Here is how I added an extension to a bicycle so I could haul boxes of vegetables from a farmer's market. The Xtends unit uses a discarded bicycle frame and some electrical tubing. My cost was under $20 but the Xtends unit could be built for free if you have a good recycle pile.
The Xtends unit can be removed to return the bicycle to its normal configuration.
The Xtends unit involves welding. For a similar conversion that does not require welding see: https://www.instructables.com/id/Homemade-Sport-Utility-Bike-SUB/. Step 9 has some suggestions on how to make the Xtends unit without welding.
NOTE: I have added Step 11 to show you a way to make the Xtends unit that is simpler and stronger. Please check it out.
This project assumes you have some experience disassembling and reassembling bike parts. See this Instructable (https://www.instructables.com/id/How-to-Build-Up-a-Bike/) to get some idea how this works if you are not already familiar with it.
This project involves welding EMT conduit. Please see my Instructable for suggestions on ways to do this with a minimum of risk: https://www.instructables.com/id/Welding-EMT-Conduit/
Step 1: Select a Bicycle
I pulled this frame from my scrap pile after it had been stripped of handlebar, brakes, chain, front fork, etc. It looks weird because I just put parts back on it that I had lying around. It does support the idea that you can use whatever bike you have as a starting point.
I will call this the "main bike" from now on.
Step 2: Remove Rear Wheel and Related Parts
Step 3: Select a Donor Frame
You might choose to keep the brakes and shifters if the donor bike has components as good as the main bike. Otherwise we will use the parts we removed in step 2.
The biggest requirement for the donor frame is that the angle between the chainstays and the down tube (shown with the red lines in figure 2) must be close to the same as the angle between the chainstays and the seatstays of the main bike (shown with the yellow lines in figure 3).
Figure 4 shows the final rig with the main bike highlighted in yellow and the Xtends unit highlighted in red.
Step 4: Prepare Donor Frame
Step 5: Add Linking Axle
Measure the distance between the rear dropouts of the donor frame (this was 5" in my bike) and subtract 1/2" for the two inside nuts. Cut the metal axle tube (at 4.5" in my case) and prepare it for welding (polish the surfaces that will be welded). See figure 2.
Polish the top of the inside of the bottom bracket shell in the donor bike and drill two 1/4" holes for the plug welds. Center the metal axle tube in the bottom bracket shell under the plug weld holes and clamp it in place. See figure 3. I used Vicegrip pliers for this.
When the welding is completed insert the threaded axle rod through the metal axle tube and tighten the two inner nuts to hold it in place. Add the two outer nuts so they are ready to tighten in the next step. See figure 1.
Step 6: Add Frame Clamp
Rotate the Xtends unit so that the main tube of the Xtends unit swings forward toward the seat tube of the main bike (see figure 2). Mark the Xtends unit down tube where it overlaps the brake bridge between the two seatstays of the main bike. This location is not critical so locate it wherever you want the clamp to be.
I used 1" angle iron for the two clamp pieces (see figure 3). I cut my clamp pieces 4.5" long and drill 1/4" holes 1/2" in from each end. The upper end of the Xtends down tube (now cut to length) is welded to the middle of one of the angle iron pieces and the other angle iron piece forms the back side of the clamp. Two 1/4" bolts that are 1.5" long are used to hold the clamp in place. Either use nylon lock nuts or regular nuts with lock washers.
I used two 3" long pieces of bike inner tube to cushion the clamp and avoid scratching the main bike seatstays. Also do not over tighten the clamp as you could crush the main bike seatstays.
Step 7: Install Rear Wheel and Related Parts
Install the rear brake and replace the main bike rear brake cable with a new longer one. Keep the old cable in case you want to remove the Xtends unit.
Step 8: Add a Cargo Platform
Step 9: To Sum Up
You might be able to build the Xtends unit without welding anything. You could use bolts for joining tubes and U-bolts for attaching the metal axle tube to the bottom bracket shell of the Xtends unit. Welding makes it a LOT stronger, though.
I would welcome any suggestions or questions you might have.
Step 10: Parts List
You will need:
A bicycle to add the Xtends unit to. See step 1.
A donor bicycle to cut up. See step 3.
A piece of 1/2" ID metal tubing 4.5" long. I got this at Tractor Supply Company.
A 1/2" rear threaded axle with nuts.
Two pieces if 1" angle iron 4.5" long. I got this at Tractor Supply Company.
Rear brake and shifter cables.
A piece of bicycle chain to extend the main bike one.
A scrap pair of upper stays.
A piece of seat post tubing.
6" of bicycle inner tube.
You will also need these 1/4" bolts:
2 ea 1" hex head bolts.
2 ea 1.25" hex head bolts.
A 2.5" carriage bolt.
Step 11: An Even Better Way
Photo #2 shows the bottom tube cut 15" long and the top tube rough-cut to match.
Photo #3 shows the spindle screws holding the Xtends unit in the main bike rear dropouts. The seatstay clamp is not yet on the bottom tube.
The clamp is 1" angle iron 4.5" long. Three 1/4" holes are drilled. Two are 3/8" from the ends and the third hole is at the midpoint. (See Photo #4)
A 1/4" hex bolt 1.5" long is used to attach the clamp to the cut end of the bottom tube (See Photo #5) and the basic Xtends unit is done. (Note the clamp can be welded on if you prefer.)
All the other previously described steps apply.