Modern Cars tend to have disc brakes .Vintage and Classic Cars ted to have drum brakes.Disc brakes perform better and therefore are deemed safer .In fact disc brake conversions are applied to some classic cars.Why is it that trucks still persist with drumbrakes?
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1Possible duplicate? ... mechanics.stackexchange.com/q/6381/4152– Pᴀᴜʟsᴛᴇʀ2 ♦Commented Jan 25, 2018 at 14:18
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3Drum brakes are better for hand brake on rear wheels. Hand brake works worse on disc brakes.– i486Commented Jan 25, 2018 at 14:22
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1@i486 - My experience lends credence to that statement. My Camaro with disc brakes only e-brake didn't like to hold on inclines. Caught my car scooting down an incline at about 6" every 15 seconds. Quite interesting to see, lol.– Pᴀᴜʟsᴛᴇʀ2 ♦Commented Jan 25, 2018 at 14:43
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2look specifically at this answer: mechanics.stackexchange.com/a/15462/15036 The main advantage to drums that is often overlooked is that they take much less activation force. In this day where we all have power brakes it doesnt matter, but was a big deal in the past and I assume still an issue for big trucks.– agentpCommented Jan 25, 2018 at 16:05
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1@Criggie Not only period correctness. Still haven't seen this reason mentioned before, but I find it an important one: your rims will stay much cleaner for much longer with drums. Especially if you have nice alloys or chrome wire wheels, it's nice to still have 'em shiny after a few brake sessions. With drums they'll barely accumulate brake dust at all, with discs, already after a few 100 km.– BartCommented Jan 26, 2018 at 10:51
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5 Answers
It's not really fair to say as a general statement that disc brakes perform better than drums.
Brake effectiveness depends largely on the "swept" or contact area. In a truck, the shoes "liners" and drums are huge - far more surface area in contact than a pair of brake pads.
It is true disc brakes offer many advantages: compact, ease of maintenance, constantly self-adjusting for wear, cooling, and reduction of unsprung and rotating mass at each wheel. Discs are the choice for modern performance vehicles.
However, provided the braking forces are equal, there is nothing inherently less "safe" using well-adjusted and maintained drum braking systems. On a truck with air brakes, the braking force is accomplished with mechanical springs. The air pressure removes the braking force and allows the truck to move. If there is a failure, the brakes default to being applied. With a hydraulic disc system, a hydraulic failure can lead to no brakes at all.
On Edit:
One more salient point: discs transfer heat a lot better into the fluid, which can boil and make braking next to useless. Drums have a lot of isolation from the fluid as the only real contact is the shoe tabs or pushrods against the cylinder pistons.
answered Jan 25, 2018 at 2:57
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4I would add that drum brakes have the capability to self-actuate. (this depends on the design of the drum break, some do not have this capability) This capability reduces or eliminates the need for a power booster. Disk breaks must have a power booster.– vini_iCommented Jan 25, 2018 at 8:49
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1Disc brakes in vehicles the size of a car or larger need assistance. Motorcycles do not need vacuum assist for brakes as they are more than able to be locked up with just the strength of your hand/foot.– MauroCommented Jan 25, 2018 at 9:08
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1@SolarMike I'm referring to the action of the leading shoe applying additional force to the trailing shoe.– vini_iCommented Jan 25, 2018 at 9:28
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1Most cars I drove with drum brakes were terrible in the rain. You had to learn to "dry" the brakes by applying them often, or you would find yourself with zero brake power when it mattered most. Commented Jan 25, 2018 at 12:20
Trucks are vehicles that carry significant loads on the bed or on towed trailers. Correctly sized leaf springs work better in those situations over coil springs. Leaf springs also negate the need for trailing rods etc, so work well in the back.
Disk brakes are better at stopping from high speed, drum brakes are better at stopping heavier loads from lower speed. (assuming everything's correctly adjusted etc)
Disks are lighter and cool down faster, which is good for a car (race or otherwise) but for a heavy vehicle.
Answer Drums are sometimes a better choice than disk brakes, all things considered.
Additional points:
Drums feature on 4WD vehicles for long while after cars went disks. Again, the requirement of stopping heavier loads from slow speed is more of an advantage than hauling down from open road speeds.
In addition, a drum can survive impacts that would damage a disk brake rotor, so less likely to be stranded in the back-country.
Last advantage is pricing - disks cost more to make than drum brakes, so particularly cost-conscious products may be specified with whatever's cheapest.
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Having seen a truck with >20t of load going down a steep hill with its brakes smoking, I wonder how faster cooling would not be good for a heavy vehicle. Commented Jan 25, 2018 at 14:55
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1Usually, "lighter and cool down faster" is directly correlated with "lighter and heats up faster" as well. Also, "cool down faster" is probably only an accurate description when the brakes are not being used.– JeutnargCommented Jan 25, 2018 at 14:59
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1@Jeutnarg I believe the statement should be interpreted as "disks are lighter and cool down faster than drums of the same weight would." Anyhow, the point is that disks dissipate heat more efficiently, not how much heat per unit mass they can absorb. Commented Jan 25, 2018 at 15:04
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1"drum brakes are better at stopping heavier loads from lower speed" - why would that be?– HobbesCommented Jan 25, 2018 at 16:33
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The "cooling" advantage is largely due to the ability to direct cooling air to the exposed disc surface, which there is a lot of. Drums are harder to cool - there's more mass, and it's kind of a sealed system. You can put all sorts of fancy fins and things on them, but you are cooling the outside surface, not the actual braking surface. Such things make your cooling a factor of the heat transfer coeffecient of cast iron (or much better Buick aluminum for a very few years which sacrificed other things) which isn't all that great. Commented Jan 26, 2018 at 3:42
One thing that hasn't been mentioned in other answers is the service life. Drum brakes that are properly set up can last multiple times longer than comparable disk brakes.
For trucks that drive longer distances, increasing the service interval is a significant cost-saving measure.
I've came back and remembered yet another thing: handbrakes. It's much harder to include a cable-operated handbrake into a disk brake system (although it's obviously been done). IIRC at least in the US the regulations require that it must be cable operated (so that a potential fluid leak couldn't let the car loose). This is one of the reasons for drum brakes persisting in the cheaper and smaller cars as well.
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Yes, but isn't changing brake pads much faster and cheaper than maintaining and adjusting a drum brake? Commented Jan 25, 2018 at 15:10
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1I'd say only marginally so. The maintenance interval greatly offsets the additional time it takes to replace shoes and other parts of the drum. A drum brake rebuild kit should not take much longer than 30 minutes to install for a moderately competent DIY mechanic, if no problems are encountered.– sleblancCommented Jan 26, 2018 at 3:27
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1@DmitryGrigoryev Recently, aftermarket parts manufacturers have been making readily assembled drum brake kits that make the process way faster and minimize the room for error. It's essentially "pop old stuff out, put the new set (as one part) in, adjust". With that, I don't think it'd take much more time for a skilled mechanic than it'd take to replace pads (and possibly rotors), especially given that you still should do some inspections of the calipers, grease the sliding surfaces etc. Commented Jan 30, 2018 at 8:49
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Handbrakes on discs are routine. Every rear disc system has one, and that's (looks outside) 90% of the cars here. Toyotas, Fords, everything but the occasional economy car. So cost, not engineering difficulty.– geoOCommented Mar 28, 2018 at 14:23
For the front wheels that perform nearly all of the braking and take the largest forces discs are what you want. For the rear drum brakes are good enough. You never, never, never see drums on the front of a modern vehicle. Never.
Disc brakes also cost more and drums are adequate. So cost cutting may just be the greatest motivator.
To add my experience to these already great answers, drum brakes are far less likely to be affected by salt and road debris. I had a small Honda SUV for a decade in Chicago, where the salt in winter would rust and seize the rear brake pads nearly every year due to receiving the brunt of the salt spray. I replaced and rebuilt the rear brakes four times compared to the front's once. Drum brakes on rear wheels on lighter vehicles are less maintenance overall, and rear brakes do far less work than the front brakes do by the nature of physics and the proportional design so they don't need to be as robust.
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You are probably going to want to source that claim. It might just be your experience.– geoOCommented Mar 28, 2018 at 14:15