DIY Cargo Bike From an Old Mtb

How to make a Cargo bike from an old 26'' mountain bike and some loose parts. It's been in daily use for 18 months now and its been broadly successful. Hoping this is useful to anyone who I would like to build something similar. I have waited awhile since building so I could add some extra details like any parts that wear out quickly, etc.

This is not a step by step guide but more of an overview of the process and pitfalls. As I assume most people taking on a build like this have some background knowledge and reasonable expectations of the finished result. Before reading on you should decide if its the best type of cargo bike for you.

To help you here is what I find decide. On a CAAD8 cannondale, I hold a comfortable 28km/h pace with a peak speed of around 43km as a solo rider.

On the DIY cargo it is around 15-20km/h on flats and 5-10km/h uphill depending on factors load/slope, a peak speed with medium load would be 25-27km/h. So be aware you will not set speed records on it. I find distances over about 25km (loaded) in a single point to point is about the max I want to ride. It tips over easier than a drunk pig when its empty, but stable as once loaded up. The heavier weight is not a problem once your moving its the start/stop that gets you. I chuck the the little critter in there with a bluetooth speaker blasting some Disney tracks and its happy days to/from school.

It carries sheep loads of stuff! Some of the junk I have carried:

1. 3 x 26'' mtb bikes on front and helmets. (I got some odd looks that day)

2. One adult and 25kgs of luggage. Total weight = 230kgs (load+bike+rider)

3. Two kids (5yrs) and school bags - 50kg ++

4. Phill and Teds Stroller

5. Tool box's

6. Even a, 3 seat reclining sofa.

In the end I spent $200 USD on bits and pieces, spent an afternoon or two tinkering in the shed to some tunes and made a car-lite solution for our commuting and shopping.

Having said all that lets get cracking. Some of the photos are dark, but you get what you pay for. ;-)

Supplies:

1 x 26'' mountain bike with rear wheel

2 x 20'' front bike wheels

Tools:

Wielder, angle grinder, drill, vice and some half baked skills.

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Step 1: Dismantle the 26'' Bike

Before dismantling the mtb take a few measurements such as the height of the crank to the ground, the length of the top tube (from seat to handlebars), etc. These measurements will help you get the sizing right as you build.

Once you strip the mtb down to the frame it's time to get cutting. I cut the bottom tube flush off from the crank to the head tube. The top tube was cut flush at the head tube, but I kept around 75mm of the top tube attached at the seat tube end. This 75mm tail is used to support an angle brace later on.

Clean up the crank case and head tube parts, so they are ready for welding/reuse.

Step 2: Front Frame/box

After I had cut up the mtb, I started to approximately lay out the design. I had an added requirement that I assume most people doing this task will not have. The finished bike had to disassemble ready for air transport, meaning that the box frame and all components had to be under the maximum allowed dimensions for the airline and the total finish weight had to be less than 32kg.

The finished box frame is 750mm wide x 550mm long. With the 110mm for each wheel subtracted I ended up with an approximate 500x500mm cargo area.The box in the photos is actually a core flute sign that I used to wrap the finished bike for transit. Once I arrived I used it to template the timber panels that I was planning on swapping it for but.....eh.

The box frame:
The frame for the box area is really basic (underneath photos later). Its a rectangle with a 'H' frame in the middle to hold the inner head tube (welded directly onto the box frame). I think I used some parts from an old desk to build it, in any case its a thin steel of approx 0.5-0.75mm thickness, 40x20mm RHS. The dropouts are from some angle that was laying around with slits cut into it for the axles. The whole box frame is attached to the bike using the standard head tube assembly from the mtb (more on that later).

Step 3: Joining Parts and Handle Bars

Now to join the two main components:

  1. The rear wheel assembly
  2. The front box frame.

To do this I used the bottom tube from the mtb and some 40x40 RHS. In essence this was simple enough, but there was some trial and error to get the headtube angle correct. Hence the bend in the 40x40mm RHS. I initially thought that having an angle on the headtube to lean the seat and rear wheel in turns would help to manage the center of gravity and prevent tipping in corners. This made for some odd cross body steering positions and extremely impractical to ride. Eventually it ended up pretty close to 90°.

RHS bottom tube:
The 40x40mm RHS bottom tube is bolted into a short length of 50x50mm RHS about 150mm in length that is welded to the bottom of the crank case (more on this later, see opening image). The same bolts were used for both the angle brace and RHS bottom tube, they are 10mm in dia. with nylon anti-vibration nuts. (I have had no problems with strength whatsoever.)

The angle brace:

Is the recycled bottom tube from the mtb. I would normally notch this to make the bolting tabs, but I grabbed some off cuts of flatbar, because well....lazy. This helped to stiffen the whole thing up a lot.

Handle bars:
This was a simple recycle from the mtb with some extra tube. It runs directly from the box frame to the hand position. This was the first area to show fatigue, but it was mostly do to the thin material of the box frame itself. Every time you brake some of your weight pushes on the handlebars and when you lean back or adjust your balance I tend to pull back on the handles a little. The back and forth of this fatigue the .5mm rusted steel pretty dang quick. You will see in the pic the joining area has had some bits added to make it more resistant to this movement.

Step 4: Steering and Thoughts on Next Time

Steering:

The steering is direct and sliding. In that you 'push' the handles left to right rather than 'twisting' them, hopefully that makes sense. It's important that you have the headtube 'pivot' point behind the axle line. If the pivot and the axle are aligned it will be very twitchy at speed, not that its a fast monster to begin with. Even with the axles backset from the front (approx 50mm), I still got a bit frustrated with bump steer and lightness of steering. To help this I put in a cheap gas strut. The type you will find at home depo or auto store.

Once I had it bashed together, I went for the ol' $2 spray job with no prepwork. I mean really at that point f-it she carries stuff and rolls. It was never going to win a beauty contest was it.

The next times:

1. Two gas struts or some other return to centre on the steering. The single strut naturally 'pulls' to one side, which is fine when riding, but pushing through a doorway or gate is a pain.

2. The head tube. I will look for a custom and heavier solution next time. I will probably use angled roller bearings and a solid shaft. The annoyance here is in the basic cheap headtube type and bearing slop. Take a another look at the underneath pics you will see a 3-way brace that was added to help reduce the flex in the headtube joints and material. The brace works well, but is not very elegant.

3. Solid welded frame. This is probably the most annoying aspect of the finished bike. Due to the bolting and transport constraints, there is 'racking' or twisting in the rear half of the frame that gets bothersome on stepper descents.

4. Keep the 750mm overall width. It works great!

5. Pay closer attention to the alignment of the frame (again transport), as it leans slightly left resulting in uneven tyre wear.

6. It will be a tilting or independent suspension design.

If you got this far, hopefully my rambling is of some use.


Later gator.


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    2 Discussions

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    Alex in NZ

    15 days ago

    Very impressive! Well done, and thank you for including the "Next Time" section:- knowing the dead-ends which have been explored should save time for anyone doing something similar :-)