A rubber O-ring. If I have nothing else with me while changing a flat on the road I'll slide the spare tire under the frame while unbolting the flat tire and slide the flat tire under the frame while putting the spare back on. I break the lugs loose with the weight on the tire and finish removing the lugs and tire after jacking it up and putting the spare under the frame. This leaves the car without a safety net of a tire rim for only a moment while I slide the spare out and slide the flat back in it's place. I can do this without placing my hands in a place of danger, once the spare is in place and the lugs finger tight I slide out the flat tire, lower the car and finish tightening the lugs.