Sole structure with progressively adaptive stiffness

1. A sole structure for an article of footwear comprising:

a sole plate including:

a foot support portion with a foot-facing surface and a ground-facing surface;

an opening extending through the foot support portion from the foot-facing surface to the ground-facing surface;

a bridge portion underlying the opening and secured to the foot support portion fore and aft of the opening;

a piston including a body and a support arm extending transversely from the body;

wherein:

the body extends through the opening; and

the support arm is supported on the bridge portion, trapped below the ground-facing surface by the foot support portion, and extends under the ground-facing surface at medial and lateral sides of the opening.

2. The sole structure of claim 1 , wherein the piston is moved relative to the sole plate by dorsiflexion of the sole plate, with the bridge portion in tension, the foot support portion in compression, and the support arm separating the bridge portion and the foot support portion.

3. The sole structure of claim 2 , wherein:

the sole plate has a guide track; and

the body of the piston has an engagement feature that engages with the guide track, ratcheting the piston incrementally along the guide track with repetitive dorsiflexion of the sole plate.

4. The sole structure of claim 3 , wherein bending stiffness of the sole structure varies with a position of the piston along the guide track.

5. The sole structure of claim 3 , wherein the guide track includes a first set of directional fibers, and the engagement feature of the piston is a second set of directional fibers that engages with the first set of directional fibers.

6. The sole structure of claim 3 , wherein the guide track has teeth, and the engagement feature of the piston is at least one tooth that engages with the teeth of the guide track.

7. The sole structure of claim 6 , wherein the teeth of the guide track have a varied spacing.

8. The sole structure of claim 7 , wherein:

the piston body includes a rear car and a front car;

the teeth of the guide track have a first spacing at a first portion of the guide track;

the teeth of the guide track have a second spacing less than the first spacing at a second portion of the guide track;

the sole plate has an obstruction blocking ratcheting of the rear car along the guide track at a predetermined position between a start position and a final position of the piston body;

the rear car abuts the front car between the start position and the predetermined position such that the front car is moved by the rear car as the rear car is ratcheted along the guide track from the start position to the predetermined position; and

the front car is ratcheted along the guide track free of the obstruction from the predetermined position to the final position.

9. The sole structure of claim 8 , wherein the teeth of the guide track are split in two transversely spaced sets at the first portion of the guide track.

10. The sole structure of claim 6 , further comprising:

a first post extending from the sole plate;

wherein:

the guide track has a first segment with a first series of teeth, and a second segment with a second series of teeth;

the second segment is oriented at a first angle with respect to the first segment;

the first post is between the first segment and the second segment;

the at least one tooth of the piston is pivotable; and

the first post contacts the at least one tooth of the piston, pivoting the at least one tooth by the first angle.

11. The sole structure of claim 10 , wherein the first series of teeth progresses in a longitudinal direction along the sole plate, and the second series of teeth progresses in a transverse direction along the sole plate.

12. The sole structure of claim 11 , further comprising:

a second post extending from the sole plate;

wherein:

the guide track has a third segment with a third series of teeth;

the third segment is oriented at a second angle with respect to the second segment;

the third series of teeth progresses in an opposite direction as the first series of teeth so that the piston is ratcheted in the opposite direction along the third series of teeth;

the second post is between the second segment and the third segment; and

the second post contacts the at least one tooth of the piston, pivoting the at least one tooth by the second angle.

13. The sole structure of claim 12 , wherein the first series of teeth progresses in a forward direction along the sole plate and the third series of teeth progresses in a rearward direction along the sole plate so that the piston is ratcheted forward along the first series of teeth, and is ratcheted rearward along the third series of teeth.

14. The sole structure of claim 6 , wherein the teeth of the guide track and the at least one tooth of the piston extend transversely relative to the sole plate.

15. The sole structure of claim 3 , wherein the guide track is curved toward a lateral side of the sole plate such that bending stiffness of the sole plate under bending in a transverse direction increases as the piston is ratcheted along the guide track.

16. A sole structure for an article of footwear comprising:

a sole plate including:

a foot-facing surface and a ground-facing surface:

a compressive portion above a neutral axis;

a tensile portion below the neutral axis; and

a guide track in the foot-facing surface having a series of protrusions;

a piston including:

a body disposed above the tensile portion;

a support arm extending from the body, resting on the tensile portion, and disposed below the compressive portion and against the ground-facing surface; and

at least one protrusion engaged with the series of protrusions of the guide track and ratcheting the piston along the guide track as the piston translates relative to the sole plate in response to dorsiflexion of the sole structure.

17. The sole structure of claim 16 , wherein the sole plate has an opening, the body of the piston extends through the opening, and the support arm extends across the opening.

18. The sole structure of claim 16 , wherein bending stiffness of the sole structure varies with a position of the piston along the guide track.

19. The sole structure of claim 16 , wherein:

the series of protrusions is a first set of directional fibers; and

the at least one protrusion of the piston is a second set of directional fibers engaged with the first set of directional fibers.

20. The sole structure of claim 16 , wherein the series of protrusions is a set of teeth, and the at least one protrusion of the piston is a tooth that engages with the set of teeth.