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Old 05-17-2011, 06:41 AM   #1
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Brake magnets

2008 25fb safari....had my brakes serviced at JC yesterday.....they replaced the magnets and that is all......trailer had about 40000 miles on it.....do these magnets operate the brake shoes.....

will someone explain the brake operation to me

thanks
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Old 05-17-2011, 06:59 AM   #2
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Here is a link to a video on brake installation. You can see the magnet and the actuator arm. When current is applied the magnet grabs the inside of the drum causing the actuator to move and the shoes to expand. Old technology, but very simple and effective.

YouTube - etrailertv's Channel
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Old 05-17-2011, 06:59 AM   #3
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The brake magnets ride along the face of the drum. When they are energized they grab the drum and the spinning of the wheel causes them to move. That movement is transferred to a cam between the two brake shoes which causes the brake shoes to move away from each other and engage the surface of the drum.
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Old 05-17-2011, 07:06 AM   #4
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I'm sure that others can give a more understandable and complete explanation than mine but here goes .......

The brakes on your trailer (other than newer trailers with disc brakes) are basically similar to standard drum brakes on a car - with one big difference.

Instead of using a hydraulic slave cylinder to force the brake shoes into contact with the brake drum, the trailer brakes employ electromagnets on a lever arrangement to force the brake shoes out.

The magnets are arranges so that they face the inside (vertical) face of of the drum.

As you apply brakes, the trailer brake controller in your car sends current to the electromagnets which causes them to be attracted to the inside face of the drum. Since the drum is rotating, it tends to drag the magnet around in the direction of rotation.

That dragging force is then translated through the lever mechanism to which the magnets are attached and causes the linkage to apply outward pressure, forcing the brake shoes in contact with the drums.

The face of the electromagnets will eventually wear due to their contact with the drum, at which time they need to be replaced. They last a long time. I've never yet had to replace them on trailers we have owned.

That's my understanding anyway! Clear as mud?!

Brian.
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Old 05-17-2011, 07:19 AM   #5
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I'm sure that others can give a more understandable and complete explanation than mine but here goes .......
Brian.
Thanks for the explanation. Even though I've done my own car brakes for 35 years and repacked/adjusted the trailer wheel bearings every year, I never quite understood how they worked. Great explanation, now I understand it.
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Old 05-17-2011, 09:06 AM   #6
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Originally Posted by kingfisher24 View Post
2008 25fb safari....had my brakes serviced at JC yesterday.....they replaced the magnets and that is all......trailer had about 40000 miles on it.....do these magnets operate the brake shoes.....

will someone explain the brake operation to me

thanks
The only problem I see, is that electric brake magnets only last about 20,000 to 25,000. At 40,000 miles I would be surprised if the drums were not scoured.

The shoes can and do usually outlast the magnets about 2 to 1, in miles.

Andy
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Old 05-17-2011, 12:10 PM   #7
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good thoughts

i thought, the more i thought that that is the way they worked....maybe 40000 is stretching the trailers mileage but 30000 is pretty darned accurate....

as i say the service center at jackson center did it and i questioned the lack of need for shoes and they said everyting else looked fine......strange to me however

and even stranger...the magnets were not really that worn...i saw them.....but they were beginning....and so i had them replaced
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Old 05-18-2011, 08:22 AM   #8
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For the untrained eye, how do you know when the magents are worn and need to be replaced?
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Old 05-18-2011, 10:19 AM   #9
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Think of the magnet as a disk brake pad, the more of the magnet that is present, the more efficient it is. On our old 63 I noticed a lot of brake loss at about 50% ware.

On our Classic the wiring gave out long before the shoes or magnets.
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Old 05-18-2011, 10:31 AM   #10
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I believe there is a aluminum philips head screw in the face of the magnet. The rule of thumb says to replace the magnets when you can no longer see the cross in the screw.
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Old 05-18-2011, 10:59 AM   #11
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cross in the screw.
Come on man...I just got back,
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Old 05-18-2011, 11:22 AM   #12
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No wonder people missed you

I can see why people were wondering where Bob Cross had gone! Excellent retort, Sir.
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