Deep rimmed aero wheels are becoming widespread, despite the fact that the experience of riding them can sometimes be tempered by unnecessary tyre inflation problems. Open-ended valve extensions that require the valve to be left unscrewed are a flawed design, and can prove very hit and miss in use.
Continental offer a selection of extensions that come in 30, 40 or 60mm lengths and are designed to fit any inner tube or tubular tyre with a replaceable valve core. Simply use the supplied tool to unscrew the valve core, then screw the core into the extension, and use the tool to screw the extension back into the valve again.
About 10mm of the extension is an internal thread that screws into the valve, so a 40mm extension will only increase the valve’s effective length by around 30mm. But pairing the right extension with valves of an appropriate size should give you the right length for your rims.
The beauty of this design is that you retain a fully functioning valve for peace of mind and security, with a nominal weight of only about 5g per valve.