h a l f b a k e r yA riddle wrapped in a mystery inside a rich, flaky crust
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Drum brakes are, on the face of it, far better brakes than disc brakes. They present a greater shoe area, require less operating force, and allow for geometries of application that are finely tunable from zero to a considerable degree of self-servo effect. However, they overheat and fade very rapidly
in even in relatively gentle use, due to the difficulty of keeping their internal components cool. This has led to the universal adoption of disc brakes for anything approaching high-performance applications.
Up to the mid-'60's various attempts were made to cool drum brakes, e.g. finned drums and air scoops on the backing plates. To my knowledge, however, no attempt has been made to arrange air currents through drum brakes, especially through the hottest area where the shoes bear on the drum.
Very effective brakes may be produced by doing so, making use of centrifugal force to draw air through vent openings in the working surface of the drum, much like a centifugal fan does. The vents are arranged in circumferential rows, to which correspond deep grooves in the faces of the shoes. Air is supplied through holes in the bottoms of the grooves. The consequent ±25% decrease in effective shoe area, as compared with conventional drums, may be compensated by making the drums wider, as wider modern wheels provide for more width. This is possible without making the drums heavier.
Airflow can be further improved by doing away with as much as possible of the drum side web, by reducing it to spokes, and the backing plate, by replacing it with a spindly bracket to hold the slave cylinders and shoe pivots. All this makes for savings in weight. Needless to say, the drums would be forged aluminium alloy, for thermal conductivity and lightness, and the friction material would be of a modern type, ceramic-metallic or otherwise.
It's a multitude-of-tweaks solution rather than a radical new concept, but in my experience such solutions often work extremely well.
Cool-looking drums
http://www.9inchfactory.com/brakes.shtml
Available aftermarket items [Ned_Ludd, Nov 29 2007]
More cool-looking drums
http://www.automedi...ool/res20051101db/1
Maserati, in this case, plus tech article [Ned_Ludd, Nov 29 2007]
[link]
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Make drum brakes LOOK cooler and you'll get another bun from me |
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I think aluminum drums would be too soft. Other than that, I'm down with this as a concept. |
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//Drum brakes are, on the face of it, far better brakes than disc brakes. They present a greater shoe area, require less operating force, and allow for geometries of application that are finely tunable from zero to a considerable degree of self-servo effect.// |
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I think I disagree with every point of your first paragraph. So lets’ work thru this and show me what I missed. |
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1. //They present a greater shoe area//
Lets assume a 15” diameter brake with 3” wide pads of each type:
Let’s say the disc brake has a center hub diameter of 12”
Disc area is 2*((15/2)^2*pi() - (10/2)^2*pi()) = 127 sq.in.
Drum swept area is 14*pi()*3 = 132 sq.in. (I assumed a .5” thick drum)
So, I’m wrong, the drums do have a slightly larger swept area.
It’s a hard comparison because drums can be made wider and disks can be stacked.
Your drums would be smaller because you allow cooling passages. |
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2. //require less operating force//
Why, because the pads have a habit of locking? Is that a selling point? |
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3.//allow for geometries of application that are finely tunable from zero to a considerable degree of self-servo effect//
This sounds like you are tuning the locking again. |
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The other problem I have is that your cooling comes from only one side, so the other side will always be hotter and thus thermally expand more. |
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I don't want to sound negative, but I used to drive a Chevette with drums and it almost killed me. |
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My Chevette almost killed me, too. The little rod keeping the pads appart snapped, & we lost all brakes. The memory is very vivid. |
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Mine like to lock all four and spinspinspinspin. |
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Make drum brakes that are easier than disc brakes for a human to install and remove. Make drum brakes that ensure the dust deposit is vented out of the chamber. Make drum brakes that can deal with the differential wear caused by running a curved pad against a curved drum more effectively. Then we may discuss cooling them more efficiently. |
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Until then, I will stick with disc brakes, and possibly consider working on cooling them internally with a line through the radiator or something. |
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[MrQED], drum brakes don't have pads, they have shoes. The relevant measure is not the disc or drum area but the pad or shoe area. In your example (which is somewhat unusually large) the disc brake's pad area is likely to be about 18-24 sq.in, and the drum brake's shoe area about 106-120 sq.in, i.e. five or six times as much. |
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Stacking discs would cause the same cooling problems as plague conventional drum brakes. |
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Real-world braking has a lot to do with pad/shoe area. I know it doesn't conform to f=mu*r, but little does in practice because actual coefficients of friction vary greatly from one spot to the next, or from one moment to the next. The greater the sample, therefore, the greater the highest likely value of mu and the less it varies. Hence, big shoes need less force. This is indeed borne out in practice, both in the case of well-sorted drum set-ups and bloody awful ones of which the Chevette seems to be a prime example. |
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Those are often the result of cheese-paring on the part of manufacturers who wish to provide finger-light pedal pressure without vacuum or hydraulic servo. This is especially the case with the "self-energising" drums so beloved of American manufacturers, where the primary shoe pivot serves as the secondary shoe activator. These are particularly prone to runaway self-servo effect and the unintended locking you describe. |
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By contrast, nicely-done drums require a hefty shove but afford lots of linear, intelligible brake feel. That means placing the pivot points to reduce the self-servo effect so that there is a perceptible little grab to tell you the brakes are happening, but little more. My Morris Minor's brakes were like that, lovely for the first two minutes. Thereafter, being the size of large bottle-tops, they would overheat and cease to exist for all practical purposes. That's why I fitted discs on the front instead. |
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[evilpenguin] and [Texticle], haven't you ever seen finned aluminium Buick drums on a traditional hot rod? Tell me that doesn't look cool, [ep]! |
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The pad size is not limited on disc brakes though, it just doesn't need to be any bigger. I'm sure you could quite easily add another pad on the opposite side of the disc to double the size. |
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What you could do is work on Stirling regenerative braking drums. |
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Small pads are good for cooling, and practical as long as one assumes vacuum/hydraulic servo levels of applying force. I've got non-servo discs on the Morris, though. They're effective but require a lot of pedal pressure. |
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How about filling the ducts not with air (which is a poor conductor of heat) but with a sodium/potassium mix ? Centripetal forces and convection would provide very efficient heat transfer, plus spectacular pyrothecnic effects during crashes if the containment fails (available with a choice of heavy metal dopants for a selection of pretty colours as the inextinguishable metal fire consumes the remians of your vehicle). |
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I can't help but notice that drums have all of the shoe surface acting at the same distance from the hub, whereas only the outermost part of a disc pad is acting with the most leverage. |
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but the pressure applied by a disc pad is much more evenly distributed over the entire surface of the pad allowing for much greater level of control and more overall friction. Cars like the new corvette actually have 6 pads and 6 pistons on the front brake rotors. Also since in a disc application the pads only operate on one side there is 2xs the amount of time for heat dissipation than in a drum application. |
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Further consideration is that the braking force on a rotor is compressive in the axis of brake application and the material is very strong in compression and suffers from minimal deflection. In a Drum the material has to overcome the radial force of the brake pressure so a certain amount of stretching or deformation of the drum out of round will occur, the material must be correspondingly thicker to offset this. thus the brakes are more massive than an equivalent disc. |
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What if you used a fairly thin drum with pads on the outside to treat it like a disk as well? It might even help to keep the drum from getting warped. And it would allow for greater stopping force from less space. |
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then it is just a more complicated disc brake. |
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This is an important discussion, because unless the laws have been changed, drum brakes are manditory equipment on semi-trailers in the USA. Probably because it is harder to get the failsafe brake application to work as easily with discs. |
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Your idea basically sounds like cross drilling, which is done on discs, but I don't think it was done on drums and it should work just as well as long as it doesn't weaken the structure. |
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It is indeed cross-drilling - and I have never seen it done on a drum brake - but it is cross-drilling not only through the drum but also through the steel backing of the brake shoes. Hence the grooved friction material. This is because the overheating problems of drum brakes are caused to some great extent by the fact that the shoes shroud the inner surface of the drum and do not allow an adequate volume of cooling air to reach that area. |
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