Choosing The Best Trailer Frame Material
What materials should I use for building my trailer frame? It’s a common question with strong biases for Rectangular Tube or I-Beam or C-Channel. Also, choices of Aluminum versus Steel. So, which trailer frame material geometry is best? In this article we’ll give some insight for function, but ultimately, the decision is yours. The needs of your circumstances should drive the picks.
While it great to talk about Aluminum vs Steel, we’ll refer that back to previous articles. Aluminum vs Steel Part 1, then the following Comparison Part 2, are both good reading with a lot of detail. Also, reading the Post on Beam Shape is great background to help in selecting trailer frame material. Honestly, the beam shape article and this one began as one article that got too long. You can think of this article as the application portion.
The intent here is to give insight on beam types as you plan your next trailer.
Review of Beam Loading
In general, there are three types of beam loading: Bending, Sheer and Torque. To understand them, here are some quick illustrations.
We won’t go into detail with these except to say that elements of all are present in a trailer frame. Bending is the big one we think about for main beams running front to back on the trailer frame. Also for cross members going side to side under the cargo.
Examples of Shear Forces include areas where the suspension attaches, or where the tongue intersects the front cross member.
While Torque loads are not as prevalent in most trailer frames, the effect is still there in some degree. Twisting often is the result of something attached to a frame member and hanging off to the side. (A side mount tongue jack for instance.) Or when the loading is bias to one corner of the bed.
To do the calculations, there are free resources online. One is at engineeringtoolbox.com. It’s good information with explanations and links, and it has free calculators, but the site is pretty cluttered.
Needs When Choosing Trailer Frame Material
Different portions of the trailer frame have different needs, and that obviously drives part of the decision about what material to use. For illustrative purposes, let’s think about a typical frame and describe the beams as well as their needs. This illustration will work for the discussion, though keep in mind that every frame is a little different.
We will refer back to the beam shapes described in the previous post for most of the below discussion. If you’re not familiar, that article is helpful.
We’ll call the beams that typically run the length of the trailer bed — front to back — the main beams. They are usually the longest beams on the trailer and hold (effectively) all the weight. Their need for strength is high, then depending on length and trailer type, stiffness comes into play. Look in the beam shapes chart for those with good vertical strength per weight.
Incidentally, these are often the heaviest pieces in the trailer frame material, but they are not usually the biggest contributor to weight.
Another common requirement of main beams is the connection with other systems. They (usually) connect the other load carrying elements — like the cross members, the tongue, and the axle(s).
The prevalent load for main beams is bending, which is illustrated in this colorful analysis image. The distribution of color shows the highest stress is on the very top and bottom of the beam. See the arrows. (Which is why we don’t recommend drilling or welding across the beam on top or on bottom. Also, see There’s More To It.)
For shorter-ish trailers, C-Channel and Rectangular Tube are common. Some considerations are ease of connection with other beams, interface with the deck, and corrosion. Some people don’t like tube because you can’t paint the inside. That is a consideration, but it’s also pretty easy to cap the ends. Some people like Channel because you can bolt directly through it without squishing it — unlike tube. Either can be a good choice.
For longer trailers, I-Beam becomes the shape of choice — particularly tall, narrow flange beams.
Cross members are not as long as main beams, nor do they each carry the full cargo load. Typically, cross members support the trailer decking floor and share the load carrying duties with the other cross members.
Typically open section beams like L-Angle or C-Channel work well for these because they are easier to bolt through. Simply drill a hole through one wall, then use a nut and bolt. When mounting the decking with bolts, these shapes work well.
Consider the trailer construction, and what loads it will carry. For something like a tractor hauler, the load is applied to the deck in just a few locations, so each cross member must be stronger for this style. However, when hauling rocks or boxes the load spreads much more, and each cross member shares the weight.
Front and Back Cross Members
These might also be called the first and last cross members, or the back one called the bumper (depending). Anyway, the cross members connecting the perimeter of the bed usually carry greater load, and therefore, have significance. The front member transfers a lot of the bed load to the tongue. The back member often acts as a bumper, but also carries the load of nearly everything that loads into or onto the trailer. Ramps and other loading rely on the back cross member.
For the reasons above, these two special cross members are usually stronger than the others. Like the main beams, C-Channel or Rectangular tube are good for this trailer frame material. One nice benefit of C-Channel in the back . . . you can set the tail lights into the channel for protection.
These front and back cross members are a good place to angle brace or gusset as well.
Cross Member Extensions (Outriggers)
With many trailers the left and right perimeter of the bed are the trailer frame main beams. On others, there are cross member extensions, or outriggers, that extend the bed on each side in front and behind the wheels.
The labeled figure (near the article beginning) illustrates a trailer frame that extends left and right beyond the main beams. It also shows the frame going up and over the wheels (not so obvious in the image). Material selection of all of these areas is important.
The photo here shows typical outriggers — as fabricated brackets that are not a standard beam type. These are an example, but this approach is just one possibility.
While beams connecting the coupler to the trailer may seem less important, tongues are usually high stress. The choice of trailer frame material in this area is critical. In particular, the area where the tongue beam(s) leave the front cross member. At this point, the loads of the main beams transition to the tongue beams. Additionally, the tongue often carries obnoxious loads like jack knifing or offset jack loads (torque). For the tongue, the ability to carry some twisting loads is important.
Rectangular tube and Channel are the frequent choices for Tongue beams. Tube carries torque loads better, but with some cross bracing C-Channel also works very well. Depending on the design of your trailer, if a single center tongue beam is used, angle braces out to the main beams are helpful. The angle beams don’t carry as much load so making them of L-Angle or C-Channel is often great. Again, all of this depends on the capacity of the trailer and the expected loads.
Other Frame Areas
Trailer frame material includes a lot of other pieces that depend on the construction and purpose. Perhaps perimeter rails, sides, tailgates, gussets and diagonal bracing. We won’t go into detail on these here, because there is a separate article about strengthening a trailer frame.
Like the outriggers above, trailer frame material includes all of these extras. It also includes brackets and other fabrication items that are not really beams. Selecting these for the application is just as important as selecting the main beams.
After the above thoughts on material type for various areas, some may think about a different beam type at every location. For DIY projects particularly, we suggest finding some commonality. For instance: Can you use the same trailer frame material for the main frame and for the front and back cross members?
Material comes in standard lengths. So, when selecting the type, make choices that best consume what you buy. This reduces scrap, and makes the project more efficient — especially for smaller parts like gussets and outriggers. It might be a little overkill in one area or another, but commonizing material will reduce the cost. For example, let’s say L-Angle is the choice for cross members. The gussets may not need to be that strong, but using scrap from the cross members might save you from purchasing another full stick of something. And, it won’t change the total weight very much.
Importance Of Beam Type
Compare the strength and stiffness characteristics of Rectangular Tube v. I-Beam v. C-Channel v. L-Angle as you make choices for trailer frame members. Main beams and Cross-members have similar, but different roles on a trailer.
Beam choices matter. Proper selection can be the difference between a solid feeling trailer and one that feels (and probably is) wimpy.
Over-built trailers are certainly stiff and capable, but heavier perhaps than necessary.
Under-built trailers tend to flex and twist and bend. They are lighter, but not up for a lot of jobs. They can also be dangerous.
Which is better? When selecting trailer frame material, it’s better to over-design and over-build than to have something dangerous on the highway. Choose your beam type based on needs and hopefully some engineering equations. Also, make sure you include appropriate safety factors.
Good luck with your build project.