Castor wheel

1. A castor wheel for a vehicle or an attachment for a vehicle, the castor wheel comprising:

a wheel having an axle about which the wheel can rotate;

an offset pivot mounting for mounting the wheel to the vehicle or vehicle attachment, wherein the offset pivot mounting is connected to the axle and defines a pivot axis perpendicular to and offset from the axle, wherein the wheel is capable of pivoting about the pivot axis; and

a restrainer for restraining the pivoting of the wheel, the restrainer comprising a first element and a second element that interact to produce a restraining force on the pivoting of the wheel, wherein the first element comprises a contact plate and the second element is an electromagnet, and the contact plate is wider than the width of the electromagnet and biased vertically away from the electromagnet.

2. A wheelchair trailer for carrying a wheelchair, the wheelchair trailer comprising a base for carrying the wheelchair, a ramp for getting the wheelchair on and off the base, and at least one castor wheel of claim 1 .

3. A wheelchair trailer of claim 2 , further comprising an actuator for moving the ramp, wherein the actuator comprises a piston rod driven by a hydraulic, an electric, a pneumatic, or a magnetic motor.

4. A wheelchair trailer of claim 3 , wherein the actuator has a thrust of up to 8000 N.

5. A wheelchair trailer of claim 3 , wherein the piston rod has a maximum speed of no more than 70 mm/s.

6. A wheelchair trailer of claim 2 , wherein the ramp comprises at least two ramp portions and a joining hinge, wherein the at least two ramp portions are joined together by the joining hinge, and wherein the joining hinge allows the ramp portions to be moved between an extended position and a folded position.

7. A wheelchair trailer of claim 2 , further comprising a hinge connecting the ramp and the base, wherein the hinge is at the end of the ramp, and wherein the hinge can move the ramp to between an upright position and a lowered position.

8. A wheelchair trailer of claim 2 , wherein the base comprises two base portions and a single sided hinge, wherein the base portions are joined together by the single sided hinge, and the single sided hinge allows the base portions to be folded into a storage position when the wheelchair trailer is not carrying a wheelchair.

9. A castor wheel of claim 1 , wherein the restrainer is offset from the pivot axis such that the distance between the restrainer and the pivot axis is 5-40 centimeters.

10. A castor wheel of claim 1 , wherein the offset pivot mounting comprises a bearing for pivoting of the wheel, wherein the pivot axis is defined by the bearing.

11. A castor wheel of claim 1 , wherein the electromagnet has a breakaway capacity of 20-100 kg.

12. A castor wheel of claim 1 , wherein the electromagnet may be switched on and off.

13. A castor wheel of claim 1 , wherein the electromagnet is mounted to the offset pivot mounting and the restrainer further comprises an adjustor for adjusting the extent to which the electromagnet extends from the offset pivot mounting.

14. A castor wheel of claim 1 , wherein the restrainer further comprises a spacer for spacing apart the first and second elements.

15. A castor wheel of claim 1 , wherein the restrainer further comprises a sensor for sensing the speed of the vehicle or vehicle attachment, whereby the electromagnet is switched off when the sensor detects that the vehicle or vehicle attachment is travelling at a certain low speed.

16. A vehicle comprising at least one castor wheel of claim 1 , wherein the vehicle is capable of travelling at a speed in excess of 10 km/hr.