best mattress showroom near thillainagar growell beds and furnitures A Bonnell spring mattress is a type of innerspring mattress that features a traditional coil system known as Bonnell coils. These mattresses have been around for decades and remain popular due to their affordability and reliable support Bonnell coils are hourglass-shaped springs made from tempered steel wire. These coils are interconnected with wires, forming a network of springs that provide support across the mattress surface. The coils are typically larger at the center and taper at the ends, giving them an hourglass shape.