Wheeled chairs have a minimum of five wheels for safety reasons. Because the chair is on wheels it is assumed that people will 'scoot' on them (for short distances) rather than get up and walk, but because the 'scooting' force is applied sideways, to the end of a long arm, the force tends to topple the chair as much as it propels it. While the wheels are on a smooth surface there are few problems but if someone hits a ridge in the floor, such a rug or a raised floor vent, the chair is much more likely to topple than roll. This is made even worse by the fact that the wheeled base can rotate freely, so once one wheel has struck a ridge and stopped, the wheeled base is likely to rotate until a second wheel hits the ridge and when this happens the effective wheelbase is no longer twice the arm length but only sin/cos 45 deg of the wheelbase i.e. it is reduced by a factor of 0.7071067. Increasing the number of wheeled arms on the base mitigates against this.
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