On In-Use requirements UNECE R55 make no distinction between performance and requirement Annex 6 lays down how the performance of coupling equipment shall be determined. In paragraph 2.11.x. you find verbal definition of the performance values (called characteristic values) In paragraph 2.11.x. you also find formulae referred to as reference values. These formulae make use of masses of towing and towed vehicles. At type approval of the coupling those masses are not known. Some argue that those formulae are needed to determine towable mass. This is not fully true. Towable mass is determined through other parameters.

On In-Use requirements Towable mass Towable mass as stated by the manufacturer is not determined using R55. From 2007/46/EU we read in regulation 1230/2012/EU Article 3 1. The manufacturer shall determine, for each version within a vehicle type, irrespective of the state of completion of the vehicle, the following masses: (b) the technically permissible maximum laden mass of the combination; (c) the technically permissible maximum towable mass; (e) the technically permissible maximum masses at the coupling point(s) taking into account the technical features of the couplings that are fitted or can be fitted to the vehicle as the case may be.

On In-Use requirements Towable mass Towable mass as stated by the manufacturer is not determined using R55. From 2007/46/EU we read in regulation 1230/2012/EU Annex 1 § 4. Hill-starting ability and gradeability 4.1. Vehicles designed to tow a trailer and laden to their technically ermissible maximum laden mass of the combination shall be capable of starting five times within five minutes at an up-hill gradient of at least 12 %. 5. Engine power 5.1. Vehicles shall provide an engine power output of at least 5 kW per tonne of the technically permissible maximum laden mass of the combination In the case of a road tractor, or a tractor unit for semi-trailer intended for the transport of indivisible loads, the engine power shall be at least 2 kW per tonne of the technically permissible maximum laden mass of the combination.

On In-Use requirements Towable mass Towable mass as stated by the manufacturer is not determined using R55. From regulation UNECE R13 we can read in Annex 7 §2.3.3.: If it is intended to attach a trailer to a power-driven vehicle, the trailer shall be represented by an energy storage device whose maximum relative pressure p (expressed in kPa / 100) is that which can be supplied through the towing vehicle's supply circuit and whose volume V, expressed in liters, is given by the formula p x V = 20 R (R being the permissible maximum mass, in tonnes, on the axles of the trailer). This gives a requirement on the capacity of the towing vehicle energy source for compressed air that is related to the size of the trailer. I.e. towable mass is dependent of the capacity of the braking system of the towing vehicle.