Tongue Strength

TONGUE STRENGTH

This page provides you with a guide to the strength of tongue you need. It is based on the requirements for small trailers of the Australian Government (thanks, Aussies!). If you want the technical details, look further down this page. For Australian users, I cannot promise that the simplifications used here would be acceptable to Australian authorities, though it would demonstrate you had tried!

Required Information

To work out the required tongue strength, you need two bits of information:

1) The total trailer weight. This is the total of the weight on the wheels and the weight on the hitch. It is the maximum laden weight (GVWR) so you need to add the weight of the camping equipment, food, stores, belongings and just plain junk that you may carry. At the planning stage, estimating this weight may be difficult. If you have no other information, you can use these figures:

Lightweight trailer, 4'x8' (Cubby, etc), lightly loaded	800-1000lb
Medium-weight trailer, larger than 4'x8', custom frame	1000-1500lb
Heavyweight trailer, 5'x10' and bigger, heavily-built, lots of equipment	1500-2000lb

2) The length of the tongue - the distance from the front of the main frame of the trailer to the center of the hitch ball. These diagrams show where the length is measured for different types of tongues:

Composite, or Y-shaped, tongues need two length measurements - one for the single tongue part and one for the complete tongue.

Calculate Required Tongue Strength

The required tongue strength (lb-in) = 0.5 .x. Trailer Weight (lb) .x. Tongue Length (in)

This strength is required both vertically and horizontally - though see below for some simplifications.

As an example, if the trailer weight is 1500lb and the tongue length is 36", the required tongue strength

= 0.5 x 1,500 x 36 = 27,000 lb-in

The tongue strength requirements for the different types of tongue design are:

Single tongue:

2 checks required

Check vertical strength and horizontal strength.

A-frame tongue:

1 check required

Check vertical strength only - the horizontal strength will automatically be good enough.

Composite tongue:

3 checks required

Check vertical and horizontal strength for single tongue part, using tongue length L1.

Check vertical strength only of complete tongue, using tongue length L2 - the horizontal strength will automatically be good enough.

Tongue Section Capacities

The table below gives the vertical and horizontal capacities and the weight for typical tongue sections. Channels are vertical - one side is open and the flanges are at top and bottom.

Section Type	Width x Height x Thickness in	Vertical Capacity lb-in	Horizontal Capacity lb-in	Weight lb/ft
Angle	1.5" x 1.5" x 1/8"	1,900	1,900	1.22
Angle	2" x 2" x 1/8"	3,400	3,400	1.65
Channel HF*	2.25" x 1.5" x 11g	12,500	4,900	2.05
Angle	2" x 2" x 3/16"	5,000	5,000	2.43
Rectangular Tube	1" x 2" x 11g (1/8")	8,900	5,900	2.20
Angle	2" x 2" x 1/4"	6,400	6,400	3.19
Channel	1" x 2" x 3/16"	9,800	2,300	2.31
Rectangular Tube	1" x 2" x 3/16"	11,200	7,000	3.04
Square Tube	2" x 2" x 14g (0.083")	11,700	11,700	2.05
Channel	1.5" x 2.5" x 1/8"	13,600	3,700	2.23
Rectangular Tube	1.5" x2.5" x 11g (1/8")	17,200	12,800	2.94
Rectangular Tube	1.5" x 3" x 11g (1/8")	22,800	15,200	3.35
Square Tube	2" x 2" x 11g (1/8")	15,600	15,600	3.05
Rectangular Tube	2" x 3" x 14g	20,600	16,500	2.67
Square Tube	2" x 2" x 3/16"	20,900	20,900	4.32
Rectangular Tube	2" x 3" x 11g (1/8")	28,000	22,300	3.90
Square Tube	2" x 2" x 1/4"	25,300	25,300	5.95
Square Tube	2.5" x 2.5" x 11g (1/8")	25,800	25,800	3.90
Channel	2" x 3" x 3/16"	30,900	9,600	4.23
Rectangular Tube	2" x 3" x 3/16"	38,800	30,600	5.59
Square Tube	2.5" x 2.5" x 3/16"	35,600	35,600	5.59

* Tongue of Harbor Freight 1,800lb trailer
Error in horizontal capacity of channel sections corrected Sep-06

When checking vertical or horizontal strength, the capacity given in the table must be at least equal to the required strength calculated from your trailer's weight and tongue length.

When checking A-frame tongues, multiply the vertical capacity by 2, because there are two tongue members.

When checking the complete tongue of composite tongues, add the vertical capacity of the main tongue member to 2 times the vertical capacity of the angled braces (because there are two of them) to get the total vertical capacity of the complete tongue.

That's it - you are now a tongue structural engineer.

Even Simpler Look-Up Table

If all that looks too complicated, here is a look-up table for some typical tongue designs. Select your tongue section on the left side and your tongue length across the top - the cell where they meet is the maximum trailer weight.

Maximum Trailer Weight, lb			Tongue Length
	24"	30"	36"	42"	48"
Tongue Section:
A-frame 2" x 2" x 1/8" angles	570	450	380	320	280
A-frame 2" x 2" x 3/16" angles	830	670	560	480	420
A-frame 2" x 2" x 1/4" angles	1,070	850	710	610	530
Single 2" x 2" x 1/8" square tube	1,300	1,040	870	740	650
Single 2" x 2" x 3/16" square tube	1,740	1,390	1,160	1,000	870
Single 2" x 3" x 1/8" rectangular tube	1,860	1,490	1,240	1,060	930
Single 2" x 3" x 3/16" rectangular tube	2,550	2,040	1,700	1,460	1,280
A-frame 2" x 2" x 1/8" square tubes	2,600	2,080	1,730	1,490	1,300
A-frame 2" x 2" x 3/16" square tubes	3,480	2,790	2,320	1,990	1,740
A-frame 2" x 3" x 1/8" rectangular tubes	4,670	3,730	3,110	2,670	2,330
A-frame 2" x 3" x 3/16" rectangular tubes	6,470	5,170	4,310	3,700	3,230

Tongue Strength Example

Dave Nathanson was kind enough to report his tongue collapse (see photo) in this thread on the T&TTT forum, so let's use his trailer to do a worked example of the tongue strength calculation.

Data for Dave's trailer:
Trailer weight: about 1100lb
Tongue length: about 40"

So, required tongue strength = 0.5 x 1100 x 40 = 22,000 lb-in

Actual tongue section: square tube 2" x 2" x 1/8". This is a single tongue, so the vertical and horizontal tongue strength needs to be checked. From the table above, horizontal and vertical capacity for this section is the same: 15,600 lb-in

So Dave's tongue was only 71% as strong as this rule requires, both horizontally and vertically. He would have to go up to a 2" x 2" x 1/4" square tube, or 2.5" x 2.5" x 1/8" square tube, to meet the required strength.

To be absolutely precise, Dave's tongue had a 3/4" hole horizontally through it just near where it exited from the receiver tube under the trailer body - that hole makes no real difference to the vertical capacity of the tongue section (15,500 lb-in), but the horizontal capacity is reduced to 10,600 lb-in. So the tongue was only 48% as strong horizontally as the Aussie rules require.

Technical Details

Source

The Australian government has published a guide to building small trailers, and it has been used to make this calculation.
The guide can be found online here: http://www.dotars.gov.au/transreg/vsb/PDF/vsb_01.pdf

The guide's strength requirements for tongues (drawbars) are:

12.1 Drawbars

The drawbars must be securely attached to a substantial portion of the trailer.

The drawbars must withstand the following forces applied at the centreline of the intended coupling without
detachment or any distortion or failure which will affect the safe drawing of the towed trailer:

Longitudinal tension and compression (N) 1.5 x 9.81 x ATM (kg)
Transverse thrust (N) 0.5 x 9.81 x ATM (kg)
Vertical tension and compression for rigid drawbar trailer (N) 0.5 x 9.81 x ATM (kg)

ATM is Aggregate Trailer Mass - the maximum permitted trailer mass (weight) in kg, equivalent to GVWR.

To summarize this, the tongue must be able to withstand 1.5 times the trailer maximum weight longitudinally and half the
trailer maximum weight both horizontally and vertically. I read those as being three separate load cases, rather than one
combined load case, though that may be wrong.

Assumptions

To produce the table of tongue section properties, several assumptions have been made:

Steel yield stress	35,000 psi (lb/in2)	Typical value for basic mild steel.
Material factor, bending, closed sections	90%	Closed sections are relatively stable so will approach yield stress before failure.
Material factor, bending, open sections	75%	Open sections likely to fail by tripping, so factor reduced.
Material factor, compression, closed sections	50%	Rough estimate of buckling stress in tongue members with typical slenderness.
Material factor, compression, open sections	33%	Lower factor reflecting lower buckling stress in less stable open sections.

These assumptions give the following maximum allowable stresses:

Maximum allowable stress, bending, closed sections	31,500 psi (lb/in2)
Maximum allowable stress, bending, open sections	26,250 psi (lb/in2)
Maximum allowable stress, compression, closed sections	17,500 psi (lb/in2)
Maximum allowable stress, compression, open sections	11,550 psi (lb/in2)

Tongue Properties

Applying these maximum allowable stresses to the properties of typical tongue sections gives these results:

Section Type	Width x Height x Thickness in	CS Area in2	Vertical Modulus in3	Horizontal Modulus in3	Longitudinal Capacity lb	Vertical Capacity lb-in	Horizontal Capacity lb-in	Weight lb/ft
Angle	1.5" x1.5" x 1/8"	0.359	0.072	0.072	4,100	1,900	1,900	1.22
Angle	2" x 2" x 1/8"	0.484	0.130	0.130	5,600	3,400	3,400	1.65
Channel HF*	2.25" x 1.5" x 11g	0.601	0.477	0.188	6,900	12,500	4,900	2.05
Angle	2" x 2" x 3/16"	0.715	0.190	0.190	8,300	5,000	5,000	2.43
Rect Tube	1" x 2" x 11g (1/8")	0.625	0.284	0.186	10,900	8,900	5,900	2.20
Angle	2" x 2" x 1/4"	0.938	0.245	0.245	10,800	6,400	6,400	3.19
Channel	1" x 2" x 3/16"	0.680	0.375	0.086	7,900	9,800	6,700	2.31
Rect Tube	1" x 2" x 3/16"	0.894	0.354	0.222	15,600	11,200	7,000	3.04
Square Tube	2" x 2" x 14g	0.619	0.373	0.373	10,800	11,700	11,700	2.10
Channel	1.5" x 2.5" x 1/8"	0.656	0.518	0.140	7,600	13,600	11,100	2.23
Rect Tube	1.5" x 2.5" x 11g (1/8")	0.902	0.546	0.407	15,800	17,200	12,800	2.94
Rect Tube	1.5" x 3" x 11g (1/8")	0.985	0.723	0.483	17,200	22,800	15,200	3.35
Square Tube	2" x 2" x 11g (1/8")	0.865	0.496	0.496	15,100	15,600	15,600	3.05
Rect Tube	2" x 3" x 14g	0.802	0.525	0.654	14,000	16,500	20,600	2.67
Square Tube	2" x 2" x 3/16"	1.269	0.662	0.662	22,200	20,900	20,900	4.32
Rect Tube	2" x 3" x 11g (1/8")	1.105	0.888	0.709	19,300	28,000	22,300	3.90
Square Tube	2" x 2" x 1/4"	1.589	0.804	0.804	27,800	25,300	25,300	5.95
Square Tube	2.5" x 2.5" x 11g (1/8")	1.105	0.818	0.818	19,300	25,800	25,800	3.90
Channel	2" x 3" x 3/16"	1.242	1.177	0.364	14,300	30,900	9,600	4.23
Rect Tube	2" x 3" x 3/16"	1.644	1.231	0.970	28,800	38,800	30,600	5.59
Square Tube	2.5" x 2.5" x 3/16"	1.644	1.131	1.131	28,800	35,600	35,600	5.59

* Tongue of Harbor Freight 1,800lb trailer
Error in horizontal modulus/capacity of channel sections corrected Sep-06

Simplifications

Some parts of this tongue strength calculation can be ignored since they will always be strong enough in all normal cases:

Longitudinal strength	The longitudinal capacity of the lightest section (normally only used in pairs) is large enough for a trailer of 3,730lb - far in excess of any reasonable teardrop. The vertical capacity of this same section would be exceeded at this trailer weight if the tongue was 2" long! So it would seem that it is unnecessary to check the longitudinal strength of any teardrop tongue.
Horizontal strength, A-frame tongues	A-frame tongues with the industry standard 50 degree angle have tensile/compressive forces in the tongue members of 2.36 times the horizontal force, which is 1.18 times the trailer weight. For the lightest section, this allows a trailer weight up to 4,750lb. At this trailer weight, the vertical capacity of a pair of these sections would be exceeded if the tongue was 1.5" long! So it would seem it is unnecessary to check the horizontal strength of A-frame tongues.
Horizontal strength, composite tongues, at base of tongue	This case is identical to that for A-frame tongues. However this only applies at the base of the tongue where the angled braces make a truss - the horizontal strength of the single tongue member still needs to be checked at the point where the angled braces meet it.

Comments

Some comments on the Australian rules and my simplification of them:

They are simple, which is a great benefit - for example, it isn't necessary to make any estimate of hitch weight or trailer balance which may turn out to be wrong when the trailer is built.
The simplification of not considering hitch weight could cause problems in the USA where very high hitch weights (20-30% of trailer weight) are often used on small trailers. For example, for a 33% hitch weight, the required vertical tongue strength is only 1.5 times the static load, whereas it is 5 times the static load with a 10% hitch weight. When used on a trailer with a high hitch weight, it would be sensible to pick a tongue section that comfortably exceeds the minimum required, say by 50%.
Requiring a horizontal tongue strength equal to the vertical tongue strength seems a bit 'over the top'. In the example of Dave Nathansons's trailer, the tongue failed vertically, even though its horizontal strength was significantly less than its vertical strength. Maybe it is acceptable to have tongue that only has a horizontal strength of 75-80% of the minimum required by the rules.
To produce a single 'strength' figure for each tongue section has required a substantial simplification of some complex factors like buckling. If long, slender tongue sections are used (say, greater than 30:1 span/depth) there is a risk that buckling will occur at stresses below those used in the table above. However long, slender tongues will probably not pass the vertical or horizontal strength test, so this will hopefully not be a problem.

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