How to Calculate Maximum Permissible Download on Trailer Hitch

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

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 = 1800 kg
	Wheelbase a = 2100 mm
	Overhang b = 600 mm
	Tire marking = 130A8
	Maximum permitted speed of tractor = 40 km/h (25 mph)
	Permissible rear axle load = 3500 kg
	Hmax. = 3500 kg (1900 kg * 2 = 3800 kg, rear axle load = 3500 kg)
S =	(3500 kg - 1800 kg) * 2100 mm 2100 mm + 600 mm	= 1322 kg

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

Trailer hitch download must not exceed the trailer hitch limit specified by the manufacturer.