In its primitive form, a wheel is a circular block of a hard and durable material at whose center has been bored a hole through which is placed an axle bearing about which the wheel rotates when torque is applied to the wheel about its axis. The wheel and axle assembly can be considered one of the six simple machines. When placed vertically under a load-bearing platform or case, the wheel turning on the horizontal axle makes it possible to transport heavy loads.
Bicycle wheels are inexpensive and universally available but support limited weight and can be difficult to mount to frames. BMX bike wheels have stronger spokes, and motorcycle wheels stronger still, for associated increases in cost.
As opposed to sourcing high end bicycle or motorcycle hardware, increasing expense and decreasing availability of supply, we've decided to standardize on low-end highway capable trailer hardware. Trailer hardware is also widely available, designed to operate at speed under heavy load, and low cost due to it's highly standardized mass production.
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