drum brake advantages

[JHZR2]

Hi,

Every car that I own has 4-wheel discs. Ive always viewed 4-wheel discs as superior. This is mainly because 1) theyre 'sportier' and more performance oriented 2) they look good and 3) (most importantly) I know how to do disc brakes and rotors in notime, but dont know how to do brake drums and adjust them properly.

For a car that isnt 'racing' and at most will need to do a strong 80-0 stop, or a number of repeated 40-0 stops, with a number of feet of rolling between (like in speedup/stop and go traffic), is there any real advantage to rear discs?

As I understand it, drum brakes have less drag, though Im not sure if thats an issue after driving, say 10 miles without touhing the brake pedal. It seems like brake drums have a lot of mass, which is good for absorbing the kinetic energy, and have a lot fo surface area for cooling.

Are there any real, notable, worthwhile advantages to having rear drums? so many cars are moving to 4-wheel discs, but then again, large trucks arent... and, not too long ago all cars had rear drums or 4-wheel drums... its just one technology to the next... not necessarily any real advantage for small, light, economy type cars.

Any ideas?

Thanks,

JMH

 

[ekpolk]

Drum brakes may have some advantages, but I don't see how they apply to passenger cars. I have the odd experience of having owned an 02 Camry I-4 with drums on the rear and an 03 V-6 with discs in the rear (the 02 was destroyed in a wreck -- don't ask about why I now have a G35...). I'm very sensitive to minute things in cars, I'll admit, but I noted a distinct difference between the two in braking performance. It's hard to verbalize, but the 4xdiscs setup just feels more controllable, connected, and powerful (maybe I should say "anti-powerful"). Years ago, I read a technical explanation of why disc brakes feel better, but I've forgotten the how and why of it. My hunch is that in the all-disc setups, the rears are actually doing more of the work* (understanding, of course, that in cars, the rears are relative slackers anyway). To me, I-4 Camrys with drums feel like they are wobbling on their tiptoes when braking hard, whereas the V-6 (actuall much heavier in the nose) feels like it's just sitting on rails regardless of how hard you mash the brakes.

EDIT: *more of the work relative to comparable drums, NOT relative to the fronts, just to clarify.

 

[Audi Junkie]

Drum's performance is not affected by wet conditions.

 

[kenw]

quote:

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Originally posted by Audi Junkie:
Drum's performance is not affected by wet conditions.

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yeah, they're just as bad wet or dry.....

 

[nel]

4-wheel discs feel more linear than systems with discs in front but drums in the rear. The reason is brake biasing. The bias is a constant split, and with 4-wheel discs that works perfectly (or closer to it).

The issue with drums is that they do not act in as linear a fashion as the discs (brake torque vs. brake pressure), and that makes the engineers' job more difficult. They have to consider the operating envelope of the vehicle, worst case conditions and even legal liabilities. I spent my last year as an engineering student at GM's Wheel & Tire Engineering at the Milford Proving Grounds. While interesting, I couldn't see myself in wheel design for long.

 

[TallPaul]

Drums don't cool as well as discs and therefore are more subject to fade with heavy use.

I thought drums were very bad after going through a puddle, but discs don't seem to be affected.

 

[jtantare]

Drums are lighter and cheaper to produce.

 

[nel]

Drums can be also more fuel efficent than discs. The return springs ensure there's no drag of the lining against the drums while there's always a little drag with discs.

As TallPaul stated, discs can be better going through puddles because discs are self-cleaning--because of the slight drag and the open path for debris to fall out.

 

[mechtech]

Many modern cars have ABS. This works much better with rear discs.
Rear drums are self energizing, and when racing can swing the rear out on turns - they are not as linear and predictable as discs.
I like having rear drums for the street.
In the shop, there are all sorts of sticking, wear, and parking brake problems with rear discs. [Maybe because they are lightly loaded and used.]

 

[Shannow]

quote:

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Originally posted by Audi Junkie:
Drum's performance is not affected by wet conditions.

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garbage.

my first car (as should everyone's first car be) had four wheel drums.

They are affected by standing water like you would not believe, taking it in turns to lock (front left, front right, etc).

The drum expands away from the linings and cylinders, meaning that with a crash stop from 80MPH you had to pump them back up when you got down to 30-25MPH...When you are emergency braking, your mind won't let you lift off, even when you have penetrated the firewall.

If you want drums (4 wheel) to actually stop, then they are adjusted to drag always. The return spring pulls them to a "sheeeeeeshh" sound as you spin the wheel.

The seals in a disk brake do a better job than the springs in a drum.

 

[moribundman]

quote:

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there's always a little drag with discs

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Well, if I spin my wheels by hand, I don't hear any drag (With brand-new pads I do, but only for the first 50 miles or so), so if there is drag, it must be minimal. When I lived in areas with snow I got rid of ice on the brake discs by braking slightly every couple minutes. If there had been drag, no ice could have formed due to mechanical abrasion and warm rotors. Some new cars (Audi) are equipped to apply the brakes automatically ever so slightly in regular intervals in wet conditions to keep the discs clean, dry, and ice-free.

 

[moribundman]

I imagine that under really dirty and sandy conditions drum brakes may be better than disc brakes in terms of longevity. What type brakes do real off-road vehicles like a Land Rover, a Jeep Wrangler, an MB-G, a Hummer, etc, use?

 

[ikeepmychevytoo]

Drums can be "self energizing" type. Real advantage is when the assist lost. To have an idea of how much power required to activate each type just compare the diameter of the cylinders on a disc and a drum. (front to front and vice versa)

Discs tend to continue grab for a little while after you release the pedal, because -indeed- seals are not doing a better job than the springs. The liner on a drum is being applied at an angle (that's what makes them self energizing). As the drum rotates converts the rotational energy to the pressure outwards. That's why they are not so progressive.

There are many more variables. Install a big finely adjusted drum and observe the repetitive 80-0 fades over a disc... On a rear drum setup rears needed to be applied more earlier for directional stability. If a poor design decided to use same proportioning/advance "optimized" for both types, surely may suffer directioanl stability or may have more front dive with rear drums.

 

[ikeepmychevytoo]

quote:

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Originally posted by moribundman:
I imagine that under really dirty and sandy conditions drum brakes may be better than disc brakes in terms of longevity. What type brakes do real off-road vehicles like a Land Rover, a Jeep Wrangler, an MB-G, a Hummer, etc, use?

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The rough terrain truck of Unimog (the only thing that could compare to Hummer, one that have axles ex-centricly higher than wheels) has four discs with equal diameters. It's load capacity was high and I was surprised to see all-discs.

 

[JHZR2]

wow... great info... thanks!

JMH

 

[Whimsey]

For the non-technical part of this discussion I grew up with 4 wheel drum brakes in such classic vehicles as a 1958 Olds 88, a 1961 Chevy Impala, a 1966 Ford Custom 500, a 1968 & 1970 Plymouth Fury, a 1972 VW Bettle and a 1972 Mustang. I can honestly say that today's vehicles with at least front disc brakes stop a h*ll of a lot better and require less maintenance

. I noticed that the new GM full size p/u trucks have drum brakes in the rear again. Was this done because rear drums with front discs are better than 4 wheel disc brakes? Or is this a cost savings for GM

?

Whimsey

 

[JHZR2]

Ill turn that around and ask, why do many large vehicles like large busses (and I guess tractor trailers) still have drum brakes on all axles?

Today's fullsize trucks are being designed to lug more and more and more... especially trailer weight. I have to wonder if its easier to make a drum that can take the beating of stopping with a significant load, yet not have too much bite, etc. when the vehicle is unloaded...

???

JMH

 

[ekpolk]

quote:

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Originally posted by JHZR2:
Ill turn that around and ask, why do many large vehicles like large busses (and I guess tractor trailers) still have drum brakes on all axles?

Today's fullsize trucks are being designed to lug more and more and more... especially trailer weight. I have to wonder if its easier to make a drum that can take the beating of stopping with a significant load, yet not have too much bite, etc. when the vehicle is unloaded...

???

JMH

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I have heard that one advantage of drums is that they can absorb massive amounts of heat without warping like discs can and will once they get too hot.

 

[darryld13]

quote:

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Originally posted by ekpolk:

quote:

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Originally posted by JHZR2:
Ill turn that around and ask, why do many large vehicles like large busses (and I guess tractor trailers) still have drum brakes on all axles?

Today's fullsize trucks are being designed to lug more and more and more... especially trailer weight. I have to wonder if its easier to make a drum that can take the beating of stopping with a significant load, yet not have too much bite, etc. when the vehicle is unloaded...

???

JMH

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I have heard that one advantage of drums is that they can absorb massive amounts of heat without warping like discs can and will once they get too hot.

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This is partially true. Potentially drum brakes can stop in a shorter distance than disc brakes because of the larger braking shoe vs. pad contact-to-surface area ratio until it comes time to get rid of the heat. Drum brake shoes contact more surface area with brake material than the smaller disc pad, so the heat does dissapte over more surface area. But the air passing over the open disc of a disc brake system carries heat off faster than the closed drum system can, negating the initial advantage. The first law of thermodynamics says that because energy is conserved, the drum brakes lose energy for braking compared to discs because of the heat retained (change in internal energy = heat added - work done by system).

[ September 23, 2005, 06:17 PM: Message edited by: darryld13 ]

 

[ekpolk]

Darryl: I agree with what you're saying here. My point, though, was more one of metalurgy and applied engineering, than it was one of thermodynamics. The bane of disc brakes is, IMO, warping. Once they've warped, I don't care how superior they felt when new, or what the relative energy handling characteristics are, they will then feel like absolute crap. Discs may shed energy better than drums, but they simply don't have this Achilles heel as the discs do.