Simon Juncal proclaimed:
Just a special case of torsion bar.
Yeah, towing with the leaf spring in a Corvette, good idea.
I'll go with tradition. Trucks use leaves, trains use coils.
You forgot propped up by a political system that killed the more efficient means of long distance transportation of heavy goods, with the added advantage that you don't get as many civilians mixed in with the long haul freighters, which increases safety for both.
OK, lets see if I've got this straight.
Coils offer a better ride, more suspension travel enable a more sophisticated and compact suspension design.
Leaf's are stronger and a leaf spring suspension can potentially put up with more abuse over the long term because of it's strength and simplicity.
The big trucks use drums because that's what they've always used.
Does that sound about right?
Jeff DeWitt
Sim>
So far in this thread, nobody has answered Jeff's question: Why do semi-trucks use drum brakes?
I also am curious - why do they use drums rather than disks?
Tom
Hazarding a guess I'd say it's because the drums actually offer more stopping power and are less susceptible to brake fade. Disks are lighter but not as powerful. For us they also handle mud better than drums do, which isn't really a consideration for the average tractor.
Of course the real FACTS may completely different than the guess I hazard. :)
Well... I guess an awful lot depends on how the rest of the suspension is designed, and of course a LOT of vehicles use them, not just weenie little cars!
Jeff DeWitt
L.W.(Bill) Hughes III wrote:
Well... drums are MORE susceptible to fade, that's one of the advantages of disks. When a drum gets hot it will expand away from the shoes, while when a disk gets hot it will get closer to the shoes.
I think all other things being equal disks are going to be more powerful too, because it's easier to squeeze things together than apart.
However with the drums you can get more swept area, so all other things AREN'T equal.
Jeff DeWitt
Well, in your case, the horse can't drink because it is a horse's *ss at both ends.
L.W.(Bill) Hughes III proclaimed:
Minor correction, Jeff. Disc brakes require MORE pressure to operate
- a lot more. Check the size of the pistons in the front calipers versus the size of the pistons in the rear cylinders (or try stopping with discs and no power assist ). Also, with drums, you can (and most modern systems do) get a mechanical boost that increases the braking pressure on a drum/shoe where you can't on discs.
Where you got it dead on is the heat issue. Disc brakes are much less suceptable to fade. They also handle water better due the the extra pressure, a larger open surface, and the wiping action of the pads.
Then there is the real reason: disc brake assemblies are lighter and cheaper for given braking power.
No surprise you're a hypocrit as well.
More like fantasys and your links OFTEN contradict your fantasys.
See Chrysler 8.25 rear axle that you refused to believe existed simply because you didn't see their listing in a PDF you posted a dozen times. Despite the fact that they were stairing right at you on page three of the PDF...
You should take that as a clue, you can be utterly 180 degrees off the mark and you have no freakin idea. You would probably be surprised at just how often you are.
This is one of those questions that has a bunch of answers which are all more or less true.
Because the industry is traditional and doesn't like change, because drums perform on Semi's pretty well in an overall sense, because changing to discs requires a lot of new technology and new untested (in trucking) systems which can fail until the bugs are worked out causing law suits. In short because the industry is cheap and as stubborn as Bill about adapting to new stuff.
Probably the major reason (and long winded one) though is mechanical; Because air brake systems works backwards from how we are used to brakes working...
The default state on a semi's brake system is LOCKED UP... Unlike hydrolic systems where a lack of fluid will result in NO BRAKES, a lack of air will result in total brakes. This is because air is used to push against the heavy springs that are ALWAYS trying to push the brake shoes against the drums. The air is used to compress these springs.
The mechanical advantage here is that in emergencies Semi's have brakes even without a running motor OR air in the system. With no motor you can still lightly brake a Semi to a safe stop. With no air it will come to a more or less screeching halt, but at least it will stop.
The same could certainly be done using discs but would require a system that is very different from our cars discs and thus lots of R&D dollars. Drums have almost no advantages over discs, they are FAR heavier, FAR less thermally efficient, they trap water, and heat, they have LESS friction surface until they are just about locked up. and they use WAY WAY more parts, they are manually adjusted (even automatic slack adjusters must be manually adjusted occasionally!)
You asked :)
I'm not sure sure about how much pressure it takes to operate disks. I imagine the pistons in drum brakes are smaller because they have to be to fit inside the drums, with disks there is no such limitation. The bigger the piston the more leverage (or something like that) and so the more pressure on the pads for less pressure on the pedal.
As a matter of fact my Studebaker Lark has non power (after market) disk brakes and its brakes are excellent... stops at least as well as my Jeep.
Most every car these days has power brakes because the Imperial Federal Government issued a decree that limits the amount of pedal pressure, and of course if you have power brakes and the booster isn't working then it's going to be a REAL challenge to stop, which may be what your thinking of.
Jeff DeWitt
They have to use proportioning valves on Jeep and most other mixed brake systems to make up for the extra push needed to run disks.
Without this valve, the rear drums will lock up way sooner than the front disks will grab. This causes you to do 360's down the road. Not a good thing....
With the valve in there, the rear brakes are prevented from locking up. Only the front's should skid.
Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.