As we’ve seen in previous entries nothing in the Brompton universe is straightforward. Inner tubes compatibility are ample proof of this.
If you eventually need to replace them, bear in mind that a regular 16″ children bike tube will not fit. This is the one you are likely to find in a rush in a bike shop and for your surprise too wide. You’ll notice is very tricky when sliding it inside the tyre. These inners will cover ranges between 1.75 to 2.125 inches (45-50mm).
On the other hand, Bromptons are equipped with up to 1.37″ (1 3/8″, 35mm) inches width tyres. Schwalbe makes good quality spares and are the ones I recommend.
Most of the Brompton compatible tubes fit tyres from 340 to 355 mm bead seat and widths from 28 to 37 mm approximately.