Drum Brakes rather than Disc Brakes

[john west]

Looking at most smaller cars i can see the rear brakes are Drum type
while at the front of the car are Disk type.
I cannot think Drums are that much cheaper to manufacture than Disc type
and i thought are less effective than Disc type. So why do manufacturers
choose to make the rear brakes Drums rather than Discs ?

[Ian Jackson]

Post by john west
Looking at most smaller cars i can see the rear brakes are Drum type
while at the front of the car are Disk type.
I cannot think Drums are that much cheaper to manufacture than Disc
type and i thought are less effective than Disc type. So why do
manufacturers choose to make the rear brakes Drums rather than Discs ?

Front disc, rear drum, became pretty standard in the 1960s. This is
because discs provide better braking, and it's the front where most
braking occurs.

When braking hard, the drums heat up. Their diameter increases and
releases the grip of the shoes. As a result, the braking action tends to
fade. With discs it's the opposite. When discs heat up, their width
expands, and they push harder against the pads - thus maintaining the
braking action.

The rear remained drum because less braking is required. They are also
considered safer, because if the handbrake is applied when the drums are
hot, as the drums cool down they contract and tighten the grip on the
shoes. Discs do the opposite, and tend to release the handbrake as they
cool down - but despite this, most cars now have discs on the rear
(presumably because of their better braking action).

[Abandoned_Trolley]

Post by Ian Jackson
The rear remained drum because less braking is required. They are also
considered safer, because if the handbrake is applied when the drums are
hot, as the drums cool down they contract and tighten the grip on the
shoes. Discs do the opposite, and tend to release the handbrake as they
cool down - but despite this, most cars now have discs on the rear
(presumably because of their better braking action).

Its a nice theory, but in reality the considerable length of the
handbrake cable will most likely have an almost constant tension to
compensate for it ?

[Dave Plowman (News)]

Post by Abandoned_Trolley

Post by Ian Jackson
The rear remained drum because less braking is required. They are also
considered safer, because if the handbrake is applied when the drums are
hot, as the drums cool down they contract and tighten the grip on the
shoes. Discs do the opposite, and tend to release the handbrake as they
cool down - but despite this, most cars now have discs on the rear
(presumably because of their better braking action).

Its a nice theory, but in reality the considerable length of the
handbrake cable will most likely have an almost constant tension to
compensate for it ?

Trying to think how many cars have a true disc handrake. Isn't a drum
handrake inside the rear disc the most common? Or is that just German cars?

But a drum also has a self servo action. If the car tries to move, it
actually increases the braking effort. Not so with discs.

[Abandoned_Trolley]

Post by Dave Plowman (News)
Trying to think how many cars have a true disc handrake. Isn't a drum
handrake inside the rear disc the most common? Or is that just German cars?
But a drum also has a self servo action. If the car tries to move, it
actually increases the braking effort. Not so with discs.

My 1966 Mark 2 Jaguar, along with all E types, S type / 420 / Mark 10
etc and all XJ saloons up to the mid 80s were fitted with a seperate
mechanical handbrake caliper and pads with its own self adjusting mechanism.

I seem to recall something similar on a 1967 Rover 2000 as well ? - it
certainly had inboard rear discs which were a pain to maintain

Back in the days when brake pads were less abrasive than they are now
(they were allowed to use asbestos), the discs would often last the life
of the car which led to another problem - if the handbrake was never
used when the vehicle was in motion then the vanishingly small amout of
wear on the pads would mean that the self adjust mechanism would seize
up before you ever got to the first tooth on the ratchet :-\

[Dave Plowman (News)]

Post by Abandoned_Trolley

Post by Dave Plowman (News)
Trying to think how many cars have a true disc handrake. Isn't a drum
handrake inside the rear disc the most common? Or is that just German cars?
But a drum also has a self servo action. If the car tries to move, it
actually increases the braking effort. Not so with discs.

My 1966 Mark 2 Jaguar, along with all E types, S type / 420 / Mark 10
etc and all XJ saloons up to the mid 80s were fitted with a seperate
mechanical handbrake caliper and pads with its own self adjusting mechanism.

Yes. Which didn't work as well as the current idea of a drum within the
rear disc.

Post by Abandoned_Trolley
I seem to recall something similar on a 1967 Rover 2000 as well ? - it
certainly had inboard rear discs which were a pain to maintain

Early P6 were the same Dunlop brakes as the Jags. Later used Girling swing
caliper rears, and the inbuilt handbrake mechanism extremely complex -
although it did work well when in good nick.

Post by Abandoned_Trolley
Back in the days when brake pads were less abrasive than they are now
(they were allowed to use asbestos), the discs would often last the life
of the car which led to another problem - if the handbrake was never
used when the vehicle was in motion then the vanishingly small amout of
wear on the pads would mean that the self adjust mechanism would seize
up before you ever got to the first tooth on the ratchet :-\

[Roger Mills]

Post by Dave Plowman (News)

Post by Abandoned_Trolley

Post by Ian Jackson
The rear remained drum because less braking is required. They are also
considered safer, because if the handbrake is applied when the drums are
hot, as the drums cool down they contract and tighten the grip on the
shoes. Discs do the opposite, and tend to release the handbrake as they
cool down - but despite this, most cars now have discs on the rear
(presumably because of their better braking action).

Its a nice theory, but in reality the considerable length of the
handbrake cable will most likely have an almost constant tension to
compensate for it ?

Trying to think how many cars have a true disc handrake. Isn't a drum
handrake inside the rear disc the most common? Or is that just German cars?
But a drum also has a self servo action. If the car tries to move, it
actually increases the braking effort. Not so with discs.

Conversely there is little braking when going backwards down a steep
hill, especially without the engine running.

For front drums, maybe - with two leading shoes, which become trailing
shoes in reverse.

But rear drums usually have one leading and one trailing shoe - so
they're equally effective in both directions.

When I was young, my father owned a 1939 Sunbeam Talbot 12 (I think) -
which had a complex Bendix braking system. The brakes on all the wheels
were rod operated from a central lateral tube which was rotated both by
the brake pedal and the hand brake. This meant that the handbrake worked
on all four wheels - which was good until it all froze up in the winter.
However, there was no side to side or front to rear compensation, so
adjusting them was a nightmare. Although each wheel had one leading and
one trailing shoe, there was a complex system of levers which
transferred force from one shoe to the other - making them act like two
leading shoes. This wasn't too bad when going forwards but was
absolutely lethal when you tried to stop when going backwards!

[Roger Mills]

Post by Dave Plowman (News)
Trying to think how many cars have a true disc handrake. Isn't a drum
handrake inside the rear disc the most common? Or is that just German cars?

No, it's not just German cars. Both my Volvo V70s (2 different vintages)
had this system - as does my Hyundai Tucson. You get the best of both
worlds with this system.

I'm surprised that no-one has mentioned transmission brakes. Landrovers
used to a parking brake which consisted of an in-line drum brake on the
propshaft. If you applied this on a slope, the vehicle would move quite
a few inches while it wound up the transmission. If you applied it while
in motion, the vehicle would rock violently backwards and forwards.

[Peter Hill]

Post by Roger Mills

Post by Dave Plowman (News)
Trying to think how many cars have a true disc handrake. Isn't a drum
handrake inside the rear disc the most common? Or is that just German cars?

No, it's not just German cars. Both my Volvo V70s (2 different vintages)
had this system - as does my Hyundai Tucson. You get the best of both
worlds with this system.

Nissan used to have drum/discs on RWD models before Renault.

[Dave Plowman (News)]

Post by Dave Plowman (News)
Trying to think how many cars have a true disc handrake. Isn't a drum
handrake inside the rear disc the most common? Or is that just German cars?
But a drum also has a self servo action. If the car tries to move, it
actually increases the braking effort. Not so with discs.

Conversely there is little braking when going backwards down a steep
hill, especially without the engine running.

Eh? No servo with any handbrake I've ever seen. And a drum handbrake is
one leading, one trailing shoe. So you get the same self servo action
forwards or backwards.

You're likely thinking of twin leading shoe front brakes which require
less effort going forwards than backwards.

[Fredxx]

Post by Dave Plowman (News)

Post by Dave Plowman (News)
Trying to think how many cars have a true disc handrake. Isn't a drum
handrake inside the rear disc the most common? Or is that just German cars?
But a drum also has a self servo action. If the car tries to move, it
actually increases the braking effort. Not so with discs.

Conversely there is little braking when going backwards down a steep
hill, especially without the engine running.

Eh? No servo with any handbrake I've ever seen.

Not sure what the current MOT standards are, but I recall a while ago a
handbrake need only be 16% efficient for a car with a split hydraulic
system. That would barely hold on a 1 in 6 hill, and if moving not slow
it down.

Post by Dave Plowman (News)
And a drum handbrake is
one leading, one trailing shoe. So you get the same self servo action
forwards or backwards.
You're likely thinking of twin leading shoe front brakes which require
less effort going forwards than backwards.

Quite, where all the breaking is. The rears are unlikely to provide more
than 25% of the overall footbrake braking force, sometimes much less.

[Peter Hill]

Post by Dave Plowman (News)

Post by Abandoned_Trolley

Post by Ian Jackson
The rear remained drum because less braking is required. They are also
considered safer, because if the handbrake is applied when the drums are
hot, as the drums cool down they contract and tighten the grip on the
shoes. Discs do the opposite, and tend to release the handbrake as they
cool down - but despite this, most cars now have discs on the rear
(presumably because of their better braking action).

Its a nice theory, but in reality the considerable length of the
handbrake cable will most likely have an almost constant tension to
compensate for it ?

Trying to think how many cars have a true disc handrake. Isn't a drum
handrake inside the rear disc the most common? Or is that just German cars?
But a drum also has a self servo action. If the car tries to move, it
actually increases the braking effort. Not so with discs.

It isn't all German cars. My bothers Audi A3 has a handbrake on the rear
caliper. Electric handbrake.

So it may only be M-B, BMW and Porsche.

[Dave Plowman (News)]

Post by john west
Looking at most smaller cars i can see the rear brakes are Drum type
while at the front of the car are Disk type.
I cannot think Drums are that much cheaper to manufacture than Disc type
and i thought are less effective than Disc type. So why do manufacturers
choose to make the rear brakes Drums rather than Discs ?

Many cars have a drum handbrake inside the rear disc.

Seems to be cheaper/better to use a drum for the handbrake.

But not impossible to make a handbrake that works on the disc.

It may be to do with making a secure parking brake. With a disc, as it
cools, the grip of a handbrake reduces. With a drum, it increases. Helps
prevent law suits in some countries.