This paper addresses the design of rigid wheels for planetary rovers in loose, granular soil. Wheel surface features, such as grousers, are known to improve tractive capability in planetary-relevant soils. However there are no comprehensive design guidelines for these wheel features. In this research, a series of intensive and extensive parametric studies were carried out in full-scale vehicle slope experiments and single-wheel tests that assess the influence of grouser count, height, orientation, and end-cap in various longitudinal and lateral slip conditions. This work also investigates the traction process of grousers based on a soil flow imaging technique. The soil motion analysis reveals that grousers reduce forward soil flow/motion resistance and increase net traction. A grouser design formula is derived from the soil flow observation, and design guidelines of rigid wheels of planetary rovers for loose soil are proposed based on these results. The proposed guidelines are applied to the modification of grouser design of the Mars Science Laboratory rover to improve tractive performance on loose terrain.