I now have PDF copies of weight regulations for Alberta, Canada and Ontario, Canada. If interested, see the attached files. The quick version of what I learned is this: you get the maximum legal weight allowance of 19,100 kg for a tandem when the spacing is 1.8 but less than 3.1 m (equal to 70.87" but less than 122").

Now, Alberta has their maximum weight for a tandem set to 17,000 kg and the spacing can be between 1.2 m and not more than 1.85 m. Notice that if your tandem spread is exactly 1.8 m, you still will be able to carry maximum weight per axle in Ontario and be under 1.85 m for Alberta!

I guess that's why so many folks here use 72" spread. So, there's no reason to use 120" spread on a stepdeck at all. Even with the smallest spread of 1.2 to 1.6 m (47.2" to 66.9") you get 18,000 kg allowance in ON.

 

yes yes, use the closed tandem or face the wrath of the provincial DOT. with a 10'1" spread you are allowed only 4500kg per axle in Alberta. actually, most of the rigs I see are tridems to maximize load potential for that area.

 

Yes, DM, after I posted this I did the homework, argued with the trailer dealer and we finally decided to go with a closed tandem. As Allan mentioned in his posts, when you have such a short deck (37' or 38'), having a big spread on trailer axles just doesn't work. It's one thing to 120" between them when you pull a 48' skateboard, but it's a totally useless thing to have for a 37' lower deck.

I also found that in ON, Canada you don't need to be 120" apart to get the maximum load rating. Anything between 72" and 120" gives you the same 19,100 kg. Even with closed tandem, you're rated at 18,000 kg, higher than in US.

Alberta doesn't care how far your tandem axles are as long as they are in the 1.2m to 1.85 m range (47.24" to 72.83"). You're rated for 17,000 kg no matter what. So, with a 72" spread you're pretty much okay in ON, Alberta and US

BUT

it's still too much for a short lower deck on a stepdeck! That's why Canadian DeckX uses for their stepdecks all closed tandems (fixed, no slider) that are in the very rear position. You get: better load distribution, less tire wear, better fuel mileage, less weight and cost (because there's no slider).

I wanted to put tandems at 49" apart but the dealer said than the center will be more than 41" away from the kingpin, so we had to go with a 61" spread. This is still closed tandem and rear center axle will sit appr. 31 inches from the rear.

If I had a 53' step, I'd probably go with a tridem on 17.5" tires (I'm using same tires on my stepdeck spec).