After trying a trailer, backpack, and rear seat, I think the bakfiets is the best way to transport children. Although there are some concerns...(see step 6 for some ideas on the safer ways of placing children in the box).
This instructable is all about how to design and build a bakfiets style cargo bike. The design tips apply to any bakfiets style cargo bike and there is no need to build it out of bamboo. I like the ride quality and uniqueness of bamboo. However, if I do it all over again I might use steel... Building a bakfiets is like building three normal bikes. The amount of epoxy and sanding seemed to add up. I think my next bike may be fillet brazed steel.
Here is the breakdown:
Cost: $600 (including components - mostly used)
Time Frame: 4 months
Larry vs. Harry Bullit
the dimensions of the bullit
Tom's Cargo Bikes
Bamboo Bike Blog
Sources for materials:
Epoxy: West Marine (I used West Systems but I think I should have tried this: http://www.entropyresins.com/products/super-sap-clr - half the price of West System 105)
Bamboo: local bamboo nursery
Drop Outs and Tube: Nova Cycle Supply : http://www.cycle-frames.com/bicycle-frame-tubing/home.php
The art of building bamboo bikes has really come a long way. I have realized that my technique from 2008 is probably not worth emulating. I strongly recommend that you look at the Bamboo Bike Blog and other instructables for the next level of building with bamboo. I particularly like this instructable by kentdvm:
So let's get started!
Step 1: Design Geometry
The design concept of the bakfiets is that you have two wheels (which makes the bike nimble and easy to steer and ride) and a low center of gravity which makes it very stable.
Wheel size - smaller theoretically means more stability. I went with a 20" wheel in the front and a 26" wheel in the back. I have heard of some people using a 20" rear wheel, although I have never seen this. There are a few different sizes that are all called 20". The front wheel I used is a 406 mm rim diameter. This seems to be the most common 20" size and there are a lot of bmx tires and recumbent tires readily available.
Bottom Bracket Height - 265 mm - this should probably be 30 to 50 mm higher. When I drew the bike out I had this as 290 mm but somehow in building the bottom bracket came down. This may have something to do with skinny tires I ended up using. I do get pedal strike when turning corners sharply or pedaling over speed bumps.
The basket is pretty low (150 mm). The lowest point however is the bottom of the steering tube (120 mm) I am pretty sure this has never struck a bump in the road.
The bike is 580 mm wide. This is no wider than many mountain bike handle bars (600-700mm).
Below is the geometry of my bike as it is built. So far the only issue with the geometry is the bottom bracket height - this really not too big a deal and I subconsciously keep my pedals up when going over speed bumps. The bike may be unnecessarily long. There seems to be a bit of spare room between my knees and the basket.
Another thing I did was slope the seat tube back to allow me to keep my butt on the seat and still have my feet reach the road. This makes stopping and starting much more comfortable.
I have long chain stays which are not really necessary. This was a half baked idea to get more tire clearance by moving the wheel back. (I ended up messing up a bit and don't have the tire clearance I would like.
I would design the bike to accept big tires even if you end up riding skinny tires.
The bike is surprisingly upright in the turns and unlike a normal bike the bakfiets is steered with the front wheel and not by leaning over. Because of this you almost always have to have at least one hand on the bars at all time and really need to to have your butt on the seat for stability.
It is slower but I am enjoying learning to take my time on the rides. One big advantage has been the added effort required to get around. My daily commute on my road bike is not much of a work out anymore. Now on the bakfiets I am getting a workout again. My 5 mile trip to work takes me just under half an hour on the road bike and just under 40 minutes on the bakfiets.
The attached pictures show the geometry of the bike and some ideas on design changes you can make if you go with cable steering instead of rod steering. I chose rod steering and a very rectilinear design - this works particularly well with bamboo. If I used steel I would definetly added some curves because curves are pretty. Josh Muir's bikes are works of art.
Steering is discussed in more detail in step 3.