 The grit is stored in the large hopper on the rear of the vehicle, with a wire mesh over the top to prevent foreign objects from entering the spreading mechanism and hence becoming jammed.The salt is generally spread across the roadway by an impeller, attached by a hydraulic drive system to a small onboard engine.However, until the 1970s, the grit was often spread manually using shovels by men riding on the back of the truck, and some older spreading mechanisms still require grit be manually loaded into the impeller from the hopper.