Tuesday, January 24, 2012

The Homebuilt Bakfiets Cargo Bike

It finally came to that time in life to have a child so my thoughts briefly turned from adventures like caving and climbing to things like "how to transport the offspring in a two-wheeled pedal-powered fashion?" I dislike trailers for child transport because the child is behind you and out of sight where cars can pounce with no warning. Child carriers perched on the bike seem a bit vulnerable if the bike falls over. The solution? A two-wheeled bakfiets cargo bike as used by thousands of mums and dads in Denmark, Holland and increasingly Seattle.

The commercial models aren't readily available in New Zealand and they are rather pricey, although still cheap compared to a car. If other people can hack one together from chopped up bikes, then why not me?

I'm not much of a welder but luckily I have a friend, Bevan, who has a TIG plant and is a dab hand at welding bike tubes, having built himself a tandem bike amongst other projects. I drew up a quick plan and procured the necessary donor bikes. In the end the rear bike was given to me by the same friend who did the welding. I wanted to put a disc brake on the back so a vertical dropout frame was preferable. The frames I had were all the older style sloping dropout so the donated frame was more suitable.

I decided to go with a 26" wheel mountain bike for the rear because most of the bikes in my stable are mountain bikes and I figured a triple chain ring could come in handy when hauling a big load. For the front I bought a trashed old kid's BMX which came with an extra beefy front wheel which I was glad of.

The front end tacked together and the brake bosses ready to go on.The front fork off the BMX was set up for a caliper brake so I shaped up some cantilever bosses (cut from another old fork) to weld onto it. For the steering arm that the steering strut attaches to I used the bottom of a fork arm with the dropout cut off. I left the conveniently M6 threaded fender mount to bolt the ball joint to.

To shape the tubes accurately for joining, I used one of the many tube notching programs available on the web. I cut the tubes with a hacksaw and then did the shaping with a half-round file. I bought a good file for the job which made it much faster.

The main tube joined to the donor bike.I was planning on getting a piece of rectangular hollow section steel for the main tube but my friend came through again (remind me to buy him some beers!) with a length of 2" mild steel exhaust tubing. The main tube is ovalised at the front and welded to the head tube cut off the BMX. The main tube is pierced by the rear head tube which is formed from the top tube of another cannibalised bike. The back of the main tube is ovalised and welded to the donor frame just in front of the bottom bracket.

The bottom of the rear head tube and the stand.A beefy kick stand was formed by cutting and welding two old steel handlebars into an H shape. The top of the H mounts into two cut down quill stems. The quill stem brackets mount onto a small diameter tube that pierces the main tube. They run on a polythene bush made from irrigation pipe.

The rear steerer tube was fabricated by cutting the steerer tube off a donor fork and extending it with some extra tube of a similar diameter. The steering arm was again made from the reused end of a donor fork.

The steering strut that joins the rear steerer tube to the front fork was cut and welded rather than bent because it was easier to do and it also matches the sharp angle of the front of the box better. Getting the positioning of the steering system correct was quite important to allow sufficient turning angle. I didn't get it quite right so that on full left the steering strut rubs on the back of the front tire. Not a big deal really as the full lock is never reached while actually riding. The steering strut has M6 nuts welded into the ends to mount the M6 threaded ball rod ends. I left the rod ends loose in the steering strut so the strut is free to pivot out of the way of the front tire when it hits.

The disc brake mount and brace.

The only other modification was to weld a disc brake mount to the rear of the main donor frame and a small reinforcing brace next to the mount.









To house the cargo I built a plywood box. The bottom and ends of the box are 9mm ply box-jointed together. The box joints were just cut by eye on the cross cut fence of my table saw, as a high precision fit was not required because the plywood is soft pine. Cutting the angled box joint was tricky but I rigged up a tilting fence which did the job. The 7mm sides of the box are joined to the bottom using aluminium angle connected with M5 countersunk screws into T nuts. The box is mounted to the frame using steel saddle clamps and M6 screws into T nuts.



After about six months of intermittent work on the cargo bike and the delivery of a new daughter to use as cargo, I was finally ready to give the whole lot (not the new daughter though) a couple of coats of gloss black enamel paint from a can. A free ride to anyone who can correctly identify all the parts in this photo.








Update 7/5/12
I weighed the bike and it comes in at a very respectable 25kg with the box. Compare with the commercial models which run high thirties.

I added a removable aluminium bar to support a weather cover. It seems to work (see photo below).

The steering bar has turned out to not be quite stiff enough. It is fine during normal riding but flexes when the wheel is pushed on hard. I'll upgrade it to 12mm or larger at some point.
Update 14/9/12
The stand retaining clip was a bit light weight and I had a couple of complaints about it so I remodelled it using a spring.

13 comments:

Andrew and Vicki said...

Nice work Felix!

One question - you used exhaust pipe for the main bottom cantelever - that's quite a long bending moment, I seem to recall exhaust pipe is pretty thin gauge, will it be stong enough??

Felix said...

Seems to be so far. I've sat in it but have yet to do a test with an adult in the box! I have a back up plan if the tube starts to bend. I'll get Bevan to weld a steel plate gusset to the tube.

Michael said...

Quite an ambitious project...if it doesn't work out, let me know and I will send you a discount for a Zigo Leader Carrier Bike (http://www.myzigo.com). If anyone deserves a bakfiets it's you.

Felix said...

That zigo looks cool, but it's a tadpole trike in the kid configuration. I wanted two wheels all the way!

janvos said...

Kia ora felix/melissa and allegra
Great to see this. Here in Holland started Green/left orientated person to bring their kids to school /kindergarten. Now a lot more parents do that. The only problem is that there is not enough parking space for those bikes. I remember that Bryan sinnot , friend of Karina at Karapiro also build some of these. before he became ill , due to cancer.
For your info: google on bakfiets.nl There you will see pics. but most populair are the ones with a wheel in front, and via bars it can be steered.
Jan and Bas from Holland

janvos said...

Sorry Felix. They started this 15-10 years ago. But also my grandfather (opa)Father of Ben Hilterman) did deliver his coals with a bakfiets
jan vos and bas from Holland

Hamish said...

Gidday Felix, hows the bike going? I'm working on one over Avonhead way and wonder how the steerings working out in ChCh? I'm concerned that the single sided push/pull rod might not be strong enough for typical kiwi curbsides - I'm making mine for 2 kids, 40kg load. Do you get much flex there? And any stress cracks where the CroMo joins plain steel? My test bits always broke not on the weld but in the CroMo after my mongrel welding overheated it. Anyway, fine fine job, looks great!
/Hamish

Felix said...

Hi Hamish,
drop me an email at felixcollins at yahoo dot com. I've had over 40kg of groceries in mine and no probs. The steering strut is well stiff enough unless you are planning on downhilling. Come over and check mine out sometime. I thought it must be unique in NZ, but glad to find out that it is not!

Kev said...

That looks great, for your next challenge.......

SUSPENSION.

Imagine the luxury for your wee ones.

Kev said...

That looks great, for your next challenge.......

SUSPENSION.

Imagine the luxury for your wee ones.

Felix said...

Hey! We are having a cargo bike rally this weekend in Christchurch. Details at

http://www.cycletrailers.co.nz/chocbar.pdf

Teo T said...

Awesome build! I'm very impressed, it looks fantastic. Really enjoyed this post, and your other cool DIY projects here! I work for a new social blogging site called glipho.com, and was just wondering if you would be interested in sharing your posts there with us? It wouldn't affect anything here at all, and I know there's a ton of people within our community who would love to read your work here. Let me know what you think!

All the best,

Teo

jk said...

Just beautifull DIY! It really inspires me.