Tandem flatbed.

One of my step decks is a closed fixed tandem.
Backs a little differently than a spread especially on uneven terrain: follows the push vs transferring which axle is directing it like a spread.
Can't load a fixed tandem trailer heavy quite the same: idk about a slider/never had one.

What specifically do you want to know?

 

The fixed closed tandem is 34k
Spread is 40k
(Depending on distance between 6' - 10')

 

Well, I'll give you an example see if it fits with what you do.

I run 48' trailers, I have a 265" wheelbase tractor. I have 8 bag kW suspension, which may effect the outcome of this example...

With an alum spread axle, 30" pin setting, 50k on the deck: fifth wheel centered between drives I have 10,000 lbs on my steer axle.
With the fifth wheel all the way forward, I have 10,500 on my steer axle.

Weight doesn't transfer well thru my fifth wheel slider, I'd imagine bc the 8 bag doesn't push weight forward much to begin with.

Now consider that if I had a closed tandem, there'd be no way to get 12/34/34 on axle groups when the steer axle doesn't take up the slack.
( I realize there's exceptions to this formula, just using it for simplicity)

With a spread I can load 34k on my drives and push 18k thru each axle on the trailer, still plenty legal as they can carry 20k each.

Now if you don't run a wheelbase like mine, and can transfer weight to your steer I suppose you may be alright to scale a legal heavy load.

If you don't run much heavy stuff, prolly fine.

If you go into and out of a lot of tight jobsites, a slider/closed tandem is a safer bet. ( I know I know, "use a dump valve"
-a dump valve doesn't save a tire on a dumped axle when it slides sideways over a sharp granite curb getting rammed sideways into a jobsite)

So really: all depends on what you do.

Can you make a tool work for a job it wasn't quite designed for? Yes. Won't always work how you want tho.

 

Get a good airbag scale setup on a spread axle and you'll never have to shell out $10.50 for an axle scale again. Until the customer requires it, in which case you just bump up the rate $100. Not having to worry about sliding axles is a huge plus as well, especially if you're looking at used trailers.

I've never had to worry about axle weight with a spread axle. Steer axle and first trailer axle is all any decent DOT officer wants to see at a single axle scale since they know if you didn't screw either of those up you're fine 99% of the time.

 

I have a spread and wish it were a tandem. I haul my own stuff most the time and it isn't that heavy. A tandem is easier to maneuver and easier on tires. If you haul heavy a spread is a blessing because you can load the trailer heavy and not really have to worry too much about axle weights if you are good on gross.

 

One of the reasons I know flats went to fixed spread is for weight savings, sliding tandems add weight. I can't remember how much, I don't think it is a significant amount. A good air gauge on sliding tandems will also relieve you of the $10.50 cat scale fee. Before I parked my flat I was considering trading for a sliding tandem flat, main reason is because it would be much easier on tires.

 

Tandem isn't the way to go. And keep the trailer at 48 feet you won't have to worry about tandems.

@peterbilt_2005 You need more then 8 feet to jump from 34 to 38k. 9 feet allows 39 and 10 feet for 40.

It's in the atlas. Under bridge formula table.

 

I threw the 6' - 10' spread in parentheses. I didn't have my bridge numbers for all the distances between 6 &10' spread. @Ruthless your post had alot more info. I should of kept digging my hole and not texting. Lol