From Personal Experience And Non-Professional Testing . . . .
The use of 75 mm twin-wheel castors on office chairs provides much better rollability and user stability on hard or carpeted floors even when using soft, rather than hard, wheel material.
And, in laboratories where high stool-type chairs are used, the 75 mm twin-wheel castors provide a significant increase in stability over the standard industry-provided 50 mm castored chairs. The increased size of the castor wheels usually increases the diameter of the base because most of the time most of the castor wheels are swung outward further away from the base’s center. And, because the 75 mm twin wheels must rotate about the vertical pin in a larger floor circumferance than the 50 mm castors, the 75 mm castors allow more user control over the chair, due to the fact that it is more difficult to initiate lateral swivel over a larger floor diameter, thus making the travel of the chair more user-intentional than that of a chair with a smaller castor system, where chair travel is oftentimes the result of unconscious fidigeting, or the result of normal work motions of the user’s upper limbs.
The two main reasons chair manufacturers stick with 50 mm castors is (1) cost; and (2) the chair seat height quota for the 5th percentile chair user is more difficult to accomplish with the resulting 12 mm overall height increase of the 75 mm castored chair.
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