How Drum Brakes Work

today I'm going to show you what's

inside these drum brakes and how they

work to stop your vehicle now nowadays

drum brakes are only found in the rear

of vehicles typically it's bolted to the

hub of the vehicle so it's also rotating

with the wheel of the car now if I

remove the drum here you can see why

they call it drum brakes is basically a

giant drum with the internal surface

here being the friction surface that

these shoes will rub against when you

apply the brakes to stop the vehicle to

get a clear access I'm going to remove

this wheel hub I'm going to unbolt these

four fourteen millimeter bolts and now I

can remove the wheel hub assembly now

there's a few main components that go

into this drum brake assembly we have

the wheel cylinder up here which is your

hydraulic piston and we have the brake

shoes on the side here to push up

against the drum then on the sides here

we have these to guide pins and that

guides the shoes in and out against the

drum and then at the bottom here we have

the parking brake cable that comes in

here we've got the parking brake lever I

know we've got a return spring over here

and then finally we have the adjuster

for the drum now when you press the

brakes these lines here fill with

hydraulic fluid and that will fill up

the wheel cylinder the wheel cylinder

will then expand and push these shoes

out against the drum and the friction

pad material here will graze up against

the drum slowing it down and turning its

kinetic energy into heat to demonstrate

how the wheel cylinder works I'm going

to go pump on the brakes and this is the

adjusting mechanism for the drum it's

basically a long screw that I can adjust

the length of and that will adjust the

park position of the shoe relative to

the drum to adjust for drum wear at the

bottom here we have the parking brake

cable and it's lever now when you pull

up on the parking brake this cable here

will retract and that will pull this arm

back which in turn will push the shoe

out against the drum stopping the wheel

to demonstrate how the parking brake

works I'm gonna pull up the parking

brake handle to get a better look I'm

going to disassemble these drum brakes

I'm gonna first start by removing these

guide pins and now with the spring free

from the bottom here we can go ahead and

remove the shoes and then now I can pull

back this spring here and release the

parking brake cable from the shoe

assembly now I'm going to remove the

brake line like that so here I've got

the drum brake assembly components laid

out here we've got this backing plate

that has this tab that's riveted on for

the parking brake to run through as

this tab here where the spring these two

shoes together at the bottom now this

here is the brake drum itself it's got a

set distance for the diameter and when

it starts to wear down or fur gets

grooves it will have to be replaced now

one of the disadvantages of drum brakes

is that it tends to keep all of the

brake dust on the inside of the drum as

opposed to disc brakes where it can just

fly off and stick to the wheel and this

here is the shoe assembly we've got this

friction material here now if you look

closely you'll notice that the leading

edge of the pad material is actually

thinner than the trailing edge at the

bottom here and that's because it's near

the wheel cylinder and that's just the

nature of how it pivots now on this side

we have the main shoe and that's got

this parking brake arm on it and that's

just held in by the c-clamp now when the

cable gets tugged this lever will move

slightly and the relative motion between

the two will push the shoe out against

the drum to lock it in place now this

here is the adjusting screw you can see

as I screw it in it gets smaller and as

I screw it out it gets larger now I know

how the adjuster installed into the shoe

here we have this little tab here that

will lock this star wheel from

retracting so it can only go in the

expanding direction which is this way

but it won't be able to retract because

it's locked into the tab here now the

wheel cylinder is the heart of the drum

brake it's basically a piston that has

these two ends here that move out and

press against the shoe on this side here

we've got this boot I'm gonna remove

that and you can see that the piston

also popped out at the same time that's

the piston and we've got a spring there

with some brake fluid so basically as

pressurized fluid comes in to the

housing here it will expand against the

spring force pushing this piston and

this piston outwards against the shoes

which will rub against the drum now

while this brakes are less complicated

drum brakes are still cheaper and easier

to make that's why you'll see them on

the back of economy cars now the main

disadvantage of drum brakes is a lack of

stopping power compared to disc brakes

and that's pretty much all the

components that go into making your drum

brake work on your car