Drum Brakes rather than Disc Brakes

In message <snibh1$t1q$ snipped-for-privacy@dont-email.me, john west snipped-for-privacy@mail.invalid writes

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).

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.

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 ?

It isn?t just small cars / vehicles. Our motorhome, plated at 3.65T, has drum brakes on the rear and disks on the front. The chassis can be up plated to at least 4T without changes to the brakes.

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 :-\

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!

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.

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

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.

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.

Yup. I had an old Bentley which had twin trailing shoes to the front brakes. With a very powerful servo. Seems training shoes are less prone to fade in use.

...and if one side was on a wet grassy verge the handbrake didn't work at all.

If you applied it while in

'''and if it was a series LR it would eventually snap the shorter rear half shaft.

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.

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

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.

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