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 |
Vertical Capacity |
Horizontal Capacity |
Weight |
|
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, remember multiply the vertical capacity of the chosen
section by 2 to get the total capacity of the tongue, 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.
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.infrastructure.gov.au/roads/vehicle_regulation/bulletin/vsb1/vsb_01_b.aspx
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 |
CS Area |
Vertical Modulus |
Horizontal Modulus |
Longitudinal Capacity |
Vertical Capacity |
Horizontal Capacity |
Weight |
|
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.