adjusters on a drum system. But only if they are installed correctly which they almost always are from the factory. I haven't heard of one from the factory that had a bad one but who knows...

EDIT: Also, if the self-adjusting mechanism on drum brakes isn't working properly then you would have excessive pedal travel. Due to the fact that it would require more fluid in the cylinder to get the pads to touch.

[tnord]

that's the explination i was looking for.

[DSC]

thx for the good info.

[240racer]

There's more to it then just heat disapation. IMO, they aren't that bad at heat disapation. The usually have heat sink fins on the outside and when they used to use them on race cars, the drum was made out of Al and had a steel (or something) liner. I think the main problem with them is weight, they have a lot of unnecessary weight, since the braking forces aren't cancelled out like they are with a caliper. The shoes push out on the drum, but nothing pushes back in, except the drum itself, so not only does it have to react to the torque produced by the brake shoes, but the radial force as well. With disk brakes, the "normal" force from each pad is counteracted by the opposing pad, so there is only a torque load on the rotor itself. Also, drums are very complicated and not reliable. One other benefit of drums is that they allow the braking surface to be at a much larger diameter then disk brakes, given the same size wheel. This means that the actual friction force can be less to achieve the same braking torque. Drums have more swept area in general, and while I don't know why it's a good thing, everybody pretty much agrees that it is. Also, drums work good for trailers, since for some reason they work worse in reverse then they do going forward (refer to misnomer's post) So when you back up a trailer, it isn't trying to stop as much.

[misnomer]

Oh yes, forgot about the wieght and other forces. . . Rotating masses, heavy wheels, etc, will really hurt power available to the ground (refer to torques for those of you with basic physics). As 240racer explained, drums need to be relatively hefty, and the full drum is rotating. With disk brakes, you have a the rotor, which will typically be significantly less mass than a drum.

[AKADriver]

actually for most automotive applications, drums are lighter (at least rear drums are). There isn't that huge mass of the caliper to deal with, though the rotating mass might be slightly more.

[240racer]

yeah, most of the drums I've had to work on have either been on suburban's or older RWD cars, so I don't know how light the rear drums are on a newer FWD car. The drums on my parent's '99 taurus are pretty small and probably aren't all that heavy, but the disk brake system with comparable braking capacity probably wouldn't weigh much more. In general calipers aren't all that heavy, but they probably weigh a little more then the drum brake internals on a newer small FWD setup. The other thing about rear disks is that nobody seems to know how to do a nice parking brake. What is the deal, it's not a hard concept. It ****es me off so bad, the integra I had and the s13 have horrible parking-brake/hydraulic calipers. Do auto manufacturers realize that they make cable calipers and they just need to add one of those to a normal floating hydraulic caliper setup. Has anybody seen the rear brakes on the new Enzo? That's how they did it there. Don't give me any cost arguements about how they can do anything they want on a ferrari either. The 300zx drum brake system is almost as bad as the 240setup. A little cable caliper mounted on the other side of the rotor that functions as a parking brake only is not expensive, like $5. If I ever upgrade my rear brakes, I can guarentee that I will have a seperate caliper for my parking brake.

[tnord]

so wait..........there's 2 calipers..........how would that effect heat dissipation of the rotor with less being exposed to the atmosphere?

remember the Enzo has carbon brakes.

[240racer]

I believe that most of the heat disapation comes from the air flowing through the vents inside the rotor. Also, the second caliper might use a pad about 1" in diameter, round pads are common in this application, so it's not losing very much surface area.
I don't think the material of the rotor matters too much, but I could be wrong.

this is the kind of caliper I had in mind http://www.wilwood.com/products/calipers/msc/index.asp

[cdlong]

i know this is an old thread, but i thought i'd add a bit to the discussion. drums are cheaper than disks, auto companies don't care that much about weight, and it makes a good parking brake. heat dissapation is a problem, which is why sports cars use four wheel disks. but there's one reason that drum brakes are no longer used as front brakes. because of the servo action that misnomer spoke of drums are more sensitive to differences in friction level. the servo action is a force multiplier so if there is any difference in the friction level of the pads there is a big difference in braking force. in the rear this isn't a problem because the rear brakes don't do much and the rear wheels don't steer the car. but in the front the brakes do a lot more and if one brake works better than the other it will pull the car to one side every time. not a good thing if you are trying to sell new cars.

btw, making a good parking brake is harder than you think, i spent almost an entire semester on co-op working on parking brakes. i could go into it more but i don't feel like it and technically i could get in trouble for discussing it in public (with the company i worked for).

[240racer]

We all figured that drums were cheaper the disks, but we don't understand why. They have more parts and they are more complex. I understand that the parking brake is easier with drums, but who cares. I just don't get why drum brakes are cheaper for car makers to use anymore. I understand why they would have been cheaper 50 years ago, because all their tooling is setup for drums. But now they have to have two types of tooling, one for drums and one for disks. In fact, some cars are available with either, which leads to even more cost. I thought that maybe with some of your background in the industry you could help us out here.

[cdlong]

Originally posted by 240racer
We all figured that drums were cheaper the disks, but we don't understand why. They have more parts and they are more complex.

i have no idea, i was mostly involved in design. it doesn't make sense tom me either.

[AKADriver]

It has to do with the typ, precision, and amount of machining necessary.

A drum is easier to cast and machine than a vented rotor.
The hydraulic parts of a drum are simpler than a caliper.
The springs and retainers and such are all fairly simple.

[AceInHole]

so uh, any explination for why some VW minibusses had drum front brakes and disc rears?

[twitchy]

my friend has an old ford torino and it has 4 wheel drums...

freakin thing stops like its no ones business though! Almost no pedal effort required too

HUGE brakes tho.

[misnomer]

Ace: 'cause it's VW.

[240racer]

Originally posted by AceInHole
so uh, any explination for why some VW minibusses had drum front brakes and disc rears?

The same reason that Porsche puts the engine behind the rear axle and then adds awd just to put some weight up front so the thing has reasonable weight dist. :-)