Since someone brought it up ...... I did a little research in the archives.
In addition to Don's notes below I found the following factors that go into
the mfg's capacity rating.
1. brakes

2. engine/clutch

3. suspension

4. mounting points

5. center of balance

I'm trying to figure out if I can tow my 73 Thing.  For extended stays it
would be great to set up camp, leave the van in place and be mobile in the
Thing.
Since it tows on all four wheels tongue weight is not a problem.  However,
it greatly exceeds the unbraked weight limit.
VW 181 Thing Curb weight ...........1920
Max unbraked trailer weight ...........1320
Difference ...................................600lbs / 45%
Does anyone know the numbers underlying the above factors and their
relationship to calculate the max?  I plan to eventually upgrade to a Subie
engine.  Brake upgrade is pretty straightforward.  I'm just not sure how
much these upgrades will buy me in increased capacity.  Given how
underpowered the vans are it would be reasonable to assume 1 & 2 carry the
highest weight, but I don't want to make assumptions and wind up in a ditch
because the back end got squirrlley on me.
Thanks for any input.
Mike
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 From the owners manual - 1986 and up:
Maximum braked trailer weight -    900kg (2,000 lb)
Maximum unbraked trailer weight - 600kg (1,320 lb)
Maximum tongue weight -                 75kg (165 lb)
Note that there is no difference shown between the Syncro and the passenger
20 van.  Not sure if the camper supplement shows a lower number or not.
Don in Reidsville, NC