I’m going to answer my own question here.
I got a really good answer from Martin Cote at Clearpath Robotics (where I purchased my Turtlebot):
Greetings James,
Thank you for contacting Clearpath
Robotics support! This is not a dumb
question at all, and actually
something that has been brought up by
a few other customers recently! Sadly
this is indeed how all Turtlebots are.
The Turtlebots weren't always like
this, but right around the time you
probably purchased your Turtlebot,
this was changed to it's current
state. As you may be aware, we're
simply a North American distributor
for the Turtlebot, and don't have too
much design input.
The Kobuki base is made by Yujin
Robot, and I reached out to them
recently about this issue, so sadly
I'm not able to help to much to
resolve this, but here is the
representatives response for your own
insight.
If I remember correctly, the only
thing that changed is the dimensions
and properties of the passive wheels.
This does cause different rocking
properties. I do not recall that we
did anything to the diff drive
mechanism for this particular problem
- that is unlikely as it would have been a significant change for
moulding/factory.
Getting the wheels right is a
pareto-optimal game for a base with
four contact points and an unknown
centre of mass (knowing the centre of
mass, you can get away with three
wheels and avoid many problems like
cleaning robots do).
Previously with all four contact
points close to each other there was
little rocking, and this works fine on
a perfectly smooth and slippery floor.
Put it on carpet though and the
passive wheels will create alot of
drag. Lay down some small obstacles
like cables, and the base won't get
over them. Since this made it
effectively useless for a large number
of users, we had to find that
pareto-optimal compromise between
stability (minimal rocking) and being
functional on many environments.
There's not much worse than having to
make decisions where you are forced to
be average rather than great.
I hope that at least answers you
questions, let me know if there's
anything else I can assist with!
Best regards, Martin
So, from Googling around it looks like the reason for the change is probably discussed here:
https://github.com/yujinrobot/kobuki/...
Apparently the Turtlebot would get easily stuck without the extra clearance.
I understand the reasoning, but I wish there were a better solution.
