When one wants to study the behavior of coarse suspensions directly, one needs a special large‐scale rheometer with a wide gap. As a first step towards the understanding of coarse suspension behavior a wide‐gap coaxial cylinder rheometer (1.2 m diameter, 0.2 m gap, sample volume: 0.5 m3) was built. This apparatus, which should be cheap and strong, was designed to be towed and used in the field to test fluids including particles whose diameters do not exceed 1 cm. The validity of hypotheses and the various theoretical problems of the rheometrical calculations in the case of a wide‐gap rheometer when used to test a coarse suspension are then examined. The main advantage of this geometry appears to be that edge effects should not be too important. Its main disadvantage, especially for coarse suspensions with a complex behavior (yield stress, thixotropy, presence of coarse particles), is the heterogeneity of shear rate from one tool to the other. Experiments with various natural materials (water–clay mixtures and water–debris flow sample mixtures) at different concentrations were carried out. Some experimental problems (settling, slip, fracture, etc.) were encountered but they could be mastered reasonably well. These results show that when the clay fraction is high enough, a Herschel–Bulkley model can well represent the flow curve of these mixtures in the range of shear rates (0.5; 50 s−1). When the clay fraction is low and the solid particle fraction is high enough, suspensions behave in a very peculiar way: the flow curve has a minimum.