The stokid python uses the front part of the pythoon replacing the rear with a new module. Thus forming a tandem where the stoker position is sized for a child, hence the name "stokid" (stoker kid, see http://sheldonbrown.com/gloss_st-z.html#stokid).
The construction of the rear part
The bottom tube is a 40x40mm tube of 1.5mm thickness.
The chain stay, seat post and bottom bracket are taken from a mountain bike. The seat post is shortened down to accommodate the inseam of the child.
The top tubes and seat stays are taken from a mixte bike (http://sheldonbrown.com/gloss_m.html#mixte).
The steering tube of the stokid is a recycled seat post from some bike - the diameter matching the standard headset diameter of 1⅛ inch.
The rear frame as a whole is quite stiff, which is important for good handling of the bike.
Building this rear part went quite easy. Once the square bottom tube was brazed to the pivot and the rear triangle, the rest was just building up on that using the square tube as a base. I was finished in less than a week even though I have to mind a day job and am not a quick builder at all.
The cranks are shortened down to accomodate to 100mm. I used some cheap cranks to 100mm. The crank size of 100mm is a little less than 1/4 the inseam of the stokid, matching about the proportions of an adult inseam and adult cranks. These cranks come with a non removable single gear chainring. Such a chainring is wider than what would fit for a 8 gear chain, but the difference is only about 0.5mm which I grinded off.
The stokid drives the rear wheel using a 7-gear dérailleur, and he is able to follow at all speeds.
I can best compare to the pythoon which is a long bike too.
The angle of the pivot on the stokid python is more shallow (57.5 instead of 60, togethyer with a slightlý shorter trail). This change has an effect that is immediately felt. For example the bike turns lighter (less self centering force to surmount), and this makes it easier to maneuver at low speeds.
I had my first accident right from the start: the CoG of the stokid is high, care must be taken when leaning into the curve at very low speeds. While with the pythoon it was possible to use leaning+acceleration to do a sharp turn when starting, I got nasty surprise trying to do the same with this bike: we fell over ---luckily no damage was done.
Because the pivot angle is slightly different from the pythoon I had to adapt to the slight different behaviour before I could start cleanly (=without a sway). In the beginning I had the tendency to over-steer when starting (front part swaying over to the left when starting with my right foot). After 100kms this is gone at the bike feels natural.
Riding at speed
We've been riding this bike in the last 2 weeks (about 100km together). Both of us are still learning.
There is interference with the steering when the stokid is pedaling. But either I am getting used to it or my son is riding smoother, because often I am unaware of this. I expect that after some time it won't be visible anymore.
When my son is not pedaling I have not noticed any PSI. The bike rides like a train.
Seen from the point of view of safety the pythoon is unbeatable. Even compared to other solutions the pythoon really shined: the bike is compact compared to an upright + trailer; the position of the passenger is really low; and even though the box was open he was very effectively protected by it.
The stokid python is not that safe for the stokid. And because of that I try to limit our speed just in case we would be involved in an accident.
Another reason to limit the speed is the need for braking. I have already been able to push the stokid forward out of his saddle just by braking strongly. He seems not to be strong enough to withstand the deceleration. Luckily the effect is that he falls down to the top-tubes instead of being flipped over.
Outside of Town our cruising speed is about 30kms (max has been 45kmh up to now). Though I limit the max speed, we are not that much slower than on the pythoon. Inside of Town speed is determined by outside factors so I can't really give any numbers.
The positive side of the stokid python is that we are much more visible: my son now wears a reflective vest and this works as a beacon for the cars.
On the pythoon my son was always looking sideways into the landscape. Now his head sticks out up over my head, so he can overview the situation in front of the bike.
Our trips vary between 5 to 15kms single way. These are quite long rides for a 5 year old and he can feel it in his bottom and arms. Just the typical problems of an upright...
The main goal was to involve my son into the ride and that has been a real success. He loves riding this bike and is now very active with both his legs and mind. He often shows direction with his arms, or tells what way I could go. Or warns me about cars coming from behind. (he even tells me that he is taking a rest from pedaling ;-)). There is also a lot coordination going on. If he wants to slip his hands he has to tell me. Or when a bump is coming up I have to call out so he can brace himself.
All these things are missing when you put a child on a half-cycle. On a half cycle pulled by an upright the child can help pedalling, but is still just being pulled along.
On the down side: Controlling the stokid python is "advanced level" and I would only recommend it for those who already now how to ride a regular python well.