This invention provides a hollow golf club head in which the ratio of the average thickness of a sole portion to that of a crown portion is 1:0.3 to 0.8. Preferably, a crown thin-walled region is formed on the face side of the crown portion, and a crown thick-walled region is formed on the back side...http://www.google.es/patents/US7513836?utm_source=gb-gplus-sharePatente US7513836 - Hollow golf club head
