Truck vs car brakes

[Old Techo]

From Quora...


Why do semi trucks have drum brakes when disk brakes provide better stopping power?

Here’s the thing about disc brakes:

A friction brake (disc, drum, whatever) works by converting rotational motion into heat energy - or, in common terms, you step on the brakes and they get hot. When your brakes cool off, you get your full stopping power back.

If you want great and consistent stopping power, you need to do one of two things: build brakes that cool off faster, or put a lot more metal in them so it takes more effort to heat-soak them.

A car doesn’t have room to make drum brakes bigger, so they use brakes that cool off faster - disc brakes. Look at the high-performance brakes they put on sports cars: they started with ventilated discs, then went to drilled discs and I wouldn’t be surprised if in a few years they figured out how to put dual discs on a car wheel. (It’s already done on motorcycle front wheels.)

Heavy-duty trucks have all the room they need, so they put really wide brake shoes and super-deep drums on the drive axles. The steer axle is a different story: Kenworths and Peterbilts all come with front discs; they’re available on other brands at an upcharge.

Given all that, there is one group of truckers who LOVE disc brakes: garbage truck drivers. Garbage trucks start and stop so frequently they don’t get the chance for drum brakes to cool. Discs cool quicker, so on a garbage truck they’re better.

[A'van]

I worked in Mansfield Victoria, in the bush, they had timber trucks with jinker trailer on the back, in between the wheels on the back they had a 250 lt. drum full of water and a pipeline and a tap to each brake drum.

When they got too the top of the hill ready to go down they would open the taps and the water would cool the drums, simple 

[nsgnomad]

I have noticed most modern train carriages have changed over to disc brakes.

I believe part of the theory behind why discs are better is that as a drum heats up, it expands in diameter, increasing the clearance between the drum and the brake shoe, whereas with a disc, as it heats up, the disc becomes a little fatter and so decreasing the clearance between the disc and the pad.

[A'van]

Disc brake pads are a lot easier to change than drum brake pads 

[Old Techo]

Roger,

I think drums vs discs is no more than heat.

As either gets hotter the coefficient of friction between brake material and metal reduces.

Drums being closed cannot dissipate the heat as quickly as an open disc.