How to Calculate Maximum Permissible Download on Trailer Hitch (2010-52-EU)

LX1048547-UN-20JUL09

Calculation of maximum permissible download at the trailer hitch in relation to Load Index (LI)

• The load index can be read on the sidewall of the tire. If the index is not provided, refer to the tire's load capacity as quoted by the tire manufacturer.
• The load index is quoted in conjunction with a Speed index (SI)
• As a rule, the load capacity of the tire in kg can be derived directly from the LI; see the following table:

LI	kg	LI	kg	LI	kg	LI	kg
90..............	600	113..............	1150	136..............	2240	159..............	4375
91..............	615	114..............	1180	137..............	2300	160..............	4500
92..............	630	115..............	1215	138..............	2360	161..............	4625
93..............	650	116..............	1250	139..............	2430	162..............	4750
94..............	670	117..............	1285	140..............	2500	163..............	4875
95..............	690	118..............	1320	141..............	2575	164..............	5000
96..............	710	119..............	1360	142..............	2650	165..............	5150
97..............	730	120..............	1400	143..............	2725	166..............	5300
98..............	750	121..............	1450	144..............	2800	167..............	5450
99..............	775	122..............	1500	145..............	2900	168..............	5600
100..............	800	123..............	1550	146..............	3000	169..............	5800
101..............	825	124..............	1600	147..............	3075	170..............	6000
102..............	850	125..............	1650	148..............	3150	171..............	6150
103..............	875	126..............	1700	149..............	3250	172..............	6300
104..............	900	127..............	1750	150..............	3350	173..............	6500
105..............	925	128..............	1800	151..............	3450	174..............	6700
106..............	950	129..............	1850	152..............	3550	175..............	6900
107..............	975	130..............	1900	153..............	3650	176..............	7100
108..............	1000	131..............	1950	154..............	3750	177..............	7300
109..............	1030	132..............	2000	155..............	3875	178..............	7500
110..............	1060	133..............	2060	156..............	4000	179..............	7750
111..............	1090	134..............	2120	157..............	4125	180..............	8000
112..............	1120	135..............	2180	158..............	4250	181..............	8250

As a general rule, SI A8 implies a top speed of 40 km/h (25 mph), while SI B implies a top speed of 50 km/h (31 mph). If the SI is different, the manufacturer's instructions apply.

Calculate maximum trailer hitch download as follows:

S =	(Hmax - LGH) * a a + b	, where
Hmax. =	the smaller value from 2*load capacity of a tire on the rear axle and the maximum permissible rear axle load in kg
LGH =	the mass in kg acting on the ground through the rear wheels (to be ascertained by weighing)
a =	the wheelbase (the horizontal distance between the front and rear axles)
b =	the rear overhang (the horizontal distance between the center of the rear axle and center of the hitch point)

Example of how to calculate maximum trailer hitch download:

Given that:	Empty mass on rear axle LGH = 6000 kg
	Wheelbase a = 3020 mm
	Overhang b = 975 mm
	Tire marking = 169A8
	Maximum permitted speed of tractor = 40 km/h (25 mph)
	Permissible rear axle load = 11500 kg
	Hmax. = 11500 kg (1900 kg * 2 = 3800 kg, rear axle load = 11500 kg)
S =	(11,500 kg - 6000 kg) * 3020 mm 3020 mm + 975 mm	= 8122 kg

CAUTION: At least 20% of the vehicle's total unladen mass must be on the front axle.

Trailer hitch download must not exceed 4000 kg.