Had to change a damper, during a service this morning, in Warsash, so I took the opportunity to take a few pics and do a basic 'how to' guide. The trailer is an Indespension Roller Coaster and the coupling is a 'straight drawbar' style rated at 2000kg. I figured this a fairly common one, so the best to demonstrate with.
The beast to be tackled....
The first thing to do is remove the coupling head. As a rule they're held on with two M12 bolts, so you'll need a half inch ratchet with a 19mm socket and a 19mm spanner. Or two 19mm spanners if you don't have a ratchet and socket set. The bolts will either be in parallel, horizontally, or 'cross bolts', which is where one bolt goes in from the top and one goes in from the side. The rear most bolt is always the one that goes through the damper eye ring.
You'll now need to jack the front of the trailer up, as you need to take the coupling off the trailer to replace the damper. This is because the damper slides out from the rear of the coupling. Some slide out the front, but this one comes out the rear. This is also an ideal time to remove the jockey wheel and take it apart and grease it up - might as well, while you're there and it also gives you a de-cluttered work area. The amount of times I've bent over a coupling and nearly poked my eye out on the jockey wheel handle...
Whilst the coupling's still bolted on, as they're hard to hold on to and work on once off the trailer, loosen the the rear damper securing bolt. This is usually a 22mm socket / spanner. If it's been there a while it'll be tight, but it will go - promise!
Now you need to undo the M10 brake rod / brake cable from the back of the coupling. This one has M10 brake rod, therefore M10 'bulldog' clamps. You'll need a 14mm socket / spanner to undo them. These ones hadn't moved, since it was first built, so had rusted up solid. A couple of turns and they snapped, which isn't unusual to be honest. I always keep a bag of them to hand for exactly that outcome.
Now undo the two coupling retaining bolts. Again, these are generally M12, so it's out with the 19mm spanners / sockets again.
And there you go - coupling off ...
Now you'll be pleased you loosened off the rear coupling retaining bolt whilst the coupling was still on the trailer, otherwise you'd be trying to chase it round the floor, or brace it between your shins. Which hurts. Honest. Take this bolt out and put it to one side to resecure the damper later. Then slide the damper out of the back of the coupling and it should look something like this: