Design and build a pedal-powered mulcher/chopper from old bikes!

This is a very open call for a challenge so I’m happy to take criticisms or suggestions on where to go from here.

I joined the community garden at Templehof Field in Berlin this year and in talking to some of the members, I learned that they need a way to chop up large amounts of plant material for their compost pile. Currently they rent gasoline powered mulchers once or twice a year, but this is costly and unsustainable.

There were plans for someone to build a pedal-powered mulcher last year, but because of budget and other issues, it never materialized. I spoke to one of the people responsible for composting the other day, and according to her the requirements are as follows:

  • handle green wood (not more than 3cm thick), plant waste, and some food stuffs
  • be safe enough that it can be used around children
  • be locked down so that no one (especially children) can use it unsupervised

I would add one item to this list of desired traits:

  • it should shred cardboard, as this is a plentiful urban waste and an excellent carbon source.

This is all in addition, of course, to the requirement that it be pedal-powered and relatively cheap to build. I think we can design a solution to this problem that would be very nearly free, open source it (and in that process further refine it), and ideally put this machine into use in community gardens all over the world.

At the moment I have a few ideas from research that I’ve done, but not enough to translate that into a working model. I would love to have an input session with anyone interested where we can discuss the requirements, design a prototype (or several!) and even try to build a working model.

Here are some links I’ve found to get started thinking about a design:

  • pedal-powered mulcher - small scale, but some good angles of how it was built, so could be replicated on a larger scale with some study
  • hand-cranked shredder - this is a really nice idea, and it’s fully flushed out as an instructable.
  • mayapedal - an orgination in Guatemala about pedal powered machines

Interesting project! It would be very useful to have something like that in the garden.

The nice thing about the mentioned projects is, that they are very easy to build, easy to assemble and easy to take apart again.

I mainly wonder, how to make them strong enough for 3cm thick branches. And i wonder if its possible to cut that thick branches with only one person pedaling…

One thought: Especially in this garden, there is a lot of rather soft/thin organic material. So a chopper, which is specialized on rather thin twigs (up to 1.5cm?) and cardboard, would be a very desirable machine, too! The chopping process should be as fast as possible.

The first design (pedal powered mulcher) seems to work quiet fast. The second one seems stronger but slower. Maybe they are different machines for different kind of materials. Maybe it would be an interesting solution to have one design for the bigger amounts of thin materials and one design for smaller amounts of thick materials!

If found some images of a similar design here:

It seems very strong an I can imagine it can chop quiet thick branches and cardboard. The problem is, of course, that its difficult and rather expensive to build. But maybe it its a good inspiration!

Hi Philipp,

Thanks for your input! The 3cm requirement is the upper limit, and if we find that we have to sacrifice a lot (much stronger material, a much higher cost, etc) to reach it, then we might be able to drop this to 1.5cm as you suggest. The important part, in my mind, is in the discussion to generate design ideas.

Those designs you’ve posted look promising and defintely some good food for thought, thank you.

admin note: I relocated the topic and discussion from the general “Questions and Challenges” category to the “Hardware” category. Tell me if you think it should be somewhere else. And continue the discussion :slight_smile:

This design uses a reel push mower blade:

Important details seem to be a proper transmission and a flywheel (is that the right word?)

A friend suggested, that a flywheel could be moulded from concrete.


yes, the concrete flywheel is how it’s done on some of the mayapedal designs. nice one, philipp, that design looks great!

Someone who has done a lot of research into this has sent me some files and designs with her learnings. I thought this design for the ‘Easy BioGrind’ is particularly interesting as it requires mostly hand tools and no welding.

Easy_BioGrind.pdf (915.8 KB)

another interesting model, this one 3D printed:

Update on this:

The challenge happened and we got two somewhat usable ideas from it, both of which have photos floating around, hopefully they will be added soon.

For me the biggest thing I took away from this was a matter of semantics. Because I had thought and talked so much about trying to build a ‘pedal-powered’ shredder and that from old bikes, the focus was really on working with bikes. Looking back that was wrong.

The focus should have been to build a shredder which uses rotational energy.

When that is accomplished, the way which it is driven (a human cranking it, an attachment which makes it run from a bike, or even a windmill) is of secondary importance.

Here’s some photos that Sebastián took:

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This challenge spread across the globe over the year and was repeated in 2017 also in Berlin. There are Tutorials and Presentations for it here:

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Any updates on this? Based on my very quick reviews of other designs and prototypes there seems to be a need for:

  • A flywheel. This ensures more continuous power delivery.
  • A mesh to ensure more uniform sizes. For composting we don’t need a really fine mesh, but some of the twigs I’ve seen put through the prototypes are pretty much chopped into smaller pieces, not really ground. Maybe a hammermill with mesh would work well enough here?
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