International Bicycle Fund Cargo Trailer:
by David Mozer
The IBF Trailer is designed to incorporate several positive features:
Note: The IBF Trailer is designed for loads up to 140kg (300 pounds) and speeds up to 30km per hour (20 mph). It has limited applications for motor cycles. The weakest element is usually the wheels.
General Assemble Procedures
The easiest sequence of assembly is to start by cutting the proper notches in the tubing to be used as the main frame. There are two types of notches notated in the instructions as type "A" and "B". Be certain to make the correct cuts. After all of the notches are cut, fold the main frame, tack weld the joint where the ends come together, square the frame and weld all of the corners. When the frame is formed measure the length for the internal braces, cut them to length and weld them in place. The main platform is then complete. After the main platform is formed build the cargo box, attach the tow bar, its braces, the hitch, the pulling handles and finally the wheel dropouts. Because variations in notch cutting style can change the size of the frame slightly, it is better to measure and cut to order everything that attaches to the main frame. Note: The main frame can also be made by cutting to length the pieces for each side, as opposed to notching a continuous piece. This requires less sawing, but can make it more difficult to get the frame flat and square.
The following tools are used to construct the IBF trailer:
Buy the following materials:
1 25mm x 25mm (1" x 1") 16 gauge square tubing or equivalent. The piece should be at least 6.5 meters (20 feet) long.
1 20mm x 20mm (3/4" x 3/4") 18 gauge square tubing or equivalent. The piece should be at least 6.5 meters (20 feet) long.
1 20mm x 20mm (3/4" x 3/4") angle iron, 32mm (1 foot) long.
1 10mm steel rod, 24mm (8 inches) long.
2 Front bicycle wheels. It is convenient if they are the same size as those on the bicycle, unless something stronger is available.
1 100cm x 50cm piece of material for the floor of the cargo box. The actual size will depend upon the actual size of the cart. The kind of material will depend upon the load anticipated. Materials to consider are: galvanized tin, heavy gauge wire screen, plywood, woven mat, etc.
Cut notches in large tubing for main frame:
Note: There are two types of notches, both are 45 degrees, but type "A" is asymmetrical off of perpendicular, and type "B" is symmetrical off of perpendicular (see figure 1). All notches face the same direction (see figure 3). DO NOT CUT ALL THE WAY THROUGH THE TUBING.
Note: It is easier to make the type "B" notches if you make a small template: Cut a piece of metal to a 45 degree angle. Orient the piece of metal so that the 45 degree is at the apex (top). Make an isosceles triangle with a height of 25mm. Score a perpendicular line on the triangle showing the height at the center (see figure 2). You can lay this on the tubing and scribe along it to figure your cuts for type "B" notches.
Note: If you want to make the trailer a different size you can do so by adjusting the length of the sides or the width. It is not recommended that the trailer be made any narrower because it will loose stability. To make the trailer wider lengthen the 50cm dimension. To make the trailer shorter decrease the 77cm dimension. It is not recommended that the trailer be enlarged too much because there will be more of a tendency to overload and break the vehicle.
Take the large tubing (25mm x 25mm) and square one end. From the squared end measure and mark sections of the following lengths, in the order given (Do not cut them): 50cm, 16.25cm, 77cm, 16.25cm, 50cm, 16.25cm, 77cm, 16.25cm. Now cut the notches. There are type "A" notches between all of the 50cm and 16.25cm sections. The 90 degree is always perpendicular to the 50cm section. The 45 degree is always sliced into the 16.25cm section (see figure 3). All of the notches between the 77cm and 16.25cm sections are symmetrical type "B" cuts. All of the notches, both type "A" and "B" face the same direction Do not cut all of the way through the tubing.
Form the main frame:
Bend the main frame tubing at the notches until it forms a rectangle with blunt corners. Tack weld the joint where the ends come together. Check to see if the frame is square. This is done by measuring from the outside of one type "A" notch, across the center of the rectangle to the outside of the opposite type "A" notch. Remember or write down this length. Now make the same kind of measurement between the two other type "A" notch corners. If the figures are equal then the frame is "square". If the frame is not square compress the two corners that had the longer measurement. Keep remeasuring and adjusting the frame until both measurements are the same. When the frame is square weld all of the corners. DO NOT WELD ALONG THE GAPS THAT WILL SHOW ON THE INSIDE OF THE TYPE "A" CORNERS.
Cut and install braces.
Note: If the trailer will be used to carry heavy loads (over 140kg, 300 pounds) additional transversal braces should be added.
The braces in the main frame are made with the large (25mm x 25mm) tubing. Due to different cutting styles the finished frame sizes generally differ slightly with the individual. It is recommended that you only cut the two inside wheel support tubes and the transversal cross brace after you have welded the main frame and can verify the measurements. The two inside wheel support tubes should be about 95cm and the cross brace should be about 45cm. When the pieces fit properly weld them into place. Measure the position for the cross brace so that it is equal distance from the ends on both sides. The main platform is now complete.
Cut and install corner posts.
Cut four 25cm sections of the small tubing (20mm x 20mm). Using a square to make certain that the posts are vertical. Weld the posts at the corners of the rectangle formed by the short side of the main frame and the inside wheels support tube. Place the posts so that they are flush with the outside edge of the rectangle. These are the corner posts of the cargo box.
Cut and attach cargo box rails.
Measure the inside dimension between the corner posts. Cut sections of the small tubing (20mm x 20 mm) to length. Two pieces of tubing should be about 96cm and two pieces should be about 46cm. Weld the rails to the post so that they are flush with the top of the post and square with the rest of the frame.
Install mid-trailer support struts.
In the middle of both sides of the cargo box, at the transversal cross brace, there are support struts between the inside wheel support tube and the top rail of the cargo box. Measure for these posts. The length should be about 23cm. Cut the strut, measure the location for the placement and using a square weld the section in place. The cargo box is now complete.
Construct tow bar.
Measure a piece of small tubing 85cm. Square one end. On one surface of the other end cut in for 25mm at 45 degree, measure a 90 degree angle back towards the end and cut out this notch (see figure 4). Measuring along the "top" from the indented flat side of the notched end, mark 50cm. Using this mark as the apex, cut a type "B" notch. The notch will be cut into the "bottom" (see figure 4). From the squared end of the tube measure in 25mm and drill a 10mm hole from top to bottom through the tube. Make the hole centered in the tube on both sides. Again from the squared end of the tube, measure in 50mm and drill a 10mm hole from side to side through the tubing. Make certain the hole is centered in the tube on both sides. Bend the tube at the notch and weld so that the tow bar has a 135o angle. The tow bar attaches, extending up at a 45 degree angle, to the main frame in the middle of either end. Measure the location and weld the tow bar so that the planes of the rails of the cargo box and the forward section of the tow bar are parallel.
Hitch pin and pulling handles.
Cut a section of 10mm rod 6cm long. This is the bicycle hitch pin. Cut a section of 10mm rod 18cm long. This is the pulling handle. Insert the bicycle hitch pin in the vertical hole in the tow bar so the the pin sticks out the bottom and is flush with top. Weld the hitch pin in place. Insert the pulling handle through the horizontal whole in the tow bar. Measure it so that it extends an equal distance to both sides and then weld it in place.
Bracing tow bar.
Measure three sections that are cut off at convergent 45 degree angles at both ends. The two lateral brace sections should be 35cm long and the third structural brace should be 20cm long. Weld the lateral braces between the top outside corners of the cargo box and the tow bar, parallel with the plane of the top rails of the cargo box. Weld the structural brace between the lateral brace, parallel with the plane of the top rails of the cargo box. Measure and cut another structural brace which has convergent 22 1/2 degree angles at both ends and which is 25cm long. Weld this brace to the bottom of the tow bar, spanning, with equal distance to both sides, the joint of the type "B" notch. The tow bar is now complete.
Cut four 8cm sections of angle iron. In the middle of each piece, 10mm from the top drill a 10mm hole. From the the top and perpendicular to the edge, make cuts down to the hole, tangent to each side of the hole, for each piece. This is a drop. Bolt one dropout to each side of each wheel with the angle iron facing away from the wheel. Turn the trailer up-side-down and set it on some blocks so that the wheels can be put in the wheel wells unhindered. Put the wheels in the wheel wells. Check to see that all of the dropouts rest flat on the main frame and inside the wheel support tube. Adjust the dropouts if necessary. Align the wheels so that they are perpendicular to the plane of the main frame, parallel to the inside wheel support tube and parallel to each other. When everything is aligned tack weld the dropouts so that they will stay in place. Carefully remove the wheels from the dropouts. Finish welding the dropouts in places.
Floor of cargo box.
Select material for the floor of the cargo box. Your choice may depend upon what is available or what you intend to haul in the trailer. Possible materials are: plywood, wire screen, bamboo slats. Cut the material to fit the floor of the cargo box and attach it. The method of attachment will depend on what material you are using for the floor and how permanently you want it attached. The floor can be fastened with wire, nylon bands, twine or welded.
Grind the welds and paint the trailer with rust prevention paint. The value and longevity of the trailer will be greatly enhanced with good finishing work. If you later notice rust developing the area should be sanded and painted to prevent weakening of the frame.
Attach the wheels to the dropouts. Align them so that they are perpendicular and parallel.
Attach trailer to bicycle.
The simplest way to attach the trailer to the bike is with a strip of inner tube. The hitch pin can be slipped over a rear rack or into a hitch attached to the bike frame (see figure 5.) The trailer should be tied so that it will not bounce up or slide forward, yet can still rotate side-to-side, up-and-down and tilt laterally in both directions.
Your IBF Trailer should now be ready to go to work for you. When loading the trailer it is important to make certain that you put more than fifty per cent of the weight is in front of the wheels -- always have a positive weight on the hitch. This will make the trailer safer and ride better. Also, always secure your load so that it cannot fall into the wheels and will not shift in the trailer while you are riding.
Treated well you trailer should give you many years of productive work.
For an unabridged copy of the article, including illustrations, please write to IBF, 4887 Columbia Dr S, Seattle WA 98108-1919 USA -- a suggest donation of at least US$5 is requested to help support the program.
The International Bicycle Fund is an independent, non-profit organization. Its primary purpose is to promote bicycle transportation. Most IBF projects and activities fall into one of four categories: planning and engineering, safety education, economic development assistance and promoting international understanding. IBF's objective is to create a sustainable, people-friendly environment by creating opportunities of the highest practicable quality for bicycle transportation. IBF is funded by private donation. Contributions are always welcome and are U.S. tax-deductible to the extent allowed by law.
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