OT: Big rig trucks: why air brakes and not electric brakes
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    Default OT: Big rig trucks: why air brakes and not electric brakes

    Big rig trucks use air brakes because hydraulic ones would have to be bleed after every hookup. Home owner trailers often use electric or electric over hydraulic brakes.
    Do any tractor trailers use electric brakes? If not why not. Is it mostly a legacy thing?
    AFAIK the Tesla Semi trucks use air brakes for the trailer.
    Bill D

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    Do electric trailer brakes use power to engage or disengaged the brake?

    Semi trailer brakes are disengaged when the air pressure is to them. If there's a loss of air pressure the brakes come on. You want the failure mode to be that you have full braking instead of no braking.

    In a semi trailer, that would require power going to the brakes at all times. Over a long haul this could cause a coil to heat up or whatever.

    The adjustment on an electric brake for an electric motor is fairly delicate. If the plate is too far away from the coil it won't pull it in. I don't think air brakes are this particular.

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    Same reason they were developed for use on trains.

    AIR = roll

    NOT AIR = no roll

    A disconnected car or trailer will automatically brake.

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    Here's some guesses:

    1. Cost

    2. Production inertia

    3. More of number 2 because most trailers are not owned by the truck driver, so there is even more pressure to remain consistent in production system standards.

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    Power density in the tight space confines of the axle/drum/disc area.

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    Years ago I got an old vietnam era military box trailer that had electric brakes, it was semi truck size with 2 dually axles. Not common but they do exist.
    Last edited by Rob F.; 03-06-2020 at 01:50 PM. Reason: Spelling

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    Some more thoughts

    Brakes are a pretty hostile working environment, in which air is bad enough - electric would be a nightmare.

    Compressed air is easily reliably and cheaply made and stored, using simple proven technology - driven (usually) by the main engine.

    Compressed air is also used for other purposes, - shifting gears / ratio sets, operating diff locks and windscreen wipers etc etc.

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    A lot of people don't know that George Westinghouse invented air brakes.

    Electricity is the energy of the future. That's because it remains invisible, for the most part, and if you can't see it it must be good. It's a 'green' solution as well as it has no smell and produces no smoke under normal operating conditions. We know it because of the holes it comes out of in our homes, but we love it because it powers our iPhones.

    For brakes, you could simply have the brakes 'ON' anytime the electric goes on. A small, 'green' windmill connected to a generator would provide the power under normal driving. The faster you drive, the more wind you create, the better the brakes work. For slow speed stops and emergencies, you could simply use a solar panel to provide the power even if there is no wind at all.

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    When the air brake was invented, electricity was complex, immature, expensive, and unreliable. The only place electricity was necessary was electric lighting.

    By contrast, compressed air technology was widely used and mature.

    A major design driver for the air brake was railroad safety issues unrelated to air or electricity.

    Westinghouse Air Brake Company - Wikipedia

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    There is the issue of modulating the braking force. The foot treadle valve/braking force is very linear in behavior.

    Electric servo applications have the output force varying to the 4th power of current. Not a bid deal for a closed loop PID servo but rather challenging for a human to do well.

    Add in the fail safe design of the air brake system with using system air to keep the spring pack off, it makes an electric system much more complicated and less human friendly.

    Also keep in mind that the traditional electric trailer brakes require the presence of electric power to energize. This is further complicated by the fact that the electric brake coil energizes a mechanical linkage and that most of the actual braking forces from the unidirectional force the rotation of the brake drum and the brake shoes generate.

    This means that basically there is negligible braking force at a stop and no braking force in reverse.

    Not saying that a workable electric brake couldn't be developed but it be very expensive to get the fail safe redundancy and linear behavior that existing air brake technology provides.

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    Lots of interesting theory here, and it is all right.

    One basic item that everyone is missing on this: electric (magnetic) brakes just don't have the power that air or hydraulic has. Changed a buddies trailer over to electric over hydraulic system and it was night and day difference in braking power.

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    Quote Originally Posted by lakedrum03 View Post
    Lots of interesting theory here, and it is all right.

    One basic item that everyone is missing on this: electric (magnetic) brakes just don't have the power that air or hydraulic has. Changed a buddies trailer over to electric over hydraulic system and it was night and day difference in braking power.
    Everyone ?

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    Quote Originally Posted by Ziggy2 View Post
    There is the issue of modulating the breaking force. The foot treadle valve/braking force is very linear in behavior.

    Electric servo applications have the output force varying to the 4th power of current. Not a bid deal for a closed loop PID servo but rather challenging for a human to do well.

    Add in the fail safe design of the air brake system with using system air to keep the spring pack off, it makes an electric system much more complicated and less human friendly.

    Also keep in mind that the traditional electric trailer brakes require the presence of electric power to energize. This is further complicated by the fact that the electric brake coil energizes a mechanical linkage and that most of the actual braking forces from the unidirectional force the rotation of the brake drum and the brake shoes generate.

    This means that basically there is negligible braking force at a stop and no braking force in reverse.

    Not saying that a workable electric brake couldn't be developed but it be very expensive to get the fail safe redundancy and linear behavior that existing air brake technology provides.
    Double leading shoe. It was the technology of choice for motorcycles and performance cars until disk brakes became common. Most of the ordinary cars still used single leading shoe on the rear drums because it was cheaper and only required a single brake cam.

    Westinghouse Air Brake, now WABCO. Prior to air brakes trains required a brakeman at each car (or several cars in each string) who operated a hand wheel to operate the brakes. It was a dangerous job and a bad brakeman could wreck a train.

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    One thing to note here.
    Train air brakes are not exactly like big rig air brakes, their is no Maxi (spring)
    applied safety brake.

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    When air brakes came out a 40 amp [automotive] generator was big. In order to have power off brakes you need the power to retract the spring that drives the brakes at full power. One would have to do some math but I think that would be quite some current

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    I do see a lot of used trailers for sale with new brakes or new wiring so I bet the wiring is hard to keep from corroding under there.
    Bill

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    [QUOTE=Scottl;3502393]Double leading shoe. It was the technology of choice for motorcycles and performance cars until disk brakes became common. Most of the ordinary cars still used single leading shoe on the rear drums because it was cheaper and only required a single brake cam.

    Salient point: double leading shoe drum setups won't hold a bike from rolling backwards on a hill. The rears are single leading on bikes for just that reason.
    Trust me I do this all the time on the way into work. (single leading setup has one leading shoe in either direction)
    (grimeca four leading shoe setup anyone?)

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    While I believe electric brakes are a very poor idea for truck/car applications, the comment of 'strength' is not valid. An electric coupling (i.e. coil) can be made plenty strong.

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    Quote Originally Posted by bloomautomatic View Post
    Semi trailer brakes are disengaged when the air pressure is to them. If there's a loss of air pressure the brakes come on. You want the failure mode to be that you have full braking instead of no braking.
    Yup. Got the most important item in one go. Came off the back of railway lessons learnt the "hard way", made George Westinghouse a wealthy man.

    Next point is that lost air or leaked air dasn't make much of a mess!

    Then, too, the replacement supply is far more easily sourced and CHEAPER than hydraulic fluid, won't burn, won't freeze if kept reasonably dry.

    Ukrainian and Russian designers facing poor supply infrastructure, tight budgets, unpredictably trained and tooled service folks, severe winters even made extensive use of AIR instead of hydraulics on everything from Mig fighters to Antonov "Colt" uber-biplane utility aircraft.

    Pretty handy, cheap, and reliable stuff, air can be.

    Even humans use it "directly".


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    Quote Originally Posted by GregSY View Post
    While I believe electric brakes are a very poor idea for truck/car applications, the comment of 'strength' is not valid. An electric coupling (i.e. coil) can be made plenty strong.
    Numbers Greg....Show me a solenoid coil (or other actuator) that packs that much power in the same size as a maxi brake canister.


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