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Old 30 December 2006, 03:18   #1
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How to replace a trailer drawbar and coupling damper?

I have to replace the drawbar and coupling damper on my trailer. I have the bits, but before I embark on this task I was hoping to get some advice on two points:
  1. My trailer is braked, however, I can't figure out how to prevent the brakes from applying when reversing!
  2. Should I pack the coupling damper in grease before slotting it inside the drawbar?
Any other guidance would be welcomed?

Rich
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Old 30 December 2006, 05:00   #2
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Hi your brakes only come on when going forward not going backwards! Not go if you want to stop a trailer rolling back down a slip!!

Yes plenty of grease in the tube, it should have 2 grease nipples give these a good squirt once or twice a year till you see grease coming out the back.

Nick
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Old 30 December 2006, 06:23   #3
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Nick

Thanks for your reply.

I thought that when you apply the vehicle brakes that the momentum of the trailer will compress the damper and apply the trailer's brakes. If this is the case, then when I reverse will the same not happen?

Rich
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Old 30 December 2006, 06:27   #4
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Old trailers have a reversing lock which you have to engage to disable the brakes (basically stops the hitch compressing). I think modern trailers have an automatic thing to do the same but I have no idea how it works...
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Old 30 December 2006, 06:36   #5
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The brakes only work when the trailer is going forward, NOT backwards so they will not come on when you are reversing even though the hitch is compressed!
Stephen I have not used one of them since I was a boy!
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Old 30 December 2006, 06:52   #6
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Quote:
Originally Posted by Nick Hearne View Post
The brakes only work when the trailer is going forward, NOT backwards so they will not come on when you are reversing even though the hitch is compressed!
Stephen I have not used one of them since I was a boy!
There are a lot of old trailers around though!

The hitch on my trailer has a manual reversing lock. Not connected to any brakes, but it has a manual reversing lock all the same
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Old 30 December 2006, 07:17   #7
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one shoe each side has a slide sorta mechanism which comes into play in reverse, unless its rusted up, and allows you to reverse without the brakes activiating.
If you reverse at all and then put the handbrake on you will find the travel is excessive to the extent that it doesnt always apply any pressure at all. If you havent reversed it should work ok but I would never rely on a trailer handbrake on a slope-seen too many resulting whoopsies which could have been very nasty had someone been behind the trailer.
One very effective way of stopping the trailer entirely and detering thieves in the process is to allow the shoes to rust sufficiently under the linings so that they drop off into the drum-now that really is efficient when you come to try and move the trailer and a little difficult persuading the drums to come off in order to remedy the matter.
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Old 31 December 2006, 06:10   #8
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I've found some answers on the net.

First, the law covering auto reverse trailer brakes is:

Any braked trailers manufactured after April 1989 must be fitted with a hydraulically damped coupling and auto reverse brakes to give braking efficiencies required by EEC Directive 71/320 (ECE13).

The operating principles of auto reverse trailer brakes is explained in section 1.2 on page 18 of:

http://www.mira.co.uk/site/ResearchR...-01_DfT_O2.pdf
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Old 31 December 2006, 06:35   #9
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Quote:
Originally Posted by Nick Hearne View Post
Hi your brakes only come on when going forward not going backwards! Not go if you want to stop a trailer rolling back down a slip!!

Yes plenty of grease in the tube, it should have 2 grease nipples give these a good squirt once or twice a year till you see grease coming out the back.

Nick
I think what he means is that when reversing the coupling compresses (As it would do when your braking normally) and puts the brakes on. This is a natural function of braked trailers and is overcome by initially reversing slowly. And if he wants to stop it rolling down a slope its probably got a hand activated brake.
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Old 31 December 2006, 14:16   #10
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Not sure what kind of systems are "normal" in the UK, but over here, there are a couple of types, and how they operate depend on a few factors:

1) straight surge brakes (disk and drum) - The coupler is floating on the trailer tongue, with a piston between the two. Slowing down causes the mass of the trailer to overrun the tow vehicle, which causes the piston to pump hydraulic fluid to the brake cylinders. Braking will also occur if you go from a stop to hard reversing, or if you are reversing up a slope (or hopping a curb, or whatever.) Lockout is accomplished by a pin, which stops the coupler from moving, or by a lever built into the coupler (does the same thing), or by an electric lockout solenoid operating off the reverse lights, that essentially reroutes any pumped brake fluid back into the reservoir (rather than to the brake cylinders.) Electric lockout requires a minimum of a 5-pin trailer connector, as teh signal for teh reverse lights has to be routed to the trailer. There is also a type of drum brake called "free-backing", which disables the brakes whenever the trailer wheels are rotating backwards (no "free-backing" disk systems, that I'm aware of.)

2) electric over hydraulic (usually disk, but can be used with drum as well): Application of the brakes turns on the tow vehicles' brake lights. This causes a small pump to apply the trailer brakes. There is usually a controller in the tow vehicle to allow you to tune the braking force applied. Some newer models have an accellerometer that tells the actuator how fast the tow vehicle is slowing, so it can respond accordingly. Lockout is usually via a lockout solenoid operating off the reverse lights. Requires a 5-pin connector or better, as reverse light signal has to get to the trailer.

3) Pure electric: Brake light signal operates small magnets on the brake backing plates, which applies the brakes directly. Lockout is unnecessary, as the trailer only brakes when the brake lights are on (i.e. backing up, per se, does not apply the trailer brakes.) Most have some sort of controller to adjust braking force. Not normally used on boat trailers, as the electric bits at the hubs don't play nice with seawater (which, of course, doesn't mean they aren't used...)

I assume you've got the first setup. Note that the piston is not a "dampener", but is, in fact, a hydraulic pump - the brake actuator. Lockout on this system is usually accomplished by means of a removable pin that prevents the coupler from moving backwards and compressing the cylinder, or by a built in reversing lever that does the same thing. You could, if you wish, fit a reverse lockout solenoid that operates pretty transparently from the drivers standpoint.

I see no reason why one would slather the cylinder in grease, though it probably won't hurt anything. The cylinder rod is lubricated to some degree by the hydraulic fluid as it compresses/expands, the external part never contacts anything. The outer part of the cylinder is just a case to house all the moving bits, and requires no lubrication. I would suspect that grease would attract and hold more dirt and stuff, possibly accellerating wear.

Hope this helps;

jky
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