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Chassis and
Tower
A pair of 12.1/2"
Angle Girders with a pair of 9" Angle
Girders form a hollow T shape, each spaced
3" apart. The lower end of the tower is a
1Girder attached by a Flat Trunnion to a
3.1/2" x 2.1/2" Flanged Plate, the weight
catching tray. A Trunnion above joins the
Girder to the chassis cross-member, via
spacers, so that the catching tray can
eventually be adjusted to clear the ground
by less than 1/16" maximising the possible
drop distance. Five 5.1/2" Strips, some
via Brackets, suspend and brace this
structure, also steadying the tower. A
24.1/2" Girder extends the tower upwards.
Being attached by Fishplates, the final
height can be finely set to the maximum
36" allowed by the competition. A 3"
Pulley is journalled in the top hole and a
3.1/2" Strip stood-off by twin pairs of
spacers. A large pulley suffers less
friction at its bearings and keeps the
weight clear of the tower, which leans
back by 1" at the top. A pair of curved
Flexible Plates is used to direct the
weight between the chassis members near
the end of its drop.
Bearings
The road wheel axles
do not pass through holes in the usual
way. Instead, pairs of Faceplates at each
corner are overlapped by 1/2", spaced by
washers to prevent fouling. At the front,
Faceplate pairs are independently
journalled in 3.1/2" pairs each side, to
overcome any minor discrepancies among
Faceplates and axle surfaces. At the rear,
Faceplates are fixed to either ends of the
same rods passing through the lugs of
3.1/2" x 1/2" Double Angle Strips. One of
the rear road wheels can rotate freely on
its axle rod. All axle rods are first
cleaned with fine steel wool, and then
rubbed with a metal polish. Faceplates are
selected for minimal wobble. They should
spin very freely before meeting the road
wheels.
Road Wheels
These are 7.1/2"
Circular Strips fixed by 7.1/2" Strips to
Bush Wheels or Double Arm Cranks at their
centres. The weight of the vehicle is
supported through the Faceplate edges onto
the polished axles linking the wheels.
Arched pairs of Curved Stepped Strips
carry the axle when the vehicle is lifted
from the floor, but must not touch it
during normal travel, of course.
Winding
Ideally we need the
vehicle to free wheel after the weight has
fully dropped. To achieve this, a small
bowline knot is tied with Meccano cord
around the driving axle. It cannot grip.
However, a small Drive Band is held nearby
on the axle, trapped beneath a Spring
Clip. The tip of the Drive Band is slid
under the bowline loop so that when the
cord is wound tight, it will compress the
rubber, gripping the axle. Sufficient cord
is provided so that when the weight has
fully descended, the few remaining turns
of cord will be unwound under the
travelling momentum of the vehicle. The
Drive Band will then withdraw from under
the cord loop, thus preventing any
rewinding of the weight.
Weight
For maximum drop, we
need a short fat weight rather than a tall
thin one. Ideally we want it to fall
unimpeded by other parts of the vehicle. I
chose to cast my own lead weight, reminded
by an article by the late Bert Love in an
old Meccano Magazine. This can be
hazardous and should not be attempted
without adult supervision. The mould
was a small terracotta plant pot. Inside
the pot, under a long
bolt’s head, I fitted a
washer, a couple of nuts and a large
washer, and on the outside, another large
washer fixed with a nut. I melted just
less than a pound of lead in a tin can
over a camping gas stove in my
well-ventilated workshop. Wear goggles
and thick gloves, just in case impurities
cause splashing.With the plant pot
stacked on two bricks so that the bolt
shank could hang downwards, I carefully
poured the molten lead into the pot and
left it overnight. In the morning, I
removed the outer nut and washer and the
new lead weight, being conical, slid from
the pot. I added a Wheel disc, a couple of
washers, and a 1/2" Bracket to the exposed
thread of the bolt, now trapped securely
in the lead. I filed the Bracket into a
hook and bent it so that when suspended,
the weight would hang vertically.
Performance
Our combination of
weight, cord, axle, bearings and road
wheels will only just work on the average
smooth floor. Once rolling, however, the
machine has plenty of momentum. In order
to get extra power at the start of a run,
the weight can be raised up around the
outside of the tower Pulley, so long as is
doesn't put the structure over the 36"
limit. This manoeuvre will almost double
the power available from the weight, since
it forces the lead weight's centre of
gravity almost twice as far as normal from
the centre of the Pulley, until the weight
has fallen a little and hangs in the usual
way. The theoretical driven distance
possible from a machine such as this may
be simply calculated as follows:
Distance = Weight drop
distance x (Road Wheel
diameter / Winding axle
diameter)
I have ignored the
fact that the cord may become wound around
itself and therefore increase the diameter
of the winding axle. In our recent
competition, this machine travelled 37
feet 4 inches on an 8" drop. The Road
Wheel/Winding axle ratio is 195/4.1 (if
you work in millimetres!) so the 8" drop
becomes magnified by a factor of 47.56,
sending the vehicle 31 feet 8 inches. The
remaining six feet (approximately) was
done freewheeling after the weight had
dropped. The competition had to reduce the
weight drops to 8" because our 39 foot
hall wouldn't cope with any of the six
competitors running on a full drop. A full
drop for my machine would be about 34" so
theoretically, a distance of 142 feet may
be possible on a longer smooth floor.
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