This comes with another problem: on rare occasions, I need to haul a motorcycle. I bought a 4X6 trailer, which I really should have opted for the slightly larger 4X8 trailer. The bike fits diagonally, but it's just a few inches too long to fit in front to back. I could buy a motorcycle trailer, for about $500 or I could buy a 6 1/2" wide motorcycle rail for $60 plus shipping.
Or, I can spend a few hours and a few dollars with power tools and welder to build what I believe to be the first convertible utility/motorcycle trailer.
Perhaps it's not the first, I could have bolted a rail to a flat bed utility trailer and unbolted it when I don't need it. That means I need to store a 85" tall seven inch wide rail and spend probably 15 minutes bolting and unbolting it when I need it or to remove it respectively.
You'll see all work is done in my back yard for this project, we got about four inches of rain over night, I do not want to drag my welder out through the yard to do this in my driveway.

So I got out my measuring tape, went down to the metal shop and for about $30 I have everything I need to build my convertible motorcycle trailer. I'll be using a jig saw to cut the metal, a grinder to smooth it out, miscellaneous clamps and vice-grips to hold things together before it's welded. You can get by with a hacksaw and files, I'd recommend against it. It will take for ever. I'd have preferred to use a band saw and sawsall, but I don't have them. The welder is designed for up to 12 gauge steel, I'll be using 1/8th inch steel. 1/8th is a small fraction larger than 12ga and I've used this welder for larger than 12 gauge with excellent results.
The trailer is a Carry-On 4X6 2000 lb GVWR utility trailer. I've had it for several years, if there was a warranty, it's expired by now and I'm sure that what I'm about to do voids the warranty. Keep this in mind if you plan to do this project.

The first step is measuring, the height of the upper rail from the floor is 9.5 inches. I'll be using 45 degree and 90 degree joints when ever possible, as these are the easiest to measure, cut and weld. The upper rail will need a 6.5" wide, 9.5" long "box" added to the front center, with a 45" brace that will also act as the tire stop. I bought eight feet of 2X2 angle iron, three feet of 1X1 angle, 2 pieces 2" long of 1 1/4 inch square tubing, and one piece of 2 1/4" long 1" square tubing. I'll also be using some miscellaneous pieces of sheet metal (18ga - 1/8 would work) for the tire stop and some 2" flat metal. If you need to buy this, get about 2 feet. You may need to buy the square tubing by the foot, but if you don't have a band saw, ask them to cut the pieces off for you. It may cost an extra dollar, it's worth it.

Level the trailer in every direction, this is optional, but it makes things much easier.
Next, you'll need to mark and cut out the area to be removed. Use caution while doing this, for some reason, there was a lot of tension on the upper rail, as I was cutting the last 1/4 inch, it grabbed the saw blade and snapped it in two. You may want to cut the last bit from the other direction or use something to push the top railing apart slightly to avoid this. If not, just use extreme caution and always wear your safety equipment, you know, gloves, goggles, etc.
You'll notice that it appears as though this is cut off center. This is because on my trailer, the tongue can move left or right about 1/2" from center, and it is slightly off center, I only noticed this after cutting of course. I measured it, and it didn't look right, so I redid my measurements. Cut twice, measure once, or something like that. It actually works out that I can reason why it's off center, because when the motorcycle is on it's side stand, it leans just to the right. But I'd recommend against repeating my mistake.
Cut out the front part of the lower rail using a grinder. A large hammer and pliers can help coax it out. Smooth off all the edges using a grinder.

Next, it's time to start installing the new pieces.
Its hard to see in this photo, but I notched out part of the 2" angle iron to mate it up with the trailer's upper rail. I had to use a 90 degree clamp because the welding magnets had a tendency to slip, especially if the trailer was bumped. It would be wise to get help from a friend for this part.
Make sure you keep everything square and level, unless ignored that leveling part, in which case, it's not too late to do it now.

At this point, take the remaining 2" angle iron and clamp it to the top rail to use as a guide so everything stays where you want it. Use at least two clamps on each side. Then weld the front section (for me, it was 10.5") to the "box". Did I mention, you'll need to cut that off before you clamp down the 2X2 angle iron? Good thing you read this front to back first, right? Oh, you didn't? Oops!
You'll see that I've placed the welder's ground clamp on this new piece, it's the easiest place to put it so you won't have to constantly move it around.

Now we'll need to add the braces. I spent a few minutes calculating how long these would need to be, using the pythagorean theorem. I wrote down how long they should be, I cut them to length at a 45 degree angle. And they were the wrong length. So measure it instead. Save yourself the headache.
You can see I put mine from the floor to the top edge of the "box". You can also weld them to the bottom of the "box", you'll use slightly less material, slightly less weight (a few ounces), but the real benefit will be another inch forward your motorcycle will fit, especially if you have the 4X6 trailer like I do. However, it will no longer be at a 45 degree angle, so you'll need to cut them to a different angle to make it fit. In my case, the new triangle would be 9.5X7.5X12.1" (at 52 degrees and 38 degrees) instead of 9.5X9.5X13.435 (at 45 degrees each non-right angle).

The next piece I put in, was a brace between the angled braces. You can see the C-Clamp still holding it in place in this photo, bottom center. If you're going for a 6.5" rail width, it would be 6.5" long.
Now that the structure is completely together, weld every connection point, inside and outside unless you're super confident in your welding ability or carry really good insurance on your bike, trailer and vehicle. If you're wrong and try to stop very quickly, you may find your soft tail in your rear windshield. So weld 'em up good.
Also, you'll see that I had a few really crappy welds in this photo. I ground them down and did them again. One last tip for this, if you're welding outside, put an old t-shirt over the back of your welder's helmet. It will block the reflection from the sun off the darkened lens.

After everything cools, shoot it with primer and then with an overcoat. Spray paint (aka rattle can) will do just fine.

A few more shots:

Check back later for part two.