Disc Brakes Vs Drum Brakes
 

Could you pls tell me the difference between Disc brakes and Drum brakes? All i know about them is that Disc brakes are better than Drum brakes.... but what's the reason behind it?



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"In less enlightened times, the best way to impress women was to own a hot car. But women wised up and realized it was better to buy their own hot cars so they wouldn't have to ride around with jerks." -Scott Adams

My understanding is that drum brakes are cheaper than disc brakes. Check out these links: disc and drum.

Actually from an MPG perspective, theoretically drum brakes will have less drag than disc brakes, for better MPG. However the difference is probably not measurable.

My old Buick originally came with drum brakes at all four corners.

The principle of operation is the same - you have a friction material pressed against a steel surface to create drag, which slows the vehicle. Disc brakes have a rotor (disc-shaped) and has a pad on each side which is squeezed against the rotor to slow down the vehicle. Drum brakes, on the other hand, have the pads on the inside of a cylindrical surface to create drag.

-BC

I meant from a manufacturer's cost perspective. Beats me on mpg.

I forgot to answer the original question, sorry...

Disc brakes are "better" because they are more exposed to atmosphere, and thus less susceptible to heat fade. So they are "better" for racecars and such, but for your average driver that isn't really a factor.

Disc brakes are easier to work on IMO. But the hardware is simpler/cheaper for OEMs.

-BC

I hate working on drums. Retracting the pads from grooves in the rotors can be a real headache, especially when the parts get rusted by all the salt that's used around here.

Apparently drums lend themselves better to being part of the emergency brake system.

Well, I too hate working on drum brakes. When I open them up they look like a crazy pile of clockwork springs/gears/levers that I'll never be able to return to their original configuration.

They're never anywhere near as simple as this:
https://www.allfordmustangs.com/forum...drum-brake.jpg

or this:
https://www.sgauto.com/photoplog/imag..._DrumBrake.gif

for me.

See how strong my bias is? :)

Drum brakes have a self energizing mechanism... Basically the action of braking with a drum brake forces the pads outward, which makes them brake even harder, which forces the pads outward which makes them brake even harder ect ect... They work great with a non-power brake system, such as the emergency brake, or old non-power brakes.

However, they are very susceptible to mechanical fade (when the drum gets so hot it expands away from the pads), and are rendered short term useless by water in them. It's the reason why engine braking was VERY necessary on long hills, because if you rode your service brakes the whole time with drums, your foot would go to the floor and you'd have no brakes at all.

Disc brakes don't provide as much braking force in relation to the input force, so they need much higher input force in order to be effective, hence the need for power brakes. However, they are far simpler, automatically self adjusting, have far better cooling properties so they're less susceptible to heat fade, are not at all susceptible to mechanical fade, and water is instantly squeegeed off so as to proved no noticeable reduction of braking ability. All in all, they are far safer, better, and cheaper than drum brakes.

The reason drum brakes are still used in the rear of cars today is due to the fact that disc brakes make very poor parking brakes. You either have to have a very complex screw/spring system in the rear caliper to actuate it when you pull the parking brake cable, or I've seen many rear discs that have a small drum brake in the hub.


Hope that helped!

^^^ That is the absolute best explanation I've seen and 100% correct.

Drum brakes actually are better for 2 reasons, they clamp harder and they are better for fuel economy because they have less resistance and rotational weight. However, the reason discs are used today is for their fade resistance. Drums do not dissipate heat as well as rotors. Take identical cars with the sole exception of front drums vs front discs and the drum brake car will stop shorter the first time. After that, the disc brake car will stop shorter.

Is it true the front wheels see more of the braking force? Is that why discs are used there? I've also heard people claim discs are less susceptible to clogging with debris as they can spin it out. Not sure about either claim.

I heard a couple years ago about a new design that made an electric disc brake viable by taking advantage of the centrifugal force available in some manner. A quick Google search didn't pull it up and I'm fuzzy on the particulars.

Front wheels see far more braking force, especially for people who brake hard.

https://www.auto-repair-help.com/auto...ning_valve.php

I doubt that discs are any more or less susceptible to clogging with debris. Drum brakes are enclosed, though rust and such could build up inside. Disc brakes are exposed but the pads are always almost rubbing the rotor so there's no room for significant debris to get in, and even if it does it's not lilkely to prevent the brake from stopping the car (though it might score the rotor).

I think they did mean rust.

Front brakes do approximately 70% of the braking. That's not just during hard braking, that's under normal braking. Under hard braking, when the weight transfer moves forward, the front brakes can do even more work. Look at a typical 4-wheel disc car and you will notice the rotors and pads are larger on the front wheels than the rears. Also, front rotors are vented while rears are solid. Vented rotors cool faster but cost more, are thicker and are heavier.

Vented rotors:
https://www.mjmautohaus.com/catalog/...K0615301AA.jpg

Solid rotors:
https://www.mjmautohaus.com/catalog/...15601C.B.2.jpg

Don't get me wrong about discs vs drums. With the use of power assist for brakes, discs are far better for a vehicle. While a drum brakes may work better when cold, the are quickly overheated and lose their effectiveness. Discs do a much better job of handling heat. Take an easy drive where you use the brakes very little. The rotors will still be hot enough to burn you when you stop. Even the wheel itself will be very hot to the touch.

Discs also shed water and brake dust easier. That's why cars with disc brakes have dirtier wheels. You will also notice the front wheels get dirtier faster than the rears. That's because the fronts do more braking work and because some cars still have rear drums that keep the brake dust inside the drum.

As for rust, discs rust faster because they braking surface is exposed to water while the draking surface on drums in inside the drums. However, the rust is quickly removed by the pads. This is why after a rain the brakes make funny sounds the first few times to use them.

I say, it depends on how you drive.

As a flatlander that drives gently, my brakes scarcely get used at all. I have over 110,000m on the factory pads on my new vehicle. Over heating is not an issue. Power is not an issue. I would do better with 4 wheel drums. I have several classics with 4 wheel drums. I prefer them. I laugh at all these guys that, when building their rods, think they need front or four wheel discs. They are blindly following the crowd.

Just because you have a flat drive and brake easy doesn't mean everyone does. Most classic car and street rod guys drive hard and need disc brakes. Disc brakes are superior to drums in just about every aspect and that is why they are used today. The only places drums have an advantage is in cost, rotational weight and they make a better emergency/parking brake.

Disc brakes definately stop better. No question about that. If you're building a car that can go over 100 MPH you need to put brakes in that can stop a car going that fast in a reasonable distance. If you're re-doing an older car for performance and you don't upgrade the brakes I think you're crazy, unless the car is a trailer queen and really doesn't get driven much.

-Jay

My drum brake cars can put you through the windshield.

Two of them can go over 100.

As pointed out, the main deficiency of drum vs disk is heat rejection.

If you don't do 10 hard stops in a row, there is no issue. If you only have to do a couple hard stops in quick succession, I don't see where drums have any deficiency. I've never been anywhere CLOSE to experiencing brake fade with the drum cars.

What hypermiler is on the brakes enough for that to be an issue? This is the econo forum right? Or should I have taken that left turn at Albuquerque?

What hot rodder drives hard enough to tax his brakes? Seriously. You aren't pulling the wool over my eyes. I've been in car clubs longer than some of you have been alive; I know 99.9% of "hot rod hardware" is for show.

The guys that actually do track events are the ones that realize the benefits of discs, but the street guys? Not so much. Edit: emphasized cuz I guess someone didn't see it the first time?

I'm not anti-disc. For high performance like motorcycles and snowmobiles, discs are way better. Trucks pulling/hauling heavy loads- eh, I don't know. Mine have drums in the back and the trailers have drums too. High performance cars should have discs too, I suppose, but I don't drive that hard on the street.

Whatever though. I know I'm not going to start any movement or hot trend for drum brake conversions! Just sayin', the "common knowledge" and the blanket statements of "disc brakes are better" have caveats.

If it wasn't so much work I'd put drums on the front of my Tempo. Clean rims, less drag.

So have I so don't assume.

Then you've been hanging around pussies, not hot rodders. Trailer queens aren't hot rods, they are nothing but automotive sculptures. Real Hot Rodders drive their cars and drive them hard because they enjoy the speed. I've had my Nova down the track hundreds of times (some legal, some not). I've driven cars with drums and cars with discs. Cars with power brakes and cars without. Disc brakes work better, period. Thinking a street car can't benefit from disc brakes is ludicrous. The benefits of drums over discs for fuel economy is too minimal to even measure but the benefits of discs over drums for safety is immeasurable.

See, that's my problem with most hardcore hypermilers. You are willing to do things that are unsafe just to get slightly better mileage. Tailgating trucks, taking turns at too high of a speed, turning off your engine while cruising etc. Changing from discs to drums is not only expensive but making your car less safe. Would you remove the airbags and seatbelts to make your car lighter in an effort to increased fuel economy?

I'm all for saving fuel but not at the cost of safety. Not just my safety but the safety of the other drivers on the road.

Not taking sides in this one, but drive a 37 Ford with the stock cable operated drums, or a 63 valiant in a downpour with a single chamber master cylinder.

Discs are better. Chrysler had them on the first 300 in 1955.

In Virginia driving your car "hard' on public streets can cost you the car, and leave you with the payment book.

regards
gary

lol my model AA truck has 4 wheel drum brakes. they are oversized majorly to account for if the truck was loaded up with its load capacty of 1.5 tons thier close to modern drum but different in the way that theres no wheel cylinder and in its place a V shaped wedge that pushes the pads out to contact the drum

others who drive thiers around say if they stomp on the brakes it can put your face into the windshield (mechanical linked rods to the brakes, no cables/ fluid to do any stopping) the time it takes to stop is equal to the force you press.

typically drum brakes (especially ye old car ones) are suspectible to locking up.

my friends car (75 maverick) has 4 wheel drums and stops mighty fast

my chevette has manual brakes, front solid rotors and rear drum. it can stop pretty fast but feel its not good enough.

my dads 60 VW Ghia has 4 wheel drum and that thing stops on a dime!

now my s-10 is annoying cuz its got rear abs (supposedly) so when i have to jam the brakes the front wheels slide and the back doesnt...

heres a pic of the model AA brakes
https://www.fuelly.com/attachments/fo...2dd7c98423.jpg
https://www.fuelly.com/attachments/fo...a4979281a9.jpg

front axel with hubs
https://www.fuelly.com/attachments/fo...9b9fb6e161.jpg

just a side note...

i don't have a link, sorry, but a radio program informed listeners here locally of inferior brake pads and shoes. you know, the $8 variety at the cheap discount auto joints and $50 installed brakes offers?

in addition to poor quality, there's the formerly banned chemicals being used such as asbestos, mercury, and such. i wouldn't want to be around those even on my car!

i prefer ceramics BTW...less heat, no dust, and shorter braking distance.

I agree. Even people driving economy cars drive hard enough that drums aren't the best choice. It's quite common.

What are the supposed benefits again? I find it hard to believe that the difference would be measurable. Anyway, the safety IS far more important. Then there's the cost of the conversion...it will never pay for itself.

Here on GasSavers.org, most of us advise against doing things that are illegal or that drivers find dangerous.

Most of us advise against tailgating trucks; not only is it dangerous (visibility, flying tire fragments, etc) and rude, but it's also ineffective. A safe 3 second following distance (far more than most other road users practice) has been found to be most effective for some people, though some report success as close as 1.5 seconds -- but any closer and FE goes down, not up. There's a link about drafting in my sig.

Taking turns at high speeds, well, I don't know how many other hypermilers do it, but I started doing that back when I took pride in being able to waste gas like no other. When talking about it, I try to express the importance of knowing your limits and leaving some wiggle room. The car's handling is NOT the only limit; much more important is visibility. If you can't see perfectly around the turn, you don't know what's there. These days, trying to save gas, I take my turns slower because I'm usually not trying to accelerate balls-to-the-wall through them.

Engine-off cruising is not as bad as it seems on first impression. I recommend against doing it anywhere that there's other traffic (vehicular, pedestrian, or wild animals), anywhere that it's illegal (and I started a thread to collect such laws, it's not as clear as you might think), or for anyone who is not comfortable with it. Steering works fine and brakes do not stop working just because the engine is off -- and in the time it takes to use up all your brake boost (2 heavy applications or 4-6 medium for my car), you can dump the clutch and have the engine running again (not to mention extra engine braking). Then there's discussions of the mechanical effects on the car, etc.

More people would remove airbags for hot rodding than would do so for hypermiling. They don't weigh much, and most cars don't get a FE benefit from the usual weight loss efforts (another link in my sig).

That attitude is in line with most GasSavers.org users. Saving gas pales in comparison to saving lives.

There seems to be some degree of willful ignorance going on here, so whatever. :rolleyes:

that requires a willful suspension of disbelief! :)

not here tho. i own stock in ceramics, so i'm willfully, intelligently pushing for my shares to do better!