Realistic weight or length limits?

You might be interested in this Ford Body Application doc

In there you'll see the typical platform lengths that you've probably encountered talking to upfitters. (pg 15)

Note on pg 9, Percentage of Payload on Front Axle By WB/CA. Let's look at the F series crew, 84CA - 204WB. That's 7' of exposed frame infront of the rear axle and ~4' behind.

With a 12' body platform (5' rear overhang aft of axle) the chart shows 10.3% of payload (water level load) falling on the front axle. Same chassis with a 14' body platform (7' rear overhang) shows 4.4%. A 16' body is not shown (7') overhang but you can probably extrapolate the effect.

Unloading the front axle is possible with a long overhang. And tempting with all that rear real estate. As mentioned the platform mfr may not know the user's every application.

It's no small thing to put your name on a product. Caution, sticking to the "norms" may be prudent.

With RVs like the ones with the massive rear overhangs, the RV builder obviously has more control on what the finished weight distribution will be, at least until the buyer gets their hands on it...

Also, you may notice that for the same approximate wb (204-205) a reg cab chassis shows an 18' platform. That's 4' more than the crew (3' for the smaller cab and 1' more overhang. That might be only with the optional frame extension, IDK. But when you think about it, most all of those rv's with the massive overhangs are reg cabs (or equivalent). So there is something at play there.

Finally, turning diameters, that 204wb F550 shows 59'2" wall to wall (presumably that's the bare chassis without the body width or rear overhang).
For comparison, the F6/750 with similar 203wb shows 28'8" wall to wall. Significant.

It might be tempting to see a med duty 5 ton delivery truck and think, he can do it, I can do it. But know the differences.

All the above is not to dissuade you, only offered as food for thought.
Wow, this is GREAT info, thanks! I looked for something like this and my Google skills failed me, so thanks so much for sharing.

In a perfect world, my intention had been to build an F-550 Crew Cab platform, 12' flatbed, and I overhang that by 2' (on a somewhere between 20-45 degree angle) to give myself a 14' floor (with a cabover). My water tanks would sit either just after the cab, or 3' after it (figuring out if I add an insulated basement or not)..

I am absolutely concerned about lifting weight off the front axle though, and I have no idea how to do the math on that. I wonder if I need to just "take Ford's recommendation" on this and reduce my living space by 2'. I'm certainly not keen to jump up to an F-650 platform.
 

rruff

Explorer
I am absolutely concerned about lifting weight off the front axle though, and I have no idea how to do the math on that.
It's easy. Locate the weight-center of each item you are hauling, and draw a vertical line there and measure how far that is from the rear axle. Each weight times the distance is the torque at the rear axle. Distances behind the rear axle are negative. Add them up.

Your box is 168" long in total, not counting the cabover. 84" in front of the axle and 84" behind. If you distribute the weight evenly along that, all the weight will be on the rear axle. So you have to bias the weight behind the axle quite a bit (enough to counter the cabover weight) before you start lightening the front wheels. In other words, a 14' box length will be fine if you bias weight towards the front.

Note that water is a highly variable weight. You'll want a +torque both empty and loaded.
 
Just to clarify, my F6/750 reference was more about maneuverability. It's one thing to make a design work on paper, but you have to drive it too. That said, lot's of folks have done what you're planning. Just better to be frustrated during the planning than after the build.

You are in control of weight placement so the front end shouldn't be a worry.

For weight distribution calculation I like to use percent front axle/rear axle. Simplifying, we know that a weight placed directly over the front axle has a FA/RA distribution of 100% / 0%. Weight placed over the rear axle is 0/100. Wt exactly mid way between axles is 50/50. All other location percentages can be calculated using the wheelbase length divided by 100 and the distance of said locations from an axle.

So for your F550, 204WB, 84CA. every, say, 2" away from an axle effects a 1% weight transfer.

Using your water tank example. Say your tank front to back dimension is 20" with it's COG exactly midway and you place it starting 4" from the cab rear wall. The COG of that tank would be ~70" forward of the rear axle (84CA-4-10). 70/2=35. So wt distribution would be 35FA/65RA. Option 2 of placing the tank 3' further back would put it's COG at ~ 34" = 17FA/83RA.

Because of the lever effect weight placed aft of rear axle actually takes weight off of the front axle and loads the rear axle more than the object weight itself. Same calculation but with a minus/plus. So if you locate a spare tire on the rear wall of your 14' box, say 96" aft of axle, the distribution would be 96/2 = -48fr/+148rr. Total percent always equals 100

If you already have a spreadsheet for your payload you can - if so inclined - itemise the objects (stuff), and using the object location percentages, estimate the axle loads.

All of this is easier in a drawing program or on a scale drawing of your camper - or entire rig - with an overlay of scale lines indicating front/rear percentages. For eg your 14' camp layout drawing. The centre of the camper is 50/50 - put a line there on your drawing. The inside front wall (behind the cab) is 40/60 ((84CA-2 gap-2wall thickness)/2)) - another line - and the inside rear wall is -40/140 - another line etc.

End of the day, with an 84CA and a little awareness you shouldn't unload your front axle from factory unladen and you should be able to share some of the total payload onto it.

Think I got that right 🤓and hope it makes some sense. 😴
this helped A LOT, thanks!

The math and physics weren’t really clicking until you spelled it all out like this. Thanks again!
 

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