Tony Charlton, Technical Manager at Michelin:
"The inner tube is installed with the conical washer sitting between the inner tube and wheel rim. On a road bike the first lock nut should then be run down to lightly touch the rim, and then backed off by half a turn. The second lock nut is run down until it meets the first one, then the two nuts should be locked together by using spanners to rotate them in opposite directions.
For off road inner tubes being used at low tyre pressures there is a possibility of tyre creep, and the inner tube could be pulled around slightly with the tyre. If the inner tube is bolted to the rim the valve can be ripped out. If the lock nuts are raised further towards the valve cap then inner tube has some leeway to move before this happens. Any movement is thus obvious as the valve stem will no longer be straight and pointing at the centre of the hub, but will be pulled round at an angle by the tyre and can then be seen and corrected."