There might be one or two Americans who want to see this, but mostly I'm posting it so that any Europeans, particularly Brits, here can find it.
This October in Britain a trailer inspection is being introduced for the first time. As we don't register trailers (they just display a copy of the tow vehicle's license plate), up until now we had no controls over home-built trailers at all, but that is all set to change with the Individual Vehicle Approval (IVA) test. Much of the IVA test is concerned with detail that has always been required like lights and reflectors, but a big new requirement is proving the trailer has enough tongue (or 'drawbar') strength, which has to pass an international standard, ISO 7641.
To help with this, I've written a spreadsheet that takes in a few dimensions and weights, and then outputs the required tongue strength, plus some suggested steel sections that are strong enough. This uses metric units but if there's any interest it would be easy to make a version in American units. I do think it's about time I redid the tongue strength web page and made it more usable, so maybe this would be a good way to do it.
Here is the web page covering the drawbar strength spreadsheet
or it can be accessed from a new IVA test section of the Design Library