Easy, tractor trailers use pneumatic brakes, which apparently are either more cost effective or work better in drum form than disc form.
I knew about the air brakes, but that doesn’t really explain why drum brakes are better for trucks but inadequate on cars. Surely it isn’t just because one system uses compressed air and the other uses hydraulic fluid. The best reason for using compressed air has more to do with easily hooking-up and unhooking the trailers. Many class B trucks use hydraulic brakes but still have drum brakes.
I don’t meen to seem ungrateful, but if the answer was really that easy, it would not include the word “apparently.”
How and why are they more cost effective or work better?
The main advantage of drum brakes other than being cost effective is that they can apply themselves without much mechanical effort (They don’t require as much vacuum assist to engage as disc brakes usually do). Basically if you were able to take two identical vehicles, one with non-power assisted drums and one with non-power assisted discs. The vehicle with the drums would initially brake harder with the same amount of force being exerted on the pedal.
Of course the longer the braking event goes the better the disc brake will do due to it’s superior heat-disapating design, and better ability to me modulated by the driver.
I was waiting for someone to mention drums in heavy duty applications. You yourself (Whitey) said "Rear drums are perfectly fine for small light weight vehicles. ".
Most trucks are equipped with rear drums. They are meant to tow/carry heavy loads. Seems like a more demanding application for brakes than any car, don’t you agree?
I’ve been happy with the rear drum brakes on all of the 6 cars I’ve owned in my life. I guess I’m biased. I have driven cars with rear disk, but the difference could never live up to Brown’s hyperbole.
Oh yes, I agree.
I was kind of being influenced by other people’s positions until I finally thought about trucks. I was also thinking about motorcycles. It seems the smaller (engine size) and lighter bikes have drums in the rear. The Honda Nighthawk 250 even has drum brakes in the front! Then once you get past the 750 CC mark, they start putting a disk brake in the rear. I figured it was a matter of vehicle weight, but it could also be a matter of cost and relative price range of the bikes, with the cheaper bikes getting a rear drum brake. Some expensive bicycles even have disk brakes now.
I only thought about truck brakes after (probably falsely) assuming that it was a weight issue. After all, my old 1969 Dodge Dart had drums in the front and the rear, and those were not power brakes, and it stopped just fine, even in wet conditions.
I guess it isn’t safe to assume disk brakes are any better than drum brakes. You can’t judge a book by its cover. It appears to be more a matter of perceived value rather than an actual difference in performance. Disk brakes shine in the light…oooh…aah.
Remember too that the drum brakes on your bike and on big rigs are enclosed and of very different design that those on the typical rear drummed car.
One advantage that properly designed drum brakes have is that the friction of the braking surfaces can actually assist the hydraulics (or mechanical linkage if you have a bike or a Nash Metropolitan) in applying pressure to the shoes. However, a major weakness of the car-type design is that water can bet between the shoes and the drum surface and just stay in there until it’s evaporated by heat. That was a not-uncommon occurance in days of old when driving through puddles.
They are usually low profile tires, so it is probably difficult to find the right size in a non-performance tire.
As a former Nighthawk rider, let me tell you that front drums stink! The problem is that you have the same basic brake, fore and aft, with similar actuation forces, only your foot (rear) is one hell of a lot stronger than your hand! Thus, you have to teach yourself to squeeze way harder than you’re stepping to get equal action.
In an ideal world, 10% of hand strength and 10% of foot strength would produce equivalent amounts of braking. That’s control harmony, and it’s a lot easier to acheive with a front disc than a front drum.
To reply to one post above, yes trucks run drums…but then, every so often, one uses the runaway ramps. Saw a rig plow through the orange barrels on I-80 to avoid hitting the car ahead (then trucker gets out and feels his hubs to figure out “which ones are working…”)
The design of pneumatic brakes is such that they are on until such time as air pressure “forces” them off. Since the “stuck on” is like an e-brake, perhaps drums are better suited to this aplication.
I was talking about PICKUP trucks and other SUV based trucks like my Trailblazer for example. They are meant to tow 5000-6000 lbs and carry heavy loads in the bed. They have drum brakes.
Alloy wheels usually weigh more than cheap steel wheels. Only the most expensive forged aluminum wheels weigh less.
On motorcycles aluminum wheels definitely weigh more than spoked wheels.
Disc brakes have better feel, but its not from no-play. When the drum is rotating and a shoe is pressed against the drum, the shoe will wedge trying to follow the rotation of the drum. This increases braking. Makes the drum brake response non-linear and harder to control. Made more complicated because in cheap drum brakes the leading edge of one shoe is driven while only the trailing edge of the other shoe. Look at ~1970 racing motorcycle front drum brakes and note two sets of levers driving two cams so that the same edge of each shoe is used.
Would someone please explain why loaded 80,000 lb. tractor trailers use drum brakes if disk brakes are so much better?
Drum brakes are perfectly fine and safe. The difference is the “feel”. Disc brake response is linear, and drum brake response is nonlinear. Brake very hard with drum brakes and they will brake even harder than the effort you apply.
My 2008 F-250 has 4 wheel disc brakes. And is only rated to tow 12,500 pounds with a GCWR of 23,000 pounds.
A tractor trailer though they have drum brakes, they are very large and a set on every axle including the trailer. The distribution of braking load is what makes proficient stopping. Exposed discs are more prone to damage in HD application. Our work tractors have disc on the rear axle but are encased in the rear differential oil for protection (don’t ask why it works). The only way we brake the fronts is through the transmission. They must be pretty good because even unassisted we can easily stop a load weight equal to twice the tractor alone.
I remember the Popular Science articles, Disc vrs Drum. My first auto shop teacher had his mind so “fixed” in a different era he wouldn’t even accept self adjusters on drum brakes.
An interesting comment from this source, leads me to believe there are other factors than we have been discussing for preference of discs over drums for safer braking and would seem to me make them an easier integration with abs, stability and traction control.
The function of leading edge of the shoe over time, and lateral braking forces that exist within drums and not on discs are discussed here and in other periodicals on this subject. They seem to indicate an inherent superiority or one type (disc) over another. These are acceptable in low load (rear) in smaller cars and in trucks where other factors (exposure) may be more important… I don’t pretend to understand the nuances of these differences, but they are discussed.
This is one comment…
"Equal Braking effect on 2-Drum Brakes on the same axle, so necessary for a straight-line stop, is difficult to achieve with reliability and consistency over a period of time. Disc Brakes have no such limitation."
http://www.indiacar.com/infobank/discbrakes_abs2.htm