Brake help - Fiberglass RV


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Old 08-15-2007, 11:41 PM   #1
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I know this topic has been discussed a while ago (with pics exactly like mine), but I need some specific help.

I took apart one of my hubs today and I can't tell whether I should scrap the idea of saving these brakes or fix them. (see pics below). The big problem I can see is that the magnet seems to have been rubbing against the hub. Not fixable right?

The bearings look ok, but I'm definately a rookie. Should I just replace them since I have them off or is that a bad idea? I've heard greasing them is difficult if you don't have the right parts.

Also, I noticed the axles seem to be attached by just the bolts. Do they typically have bolts and welding together?

Sorry, one last question....is there any way to test a heater without hooking it back up to the trailer? I realize the lighting mechanism can't be tested without the propane, but what about the fans and other parts?
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Old 08-15-2007, 11:54 PM   #2
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I took apart one of my hubs today and I can't tell whether I should scrap the idea of saving these brakes or fix them. (see pics below). The big problem I can see is that the magnet seems to have been rubbing against the hub. Not fixable right?
When the brakes are applied the magnet is supposed to rub against the drum. The harder the brakes are applied the more friction between the magnets and hubs. This forces the shoes out against the drums.
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Old 08-16-2007, 12:15 AM   #3
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Byron is right, when you apply the brake and the power is sent to the electro magnets they attach to the brake drum. It looks like you have a couple of grooves in the face where the magnet attaches to the drum, If they are not to deep they are OK. To be safe it might be a good idea to take them to a brake repaiar shop and have them check the drums. Under the magnet you can see a small star wheel. There will be and oval hole in the backing plate, that is your brake adjustment.
Before you replace the drum on the axel make sure you know which way to turn the star wheel to adjust the brake shoes in or out.
My advice would be to have the bearings packed by a shop if you have never done it before. It does not cost that much and it can get very expensive to replace a spindle and bearings. Trouble never seems to happen close to home.
If you know a mechanic that you are comfortable with have him pack the bearings. He will have the wheels off and you can get his opinion as to the condition of the brakes. He will probably adjust the brakes as part of the bearing pack.
Good luck,
John
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Old 08-16-2007, 10:56 AM   #4
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On the face of the magnet you can see inner and outer rings of metal, and an area in between. The metal faces are the poles of the magnet, and in between is the coil of wire. In the Dexter maintenance book, it says that as long as wear has not exposed any of the wire in the coil, and everything is wearing flat and square, you're fine.

I think it is bizarre that the magnet actually drags on the drum. When I first heard of electric trailer brakes, I assumed the magnet depended on eddy current drag... but no, it's actually friction. The more voltage is applied by the brake controller, the higher the current; the higher the current, the greater the attraction force of magnet to drum; the higher the force, the greater the friction.
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Old 08-16-2007, 12:47 PM   #5
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I'm guessing you saw the thread about my Burro brakes. They looked a lot like yours as far as magnet wear and all that.
I took everything off and cleaned up everything and wire brushed parts and what not. I also got a couple of those little brake greas packets from Autozone and greased the areas where the brake shoes ride on the back of the brake plate or what ever you call it on the back.
I took my trailer to my mechanic and had him grease the wheel bearings for me. I have never done it before and it looks like you have to get your hands ALL greasy to do it so that was a $35.00 charge for half hour labor. Worth it.
On the brakes everything comes apart very easily and goes back together. I took pictures before I did it and found I was glad I did as I could not remember which way the manual brake adjuster thing with the star wheel on it went back in on each side. You may want to mark the side of the brake shoe where the star part of the adjuster goes. I also made sure the adjuster turned freely. I unscrewed the parts and wire brushed the threads and put some of the brake grease on the threads and put them back together.
It is no fun crawling under the trailer to adjust these things by the way. They are hard to get to. That brings to mind that the damn rubber cover to these adjusters was hard to get off while crawling under too. If you are going to adjust these like I did in my driveway with the Burro somewhat jacked up and on stands I suggest getting these rubbers off while you have the brakes apart like you do now. Once you have everything back together and the adjustment done then get the rubbers back on. How's your brake wires? Take a good look at them also and their connection at the wheels.
Dave
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Old 08-16-2007, 05:48 PM   #6
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Were I you, I would call Dexter and get the minimum thickness specs for the wall of the drum; an automotive brake shop may not have seen this style of brake before.

Unfortunately, the drums are the most expensive part of the system by far...

BTW, the least expensive way to get brake hardware like shoes or magnets is by just buying the complete assembly on the mounting plate.
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Old 08-16-2007, 09:23 PM   #7
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THANKS EVERYONE! I really appreciate your helpful advice.
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Old 08-17-2007, 08:55 PM   #8
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My advice would be to have the bearings packed by a shop if you have never done it before. It does not cost that much and it can get very expensive to replace a spindle and bearings. Trouble never seems to happen close to home.
If you know a mechanic that you are comfortable with have him pack the bearings. He will have the wheels off and you can get his opinion as to the condition of the brakes. He will probably adjust the brakes as part of the bearing pack.
At least I know enough to know when I need professional help. It had been 2 years since the last bearing repack and brake adjustment.
My brakes were worn enough to expose the wire in the coil. My 30 year old brakes no longer had parts available, since the designs have changed. The shop will change the mounting plates on the axle and install all new Dexter Brakes when the parts come in next week. I'm also getting new bearings.
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Old 08-23-2007, 11:02 PM   #9
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When the brakes are applied the magnet is supposed to rub against the drum. The harder the brakes are applied the more friction between the magnets and hubs. This forces the shoes out against the drums.
Byron,

If I can add to your explanation for those who didn't read between the lines...

The more the current, the more the electro-magnet is attracted to face of the drum against which it rubs*.

The turning action of the drum pulls the magnet in the direction of drum rotation, pulling on the LEVER ARM that the magnet is attached to, which rotates a cam at the other end of the arm that forces the brake shoes to expand harder against the brake drum friction surface.

Really Rube Goldberg, but quite effective!

Bob
*Under no braking, the magnets barely touch the inside surface of the drum, but with activation, they are drawn more strongly and rub much harder, and in proportion to the magnetic attraction.
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Old 08-23-2007, 11:27 PM   #10
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Byron,

If I can add to your explanation for those who didn't read between the lines...

The more the current, the more the electro-magnet is attracted to face of the drum against which it rubs*.

The turning action of the drum pulls the magnet in the direction of drum rotation, pulling on the LEVER ARM that the magnet is attached to, which rotates a cam at the other end of the arm that forces the brake shoes to expand harder against the brake drum friction surface.

Really Rube Goldberg, but quite effective!

Bob
*Under no braking, the magnets barely touch the inside surface of the drum, but with activation, they are drawn more strongly and rub much harder, and in proportion to the magnetic attraction.
Yup, you got it.
The only thing that's really changed in the past 50 years is the controller in the tow vehicle. Been big improvements there.
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Old 08-24-2007, 11:32 AM   #11
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The more the current, the more the electro-magnet is attracted to face of the drum against which it rubs.

*Under no braking, the magnets barely touch the inside surface of the drum, but with activation, they are drawn more strongly and rub much harder, and in proportion to the magnetic attraction.
That's what I was saying in my earlier post...
Quote:
...The more voltage is applied by the brake controller, the higher the current; the higher the current, the greater the attraction force of magnet to drum; the higher the force, the greater the friction.
... but perhaps it was still too cryptic!

Quote:
The turning action of the drum pulls the magnet in the direction of drum rotation, pulling on the LEVER ARM that the magnet is attached to, which rotates a cam at the other end of the arm that forces the brake shoes to expand harder against the brake drum friction surface.
Yes, and one effect of this approach is that there is no braking force unless there is some rotation: if you jack up a wheel of the trailer, the apply power to the brakes, the wheel will easily turn a bit (until the free play is taken up) making it seem like the brakes don't work. The better test method is thus to spin the wheel, then apply the brake power: the magnet "hits" and the wheel stops turning quite suddenly.

While I find the idea of an electromagnetic coil as a friction surface to be strange, I don't think this method is particularly complex or indirect. It is virtually the same as mounting the magnet on the end of the parking brake lever in a conventional automotive drum brake. The cam action is similar to both those parking brakes and the linkage for air brakes in large trucks.

I agree with Byron that it is the controller side which has advanced in the electrically operated drum system. To see real advancement on the trailer side, I would point to hydraulic disk brakes with electric-over-hydraulic actuators. Outside of North America, these systems are routinely available with anti-lock control systems (ABS); here, they are used on some of the heaviest premium RV and boat trailers. We don't see these on our trailers because the brake system would be a huge part of the trailer cost.
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