I've tried both the "mount to top-tube- hold the handle-bar" style and the "behind the seat" style and ended up with a highly modified "behind the seat" style one. Best bet is used - this isn't an item that gets used for more than 1-3 years and lots of people buy them, use them once, stick them in the garage and then sell them three years later.
Mount to Top-tube - sit in front of you
Pro's
- you can be more interactive with your child
- you can wear a back pack
- your child is "technically" more protected if you fall
Con's
- can be awfully cramped for pedalling - I borrowed one and only used it a couple of times because I had to pedal with my knees going out at a really uncomfortable angle
- you are awfully top heavy
- not the most comfy for baby
Back of the bike - behind the seat
Con's
- existing seats SUCK - they all have poorly designed connection systems and don't integrate well with fully suspended bikes (thus my custom job).
- can't see or talk to your child as easily
- rear-bike weight bias (manualling is easy!)
- you can't wear a back-pack
- less prtection for child if you fall
Pro's
- easiest for you pedalling
- more "independent" for your child
- doesn't raise centre of gravity as much
- easier to go on more trails
- I have lost track of the number of times my son has fallen asleep while riding behind me
Anyways - working on a second generation prototype of my modified "suspension " seat that will solve some of the problems - move child further back, lower their weight, solve clearance issues with disk brakes etc. Watch for a front page article coming up.
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