GMC trailer

[johnny]

I'm fixing a dual axle trailer for my GMC, and to haul at least 23' GMCs. It has electric brakes, of a type I've never seen - my experience with
trailer brakes being very limited. Attached are two photos, one of the drum which has three metal plates attached to it. They aren't electrically
isolated. The other is of the backing plate. In the center is a honkin' big electromagnet which you feed twelve wolts to activate. Around it are/is
the brake shoe. It's a single piece with two shoes riveted on, Obviously it expands against the inside of the drum, sliding on the greased clips,
and is rethracted bu the two springs at the top. The shoe is apparently the only moving part other than the roatating drum with the wheel attaced to
the other side like any drum brake. My aqquestion is, how the hell does thios setup work? The magnet which is actually cup shaped has a break in the
steel in the back - a gap all the way around concentric to the housing i.e. the housing is in two piences. The front of it, which you see, is a
mostly sand and epoxy potting compound.

Obviously the brake expands and stops the wheel, but why/how? AQnd since the forum isn't currently letting me upload I'll put the pics on the photo
site as trailer brakes.

--johnny
--
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell

 

[john r. lebetski]

Call etrailer and see if they can tell you what you have from verbal or a picture. I have Dexters on my tandem. When I needed shoes as bonding let
go, I took them to NAPA pulled up a stool. The middle aged parts guy pulled the 4” thick brake book off the shelf. Together we browsed and
eventually found the photo that matched. He ordered and them had next day. All fixed.
--
John Lebetski
Woodstock, IL
77 Eleganza II

 

[johnny]

Matt Colie and Tom Pryor 'splained it. Thanks, guys!

--johnny
--
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell

 

[1976GMC]

The picture attachments did not come through.
I've dealt with a lot of different electric brakes on my work trailers. The most common way they work is this:
Electromagnet (usually placed near the bottom of the assembly) is connected via a long arm to a pivot point between the brake shoes. Once magnet is
energized, it is attracted to the side of the spinning brake drum (flat part facing the road). This attraction coupled with wheel rotating, pulls the
metal arm rearward and turns the cam between the shoes. This action moves front shoe forward. As this happens, the shoe "grabs" on the brake surface
and forces the whole brake assembly to rotate a little more, thus engaging the back shoe. The amount of power supplied to the electromagnet controls
how much this magnet moves and how much braking power is applied.

--
Vadim Jitkov
'76 Glenbrook 26'
Pullman, WA

 

[johnny]

This one's different. There is a ring electromagnet surrounding the axle on the backing plate, held in place with a spring. It can move outward
toward the hub/drum an inch or two. On the inside of the hub/drum are three large metal platres which rotate with the drum. When the magnet is
energized, it is attracted to the metal plates, and tries to ruen with them. It is limited in its turn to a few degrees by a tab at the top. As this
tab moves, it pushes the brake shoe (one, circular) out against the drum. The amount of push is directly related to the current in the magnet which
determines the amount of force between the magnet and the rotating plates. i.e. Instead of a cam and linkage, the magnet itself contacts the spinning
drum and is forced in the direction of wheel rotatiion, forcing the shoe against the drum.

--johnny
--
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell