Originally Posted by tudorowen1

Why not fit a rear derailleur to your Brompton instead of the old chain tensioner..and then you could change between the two rear sprockets..You can buy cheap rear derailleurs that bolt on to the rear axle ..or you could find a frame builder to weld on a proper gear hanger...Chain might come off when you fold it possibly if it cannot take up all the slack in the chain ..but at least you could change gear OK..