Brake Drum Temperatures

[tonyw3026]

One of my electric brake drums often runs at about 220F while the other seldom gets above 140F. The corresponding bearing temperatures are 140F and 100F. I don't think I have a bearing problem because both bearings run at 90F when I disconnect the brakes.

I have adjusted the brakes so that there is no excess drag and the brake controller is set fairly light - the wheels are not locking up. The brakes pull evenly if I press the controller button in the tow vehicle. I even tried backing off the brake adjuster by 6 more clicks than the cool side but the hot side remains the same temperature.

So what's causing the temperature difference? Perhaps it doesn't matter since the bearing temperatures are not really too high?

Thanks
Tony Wright

[wavery]

One of my electric brake drums often runs at about 220F while the other seldom gets above 140F. The corresponding bearing temperatures are 140F and 100F. I don't think I have a bearing problem because both bearings run at 90F when I disconnect the brakes.

I have adjusted the brakes so that there is no excess drag and the brake controller is set fairly light - the wheels are not locking up. The brakes pull evenly if I press the controller button in the tow vehicle. I even tried backing off the brake adjuster by 6 more clicks than the cool side but the hot side remains the same temperature.

So what's causing the temperature difference? Perhaps it doesn't matter since the bearing temperatures are not really too high?

Thanks
Tony Wright

220 is way too high....in fact 140 is high (unless it's 100 outside and the road is 130).

I'd pull off the drums to see if the magnet is dragging. While you have the drums off, you could inspect the bearings and re-pack them.

General rule of thumb is that you should be able to touch the hub at any time without burning yourself. 220 will definitely burn you 140 will cause you to pull back pretty fast. 140 is marginally acceptable (depending on outside temp) 220 will boil the grease right out of the hub.......not good.

[hoppy]

I never had my PU brakes apart, but back in the day when I was an auto- mechanic, we would be checking for weak return springs for this condition.

[Jim K in PA]

I agree with Wavery that the magnet on the warmer side is dragging.  In fact, it sounds like both sides have some voltage bleeding to the magnet.  The clue that confirms this is the fact that unplugging the brake connector results in no heat build up.  Look for a partial short to ground on the blue (hot) wire for the brakes.  If the wires run in the axle tube, pull them and check for chafing.

The bearing grease should not boil away at 220F if it is high quality lithium grease or synthetic.  Disc brakes easily reach temps of 500F+ without liquifying the bearing grease.  Constant heat soaking is never good for the brake linings or the bearings.  If your brake shoes are dragging, you likely have a good glaze built up on the linings.  Pull the drums and check, as glazed linings have a reduced Coefficient of Friction, reducing your brake effectiveness.

I spent many years racing cars with poor brakes.

[treephiz]

I agree with Waverly and Jim K, 220 is very high and the rule of thumb is to be able to touch any part of the tire, rim and or hub and not have uncomfortable heat.  There is definately something going on to be causing the problem you are having.  It is a problem that needs to be addressed especially if the tow unit is on the smaller size.  Food luck!

[tonyw3026]

Jim K

Could you spell out that explanation for me.

As I understand it the controller applies 12v to the wiring connected to the electromagnet which is then attracted to the rotating brake drum. I assume you are suggesting that the magnet is being attracted to the drum even when I am not braking causing friction all the time.
When I am not braking there should be no voltage in the brake wiring right? So how does a short in the wiring result in the magnet dragging?

BTW, there is a small spring that seems to push the magnet against the drum all the time but I assume that is not enough to generate heat.

What am I missing

Thanks
Tony

[flyfisherman]

This site has some pretty good basic brake info ...


http://www.championtrailers.com/brkart.html#electric_brakes




Fly

[GlennS]

Tony,

I think you may be right that a short to ground would not cause the problem you're seeing. I would think that would cause a lack of braking power.

Your major clue seems to be that the heating doesn't occur when the brake controller is disconnected.  When you "disconnected" the brake controller did you do it at the controller or by disconnecting the TV-to-PUP connector? If you disconnected the connector, you might also check to see if you've got a short from another line on the connector to the brake line. E.g. if this problem occurrs only when you've got the lights on, you may have a short between the lights and brakes. Should blow the light fuse but maybe not.

If you disconnected at the controller itself, then use a voltmeter and check the voltage coming from the controller. If you do not have the brakes applied, there should be zero volts going to the magnets.  Since there's only only hot line from the controller, you can check it up near the hitch where it's easy to get to.

Good luck with it.

Glenn

[tonyw3026]

Glenn

Thanks you for your detailed reply.

I have now measured the voltage right at the drum backplate and it is zero with the contoller slider off and increases to about 3V when I press the slider fully or press the brake pedal - it gets higher if I increase the controller output. There is no difference with the lights on.

On thinking about it, the problem is unlikely due to a controller/voltage issue because the wiring is common to both sides but only one side is heating up. Also I still don't think I have a bearing problem because the drum is so much hotter than the bearing hub.

I am going to do some more road tesing then take the drum off again and have another look for a friction issue with the shoes or the magnet. Perhaps the shoes are not centering every time.

Thanks
Tony

[wavery]

I think that if I were to be taking that drum off again, I'd replace the return springs on that side. It sure won't hurt. You can probably buy a spring kit pretty cheap.

[aw738]

I checked the previous post and didn't see any reference to it, but could that side be adjusted to tight?

[Jim K in PA]

Jim K

Could you spell out that explanation for me.

As I understand it the controller applies 12v to the wiring connected to the electromagnet which is then attracted to the rotating brake drum. I assume you are suggesting that the magnet is being attracted to the drum even when I am not braking causing friction all the time.
When I am not braking there should be no voltage in the brake wiring right? So how does a short in the wiring result in the magnet dragging?

BTW, there is a small spring that seems to push the magnet against the drum all the time but I assume that is not enough to generate heat.

What am I missing

Thanks
Tony

Tony - I am sorry that my response was confusing (I re-read it).  What I meant be "voltage bleeding to the magnet" was that if you have a 12V power line to the trailer from the TV, and there is a short between the Blue brake wire and the 12V wire, you could be energizing the trailer brakes without any voltage being supplied by the controller.  I mentioned the chafed wires in the axle tube just as another possible point of failure, but you are correct, a short to ground would not increase braking voltage, it would decrease it.

If you disconnect the brake controller altogther and the temp rise still occurs, you are not dealing with an electrical cause, it is mechanical (weak return spring, etc.).

I once had a very annoying problem where the trailer brakes would activate sporadically and briefly for no apparent reason.  After MUCH troubleshooting, I found that water was getting into the factory harness connector on the back of the 7-pin connector (2004 GMC Envoy).  The little seal was folded and allowed rain water to get into the connector, and the water would short the pins for the brake circuit and the 12V supply, but only on bumps which made the water slosh that way.  I dried the connector, fixed the seal and the problem never reappeared.

[tonyw3026]

I finally solved the problem.

The fault was with the cold side not the hot side!
The cold side was doing far less braking than the hot side because the surface of the drum where the magnet rubs was grooved therefore, even though both magnets were getting the same voltage, the cold side magnet was not being attracted to the drum as strongly. I was adjusting the controller to give the correct overall braking effect but all the braking was coming from one side so naturally it was much hotter. I had the drums machined and now both sides are very close in drum temperature.

I have also realized there is absolutely nothing wrong with drum temperatures over 200 deg. In fact if your drums are much less than this I suggest they are not doing much braking. Otherwise where is all that energy going when you convert kinetic to heat energy? Car brakes get hot so why not trailer brakes?

After machining the drums I gradually increased the controller output and kept checking the drum temperatures. As soon as I could feel a braking effect the drum temperature started climbing and both sides are now often above 200 deg. I set the controller to just lock both the brakes at 20 mph on a gravel road.

Thanks for the responses
Tony Wright