I hope someday in the nearish future my child will want to ride a bike with me badly enough to necessitate the itchair for my Brompton. Until that day, I’m saving up my pennies, because at $360 it ain’t cheap.
In the meantime, she’s small enough and light enough to use the Bobike Mini+, which I was able to find locally on CraigsList for $40 in a decent color and pattern. The factory-supplied mount works great on my beater bike, but the mounting bracket was far too narrow to fit on my Brompton frame and it would have been too bulky to allow the bike to be folded anyway. So I began work on a custom made mount.
I contacted a number of local metal workers to build a small bracket with mounting tubes to accommodate the Bobike, but the job was too small for most of them to take it up. Others, though interested, priced me out of the market (one guy quoted $300-$500 for the part). I’ve still got a couple of leads to follow up regarding a “production” version of the clamp, but on Thursday I fabricated a first prototype mount and last night I completed the second prototype.
The construction is simple: the bracket is held in place with a U-clamp that has been covered in plastic tubing. The bracket itself is a piece of 1-1/4″ heavy-duty steel that’s been folded over on itself and drilled to accommodate the clamp. There is a small spacer (made of wood) to keep the bracket spaced properly and to keep the Bobike’s pegs sit firmly in place.
UPDATE 8/2012: I’ve gotten some comments about this post and looking back on it, I’d probably recommend against trying this. The chair itself won’t accommodate a lot of weight (it’s rated to something like 30 lbs). I used this set up pretty regularly with my daughter until she was ~26 lbs. But after that, her weight started to affect the bike’s steering negatively (especially when she moved around a lot, which toddlers tend to do).
In New York you must wait until your child is 1 year old to put him/her in a bike seat. One year olds usually weigh in at ~20 lbs. This means that there’s a relatively short window in which you can realistically use a seat like this. And even then, the safety is a bit dubious (especially in my case with my shoddy workmanship).