Check the documentation for your vehicle – there indicate optimal weight, which will carry car and specified capacity, beyond which you can't leave. The developers carefully calculate the load on every detail. Therefore, in the data sheet specified the payload is checked, and loading of transport over that figure can lead to breakage and failure of main components.
Vehicle weight and, consequently, the capacity consists of two components: the front axle load and rear axle load. For example, for a two-axle truck "Gazelle" total mass of 3500 kg is the amount of 1200 kg and 2,300 kg.
For vehicles that have three axles, because they have the middle and rear axle have the Association in the cart, the mass is the sum of the load on the front axle and rear bogie. An example is the calculation of the mass for the truck "KAMAZ": total weight 19650 kg is defined as the sum of the loads in 4420 and 15230 kg.
Remember that the capacity of the car and trunk are completely different things. Loading the trunk, you endanger the suspension, because the load is not dispersed through the cabin, and assembled in one place. Evenly distribute the transported material across the machine to avoid breakdowns.