How are the commercial camper axles so far back?

[GSP848]

Hello everyone, this is my first post here and thanks for having me as I am researching a diy camper. There is a lot of good info here and I really appreciate it. That said, I can't seem to find an answer to my specific question. I have seen the rule of thumb of 60/40 or 65/35 for axle placement but the commercial campers seem to have their axle VERY far back on the frame...way past 60/40....Like, in the last foot or two of the frame. Are they basing the axle placement off of the box on top and not the actual frame?

I would like to have something similar on my build. I'm planning on having a custom trailer frame built and I really like that overhang on the back and the angle for departure over obstacles but I want the sleeping cabin to be removable so the frame could also function as a flat bed utility trailer when the cabin is removed. If the axle placement includes the box length in the 60/40 rule then this would put the axle way too far back on the trailer when I remove the box, correct? Is there an axle placement that could give me that overhang but not be too far back when the box and associated overhang is removed?

If I'm being honest, this won't be for TRUE offroad use but rather just your typical forest service roads and such so I don't NEED that departure angle but without it the trailer just looks way too boxy (see attached picture labeled 'trailer 2').

I also sketched out a 7 foot trailer with a 65/35 axle placement when used as a stand alone trailer. If you are supposed to go off the length with the cabin included then that axle placement would only be about 55/45 when including the cabin length of 8 feet. I could also reverse that equation and have a 65/35 ratio based on the 8 foot box but that would be 75/25 ratio placement on the 7 foot stand alone trailer when the box is removed. (See picture labeled 'Trailer 3'.) Is one of those placements better than the other? Are either even acceptable? Any suggestions on how to do this with a removable cabin, if it's even possible, would be appreciated.

P.S.

I also thought of having shifting the cabin forward and having an overhang storage area on the front and break up that boxy look but would this be too much tongue weight? Maybe even some dangerous up-lifting action with the wind on the highway? The roof is an 8' fiberglass truck topper just for reference. See picture labeled trailer 1.

The box length must be 8 foot for all designs since I am using a truck topper as the top half of the cabin.

I attached a couple sketches of my ideas. Thanks for your help.

 

[Peter_n_Margaret]

The further back the axle, the more stable the towing, provided the hitch weight does not get too high.
In Oz, it is a legal requirement that the load length in front of the axle is always greater than the load length behind the axle.
Cheers,
Peter
OKA196 motorhome

 

[GSP848]

Is there an equation for figuring hitch weight while still on the drawing board?

Also I guess if I did a 60/40 ratio based on 8 foot cabin length then 7 foot stand alone trailer would be 69/31 call it 70/30. Might that work okay?

 

[kcshoots]

There is no rule for Axle placement. It is really driven by desired departure angle, towing capability, wheelbase, suspension design, payload per axle, manufacturing and other drivability characteristics. If coil or air sprung, axles can be placed closer to the edges of vehicle length, whereas leaf springs require more length on each side of the axle for the springs. Pinzgaurs, Land Rover 101s, HMMVVs, Unimogs, Defenders, and other vehicles tend to have limited vehicle length behind the rear axles compared with most consumer vehicles. You can calculate your estimated vehicle and trailer weights and their position and calculate your weight bias front of or rear of the axle. Best to balance it or slightly bias forward.

 

[NatersXJ6]

I think the specific answer to your question is that the whole box is considered in axle placement. The subframe is only necessary to hold the axles, so many types of trailers have a tongue and integrated subframe that goes back to the rearmost axle mount and no further, then the weight is carried above that by a box, deck, etc… that extends farther back.

 

[ripperj]

Impressive approach and departure angles only prove useful when the ground clearance and wheel base are such that you don’t high center on a curb


Sent from my iPhone using Tapatalk Pro

 

[billiebob]

The 60/40 is based on uniform loading but if the front 2/3s of the trailer is just an empty sleeper box and all the weight/storage is in the teardrop kitchen at the back, the 60/40 weight distribution will likely shift the axle back closer to 70/30.

Where you place the axle depends on where you load the weight.

Anyone with a large storage box, battery box, etc on the tongue should likely run close to the 60/40 rule.

 

[4000lbsOfGoat]

The ideal for towing a trailer in tight places (such that you might run into on FS roads) is to have the distance between the TV wheels match the distance between the rear wheel on the TV and the wheel on the trailer. This will let the trailer track on pretty much the exact same line as the TV.

Yes, as mentioned, weight distribution needs to be factored when building out but if I was building a trailer from scratch with a known TV I'd probably just match up the wheelbases.