The swing arm is the carbon moulding that supports the rear wheel of the trike. Sometimes I might call it the rear fork, or just the fork, since there is no front fork.
Building the rear arm is a matter of bonding and screwing, to secure the metal dropouts and pivot bushes to this moulding - a belt-and-braces approach guaranteed to result in a secure fixing. Care is required however with both these operations to ensure they are done accurately.
Securing the pivot bushes
The rear swing arm rotates, for suspension and folding, on 8mm countersunk-head machine screws. These are threaded into stainless-steel bushes that you bond into the front of the moulding. This arrangement enables the pivots to be precisely adjusted to eliminate free play. You can see the bushes in the picture (below right).
Holes for the bushes are pre-drilled in the moulding, but these need to be enlarged from 10mm to 12mm diameter. The best way I have found to do this is with an abrasive rod, as used in the previous section on the boom clamps.
When enlarging the holes, you want to ensure that the bushes will nestle snugly inside the corner of the moulding. The most important thing though is that the two bushes should be in line. You can check this by screwing a length of 8mm studding - threaded rod, available from DIY stores and engineers' suppliers - into each bush, as in the picture. Screw it in far enough to remove angular wobble. If the studding won't line up, remove carbon from the appropriate part of the periphery of the hole. Try not to make the holes for the pivot bushes bigger than they need to be.
When you have done both holes on both sides, and made sure they can line up, get some fine emery paper and remove the gloss from around the ends of the holes, so that the epoxy will adhere well. Then you are ready to bond the bushes in place.
The heads of the bushes go on the outside of the forks, and the thin nuts on the insides. Keep the studding inserted to indicate alignment while the epoxy is setting, but try not get it stuck in. It is best to bond one bush at a time. Get epoxy under the heads of the bush, and on the thread so that the thin nut is bonded to the bolt as well as the moulding.
In my experience it is easier to get epoxy where you don't want to that not get it where you do, but I'm a messy sort of person. You may want to use masking tape around the holes. Some sorts of ordinary sticky tape leave a residue that is difficult to remove from surfaces. Release agent remaining on the moulding from lay-up will inhibit adhesion, where you have not used emery, but don't rely on it..
You need something to support the studding while the epoxy is setting. You do not need anything as fancy as the stand in the photo; a lump of clay on a piece of wood would do. When doing the second bush, make sure that the studding is not so long that you cannot remove it.
Bonding the dropouts
Since the dropouts are much better supported in the trike than in their conventional use in a bicycle frame, you can remove metal from them. The picture shows the web partly drilled away on the face that is bonded to the fork. These holes do not go right through the metal, so they are not visible on the finished fork. Note also that the tangs of the dropouts - the tongues that in a steel bicycle frame would be brazed into the seatstay and chainstay tubes - have been cut off. You can do this with a hacksaw.
To hold the dropouts during epoxying, you need a length of 10mm studding to keep them parallel and the correct distance apart. You also need four 10mm nuts to fit the studding. 10mm is the diameter of a standard rear axle (although the studding will have a coarser thread pitch).
The effective length of rear axles is specified as the 'over lock-nut' dimension. This is the same as the distance between the inside faces of the dropouts. The axle length required for the fork is 135mm, the commonest length available for derallieur rear hubs these days. (You will need to specify this when you buy a rear hub.)
So, you should mount the dropouts on the studding at a distance of exactly 135mm apart - remember this is the distance between the outer faces of the inner nuts - tightening the outer nuts once the dropouts are aligned on the studding. You might want to bond the inner nuts to the studding to keep them the correct distance apart - with epoxy or a thread-locking compound if you have some. Use a flat surface to check the angular alignment of the dropouts on the studding, as in the picture above. Make sure the studding is right up to the ends of both dropouts. If not, the wheel won't sit happily in the finished fork, so double-check this as it is easy to overlook.
The dropout mounting holes in the moulding will have been jig drilled to 5mm diameter. You will probably need to enlarge these to about 5.5mm, to give yourself a bit of free movement. The picture shows one way of holding the fork for this operation; note the wooden chassis grips on the workbench.
The next thing is check that the assembly of the studding and dropouts fits the fork. It should go in freely: the fork should not splay apart noticeably when it is inserted. If it does you will need to relieve the inside of the carbon moulding, but this is unlikely. Any gap between the dropouts and the moulding will be filled with epoxy. Check that you can get all three screws through their holes in the moulding into each dropout.
Note the long screws sticking out in the picture; you do not want these tight during bonding, and you always want to ensure that you can remove them. Initially you need these screws in place to ensure that the holes line up; after the first bonding keeping them there helps stop the holes from clogging with epoxy. A light spray of aerosol lubricant on the screws will assist release, if you do not have proper release agent.
When the dropouts fit OK, and you have cleaned them, you are ready to start bonding. I say start bonding as you will probably need to apply the epoxy in several stages, using gravity to help you. You want to end up with epoxy all the way round the gap between the dropouts and the fork moulding. A coating of epoxy on the inside surfaces of the dropouts will save you from having to paint them, but keep epoxy off the contact surfaces of the dropouts; it is hard to remove when set. Have the fork sitting upside down on a flat surface when you do the first bonding.
So, first make an initial fix between the fork and the dropouts, but taking care not to get the goo on the screws. If you are a welder, then what you are doing is like tacking your joint together before doing the serious weld. Try to get a little blob of bond in two or three widely separated points on each dropout. When the epoxy has set you should carefully remove the axle, and check that everything looks OK. Are the dropout faces parallel? They should be, if you have followed these instructions. If you have the back wheel, gently try it in the dropout. If you are not happy, prise off the dropouts and start again. Otherwise, build up a layer of epoxy in the gap between dropouts and forks.
If you do get epoxy inside the screw holes in the carbon, you can go through with a 5.5mm drill, preferably before the stuff has set properly, using a 5mm screw inserted from the inside of the dropout to protect the thread, which you don't want to damage. Epoxy in the threaded holes in the dropout can be cleaned out with a 4mm drill, and then a 5mm (taper) tap.
According to my moulder, when you have finished with the epoxy you shouldn't need the screws. I don't take the chance. Stainless-steel button-head screws, 10mm long, look nice here. If you want a rear mudguard - not a bad idea - you could replace one of these with a short piece of 5mm studding, for mounting the stays, else drill another hole , through the carbon behind the dropout.