As winter approaches I have been trying to source some emergency snow chains. I was shown these gadgets from someone who had them in Sweden. They are very simple to use, and effectively are two metal rods with curved ends which open out to go over the wheel at four points. They have a centre screw and can be put on and taken off with ease. I met a guy who said that they are issued with hire cars in the US in snowy areas. The trouble is that I don't know their proper name and I can't source them on the net. Any ideas?