An "On Centre" valve stem inner tube, very thick and made from butyl for use on split rim wheels on the large 20 and 21" rims. This tube has a metal TR11 valve stem. 

A heavy duty robust correctly dimensioned inner tube, which we use in our Blockley 700x20 tyre, unlike anything else available. If a longer stem is required it is possible to put an extension on the metal valve stem.