From: wbradley
To: cyndecal
Subject: Lego:  Tri-Star Wheels
Date: Monday, October 09, 1995 12:24PM

(some deleted sections)

One additional problem I can speculate about your implementation is
this:  To make the wheels turn "forward", your drive shaft has to run
"backward".  In a low-friction environment (such as water), I think this
would torque the whole "meta-wheel" the wrong way.  Thus, the tank would
drive forward, but paddle-swim backward (if at all...spinning the wheels
forward while torquing the meta-wheel backward would probably result in
the tank floating in place).

The best solution to this would be to turn the entire meta-wheel from
the drive shaft, and put in some form of a differential to turn the
individual wheels when the meta-wheel encounters too much resistance
("too much resistance" being normal operating conditions).  This in
fact *further* enables the tank to traverse obstacles.  When an
obstacle is encountered, the torque on the meta-wheel will help to
"throw" the meta-wheel forward.  This also allows the meta-wheel to
turn freely in a low-friction environment (i.e. water).

Unfortunately, I can't think of any way of implementing this in a
small scale.  The best I can think to do is provide separate power
to the wheels and meta-wheel:  an axle connected into the Y-frame
can act as a meta-wheel drive shaft, and a free-spinning gear (one
resting on, but not powered by, this shaft) can supply power to the
individual wheels.  I'm not quite sure how to implement the finer
details, though.

Also, the reference mentions providing power to both "halves" of a
jointed vehicle using a flexible cable.  Power transmission through
*torsion* is pretty hard on the hardware.  You'd be better off using
a U-joint around the point of flexion(and another diff, of course).

(some deleted text)

-------
From: wbradley
To: CyndeCal
Subject: Re: Lego:  Tri-Star Wheels
Date: Monday, October 09, 1995 5:09PM

(some deleted text)

I agree that there are a number of problems with a LEGO implementation
of this design:

     o  Application of power:  there was to be some connection between
        the meta-wheel and the individual wheels such that some "slip"
        is involved.  The only LEGO element that can introduce that is
        a differential, and the problem with diffs is that the two
        outputs go off in opposite directions.  This makes connecting
        the meta-wheel and the individual wheels via a diff hard to do.

        Plus, when the design is done, you'd have to a lot of tweaking
        to make sure you have the correct *amount* of slip...

     o  Limited gear sizes/ratios:  I'm not intimately familiar with
        tri-star wheels, but it would seem that for the operation to
        be the smoothest (least noticeable to the passengers), the
        meta-wheel would have to turn at a reduced speed when compared
        to the individual wheels (this ratio would be (wheel radius) to
        (wheel radius + meta-wheel "radius")).  This would allow the
        ground speed to be the same, whether the wheels are turning or
        the meta-wheel is turning.

        I doubt you could get the right gear ratios using LEGO gears.

     o  You'd probably need a pretty cushy suspension, too...

> I really want to tryout your idea, but I need a little clarification (and 
> sorry if I'm bugging you=) )  What do you mean by a free-spinning gear...

I just went back and looked at your implementation again, and never mind
what I just said.  The meta-wheel axle and the axle which provides power
to the wheels come from different sides of the tri-star.  This actually
would work well for a LEGO implementation; you can just connect the
"wheel driveshaft" and the "meta-wheel driveshaft" to independent
motors...or build a transmission for some relation between the two!

I like the transmission idea.  If you're on the flats, you can have a
"wheels only" gear that provides all the power to the wheels.  If you're
in the water, you can have a "meta-wheel only" gear that provides all
the power to the meta-wheels.  If you're on an obstacle, you can have
a "half-and-half" gear or something else.  Alternately, you could have
*two* transmissions dictating how much power each part gets.

(Of course, this is getting pretty complex.  I'd say two independent
motors would be a good approach to toy with.)

(some deleted text)

Let me correct myself.  Power transmission through torsion isn't that
bad.  In fact, that's what you do when you turn an axle!  However,
power transmission via torsion of a flexible object is really
inefficient.  For example, take a piece of rope.  Begin twisting one
end.  Note how much you have to twist it before the other end starts to
turn?  All that stored energy is wasted, as far as you're concerned.
Using half-axles with a U-joint on the line of flexion is much better.

(some deleted text)