As well as painting/marking directional arrows on the disc carrier/hub, put a blob of green paint on the exposed part (the bit you can see when the wheel is fitted) of the right hand spacer and red paint on the left (port and starboard). Makes for easy identification at a glance. You could also put a spot of the same colour paint on the inside of each fork leg and on the hub too (if you can't remember which is port/starboard).

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