Building the Frame – Part II

Hello Again, Naturally.


I have been doing a lot of design reviews lately and the actual work on the trailer took a back seat a little. It is often beneficial to, once in a while, pause what you are doing and review the progress, take notes of lessons learned, and they are plenty.

The Economic Costs of Owning an RV

One of the most often asked question from my friends and readers would be: Why build a travel trailer? Travel trailer is probably the most cost effective way of owning an RV. Yet, travel trailers conjure up an unflatter image of neglected dwellings as we often heard the derogatory term “trailer trash”. Please accept my apology in advance for the term mentioned if it has offended you. Believe me, that wasn’t the intention.

Let’s get back to the question why a travel trailer? Like I have mentioned in the previous post, the travel trailer is designed to be the “home” part of the RV. Unfortunately, not a lot of innovation has happened to travel trailer design as those of Class A RVs. I believe it is simply a matter of return on investment. Travel trailers are considered low margin goods; therefore, not a lot of investments were made to make them better. I do, on the other hand, think otherwise. A well designed and properly manufactured trailer will serve your travelling needs well and will retain the value much longer than a motorized RV. The engine of a motorized RV costs as much or more than your family car, but it can only be used for a single purpose, RV travelling. In our previous searches for our own travel trailer, I’ve looked at some of the high end travel trailers with Aluminum design. While we were smitten with the retro good looks, we had wished that the design is more updated with modern conveniences such as slide-outs and residential refrigerator. But enough of that, let’s get back to building the frame because it’s more fun.


I built the second layer of the frame then clamped it onto the main frame. This is the tricky part. It was really hard to properly keep the corners squared in three dimensions. Especially when you started to weld, the heat from the welding caused the frame to warp slightly despite the fact that the entire second layer was tacked welded completely to prevent such warping. I had to weld at alternate ends so that the metals would have time to cool off. A whole lot of clamps were used. You can’t have too many clamps!


After the second layer of the frame has been welded together. This now became the floor for the slide-out to reside on. The next step is to wire up the trailer in accordance with federal and state laws then taking the frame out for numerous test drives. You really do not want to proceed further until the frame has proven itself to be reliable and correctly built. There is another purpose for pausing here as well: I need a general purpose utility trailer in order to purchase more materials. I am tired of paying for “Lift gate” services for home deliveries. They were extremely expensive. For example, to carry a certain component from the side walk of my house into the garage (a whole distance of about 40 feet), a certain well known carrier has charged me for $ 300 USD for it!

Trailer Wiring to Meet Federal Mandated Safe Operation

Let’s start with what we know about our trailer: The GVWR is 7,300 Lbs. and the width is 98 inches. Looking at the Federal Motor Carrier Safety Administration (FMCSA) requirements, two parts apply:

FMCSA Part 393.43 Breakaway and emergency braking:

§ 393.43: Breakaway and emergency braking.

(a) Towing vehicle protection system. Every motor vehicle, if used to tow a trailer equipped with brakes, shall be equipped with a means for providing that in the case of a breakaway of the trailer, the service brakes on the towing vehicle will be capable of stopping the towing vehicle. For air braked towing units, the tractor protection valve or similar device shall operate automatically when the air pressure on the towing vehicle is between 138 kPa and 310 kPa (20 psi and 45 psi).

FMCSA Part 393.11 Lamps and Reflective Devices:

§ 393.11: Lamps and reflective devices.

(a)(1) Lamps and reflex reflectors. Table 1 specifies the requirements for lamps, reflective devices and associated equipment by the type of commercial motor vehicle. The diagrams in this section illustrate the position of the lamps, reflective devices and associated equipment specified in Table 1. All commercial motor vehicles manufactured on or after December 25, 1968, must, at a minimum, meet the applicable requirements of 49 CFR 571.108 (FMVSS No. 108) in effect at the time of manufacture of the vehicle. Commercial motor vehicles manufactured before December 25, 1968, must, at a minimum, meet the requirements of subpart B of part 393 in effect at the time of manufacture.

Given the above requirements, here are what I’ve decided to do:

  1. The trailer will have 7-way wiring scheme
  2. All four wheels of both axles will have electric brakes.
  3. There will be a Tekonsha Prodigy RF brake controller mounted right on the tongue. I like the design of the Tekonsha Prodigy RF a lot. It allows me to carry just a small RF controller module in the tow vehicle.
  4. There will be a Hopkins Break-away Engager to satisfy the FMCSA Part 393.43 Breakaway and emergency braking

Here’s what the wiring plan looks like:

Trailer Wiring Plan

These are the components that I’ve used for this project. Each picture is a property of the respective owner(s) which I have captured and embedded into the pictures themselves.

Trailer Lighting Wiring for FMCSA  § 393.11: Lamps and reflective devices

Part § 393.11 calls out Federal Motor Vehicle Safety Standards 49 CFR 571.108 (commonly known as FMVSS No. 108). The Federal Motor Vehicle Safety Standards 49 CFR 571.108 lighting requirements for trailers can be found here:

You can cut through a lot of jumbo mumbo if you just look at these two Figures from the FMCSA Part Part § 393.11:

FMCSA Figure 8 through 18.gif

In the Figures 8 through 18, additional equipment for trailers exceeding 2.032 m (80 in.) applies. So in addition to the Areas 1 – 4a and 4b, we will need lighting for areas 6, 7 and 8.

FMCSA Figure 14-15

Figure 15 shows the locations of applicable lights and reflective devices to our trailer. The wiring took about a couple of mornings. The tedious part of the job was cutting and stripping the wires then bundling them up into nice and neat harnesses. All those years of designing wire harnesses for airplanes have ingrained in my brain regarding wire chaffing due to vibrations. You simply can not stop chaffing from vibration. You minimize their impacts over the life of your vehicle is the objective here. I used split wire looms on all wire harnesses. I’ve also welded metal wire clips along the trailer chassis where the wire harnesses will travel then secure the wire harnesses to them. So the approach is pretty straight forward:

  1. Bundle all wires into harnesses depends on their travel paths. Multiple wire stiffens the overall wire harness
  2. Keep all distances shortest possible in order to minimize voltage drop
  3. Don’t mix high voltage and low voltage wires into the same bundle. Luckily they were all 12 VDC wires
  4. Use correct wire gauges for the expected maximum current loads
  5. Use black split looms or similar wire sleeves to protect the harnesses from chaffing
  6. Weld metal clips along wire harness travels to hold them in place

Now if you have gotten this far, you will either appreciate the geeky things that engineers will do or you will have turned glassy eyes. So, how about some nice pictures of Yellowstone scenery to make up for it?

When all wiring is done, I took the trailer frame to DMV for a temporary license. By doing so, I am also committing to finish the trailer for the final inspection in three months. In the meantime, I can freely use the trailer as a utility trailer.


Here’s the trailer frame with the “utility box” in the back.


Here I am at the local metal store. I’ve used the trailer to carry a couple of thousand lbs. of metal. So, the frame has gotten a thorough checkout. I was quite satisfied with the load carrying capacity and the tire installation. There was no premature worn out, a sure telltale of improper axel alignment issue.

Thank you for staying with me through this bone dry post. If you think reading about it is bad, try to actually strip the wires and hook them all up. It was not my most favorite thing to do, but it had to be done. Next post, building the rest of the frame and the roof.


One thought on “Building the Frame – Part II”

  1. FYI, your GVWR can be more than what your axles dictate. For example, my factory built TT has a GVWR of 10,000. Guess what my axle ratings are? 4300# each. The factory allows for some of the weight to be carried by the tongue, so they stick the lightest weight axles under it they can.

    However, I do agree with having way more axle than needed. My old trailer also had a GVWR of 10,000, but it actually had axles rated at 5200# each. The difference in ride between my old TT and my current TT are significantly different.

    Keep updating your thread on when you have blog updates, and I’ll keep checking in on the project. Coming along nicely!


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