A four-bar linkage is a fundamental kinematic chain used in many different systems both artificial and natural. This kinematic chain is found in nature in many animal skeletal structures, including the human knee [32] [33]. Four bar linkages are also used in mountain bike rear suspensions, and are the basis of many other suspensions such as the double wishbone [34] [35].

There are 4 configurations for this assembly, each of which will teach a different lesson to the user.
1. Rocker-Rocker
2. Rocker-Crank
3. Crank-Rocker
4. Crank-Crank

Kinematics Associated with this Device:
1. Bar Linkages: In engineering there is a standard classification of four-bar linkages called the Grashof condition. A four-bar linkage satisfies the Grashof condition when Equation 4 is shown below;
s l < p q (4)
Four-bar linkages can also exist in states that do not meet the Grashof Condition, which are considered non-Grashof. Non-Grashof linkages are Rocker-Rocker when Equation 5 is true.
s l > p q (5)
Non-Grashof linkages are also possible when the sum of the lengths of the shortest and longest links is equal to the sum of the other two links, p and q. When this is true, the four-bar linkage can be found in any of the four Grashof states but will have a point of rotation in which all pivots will be collinear, which is referred to as the change point. A special case of this type of linkage is the parallel bar linkage [35] [36].

The different components of the device are provided in the BOM table displayed

Post Processing note: If you are printing with an FDM printer you should expect small ridges on parts where the extruder last touches the piece. Carefully inspect each piece after they are printed for these small imperfections as they will hinder the operation of your device.

References:
[32] M. M. Wachowski, T. A. Walde, P. Balcarek, J. P. Schüttrumpf, S. Frosch, C. Stauffenberg, K.-H. Frosch, C. Fiedler, J. Fanghänel, D. Kubein-Meesenburg and H. Nägerl, "Total Knee Replacement with Naturl Rollback," Annals of Anatomy - Anatomischer Anzeiger, vol. 194, no. 2, pp. 195-199, 2012.
[33] M. Muller, "A Novel Classification of Planar Four-Bar Linkages and its Application to the Mechanical Analysis of Animal Systems," Philosophical Transactions: Biological Sciences, vol. 351, no. 1340, pp. 689-720, 1996.
[34] P. '. Hayes, S. Young and Doddy, "Buyer's Guide to Mountain Bike Suspension, Part 2," BikeRadar, 2010. [Online]. Available: http://www.bikeradar.com/gear/article/buyers-guid... [Accessed 2 October 2013].
[35] Wikipedia Contributors, "Four-bar Linkage," Wikipedia, The Free Encyclopedia, 5 September 2013. [Online]. Available: http://en.wikipedia.org/w/index.php?title=Four-ba... [Accessed 2 October 2013].
[36] E. Söylemez, Mechanisms, Ankara: METU Publication #64, 1999.
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ddang12312311 months ago
Hey. Where are the files?
rbrodie1 year ago
I would like to print one of these and use it in my classroom. Where can I find the 13 stl files to download?
thanks
NUCAP_KDM (author)  rbrodie1 year ago
Yes! sorry about that i completely forgot to put the .stls up for this one my bad, you should now find it as a zip file with all necessary parts in the file.