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
Almost any bicycle can be fitted with an Xtends unit. Some bikes are probably better for this project however. I used a 26" bike with a heavy frame for strength. The stays in the rear fork are going to be carrying the load so it helps if they are sturdy.
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
Once you have selected your main bike remove the rear wheel, chain, derailleur and rear brake. These parts will all be moved back to the Xtends unit in step 7.
Step 3: Select a Donor Frame
Now we need a bike frame to cut up. The frame should use the same size wheels as the main bike. That is, if you used a main bike with 26" wheels you will need a donor bike that uses 26" wheels. Only the rear fork of the donor frame will be used so don't worry about the condition of the front fork. You don't need wheels or pedals for the donor bike. In fact you need to remove them if they are present.
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
Remove the seat, wheels and pedals if your donor frame has them. Cut the head tube off the donor frame as shown here. These are rough cuts for now as we will give them a final trim later. Discard the head tube.
Step 5: Add Linking Axle
I made the linking axle from a scrap 1/2" diameter rear axle and 1/2" ID metal tubing.
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
The Xtends unit (donor frame with linking axle) is attached by inserting the linking axle into the rear dropouts of the main bike and tightening the two outer nuts "sorta tight" (tight enough to hold the linking axle in place but allow some rotation. See figure 1. (I have the rear tire is in place in the picture but it is not really needed yet.)
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 wheel in the Xtends dropouts. Add the derailleur and chain. You need to extend the chain to almost double what the main bike used. (I chose to set my bike up as a single speed rig so there is no derailleur.) You can reuse the parts you removed from the main bike in step 2.
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
There are lots of options for cargo carrying. I chose a simple platform as shown in figure 1. I attached some scrap seat stays to the clamp and to the top tube of the Xtends unit (see figure 2). I also added a piece of tubing into the seat tube of the Xtends unit (figure 3). I welded together a platform out of 1" electrical tubing and welded it to the Xtend seatstays and seat tube. This attachment is removable in case I ever want to reconfigure the cargo format.
Step 9: To Sum Up
This was a cheap and reasonably simple conversion. The main bike was not altered in any way that is not reversible. The new long wheelbase makes this bike a joy to ride. Now I just need to paint the Xtends unit!
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
This is an even better way to make the basic Xtends unit. I used a donor bike that had removable crank arms and a squaretaper crankarm spindle. The spindle was 4.5" end-to-end and just fit the rear dropouts of the main bike (Photo #1).
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.