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Fig. 7. The stability margin (top) and internal force (bottom) required for the
gecko model to maintain a minimum stability margin of 25% body weight.
Estimated force data from climbing house geckos, Hemidactylus
garnotii (Autumn et al.,
2006a), yielded a safety margin of approximately 36% body weight,
assuming an 
* of 25°. Before point A, the frictional
adhesion and Kendall peel models dictate the gecko orient its feet opposite of
each other to maintain the specified stability margin. From point A to points
B (frictional adhesion) and C (Kendall peel), the gecko model orients both
feet with gravity since gravity naturally loads the contacts in their
preferred adhesive direction and achieves greater than 25% stability margin
(point D) without applying internal forces. As the surface becomes vertical
and overhanging, the front foot must sustain more adhesion than the rear. In
the JKR model, increasing adhesion is only possible by decreasing shear; thus,
it is preferable for the rear foot to bear more shear load than the front. In
the anisotropic models, the opposite is true. The front foot bears more shear
load than the rear, because this tends to increase maximum adhesion. After
points B and C, the respective anisotropic models only maintain the specified
stability by reversing the rear foot and pulling inward with both feet. Point
E indicates where both the peel and JKR models can no longer maintain the
specified stability using any amount of internal force. This is in part due to
the particular parameters chosen, but also due to the eventual decrease in
adhesive forces when shear forces become too large.
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