by angib » Thu Aug 08, 2013 10:06 am
Fred, the Australian rules are so severe that I don't think braking loads make any difference. And there is a valid reason for this, as they have so many washboard dirt roads which are the perfect place to 'grow' a fatigue crack in a trailer tongue until it snaps.
working-on-it, you don't have stacked tubes on your tongue! What matters is the tongue that sticks forward of the main frame and you have only one tube that does that, so that one must pass the strength test on its own. The rest of your frame provides ample support to the tongue but it can't make it any stronger - the 'weakest link' is the tongue itself.
Your tongue is 45" long (assuming you measured that from the first cross-member to the coupler ball), so the 3"x3"x3/16" tube is good for a trailer weight of 3300 pounds unbraked on-road and 1600 pounds off-road.
The 'three cross-members' issue is worth addressing on its own. The middle one of the three will be doing almost no good, but the spacing between the first and the third is good - that easily passes the half-tongue-length test I've added to the web page.
In your particular design only (this does not apply to any other trailer), the centreline longitudinal within the main frame - what you have called one of the 'stacked tubes' - provides so much support to the tongue that it doesn't really matter about most of the cross-members.
Tom, when are you getting these ZIP files? None of the current spreadsheets are zipped, so they don't need to be extracted. However they are in Excel 2007 format, which can't be opened by earlier versions of Excel. I don't know if that's your problem, but other folk may get caught by it, so I've added spreadsheet versions saved in Excel 97/03 format.