A lightweight, thin idler to take up slack - e.g., when a Brompton is folded.
The width of the idler is 5 mm. The idler takes a 13 mm OD, 5 mm ID, 4 mm wide bearing, for example, 695-ZZ:
http://www.fasteddybearings.com/5x13x4-metal-shielded-bearing-695-zz.
Small lips at each edge of the center hole provide a press fit.