UNIFIED TONGUE STRENGTH

This is intended as a simple method to work out the tongue strength required for teardrop trailers, although it could be used for other types of trailers.

It is a simplified combination of two official tongue strength guides but was not created by an industry professional, so there are no guarantees about its accuracy.

There is an Excel spreadsheet you can download to work out an exact answer for your trailer design or, even simpler, a one-page look-up table to use for common tongue sections.

To use this method, three main bits of information are required:

1) where/how the trailer will be used;

2) the type and length of the tongue;

3) the maximum loaded weight of the trailer.

Dave Nathanson was kind enough to report his tongue collapse and give permission to use this photo.

Using the off-road category, an estimated 40 inch tongue length and 1100 pound trailer weight shows the original 2”x2”x1/8” square tube tongue was only about 60% of the required strength.

As it took quite a bit of off-road travel before the tongue broke, this suggests the result is about right.

1: Type/Use

Select the category of use that matches how and where the trailer will be used.

Teardrop trailers with the axle well to the rear will have a higher than usual tongue weight, and for these consider moving ‘up’ one category in strength.

Off-Road

(strongest category)

- for braked or unbraked trailers that will be used off-road;

- for trailers where the maximum strength is wanted;

- for trailers such as utility or plant trailers that may be subjected to operator abuse;

- consider for braked road trailers that have a high tongue weight.

Braked On-Road

(intermediate category)

- for trailers used mainly on-road fitted with brakes;

- consider for unbraked road trailers that have a high tongue weight.

Unbraked On-Road

(weakest category)

- for trailers used mainly on-road not fitted with brakes.

2: Type/Length of Tongue

Three different type of tongue design are allowed for and the diagram shows where the tongue length is measured – twice for the composite tongue, where the strength needs to be checked in two places.

The A-frame tongue is structurally the best as it has great horizontal stiffness and it transfers the hitch load directly from the coupler to the side rails. For the other two designs, an extra cross-member is required in or under the main frame.

3: Weight

In order to calculate the tongue strength, you need to have an estimate of the maximum loaded weight of the trailer (GVWR).

Here are some suggestions for typical teardrop trailers – not applicable to other trailer types.

Heavyweight – 1500-2000 pounds

- larger teardrops

- teardrops built mainly with 3/4” ply

- lots of fitted equipment

- carrying lots of supplies/equipment

Medium-weight – 1000-1500 pounds

- a compromise for teardrops that are built reasonably but not obsessively light

- not much equipment or stores

Lightweight – 800-1000 pounds

- limited to smaller 4x8 or maybe 5x8 trailers

- trailers must be built with lightest components throughout

- almost no fitted equipment

- very little stores carried

4: Calculation/Result:

for use in America – inch and pound units

For the simple method, find the maximum weight in:

Tongue Strength Look-up Table US

For a more detailed calculation, download the Excel spreadsheet:

Tongue Strength Spreadsheet US

(alternative version for Excel 97/03)

The tables include common tongue sections, but if you have unusual one, download:

Section Properties Spreadsheet

(alternative version for Excel 97/03)

5: Calculation/Result:

for use in Britain – mm and kg units

For the simple method, find the maximum weight in:

Tongue Strength Look-up Table UK

For a more detailed calculation, download the Excel spreadsheet:

Tongue Strength Spreadsheet UK

Other things to think about

Horizontal Stiffness

The Australian trailer regulations used for the off-road section of this tongue strength method require the same strength horizontally as vertically and there is some evidence that trailers with tongues which are not stiff enough can suffer from ‘shimmy’ (side-to-side oscillation).

So it makes sense to design the tongue so that it has about the same strength sideways as vertically. A-frames are always stiffer/stronger sideways than vertically, but single tongues are at risk, so pick a tongue section that has about the same horizontal and vertical strength.

To help this, the single tongue section of the spreadsheet shows horizontal strength as a percentage of vertical strength and values below 80% are in red.

Single tongue trailers that suffer from shimmy can usually be cured by adding diagonal braces to convert it to a composite tongue.

Tongue Support

The connection to the main frame needs to be as strong as the tongue and this can be a problem for single tongue trailers.

A couple of ‘rules of thumb’ that will help avoid this are:

- The ‘bury’, B, of the tongue into the frame should be at least half of either its exposed length, L, or the frame width, W.

- The section used for the second cross-member should be of roughly similar strength to the tongue – or at least not a lot weaker.

Background

The previous teardrop tongue strength page used the Australian trailer regulations. Feedback has shown that these are plenty strong enough, but also stronger than commercial teardrops that don’t suffer from tongue failures. So the idea developed that a tongue half as strong as the Australian rules was OK for a teardrop, which won’t get abused like the utility trailers which the rules may have had in mind.

There is an International Standard, ISO 7641, for tongue strength and this appears to be more suited to on-road trailers, so the two methods have been combined.

Only the vertical strength part of each method has been used as this seems to be the main issue with tongue strength. For the record, the Australian rules want the same strength horizontally side-to-side and three times the strength horizontally fore-and-aft, compared to the vertical strength.

To compare the three categories, think of a helpful giant who is holding your teardrop body/frame gently in his hand (which hasn’t been drawn as it would be so big and hairy that it would unsettle those of a nervous disposition). Then the rules require the following force, from a quarter to a half of the trailer’s loaded weight, to be applied as a vertical force to the coupler:

*Type/Use Category*		*Source*
Off-Road		Australian trailer rules for design load, but with ISO 7641 calculation method used.
Braked On-Road		ISO 7641
Unbraked On-Road		ISO 7641

It’s worth pointing out that ISO 7641 says that it can only be used on trailers with tongue weight ratios of up to 10%. Many teardrop trailers will have tongue weight ratios that are higher than this.

So I have suggested that for trailers with a high tongue weight ratio, users should consider using the next category ‘up’.

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