Motorcycle Cargo Trailer

Made Simple


Authored by Brian Budlong, brianbl101@intergate.comm

Do not distribute without permission. 

No warranty is expressed or implied.






These “plans” are a generalized “how-to” for a simple, but sturdy, motorcycle cargo trailer. The ideas came from a number of sources, but in particular, from my good friend Kelly, who’s previous experience was a great help in figuring out the small stuff.


The trailer dimensions are tailored for the car top carrier made by Karrite, and sold by Sears ($99) and BiMart ($89), but the dimensions can be changed to suit the carrier of your choice, or for a custom trailer.


The trailer frame is fabricated from 2” square tube.  The outer dimensions are 30.5” wide and 41” long.  The dimensions were laid out and checked before ordering metal.  The metal shop where I purchased the material offered to cut the pieces to specified lengths and angles for a mere $15, which greatly reduced my labor time for a very reasonable expense.  All corner cuts are at 45 degrees, the remaining cuts are all at 90 degrees.


Confirm all dimensions before cutting or ordering. 


The angle iron was originally purchased as extra bracing material to aid in attachment of the car top carrier, but was not deemed necessary.  Instead, the angle was only used to support and position the fenders.


The trailer frame is a rectangle with one cross piece at the mid-point, and a tongue piece that extends “through” the front frame pieces and is welded to the cross piece.  It is important that the pieces are cut to exact lengths and angles to maintain a straight and square frame.



Lay the frame tubing out on a flat surface and confirm the fit of the pieces.  Tack weld in place, then proceed with finish welding.


In a nutshell, here’s the order of construction.


  1. Weld the frame together.  Confirm dimensions and make sure the frame is square.


Drill or weld holes in the cross piece so that the wiring can be threaded through the tongue and cross piece to the lights at rear.  Make the holes before assembling the frame.  Holes made with the welder will generally be less sharp on the edges and will keep the wires from being cut as you snake the wires through the frame.


  1. Position and weld the hanger and spring mountings.  The  squared U-shaped support hangers go towards the front.  Make sure that the spring hangers are located at the exact same distance for the front of the frame and from the tongue (diagonally measured).  This is important because the location of these supports determines the location of the springs, and ultimately the position of the axle.  These must be positioned properly so that the axle is square and the trailer pulls straight when finished.  Mine were back 7 ¾” from the leading edge of the front frame.




  1. The axle, a 1 ¾” steel pipe, will have to be cut to the proper length to allow the wheels and covering fenders to clear the sides of the carrier.  The correct position of the spindle in the axle calls for about ½” of the spindle body to protrude from the axle shaft.  The spindle ends should be taped off during welding to keep any slag from mucking up the spindle.


Once the correct length of the axle is determined, position the spindles in the axle and weld them in place.  This can be tricky, as the axle’s inner dimension is slightly larger than the spindle outer diameter.  To get a more precise position, drill three holes in the axle about 4” from the end, equidistant around.  The holes can be small and threaded.  With the spindle inserted, thread bolts into each hole, and, run the bolts in to the same amount, thus centering the spindle in the axle shaft.  With the spindle thus positioned, weld the end of the axle to the spindle completely around.  Once this weld cools, remove the bolts from the axle and weld the holes to the spindle.  The spindle should now be straight within the axle.


To position the axle, turn the trailer right side up with the springs and hangers installed.  Position the spring saddles on the base and lay the axle on the saddles to determine where they should be welded to the axle. 


Axle looking on end with bolts threaded

to position the spindles for centering.


The biggest challenge for me was hanging the springs and axle.  I hadn’t done this before, and a little help from my friend Kelly was invaluable.  Once I did it, I realized how easy it really was, if you have all the hardware. We originally forgot the axle / spring saddles and had to go back and get them.


In the picture below, you can see where the weld on the spindle goes around the axle end (the holes to align the spindle aren’t seen, but would be hidden by the spring assy).  The rear hanger is welded so that the pieces supporting the spring are vertical (remember to position the front hanger pieces first).




The light brackets can be welded or bolted into place.  I chose to weld them.



  1. Fenders:  Here’s another view of the trailer frame detail with the hangers, spring, and axle.  Note the positioning of the fenders.  The 1 ½” angle was used for the fender supports.  One length was welded on the edge of the fender, and two short 3” pieces were welded to that piece for attaching to the frame.  The results were very stout and you can customize the fender distance by varying the length of the two short pieces.  Check for height to determine where to weld the cross piece to the fenders.


The trick here is to get the axle width and spindle widths correct for your overall trailer width, keeping in mind the fender clearance.  This was done by putting the pieces together without welding, and lots of measuring of the car top carrier and frame. 


  1. The coupler can be welded or bolted into place.  I chose to bolt it, by pre-drilling the holes before welding to the frame.  Once the frame was completed, I bolted it on and then welded the nuts secure to the bolts (they aren’t coming off).  I then added a third bolt downward and double nutted it to anchor the safety chains.





I’m using a Dauntless Hitch.  Because I have ABS brakes, I elected to go with an “isolator” type harness.  The Dauntless hitch came with a standard harness, so if you’re not using ABS, you should be able to get by fine with the one supplied.  The common thought on ABS is that an Isolator type harness will keep from interfering with the ABS circuit (don’t know if it’s true, but I thought the $49 wasn’t too much to pay for the added insurance).


The Isolator harness consists of a separate relay for each trailer light circuit.  In effect, the bike’s lights only activate the relay for the corresponding trailer light.  The relay provides direct current from the battery to the light (i.e., the bike light circuits only trip the relays, so that little or no current is drawn from them, keeping the load on the bike’s lights to a minimum).


Trailer lights; THE CONVERTOR


There are two common types of lighting systems used on vehicles in the USA.  These are the more common three wire systems (American), and the four wire systems (European). 


The “American” three wire systems use the same filament/bulb for the brake lights and turn signals.  When engaged, the turn signals cycle the appropriate right or left brake light, so that this brake light (right or left) flashes whether or not the brakes are on.  This system requires three wires (not including ground) to function (tail/2brakes).  This type of system has been around forever, and was the most common on vehicle produced in the US until the influx of foreign cooperative efforts of automakers.


The “European” system uses a separate turn signal bulb (usually amber), so that the brake lights operate independently from the turn signals.  This system requires four wires (not including ground) to function (tail/brake/Rturn/Lturn).  This lighting system is more common on today’s cars, especially imports and motorcycles.


The bad news: 

To convert from a European (GL1800) to the “American” type system a converter must be used that converts signals.  Unfortunately, there appears to be some problem for standard or basic converters in dealing with the GL1800.  I’ve tried 5 different types and brands of converters and none worked.  I spoke with the technicians at two of the electrical manufacturers that make the converters about the problem, and both told me that most converters don’t seem to work on motorcycles.  They didn’t know why, but they suspected that the ground system “floated” or had some effect that prevent the electronics from working properly.   Their recommendation was to try a converter that incorporated a ground wire.  The one tech told me that he knew that only about 20% of the applications on Goldwings worked properly, and he didn’t know why.  I found that on the five different converters, 4 which also included a ground, none worked properly.  It really was puzzling, as the relays appeared to be working fine, providing a nice clean and strong current to the converters, but they refused to properly work.  The most common symptom was tail lights worked, but no brake/turn worked.


The good news:

I happened to try a converter that was carried by the trailer parts house, which advertised that it worked specifically on motorcycles as well as cars.  It was a $14.75 gamble the paid off.  On trying this one, my sixth, it worked from the beginning… no worries.  The brand and part number are included in the parts lists (Wesbar).  I know that as of July 02, Autowheel was carrying them.


Why a converter?

I didn’t find any off the shelf trailer lights that supported the European system.  All the trailer lights I found were designed for the 3 wire.  Since this is a standard among auto/trailer applications, and I conceivably might pull the trailer behind another vehicle, I wanted to use the 3 wire system on the trailer.  That’s the only reason.  If you’ve got the lights for a 4 wire trailer and you don’t plan on pulling it conventionally, by all means, skip the converter.




The 2” square tubing (2x1/8 ST) used for the frame is very stout.  After seeing how heavy it is, I considered that using 1½ “ or even 1” might work fine for the load I planned on using with this trailer.  I figure that it would probably save a fair amount of weight, but as I had already finished, I don’t think I’ll be making a new frame any time soon.  The only drawback with using smaller tubing is that the alxe/spring hangers are too wide so you might have to be creative when positioning the hangers on the frame.


Weight savings:

I believe some of the pro trailer outfits are using aluminum tubing on their trailers.  I wasn’t about to try and weld aluminum as I hadn’t done it and didn’t have the equipment.  But, that would be a pretty good way to go to keep the frame very stout and reduce weight.  As finished, the trailer weighs about 160#.  Very acceptable (comparable pro trailers weigh about 145#).


Front Jack-stand:

The front jack-stand is of course optional, as no stand is probably necessary, really.  I saw one on an older trailer and thought it looked great and might add functionality, so I included it in mine.  Cost was about $13.


Prepping and painting:

I prepped the entire trailer by grinding and sanding.  I then used acetone liberally to remove any grease or other foreign stuff prior to painting.  I then “rattle-canned” it with a good quality gloss black.  It took just under two cans, even with a third coat, and it looks good.   I don’t think you would have guessed it was a rattle can job.



I used a 3/8” piece of plywood I had on hand, cut to match the frame, and painted it before attaching.  I used a countersink and 1” machine screws with a counter-sink head to achieve a flush fit.  I pre-drilled the holes slightly smaller than the screws, and used about 12 overall, probably overkill.


Safety Chains:

Add two chains and “S” hooks for safety chains.  One 2’ length with the center link bolted to the under side of the coupler provided the two safety chains required by law.


Spare Tire

Although I haven’t put it on yet, I plan on attaching a spare tire/wheel to the undercarriage for storage.  I was going to use a U-shaped bolt that matched the hole pattern of the wheel, and mount the bolt over the tongue between the frame midpoint cross member and the front frame cross member.  It would then hide under the trailer body until needed.  But, that would have required mounting the plywood deck over this piece and would have caused a bulge, so I revised my plan.  I also had trouble locating a U-bolt with the specific width to fit the hole pattern of a 4-hole trailer wheel.  


I now plan on using two bolt shanks (cut from the U-bolt mentioned) welded along the tongue to match the hole pattern, extending down far enough to catch the wheel.  This will leave the top of the frame clear and not interfere with the deck, and provide easy mounting of the wheel from underneath (hidden until needed).



I purchased some cheap stick on reflectors that I cut into smaller pieces and mounted on the four corner points.  I don’t know if there’s a specific requirement for reflectors on a trailer of this small size, but they add a nice finishing touch to the trailer.



In my state, trailers under 2000 lb gross do not require a license plate.  I’m not sure about the “title”, but I believe one isn’t required also on trailers of this small size.  You should consult your local DMV for clarification.


Cooler carrier, front cargo area:

Some have suggested creating a front shelf area for storing a cooler or some other gear, like fuel cans.  Using the 1½ “ angle, a person could weld this as shown below and finish off with plywood or some other material.  One neat product sold at the metal fab shop I considered is “expanded” metal.  There is a type of flat expanded metal that would make a great “floor” for the shelf and be very useful.  You must have the shop cut (shear really) this to fit your space, then weld it down at each contact point.  There was a ¾-10 type I believe that looked like the right size and was only about $1 per sq foot.


An alternate method I plan on using is to create a cooler “carrier” which will bolt to the tongue.  I think this will look nicer and has the added option of being removable.

Well, that’s about it.  I realize I haven’t provided a real step by step on this, but I don’t think common sense assembly will really require it.  Laying out and welding the tubing is probably the most time consuming part.  Next is the hangers and axle.  Welding the spindle in the axles is probably the most complex, but it doesn’t have to be perfect.  Don’t forget that you must pack the wheel bearings in grease prior to using.  If any of this seems a bit difficult, there are a number of books available on trailer building basics.   You also might do what I did and seek out a willing friend who has done this before.


If you have any questions, feel free to drop me an email and I’d be happy to fill in any gaps.  If you make a trailer, let me know and send me a picture, I’d love to see it.  Maybe we could even take our trailers out for a drive some time?  Good luck. 

Brian (



Parts list

As used by me. Your quantities may vary


Description                                                       Source                               Cost

Karrite Car top carrier                                      Sears, Bi-Mart                   $95


Trailer Parts                                                   Autowheel                              

Coupler, 1-7/8, 2 2000lbs                                                     DL940-P1                                             7.17

Bracket, Oval lamps                                                             TL60720-3  (2)@ 2.70ea                     5.40

MR&W Spindle                                                                   R116BT8 (pair)                                  15.00

Spring                                                                                    AS2 (pair)                                          18.25

Wheels 8x3.75 4-4 w/480-8 B TI                                         DC844W/T 2@18.65ea                    37.30

4-4.0 Hub Grp for BT8                                                         H44-9                                                  32.17

Attaching Parts                                                                    FR175                                                 16.24

Fenders                                                                                  T65S (pair)                                         16.46

Wesbar Flat 4-Wire 25’ kit                                                  WE107725                                           4.90

Lamp Kit S/T Oval                                                               TL60002R3 7.35ea                            14.70

Atwood Jack Stabilizier                                                       AT82301                                            12.60

Wesbar Converter                                                              WE7295                                           14.75

Spring saddles 1.75” for 2000 lb. Straight axle                 3-49                                                       6.00





2x2 1/8” square tube                                                                                                                        31.00

1 ½ x 1 ½” x 1/8 Angle                                                                                                                     11.00

labor for cut                                                                                                                                      15.00




Axle 36” black pipe, 1¾”                                 Ace hardware                         6.00

Gloss black spray paint 3 cans                                                                        9.00

Plywood deck 3’x 4’                                                                                    10.00

Coupling bolts 2 ½” 5/16 bolts and nuts (3)                                                     3.00




Total cost                                                                                                $380.94



Spare wheel/tire

Spare tire hanger

Cooler carrier frame and square bolts


Auto Wheel Service Inc.

1400 NW Raleigh

Portland, Or  97209

(503) 228-9346

(800) 445-6193

website ???






class=MsoNormal style='tab-stops:3.0in decimal 5.0in'>Spare tire hanger

Cooler carrier frame and square bolts


Auto Wheel Service Inc.

1400 NW Raleigh

Portland, Or  97209

(503) 228-9346

(800) 445-6193

website ???