drum brake advantages

[Tosh]

Disc and drum brakes can both be well-designed for their intended purpose, as long as they're also well-maintained. I always assumed that discs were better for linearity (feel), fade resistance, reliability (simplicity), and light weight, at the expense of higher cost. In these times of high precision machine tools being common, it's hard to imagine that the extra complexity of drums is still cheaper than discs, but this must reflect some sort of supplier economics oddity.
There's also the marketing aspect of discs being perceived as safer and sportier, and one often sees rear drums on the low trim models and discs on the higher-up models (even with the same engine, a la Accord I4 or Camry I4). I don't look forward to ever having drum brakes again, though, as they're troublesome, ugly, and old-fashioned. (Having said that, the front discs on my upscale Honda keep warping, but that's another story.)

 

[Mitch]

Well lets see,At my work we have several c1500 chev trucks.The '99 and '00's have disc/drum and the '02 and '03's have disc/disc and finally the '04's have disc /drum again.Makes you wonder why GM did that? Jeep Wranglers in'03 and later have the option of 4 wheel disc.The Rubicon edition has 4wheel disc standard

 

[John K]

Often the discs are warped due to improper torqueing of the lug nuts. The only disadvantage I am aware of with discs is the complexity of providing emergency brakes(and sometimes noise). Often they are a small drum. I think the way they compare disc and drum is to measure the swept area.

 

[ekpolk]

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Originally posted by John K:
Often the discs are warped due to improper torqueing of the lug nuts. {snip}

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I've been lucky enough to have them warped both thermally and by gross overtorque. That was the last time the geniuses at PoopBoys, or is that Pepboys, ever touched my car.

 

[moribundman]

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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.

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Which Unimog? The older 400 series had drum brakes, some of them only single circuit. The latest Unimog, U3000/4000/5000, as well as the U300/400/500 with the huge glass canopy may well have disc brakes.

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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.

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I know that on my car the caliper guide pins will make the caliper slide away from the disc after releasing the brake. The guide pins always push against trapped air inside the caliper guide holes, which makes the caliper and guide pins behave as if they were spring-loaded.

 

[Papa Bear]

Been driving since 1960 (1st car '51 ford). The nicest brakes that I ever had were drums on my '68 Riviera by far.
P.B.

 

[labman]

I think rear drums make a lot of sense for FWD cars where the rear brakes do very little of the stopping because of the lack of load. The rear end is a good place to reduce both initial cost and upkeep by using simpler components in FWD cars. The performance of my Cavalier and the Grand AM and Phoenix I had before it, suffer little for the beam axle and drums. Yet all 3 cars were low priced and cheap to maintain while providing good performance for the dollar. I have never had much luck with self adjusters, but have been spared the problems with disk parking brakes. Since the parking brake is the main function of the rear brakes, why not go with the more workable drum? ABS reduces some of the problems with non linearity and self actuation.

Unfortunately drums have the same problem with warping and over torquing

Trucks may be sticking with their drums because their huge wheels leave plenty of room for brakes big enough to handle the heat.

 

[brianl703]

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Originally posted by ekpolk:
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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Discs don't necessarily warp just because they've gotten hot..I believe they warp because they've gotten hot and cooled unevenly.

My mom's old car was making a squeaking noise sometimes when braking. I checked the pads and there was plenty left.

I fixed that problem by doing about 10-20 hard stops from 55MPH. When I smelled the brakes, I stopped. I wonder how hot the rotor was at that point? Then I drove it for a couple of miles at 55MPH without stopping.

That did not warp the rotors, and got rid of the squeal. I suspect that if I'd stopped with the rotors as hot as they were, yes, they would have warped due to uneven cooling.

 

[Mitch Alsup]

Disks are superior because they are stiffer in several aspects of operation. A) the calipers clamp the pads directly to the rotor, B) the outside of the rotor to inside of the rotor is stiff, C) the pads carry the clamping forces through a short distance (backing+pad material itself), D) really good disks (like Brembos) apply clamping forces from both sides and the calipers are very stiff indeed. All leading to a linear relationship between pedal pressure and rate of retardation.

The stiffness of the situation is capable of removing a thin film of water in a short fraction of a second from the surface of the rotor.

In addition, disks with internal air directing vanes dispate (convect) heat much better and prevent heat from getting to the fluid (conduction) and when they ger really hot, there are quite efficient in radiative heat transport (glowing orange rotors).

Finally, discs 'decorate' the wheels with their expended pad material rather than filling up the drum housing.

[ September 23, 2005, 12:57 PM: Message edited by: JHZR2 ]

 

[Shannow]

Also, the air braking systems of big trucks lend themselves to drum brakes.

 

[ekpolk]

quote:

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Originally posted by brianl703:
{snip}

Discs don't necessarily warp just because they've gotten hot..I believe they warp because they've gotten hot and cooled unevenly.

{snip}

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This is, I suppose the usual non-torque-related way warping happens in a car. Truthfully, I don't know if it happens often in passenger cars, or ever, but heat alone can deform brakes once you reach a certain point. I experienced this frightening phenomenon during my flying days. In aviation, to save weight, brakes are designed with the aircraft's typical landing weight in mind (a big reason why most jets are designed so they can dump fuel in an emergency). The EA-6 which I flew would typically land in the neighborhood of 40,000 lbs (it's about 34k empty). In contrast, max gross takeoff was 61,500 or more than ten tons over normal landing wt. High-speed aborted takeoffs, when they happened, were a challenge. They could easily result in outright brake fires, or if you were lucky, merely physically destroyed brakes.

Obviously, in a passenger car it would be difficult to introduce the massive overdose of energy described above, but the point is, as a certain point, heat will cause the metal to begin losing its structural integrity and warping.

 

[ekpolk]

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Originally posted by labman:
I think rear drums make a lot of sense for FWD cars where the rear brakes do very little of the stopping because of the lack of load. The rear end is a good place to reduce both initial cost and upkeep by using simpler components in FWD cars. The performance of my Cavalier and the Grand AM and Phoenix I had before it, suffer little for the beam axle and drums. Yet all 3 cars were low priced and cheap to maintain while providing good performance for the dollar. I have never had much luck with self adjusters, but have been spared the problems with disk parking brakes. Since the parking brake is the main function of the rear brakes, why not go with the more workable drum? ABS reduces some of the problems with non linearity and self actuation.

Unfortunately drums have the same problem with warping and over torquing

Trucks may be sticking with their drums because their huge wheels leave plenty of room for brakes big enough to handle the heat.

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Good points with respect to the economics. Undoubtedly why drum brakes are still around in the first place (on cars anyway).

Giving it some thought, I had assumed that drums were not susceptible to warping for two reasons. I've owned several rear-drum cars and never seen it myself. Second, at least 20 years ago, the first time I had to get rotors turned, I remember the mech going on and on grumbling about the hassles of disc brakes and how this never would have happened with good old fashioned drums. Shoot, it's 0730 in the morning and I've already learned something new today. . .

 

[John K]

I had a 95 Ranger w/discs frt drums rear. It had ABS, and it was the loudest abs pump I had ever heard. I figured it was due to the distance the drum brakes had to travel when abs was activated. There have been many complaints about GM abs systems, and I think it is due to the disc/drum combination. A good system with 4 wheel discs operates much better in my opinion. Disc brakes have been made affordable by such innovations as floating calipers, but with that often comes uneven pad wear. My 79 Volvo had 4 wheel discs, and had 4 pistons on the front brakes and 2 pistons on the rears. That was too expensive for Detroit, but it was a good performer. Many of these innovations are only utilized by Detroit after they find ways to produce it much less expensively. That is both a good thing and a bad thing at the same time. I am always leary of the impact of the accountants, which imho, have too big of an impact on American iron. My $0.02 worth.

 

[ikeepmychevytoo]

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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 think this is easy... They are self energizing, like having a spare booster. Secondly all the swept area is located at the biggest diameter for better leverage. I'd expect the feel to improve as the diameter increase. Also drums may have two clylinders (one for each liner) for better progressiveness.

Having the assist lost is not all about tearing one line... one of those days that my mind was in the air I simply forgot to get gas and found myself with no assist on the left lane prior to the fist red light on the city entrance. I can't verify what if the rears were discs, but although the feel was crappy that drums were graabing easily. More than 3500lbs and muscle power is causing some alertness in my mind.

So I perceive this more than anything as a "design preference" not a technological achievement or a sole ecomnomical constraint.

 

[ikeepmychevytoo]

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Originally posted by moribundman:
Which Unimog? The older 400 series had drum brakes, some of them only single circuit. The latest Unimog, U3000/4000/5000, as well as the U300/400/500 with the huge glass canopy may well have disc brakes.

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Not really sure. They were looking like older series of U3000, very similar apart from grille style. Was syled like the ones in the 80's.

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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.

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I know that on my car the caliper guide pins will make the caliper slide away from the disc after releasing the brake. The guide pins always push against trapped air inside the caliper guide holes, which makes the caliper and guide pins behave as if they were spring-loaded.

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Hmmm... may be the rubber caps on the sliders' bolt heads were probably for better air sealing ?.. I was curious why there were rubber boots, and if for rust, why no boot for the bleeder screw.

 

[darryld13]

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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This informative ArvinMeritor presentation points out that the first air disc brake wasn't released in North America until 1981. As of 2002, Europe had migrated 69% of their air braked vehicle production to discs, so perhaps the trend in the U.S. will be toward discs as well.

The presentation does a good job of explaining the technical advantages of air disc brakes for trucks and buses.

 

[SWS]

I have to agree with John K that 4-wheel disc brakes will probably do better than rear-drums with ABS. I also think that the rather new Vehicle Stability Control (VSC) on newer vehicles would probably perform better than discs. I am not a truck shopper, but on cars with VSC I have noticed that they are all 4-wheel disc - this may or may not be significant. Anyway, it makes sense to me that discs, by design, would provide FASTER braking response (engage & release) when needed for ABS & VSC. Of course, the brake fluid pressures must be tremendous!

One reason that rear-drums may still be cheaper is that integration of the Parking Brake is easier. Disc-brakes do not make great Parking Brakes, so many times there is a separate little drum brake in the center of the rear hubs. However, note that I had a '92 Ford Taurus with 4-wheel discs that actually used the rear discs as the parking brake. It held on steep grades well, but the parking cable & lever assembly at the disc caliper needed attention to keep from the disc pads from dragging / binding. I think that Ford changed to the separate drums for the Parking Brake function in the mid-'90s - not sure.

In general drum brakes have been a dissapointment to me as Parking Brakes since they don't hold in reverse very well, even when new! I was told over the years that this was because of the self-adjusting mechanism, which also don't work on drum brakes very well !

Rear-drums can be slightly warped if you apply the Parking Brake pretty hard when the brakes are still hot. I did this on 2 cars in the '70's. However, this does not seem to be a problem with my '97 Camry and '00 & '03 Hondas, all of which have rear drums.

Finally, the brake-dust collection within the rear drums is an on-going problem. I generally just have the rear drums pulled every time that I rotate the tires so that the dust can be dumped out. They always work so much better after this simple step!

 

[Audi Junkie]

So, wet brake componets react just as fast as dry ones? Mmm..ok.

Stoptech claims rotors don't often warp. The shaking is due to pad material transfer and build-up. It's worth reading their tech section.

 

[moribundman]

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So, wet brake componets react just as fast as dry ones? Mmm..ok.

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Drum brakes are not sealed that well and water ingress happens. Water will probably vaporize less quickly in a drum brake than the water on a disc.

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Stoptech claims rotors don't often warp. The shaking is due to pad material transfer and build-up. It's worth reading their tech section.

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I've never warped a rotor on any car, but I've only used OEM parts. Cross-drilled rotors appear to more often warp and crack than plain rotors. I like slotted rotors, which offer advantages similar to cross-drilled without the added cost and possible problems.

 

[John K]

SWS - That was something I had forgotten about, emergency brakes not working well in reverse. On the Ranger, I always set the emergency brake, and I could back out of a parking space with no difficulty w/o releasing the emergency brake, and as soon as I tried to go forward I of course knew I had forgotten. The difference was amazing. Even without the self energizing aspect, it seems if you set the brake firmly it should hold in either direction!