Whats the purpose for drum brakes?

mezuk04a · Post by **mezuk04a** » 08 Jul 2007, 16:22

Just out of curiousty.

They seem to be more time consuming to fix, a lot more messy/complicated to fix and are less reliable and you cant have a quick reasonable glance to see the condition of them.

So whats the reason why disc brakes were never used for the rear of cars, although they are now to a certain extent. Why wouldnt they use them, from my little knowledge it looks cheaper for the manufacturer to put discs on the back.

Regards
Chris

AndersDK · Post by **AndersDK** » 08 Jul 2007, 16:41

Drum brakes were the first brake types developed that showed adequate friction and reliability.
Its all about technical & economic evolution. The drum brake type has been refined during many years to reach a state of most economic & reliable type. But has since been abandoned in favour of the disc brake system.

Its not more than some 25-30 years ago (in my youth) where it still was legitimate to spend hours dismantling, cleaning, lubing and reassembling drum brakes as routine jobs in garages.
Because of the economic evolution, lots of methods, techniques and ideas have been abandoned - solely for cost reasons.

Its been an ever faster growing ping-pong between economic & technical ecvolution the last 50 years or so. The more money you can get in, the more you can spend on technical evolution and cost effectiveness, returning even higher revenues, to spend even more ... etc. etc.

Remember the first PC you ever saw in your life - the cost of it ? How does it compare to todays state of the art

Post by **CitroJim** » 08 Jul 2007, 19:56

Discs, although simpler in concept and construction, took a long time to mature into a cheap and effective product. Pad material composition and disc material and casting techniques are complex and took a long time to get right, especially wet braking performance and long life.

The big advantage of drums is that you can get a massive area of lining (pad) material in and the effort to make them work well from the brake pedal (or bike lever) is relatively small. Discs on the other hand, have a relatively tiny area of pad in contact with the disc and therefore need a very high braking pressure applied, hence the universal fitment of servos. These high pressures taxed the metallurgists to come up with a suitable disc material that had a long life and did not warp under heat. A tough task. Ditto the people making friction material.

The big problem with drums was fade due to the drum being unable to dissipate the heat generated under heavy braking. Some may recall finned drums you could once get for Minis. Discs, being open are very easy to keep cool and can run very hot and still work well.

Back when I were a lad I was into my motorbikes. I had a Suzuki T500 with a twin leading shoe drum brake on the front. It took a bit of effort to set up but it was great. A really fantastic anchor, wet or dry. The new model GT500 that replaced it had a disc on the front and it was a shocker. The disc itself was made of heavy stainless steel and worked terribly in the dry and not at all in the wet

It took the bike makers a while before they got it right.

Personally, on a car I prefer rear drums. They don't suffer all the problems of the Xantia ones (and rear discs in general) and as rear brakes don't get used that hard, drums are more than adequate and give a supurb handbrake very easily. Only Xantias seem to have a brillinat handbrake on discs but then look at the pad area and the length of the lever used to apply them.

jeremy · Post by **jeremy** » 08 Jul 2007, 20:35

The simple answer here is that by the 1950's drum brakes were getting an embarrassment on some cars and something drastic was needed to combat the fade problem. What I have in mind are sports cars like XK120 and XK 140 which would get to 100 MPH on a very short stretch of road and had to stop again.

Some fought against discs like Rolls Royce who used a 3 trailing shoe arrangement on the Silver Cloud front drums but they were a bit different anyway.

I know what Jim means - a good drum setup feels nice and sharp largely due to the self servo effect pulling the leading edge of the shoe into the drum - and using this meant that a servo could be avoided. I suppose the last and best drum setup I has would have been on a Morris Oxford (yes that long ago) which had twin leading shoe fronts and would stop very well within its limitations. hose limitations were that it would fade if driven hard through a series of roundabouts and one good stop from 70 would give a noticeable build up of pressure.

The good old motor trade had difficulty getting the handbrake to work with discs and so the disc/drum system became popular - partly as a method to get a working handbrake.

My complaint about mixed systems is the lack of progression and strange light pedal braking caused by the force needed to overcome the pull off springs on the rear drums. With a nice all disc ststem I can get a nice retardation by resting my foot on the pedal - with a disc drum I have to stretch the springs to get the back brakes to work at all.

My own feeling is that after 50 years of mass production the cost advantage is probably very small and as I have to look after the things - the disc is much easier as well as being better in use.

Weight may well become an issue in the future as more emphasis gets placed on fuel consumption - in which case the drum brake with its self servo effect could make a comeback and allow the servo to be deleted but to be honest I think this will be some time off, and if hybrid drives become the norm we will get regenerative braking anyway.

bxbodger · Post by **bxbodger** » 08 Jul 2007, 20:44

Oh yes....I recall the drama of discs in the wet on bikes in the seventies and early eighties

: the holy grail of bike braking back then was discs and pads that would work in the wet. You're right: you could rely on a properly set up TLS drum to stop you whatever the weather.

I suppose drums linger on in cars because the factories are tooled up for them, and they must cost next to nothing per unit because all they need do is transfer the stub axle design, drum, and handbrake mechanism,straight onto the next model with no changes and no development costs. Plus the hydraulics will be simpler, as with many disc/drum set-ups there's no need for a load compensator as the front disc is so much more powerful than the rear drum.

I wouldn't say that a drum brake is any more complex than a disc to service: they all self-adjust these days so there's none of that fiddling with a seized square-head adjuster, and there's no sticky or worn caliper slides to bother about. Generally, a drum brake needs no attention other than an occasional shoe change.

AndersDK · Post by **AndersDK** » 08 Jul 2007, 20:54

bxbodger wrote:Generally, a drum brake needs no attention other than an occasional shoe change.

Eeeerrr -
What about the dreaded dust build up in drum brakes
- the usual cause to un-even rear braking ?

jeremy · Post by **jeremy** » 08 Jul 2007, 21:23

You're right Anders. It seems that all modern drum brakes require the bearing to be dismantled simply to inspect the linings, the self adjusters are delicate and seize with monotonous regularity and the handbrake mechanism is none too clever either.

Best drum handbrake was the old Lockheed one used on Triumphs and Hillman Hunters of about 1970 vintage - nice simple lever system with a self adjuster which was a ratchet plate rather than some screw system which seizes.

Sl4yer · Post by **Sl4yer** » 09 Jul 2007, 01:18

It always amazes me to see drums on the back of a modern car, especially when the 'performance' version will have discs. I have no experience of front drum brakes on cars, but what I have heard over the years is not too encouraging!

Do drums have a problem with ABS? I know that discs were fitted to all ZXs with ABS. I thought this was because drums were unsuitable for ABS, until my father got his Laguna in 2000.

James

jeremy · Post by **jeremy** » 09 Jul 2007, 07:55

I don't think there is any fundamental objection to using ABS with drum brakes - just a case that no-one had bothered to develop it.

ABS was originally installed on things like BMW (who claim to have invented the current systems and allowed all to use it in the interests of safety - which was probably a reaction to being told they couldn't patent it) and these cars would have had discs all round.

As an option it used to be quite expensive (£800 or so on a 1989 BX) and so was probably only really of interest to the buyers of the higher end models.

Now I think its compulsory so manufacturers have to fit it to the cheapies as well.

Homer · Post by **Homer** » 14 Jul 2007, 01:11

The handbrake issue - drum brakes are designed so that if a wheel turns with the handbrake applied the turning force will drag the brake shoes outward providing extra braking force, which makes handbrakes much more efficient.

You don't get that effect with disk brakes.

Disks are much better at dissipating heat, and work well with servo or power assistance.

dnsey · Post by **dnsey** » 14 Jul 2007, 03:07

Who remembers the 'Caution - Disc Brakes' stickers

?
More marketing hype than necessary warning, probably!

reblack68 · Post by **reblack68** » 14 Jul 2007, 08:49

When the Mk4 Escort came out Ford were very proud that it had the option of ABS. I don't think I've ever seen a Mk4 Escort with ABS but I'm sure they didn't put discs on the back of them.

1986 ad break including Escort ad

jeremy · Post by **jeremy** » 14 Jul 2007, 09:47

Early disc brakes could be very efficient and I should think were getting around 0.9g by the mid 50's. I doubt if many drum systems were capable of that and could also suffer from fade.

But largely a gimmick - discs were optional on some cars at the beginning - so having spent all that money on them you wanted something to show for it - just like ABS, automatic, EFi, TD etc.

Homer · Post by **Homer** » 14 Jul 2007, 21:00

reblack68 wrote:When the Mk4 Escort came out Ford were very proud that it had the option of ABS. I don't think I've ever seen a Mk4 Escort with ABS but I'm sure they didn't put discs on the back of them.

The XR3 (Mk3 escort) models certainly had disks on the rear because owners moaned at how poor the handbrake was because they couldn't pull handbrake turns in it.

nick · Post by **nick** » 14 Jul 2007, 23:23

I've just had a quick look at a Ford brochure I have from Dec 1987. All versions of the Escort mk4 had drums on the back by that stage, even the XR3i & those fitted with optional ABS.