Carbon Fiber/Aluminum build for a family of 5

[Andrew Herrick]

Just to call attention to something the OP is doing correctly:

He appears to be sizing the bolt such that nothing is "short-bolted" - that is, there's at least one full bolt thread extending past the nut. That's extremely important for fatigue resistance. "Short-bolting" increases the chance that a bolt or nut will break due to low-force cycling over time. So, +1!

[John61CT]

But a similar design rated to carry 15,000 lb? There isn't much like that on the market. So there's less to borrow off of.

And sadly, you can't "scale" a trailer design. The loads don't increase proportionally. You're really talking an entirely new trailer.

The fact that it's aluminum adds a layer of complexity few hobbyists are familiar with. For instance, welding 6061-T6 aluminum anneals the metal in the HAZ from a T6 to a T0 temper, meaning it loses almost 80% of its yield strength. And the fatigue strength of structural aluminum is around 11 ksi ...

I'm not saying you can't build a bolted aluminum trailer rated for 15,000 lbs.
…
may as well add 10% for expert help!

I'm just talking a similar frame layout, which is bog standard.

Say the base platform costs $7000 in materials.

Do you really think a qualified US-based engineer would be willing to spec those materials and rough-sketch the bolt / reinforcing details for $700? Maybe pay for one week of E&O insurance? ;-)

If so, you're right, well worth it.

Of course maybe a sharp overseas engineering grad student online would do the same for $100, without any liability exposure.

The key I guess is choosing a state to register and insure.

[Andrew Herrick]

But a similar design rated to carry 15,000 lb? There isn't much like that on the market. So there's less to borrow off of.

And sadly, you can't "scale" a trailer design. The loads don't increase proportionally. You're really talking an entirely new trailer.

The fact that it's aluminum adds a layer of complexity few hobbyists are familiar with. For instance, welding 6061-T6 aluminum anneals the metal in the HAZ from a T6 to a T0 temper, meaning it loses almost 80% of its yield strength. And the fatigue strength of structural aluminum is around 11 ksi ...

I'm not saying you can't build a bolted aluminum trailer rated for 15,000 lbs.
…
may as well add 10% for expert help!

I'm just talking a similar frame layout, which is bog standard.

Say the base platform costs $7000 in materials.

Do you really think a qualified US-based engineer would be willing to spec those materials and rough-sketch the bolt / reinforcing details for $700? Maybe pay for one week of E&O insurance? ;-)

If so, you're right, well worth it.

Of course maybe a sharp overseas engineering grad student online would do the same for $100, without any liability exposure.

The key I guess is choosing a state to register and insure.

I'm not a PE myself, so I can't guarantee anything ... but I've heard of similar prices for similar services A PE can do what a layman either can't do or would take 20x longer to do. This is assuming you've prepared at least a meaty skeleton of the plans for review. If the PE has to generate the product from scratch, I'm sure that's more expensive ...

[StrongFeather]

Just to call attention to something the OP is doing correctly:

He appears to be sizing the bolt such that nothing is "short-bolted" - that is, there's at least one full bolt thread extending past the nut. That's extremely important for fatigue resistance. "Short-bolting" increases the chance that a bolt or nut will break due to low-force cycling over time. So, +1!

Thanks for the complements, Andy. It means a lot coming from someone with your education and experience.
Steve

[StrongFeather]

For those that are interested, I just started a thread specifically to discuss the composite panel construction technique that I will be using on this build.

viewtopic.php?f=21&t=71192

Steve

[John61CT]

I'm not a PE myself, so I can't guarantee anything ... but I've heard of similar prices for similar services A PE can do what a layman either can't do or would take 20x longer to do. This is assuming you've prepared at least a meaty skeleton of the plans for review. If the PE has to generate the product from scratch, I'm sure that's more expensive ...

Yes, a gross-dimensioned SketchUp model, just looking for specs / sources on buying the materials, and details on the bolt / plate joins, or where welding would make more sense.

Suggestions on how to go about identifying such PEs, qualified yet not uptight about the liability issue?

[StrongFeather]

Pretty slow lately, but here's some pictures...

I clamped the frame to the floor, traced the inside perimeter of the frame onto the floor...

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...and cut along the line so the floor will drop into the frame, leaving the top of the floor flush with the top of the frame.

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And I added some angle aluminum to the front and rear frame rails to support the floor...

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Steve

[StrongFeather]

Test fit the floor today. Probably start skinning it with fiberglass/epoxy tomorrow.

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[redbicycle]

That's going to come out great. You have a lot of hours into this build already.


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[StrongFeather]

That's going to come out great. You have a lot of hours into this build already.

Thank you, redbicycle. I'm really enjoying it, too. I started skinning the floor this morning. Not exactly my favorite part so far, but it's still progress.

Steve

[StrongFeather]

I was finally able to make some progress this weekend after being held hostage by cold weather for the better part of the week. I skinned the bottom of the floor with epoxy and fiberglass, and I had to wait for the epoxy to get hard enough to put the heating blanket on it without the blanket becoming part of the floor.

All of the metal fabrication is finally done and the floor is bolted down. Milestones! The last bit of metal work was the tongue box frame work. Even though it's a pretty small area, there were a lot of miter cuts and test fitting, so it took a little longer than I had hoped. But, I'm pretty happy with the way it turned out. Here are some pictures...

I'll try to explain a few things about what's going on here...
There will be a solid bulkhead separating the cabin and the TB. It will be located above the front frame rail. The TB will hold the batteries, electrical distribution, charger, inverter, etc... and the air conditioner. In the picture, you can see the angle aluminum "ledge" that the TB floor (1/2" ply) will sit on. The blocks sitting on the tongue beams are simply to support the weight of the batteries. The TB will be as tall as the rest of the trailer and have a large door on each side. I still need to figure out how I'm going to do the ventilation for the AC and electronics.

DSC09911.JPG (164.69 KiB) Viewed 3100 times

Just a rear view shot of the floor blocks and front miter cuts.

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A close up of the front of the TB framework.

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If I didn't say this before, I'm using a very old chop saw (with a non-ferrous blade) that is very inaccurate. This makes things a little challenging because I have to essentially rough cut everything and then hand file to fit.


Steve

[StrongFeather]

...and here is the floor bolted to the frame. Yay!
I'm 6 bolts short, so I'll be held hostage again for a few days waiting for a McMaster Carr delivery. Once the bolts come in and the floor is completely bolted down, I'll epoxy and fiberglass the top and the floor will be complete.

DSC09921.JPG (181.1 KiB) Viewed 3096 times

I tried to get a picture of the slot that I added to allow the axle mounting plate to slide fore and aft along the sister rails. Obviously the mounting plate is just sitting on top in this picture, but you can see how much adjustment I'll have when it comes down to mounting the axle. Initially I was going to start with the axle placement at 60/40, but I decided to start 3" forward of that because so much of the weight will be in the tongue box.

DSC09927.JPG (139.62 KiB) Viewed 3096 times

Steve

[StrongFeather]

It's been a slow two weeks between waiting for epoxy to cure (in cold weather) and several more hours of final decision making research for the composite panels. Nevertheless, progress has been made. Some major milestones, in fact. The floor is done, it sat on it's own three wheels for the first time and I even took it on a low speed maiden voyage. Here are some pictures...

The axle mounting plate clamped in place (with some help from 5/5). Getting ready to levitate (5/5 has special powers) the axle into it's home.

DSC09928.JPG (159.71 KiB) Viewed 3011 times

And the axle mounted to the plate. The two holes in the main frame rail above the axle bracket are for the floor crossmember. It will get bolted when the final axle placement has been determined. Also, I plan on making the fender back come down to the bottom of bracket to tie the outside frame rail, axle bracket and fender together.

DSC09940.JPG (131.59 KiB) Viewed 3011 times

The back side of the axle mounting. If you look on the far left side of the mounting plate, you can see where I notched 3" out of it to allow the axle to slide forward of the 60/40 point. Honestly, I can see it going forward by another 2-3" when it's all said and done. But for now, and as long as I can, it'll stay where it sits, clamped in place.
When it does get permanently mounted, 1/2" bolts will go through the plate, the sister rail, main rail and the fender back. The bolts for the floor crossmember just aft of the axle will be done the same way.

DSC09944.JPG (158.61 KiB) Viewed 3011 times

And finally, the brakes, drums and wheels were installed and it sat on it's own three wheels for the first time.

Estimated weight right now is as follows:
Floor:
1/4" plywood (4x16lbs) 64
1" XPS form (2x4lbs) 8
Epoxy (estimated) 8
Fiberglass (estimated) 2
Hardware (estimated) 18
Brakes 48
Coupler 3
Tongue Jack 15
Stabilizer Jacks 8
Axle 55
For a total of 229 pounds

[StrongFeather]

Hitched up and ready to go on it's maiden voyage. A very slow trip to the next cul-de-sac and back.

DSC09957.JPG (145.69 KiB) Viewed 3009 times

I'm very pleased with the tongue length, turning radius and ride height at this point. Plenty of room to open the rear hatch of the van and probably around 80° turning radius.

DSC09959.JPG (210.7 KiB) Viewed 3009 times

I also clamped a fender on to see how it was going to look. I decided to go with 15x6 rims and had a hard time finding a wide fender with a smallish radius, in the end, I thought I settled for a set that was wide enough to clear the outside of the wheel but too round. Luckily I was pleasantly surprised and happy with how they fit. I also took some measurements for the door(s) while the fender was in place. As the axle sits now, I have 4'8" in front of the fender. So, the plan is a 24" wide door opening that starts 24" from the front corner. This will allow the door to swing 180° and sit flush against the wall without going past the front corner and interfering with the tongue box doors. It'll also allow some room between the door and fender and I still have plenty of room to slide the axle forward if necessary.

Now on to composite panel prototyping


Steve

[FM82]

Looking really good, SF! Definitely a milestone for your build.

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