Of Spokes Lugs and Movies

1961 Westside Dr., Rochester, New York 14624.

‘Remember those movies we took of the tractors at the
Reunion? Well, they are good except the wheels are going backwards
like crazy!’ Be not discouraged. The professionals have the
same problem. In Western movies the wheels of the stage coaches
usually appear to go backwards some of the time.

Whenever we view by stroboscopic (intermittent) vision, a
rotating member that has a pattern which is repetitive with
rotation such as a wheel with spokes or lugs, curious effects are
usually observed. Movie cameras do not record continuously but snap
‘stills’ at discrete intervals. The time intervals between
‘stills’ are utilized to advance the film to the next
frame. Let us examine in sequence the effects that may be observed
as when a wheel with spokes for example, is viewed in movies as it
is accelerated from a position of rest. As the wheel starts to
turn, the spokes as viewed, advance in position in successive
frames and the direction and rotation rate are as observed. As soon
however as the rotation is fast enough to bring the positions of
the spokes in a frame to more than half way to the positions of the
NEXT spokes as they were shown in the PRECEDING frame, the motion
of the wheel appears to change from rapidly ahead to rapidly
backwards.

The observed (not actual) reversal of motion occurs because the
eye in interpreting what it sees, automatically identified
(relates) positions of spokes in one frame, not with the positions
occupied by them in the preceding frame, but with the positions of
the NEXT spokes because they are closer to the positions the next
ones had in the preceding frame. The spokes therefore seem to move
backwards. As the rotational speed continues to increase, the
backwards displacement between successive frames lessens and the
apparent backward speed is thereby reduced. When the wheel rotates
rapidly enough so that one spoke occupies exactly the same position
as the one preceding it did in the preceding frame then the wheel
appears to stop.

The cycle of the sequence of apparent motion i.e. acceleration
forward, reversal, and deceleration to a stop, may be repeated
several times as the wheel gains speed. The repetition of the
sequence is caused by spokes becoming identified not with
themselves and the next ones as detailed herein, but with the 2nd
& 3rd, the 3rd & 4th spokes ahead, etc. At no time is the
apparent speed faster in either direction than half a pattern of
one spoke to the next per frame. Successive ‘stops’
indicate the rotational speeds have become two, three or more,
whole intervals of spoke displacement per time lapse between
successive frames.

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