CASTER

1 . A caster assembly comprising:

a primary wheel; and

a secondary wheel;

wherein the secondary wheel is positioned to contact an obstacle and lift the primary wheel before the primary wheel contacts the obstacle as the caster assembly casters over the obstacle.

2 . The caster assembly of claim 1 further comprising:

a second secondary wheel;

wherein the secondary wheels are positioned one on each side of the primary wheel; and

wherein the secondary wheels are positioned to rotate coaxially.

3 . The caster assembly of claim 2 further comprising:

a caster swivel joint that defines a caster swivel axis;

wherein the axis of rotation of the primary wheel and the axis of rotation of the secondary wheel are both offset behind the caster swivel axis as the caster assembly casters over the obstacle.

4 . The caster assembly of claim 2 further comprising:

a caster swivel joint that defines a caster swivel axis;

wherein the secondary wheels are positioned to remain within a sweep volume of the primary wheel as the caster assembly swivels around the caster swivel axis.

5 . The caster assembly of claim 2 :

wherein the secondary wheels have a generally flat tread.

6 . The caster assembly of claim 2 :

wherein the secondary wheels have a generally rounded tread.

7 . The caster assembly of claim 2 :

wherein the secondary wheels each have a continuously curved outside surface facing away from the primary wheel.

8 . The caster assembly of claim 1 further comprising:

a second primary wheel;

wherein the primary wheels are positioned to rotate coaxially.

9 . the caster assembly of claim 1 :

wherein the primary and secondary wheels are aligned in tandem.

10 . The caster assembly of claim 9 :

wherein the tread of the secondary wheel is circumferentially channeled and provides two rolling surfaces; and

wherein the two rolling surfaces are positioned one on each side of the primary wheel.

11 . The caster assembly of claim 1 further comprising:

a caster swivel joint that defines a caster swivel axis;

wherein the axis of rotation of the primary wheel and the axis of rotation of the secondary wheel are both offset behind the caster swivel axis as the caster assembly casters over the obstacle.

12 . The caster assembly of claim 11 :

wherein the secondary wheel is positioned to remain within a sweep volume of the primary wheel as the caster assembly swivels around the caster swivel axis.

13 . A caster assembly comprising:

a caster base;

a primary wheel coupled to the caster base; and

a secondary wheel coupled to the caster base;

wherein a vertical distance between the caster base and a lowermost tread position of the primary wheel is larger than a vertical distance between the caster base and a lowermost tread position of the secondary wheel; and

wherein at least a portion of the tread of the secondary wheel is positioned outside a bottom half of a cylinder surface defined by the tread of the primary wheel.

14 . The caster assembly of claim 13 further comprising:

a caster swivel joint coupled to the caster base;

wherein the caster swivel joint defines a caster swivel axis around which the caster swivel joint swivels; and

wherein the caster swivel joint is positioned to provide a positive caster angle for the primary and secondary wheels.

15 . The caster assembly of claim 14 :

wherein the secondary wheel is positioned to remain within a sweep volume of the primary wheel as the caster assembly swivels around the caster swivel axis.

16 . The caster assembly of claim 13 further comprising:

a caster swivel joint coupled to the caster base;

a second primary wheel coupled to the caster base;

wherein the primary wheel and the second primary wheel rotate around substantially the same axis; and

wherein the primary wheel and the second primary wheel are substantially the same size.

17 . The caster assembly of claim 13 further comprising:

a caster swivel joint coupled to the caster base; and

a second secondary wheel coupled to the caster base;

wherein the secondary wheel and the second secondary wheel rotate around substantially the same axis;

wherein the secondary wheel and the second secondary wheel are substantially the same size; and

wherein the secondary wheel is positioned on one side of the primary wheel and the second secondary wheel is positioned on the opposite side of the primary wheel.

18 . The caster assembly of claim 13 :

wherein a non-tread outer surface of the secondary wheel comprises a portion of a continuously curving surface.

19 . The caster assembly of claim 13 :

wherein the primary wheel and the secondary wheel are positioned in tandem.

20 . A method of facilitating the rolling of a supported object over an obstacle threshold, comprising the act of:

positioning a secondary wheel of a caster assembly to contact the obstacle and lift a primary wheel of the caster assembly before the primary wheel of the caster assembly contacts the obstacle.

21 . The method of claim 20 further comprising:

positioning the secondary wheel to remain within a sweep volume of the primary wheel as the caster assembly swivels around a caster swivel axis.