The Beginnings of Something Different - Car Hauler RTT Camper

[NC LX]

Looks like you’ve done your homework. From a Structural viewpoint your material is strong enough to support Your X Y Z static forces when loaded and and unloaded. When parked, The weak points I would worry about that have unforeseen variables is your welds at each joint.

When in movement, the elevated weight (200+lbs) at that height would multiply force several times when you stop/start. The extra leverage from the force of weight will fatigue on the welds and and your solution for mounting.

I think acceleration concerns would be minimal. My concern would be your forces exerted on the frame in the “Y” direction when braking.

 

[S4Cruiser]

Looks like you’ve done your homework. From a Structural viewpoint your material is strong enough to support Your X Y Z static forces when loaded and and unloaded. When parked, The weak points I would worry about that have unforeseen variables is your welds at each joint.

When in movement, the elevated weight (200+lbs) at that height would multiply force several times when you stop/start. The extra leverage from the force of weight will fatigue on the welds and and your solution for mounting.

I think acceleration concerns would be minimal. My concern would be your forces exerted on the frame in the “Y” direction when braking.

This makes sense.

If I wanted to semi educatedly guess on the forces when braking...

F = m x a where F is force, m is mass (220ish lbs) and a is acceleration (or deceleration in this example)

Need to find ‘a’ (acceleration) which = v / t where v is velocity and t is time.

For sake of simplicity, a 2014 Chevy 2500 brakes from 0 - 60mph in 3.4s (braking distance and time increases when trailering). Convert 60mph to fps = 88fps and b/c most math is in metrics again convert fps to m/s which = 26.8224m/s.

a = 26 / 3.4 so a = 7.89

Let’s then convert the 220ish lb RTT to kilograms = 99.7903kg

Ready to determine force (or Newton).

F = 99.7903 x 7.89 so force = 787.345N

Then we need to convert force to weight. W = N / 9.81 where W = weight N = Newtons

W = 787.345 / 9.81 so weight is 80.34kg when converted to lbs = 177.12

Does this mean the forward weight applied to the box when in an emergency braking scenario is only 177.12lbs?

 

[NCFJ]

By the time you get done calculating you could have burned in some gussets or bracing.

 

[NC LX]

This makes sense.

If I wanted to semi educatedly guess on the forces when braking...

F = m x a where F is force, m is mass (220ish lbs) and a is acceleration (or deceleration in this example)

Need to find ‘a’ (acceleration) which = v / t where v is velocity and t is time.

For sake of simplicity, a 2014 Chevy 2500 brakes from 0 - 60mph in 3.4s (braking distance and time increases when trailering). Convert 60mph to fps = 88fps and b/c most math is in metrics again convert fps to m/s which = 26.8224m/s.

a = 26 / 3.4 so a = 7.89

Let’s then convert the 220ish lb RTT to kilograms = 99.7903kg

Ready to determine force (or Newton).

F = 99.7903 x 7.89 so force = 787.345N

Then we need to convert force to weight. W = N / 9.81 where W = weight N = Newtons

W = 787.345 / 9.81 so weight is 80.34kg when converted to lbs = 177.12

Does this mean the forward weight applied to the box when in an emergency braking scenario is only 177.12lbs?

I was actually looking through my Dynamics textbook from college to do what you did above. One thing I remember from college is if the answer doesn't make sense it probably isn't.

A 220lb tent coming to a stop wouldn't weigh less, it would weigh more. What could happen I suppose is the Z force downward is less, because the momentum of the weight is moving in the Y direction.

Im not sure if 177.12 is correct but it could be the correct answer for force in the Z plane.

 

[S4Cruiser]

By the time you get done calculating you could have burned in some gussets or bracing.

1k% true! I’ll probably still add bracing. Just was trying to understand / maybe learn something new.

 

[S4Cruiser]

I was actually looking through my Dynamics textbook from college to do what you did above. One thing I remember from college is if the answer doesn't make sense it probably isn't.

A 220lb tent coming to a stop wouldn't weigh less, it would weigh more. What could happen I suppose is the Z force downward is less, because the momentum of the weight is moving in the Y direction.

Agreed. I was expecting it to be more weight force then the actual weight.

I also have zero clue if my calculations are at all correct !

 

[NC LX]

When I get back later, I’ll sit down and see what I can come up with. I’m curious about it now.

 

[S4Cruiser]

When I get back later, I’ll sit down and see what I can come up with. I’m curious about it now.

Ok...so I think I’ve rationalized how the 177 number is technically correct however doesn’t tell the entire story.

For example, say you have a 50lb box in your passenger seat and slam on brakes. It doesn’t take 50 lbs of reverse force to keep the box from sliding off the seat.

The question now though is that if the top of the box has 177lbs of force applied, then how much additional force is applied to the bottom joints of the box as the result of leverage.

 

[S4Cruiser]

So the torque applied to the bottom joint would be 487ish lbs total or 122ish lbs per joint (4 joints).

 

[mtweller]

Funny, I had done a Free Body Diagram and calculated the forces for a spare swing arm for my 40 one time long ago. I won't go into any long detail about the design so as not to derail here, but the way I calculated it, with some help of a friend who is a mechanical engineer by trade, my design had ~200% safety factor and I couldn't convince any of the fabricators on Mud it was a solid design. I wound up scrapping it anyway, but I found out quickly that old school offroaders often don't believe math and metallurgy. Long story short, expect considerable resistance to your logic for calculating this, should you decide to crosspost this on a main forum.

 

[GLTHFJ60]

What about wind loads on the tent, when two people are sleeping in there? Add another 300# to start.

If it were me, I'd put some gussets in there, but you'll likely be fine. Loading/unloading people I'd imagine would put more strain on it then braking forces, but I'm not a structural engineer. For that reason, I like overbuilding everything.

 

[S4Cruiser]

What about wind loads on the tent, when two people are sleeping in there? Add another 300# to start.

If it were me, I'd put some gussets in there, but you'll likely be fine. Loading/unloading people I'd imagine would put more strain on it then braking forces, but I'm not a structural engineer. For that reason, I like overbuilding everything.

I did enough maff yesterday to not be interested in calculating wind load, but I could definitely see it being more force then braking (e.g. 70mph travel speed).

It's kinda back to Jonathan's point...bracing makes it better with zero downside. I also think that it would be fine...not based solely on my calculations (NOT an engineer either) but when also comparing to other 'off the shelf' racks. For example, I was looking at one of Leonard's truck ladder racks. Built out of 1.5" 14 gauge tube and claimed a 1k load rating. Zero gussets. I also trust my welds more then a likely offshore made rack.

 

[mtweller]

adding a frame for drawers will inherently strengthen the whole structure significantly. I wouldn't worry beyond that if it were me.

 

[NCFJ]

Over the years I have had numerous requests for CAD drawings of the bumper, slider etc I was making. Many folks could not grasp that many fabricators work without CAD. Most go out in the shop with an idea, perhaps a drawing of some sort and start their process. Many build first from cardboard or similar, myself included. When working with tubing, I always buy a lighter tube of the same OD for use during initial mock up, cheaper than messing up expensive DOM material.

The point being that while CAD is a great tool, it is time consuming and at times inaccurate if all the initial parameters are not correct. I am all for rendering in CAD for a mass produced item, but for one offs or small run builds it can be detrimental to the cost.

I am not anti CAD. I earned a good living as an architect using CAD to produce blueprints. I just think it is over used in many occasions. Sometimes the Zen comes only from sitting and staring at the project in real time and in the case of something like a tube bumper, holding up various test pieces with different bends etc.

The fact that people contact a vendor requesting CAD drawings for their work product is an entirely different subject.

 

[mtweller]

Over the years I have had numerous requests for CAD drawings of the bumper, slider etc I was making. Many folks could not grasp that many fabricators work without CAD. Most go out in the shop with an idea, perhaps a drawing of some sort and start their process. Many build first from cardboard or similar, myself included. When working with tubing, I always buy a lighter tube of the same OD for use during initial mock up, cheaper than messing up expensive DOM material.

The point being that while CAD is a great tool, it is time consuming and at times inaccurate if all the initial parameters are not correct. I am all for rendering in CAD for a mass produced item, but for one offs or small run builds it can be detrimental to the cost.

I am not anti CAD. I earned a good living as an architect using CAD to produce blueprints. I just think it is over used in many occasions. Sometimes the Zen comes only from sitting and staring at the project in real time and in the case of something like a tube bumper, holding up various test pieces with different bends etc.

The fact that people contact a vendor requesting CAD drawings for their work product is an entirely different subject.

Don't most shops just use templates and jigs for lower volume pieces? Besides that, CAD drawings are intellectual property anyway. With that said, I'm a bit of a hypocrite since I got a drawing from Tuffy so I could try and figure out my seat layout. I knew they'd have one since they make a bazillion products on mass scale, but was actually surprised they were willing to share it. There's obviously some break-over point where it makes sense to use it for sure.

 

[lumbee1]

I've been out of town but it looks like others provided answers. @S4Cruiser triangulation makes everything stronger, whether it is a gusset or a full sized cross brace. The rectangle or square is inherently weak when the bottom is fixed and a lateral load is applied to the top edge. Your structure is a tall rectangular box with the weight of the RTT on top. The structure will be subjected to forces in the x and y planes when the vehicle is in motion. The z plane would only cover compression forces from the RTT. With acceleration and braking, the top and bottom edges of the sides will want to move parallel to each other. When turning, the top and bottom edges of the front and back of the structure will want to move parallel to each other. A fixed cross brace stops these movements through compression and expansion.

While gussets would help, a fixed cross brace would provide the most strength even if it was small.

I wish I had a better demonstration but this is the best I could find.

 

[jynx]

Late to the party, but curious if you still have this arrangement and if so, how it was doing? I need a new trailer, looking at a gooseneck, but not ready to go all in on a camper too, so thinking of putting a rtt on the raised gooseneck section and set it up to open onto the trailer deck.

P.S. we already have an rtt, so just have to work out a mount for it.

 

[S4Cruiser]

Late to the party, but curious if you still have this arrangement and if so, how it was doing? I need a new trailer, looking at a gooseneck, but not ready to go all in on a camper too, so thinking of putting a rtt on the raised gooseneck section and set it up to open onto the trailer deck.

P.S. we already have an rtt, so just have to work out a mount for it.

Yes, still have it but has t been used much in the past year...cause covid and s***. It’s worked great as is but I plan to add some additional bracing when group camping is back in vogue. I think if you’re using the gooseneck deck, you’d be set. The one benefit of it being the height I set it at is being able to pic up and put dog into the rtt. If we didn’t have said dog then the height of the gooseneck neck would be no issue.

 

[S4Cruiser]

You could easily build a platform to mount on top of the gooseneck and then mount rtt onto it. Your pin weight for towing wouldn’t increase much at all - weight of rtt (~200#). Do it so your rtt ladder comes down onto the trailer deck.