Comparison of crank based leg mechanism/Jansen linkage Modifications

Wrong Angle / Missing measurement / Tilt

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  Strandbeest with correct dimensions
  a = 38.0
l = 7.8
  locus correct and grounded
• 
  Strandbeest with incorrect dimensions
  a = 38.0
l = 0
  locus extraged and tilted
• 
  Strandbeest with incorrect dimensions
  a = 38.79
l = 0
  locus correct, but tilted

Examples:

• Klann R & D, Joe Klann, Jansen Linkage - Klann Linkage Comparison
  • https://web.archive.org/web/20160304035711/http://www.mechanicalspider.com/comparison.html
• Adam Savage's One Day Builds: Pedal-Powered Strandbeest!
  • https://www.youtube.com/watch?v=G96YQQ7vW8w?t=740
  • https://www.youtube.com/watch?v=G96YQQ7vW8w?t=1018

Note that the examples are not to mock the creators for their mistake, but praising them for sharing their experience so that we all can learn from them.

This mistake is made more likely by speaking about the eleven magic numbers instead of the 13 magic number.

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(There are elven rods, but the two fixed points are a and l apart from each other)

Triangle

Traditionally, the leg is formed by just using rods:

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When manufacturing from flat stock like paper or wood, realizing that that one leg can also be constructed from 2 triangles and 5 rods can simplify the assembly and make the mechanism more robust

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here some practical examples:

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  Leg mechanism made from card board
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  Mini Strandbeest

Model ready for 3D-Printing

A fully-parametric strandbeest implemented in OpenSCAD, with assembly instructions, is available for 3D printing.

Using Rod of the same length

f=39,4 and c=39,3 are so close in length, that using rods of the same length can simplify the assembly without affecting the gait to much.

Note that in practice, the whole mechanism as play and flex, undergrounds are not perfectly flat and lengths and holes have manufacturing inducted errors.