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An air pump, mounted on a wheel, which pumps ambient air into the tire chamber to automatically pump up the tire as the wheel rotates. The air pump generally comprises a cylinder and a piston, which relatively reciprocate to effect an intake stroke and a compression stroke. The intake stroke draws a

An air pump, mounted on a wheel, which pumps ambient air into the tire chamber to automatically pump up the tire as the wheel rotates. The air pump generally comprises a cylinder and a piston, which relatively reciprocate to effect an intake stroke and a compression stroke. The intake stroke draws ambient air into a variable volume chamber. The pressurized air within the tire acts on the cylinder to effect the compression stroke, which forces the ambient air from the variable volume chamber into the tire chamber to pump up the tire.

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What is claimed is: 1. A tire assembly comprising: a wheel configured to rotate about a rotational axis above a contact surface; a tire mounted to the wheel and configured to deflect when in contact with the contact surface, the tire at least partially defining a tire chamber configured to hold air

What is claimed is: 1. A tire assembly comprising: a wheel configured to rotate about a rotational axis above a contact surface; a tire mounted to the wheel and configured to deflect when in contact with the contact surface, the tire at least partially defining a tire chamber configured to hold air pressurized relative to the ambient air; and an air pump carried by the wheel and comprising: first and second elements coupled together for relative reciprocation between an intake stroke, during which the first and second elements are reciprocally collapsed, and a compression stroke, during which the first and second elements are reciprocally extended; a variable volume chamber defined by the first and second elements and selectively fluidly coupled to the ambient air and to the tire chamber, and the variable volume chamber is configured to increase in volume during the intake stroke to draw ambient air into the variable voluble chamber and decrease in volume during the compression stroke to force the ambient air in the variable volume chamber into the tire chamber; and a portion of one of the first and second elements fluidly separating the variable volume chamber from the tire chamber such that one side of the portion is exposed to the air in the variable volume chamber and another side of the portion, opposite the one side, is exposed to the air in the tire chamber, and defines a pressure differential across the portion based on the air pressure in the tire chamber and the air pressure in the variable volume chamber; wherein as the wheel rotates, one of the first and second elements responds to the deflection of the tire to effect the intake stroke and draw ambient air into the variable volume chamber, and the continued rotation of the wheel frees the one of the first and second elements from the deflection of the tire and the pressure differential provides the force to reciprocate the first and second elements to effect the compression stroke and force the ambient air from the variable volume chamber into the tire chamber to pump up the tire. 2. The tire assembly according to claim 1 wherein the one of the first and second elements comprises a piston. 3. The tire assembly according to claim 2 wherein the piston is fixedly mounted to the wheel and further comprising a cylinder reciprocally mounted to the piston and a rod having a top forming the portion. 4. The tire assembly according to claim 3 wherein the cylinder comprises a collapsible member configured to collapse in response to a predetermined deflection of the tire. 5. The tire assembly according to claim 2 wherein the other of the first and second elements comprises a housing and the piston is slidably received within the housing to define the variable volume chamber between the piston and the housing. 6. The tire assembly according to claim 5 wherein the piston defines the portion and is in sealing engagement with the housing to partially define the variable volume chamber between the head and the housing. 7. The tire assembly according to claim 1 further comprising a relief valve fluidly coupling the variable volume chamber to the tire chamber and configured to open when the pressurized air in the tire reaches a predetermined pressure to prevent the further pumping of air into the tire when the pressurized air reaches the predetermined pressure. 8. The tire assembly according to claim 1 further comprising a check valve fluidly coupling the variable volume chamber to the tire chamber to permit air to flow from the variable volume chamber into the tire chamber while preventing air from flowing from the tire chamber into the variable volume chamber. 9. The tire assembly according to claim 1 further comprising a lock-out device operably coupled to the first and second elements to prevent their relative reciprocating movement. 10. The tire assembly according to claim 9 wherein the lock-out device is a centrifugal lock-out device responsive to centrifugal force generated by the rotating wheel and configured to prevent the relative reciprocation above a predetermined rate of rotation for the wheel. 11. The tire assembly according to claim 1 wherein at least one of the first and second elements comprises a collapsible member configured to collapse between a normal condition and a collapsed condition in response to predetermined deflection of the tire to prevent damage to the first and second elements. 12. The tire assembly according to claim 11 wherein the collapsible member is configured to buckle to effect the collapsing and the transition between the normal condition and the collapsed condition. 13. The tire assembly according to claim 11 wherein the collapsible member is resilient and returns to normal condition from the collapsed condition after the removal of the predetermined deflection of the tire. 14. The tire assembly according to claim 11 wherein the first element is fixedly mounted to the wheel and the second reciprocating element reciprocates relative to the first element. 15. The tire assembly according to claim 14 wherein the second element comprises the collapsible member. 16. The tire assembly according to claim 15 wherein second element is made from resilient material enabling the second element to operate between a normal and collapsed condition to form the collapsible member. 17. The tire assembly according to claim 16 wherein the second element is a housing and the first element is a piston. 18. A pump for a tire assembly comprising a wheel configured to rotate about a rotational axis above a contact surface and a tire mounted to the wheel and configured to deflect when in contact with the contact surface, the tire at least partially defining a tire chamber configured to hold air pressurized relative to the ambient air; the pump comprising: first and second elements coupled together for relative reciprocation between an intake stroke, during which the first and second reciprocating elements are reciprocally collapsed, and a compression stroke, during which the first and second reciprocating elements are reciprocally extended; a variable volume chamber defined by the first and second elements and selectively fluidly coupled to the ambient air and to the tire chamber, and the variable volume chamber is configured to increase in volume during the intake stroke to draw ambient air into the variable volume chamber and decrease in volume during the compression stroke to force the ambient air in the variable volume chamber into the tire chamber; and a portion of one of the first and second elements fluidly separating the variable volume chamber from the tire chamber such that one side of the portion is exposed to the air in the variable volume chamber and another side of the portion, opposite the one side, is exposed to the air in the tire chamber when the pump is mounted to the tire assembly, and defines a pressure differential across the portion based on the air pressure in the tire chamber and the air pressure in the variable volume chamber; wherein as the wheel rotates, one of the first and second elements responds to the deflection of the tire to effect the intake stroke and draw ambient air into the variable volume chamber, and the continued rotation of the wheel frees the one of the first and second elements from the deflection of the tire and the pressure differential provides the force to reciprocate the first and second elements to effect the compression stroke and force the ambient air from the variable volume chamber into the tire chamber to pump up the tire. 19. The pump according to claim 18 wherein the first reciprocating element is configured to be fixedly mounted to the wheel and the air pressure biasing element is carried by the second reciprocating element. 20. The pump according to claim 18 wherein the second reciprocating element comprises a collapsible member configured to collapse in response to a predetermined deflection of the tire.