Disc Brakes Vs Drum Brakes

[wendywindy]

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

[GasSavers_maximilian]

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

[bobc455]

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

[GasSavers_maximilian]

Originally Posted by bobc455

Actually from an MPG perspective, theoretically drum brakes will have less drag than disc brakes, for better MPG.

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

[bobc455]

Originally Posted by wendywindy

but what's the reason behind it?

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

[GasSavers_maximilian]

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.

[theholycow]

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:


or this:


for me.

[GasSavers_maximilian]

Originally Posted by theclencher

Drums don't have pads and rotors LOL

See how strong my bias is?

[Wyldesoul]

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!

[GasSavers_NovaResource]

Originally Posted by Biffmeistro

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.