I'm planning a rack for solar panels on the roof of a cargo trailer. I will mount the panels to a pair of 15 series 80/20 aluminum extrusion rails. I will mount the rails to the roof of the trailer using aluminum angles. Here's a view from a Sketchup model giving a rough idea:

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My question revolves around mounting the set of aluminum angles to the trailer's steel roof beams. Here is a rough model showing the relevant parts:

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The aluminum angle is 1/4" 6061-T6 with 2" legs cut to 2" wide. The steel beam in the trailer roof is 1" wide, 1-1/4" tall, and about 1/8" thick. Between the angle and the beam is the thin aluminum skin of the trailer.

The interior of the trailer is finished and prevents any access to the beams from the interior so I can't use through-bolts to attach the angle to the beam.

So my current plan is to drill and tap, from the outside, the top of the beam. Then I can attach the angle to the beam using two bolts that screw into the top of the beam. I plan to use 1/4"-28 bolts. I also plan to use VHB tape between the aluminum angle and the aluminum roof skin.


Finally, the actual question. Since the steel beam is only 1/8" thick, is the choice of using 1/4"-28 bolts strong enough? Or would 1/4"-20 be better? My thought is finer threads gives me a better grip over courser threads.

Side question (that may be off-topic) - is there a better, stronger way to attach the aluminum angle to the steel roof beams than screwing in bolts given that the steel is only 1/8" thick but I can't use through-bolts?

  • 2
    $\begingroup$ Use rivnuts - they can be strong if installed correctly. $\endgroup$ – Solar Mike Feb 17 at 20:34
  • $\begingroup$ Its also possible that there is some collapsable anchor structure that could work out here. Must research later. $\endgroup$ – joojaa Feb 17 at 21:25
  • $\begingroup$ That would be 3.5 threads at nominal thickness, which is marginal. I wouldn't do it with just 2 screws at each connection. Plus it's a vehicle, so plenty of flexing, vibration, wind forces, salt spray, corrosion, and thermal expansions from outdoor service. A forming tap may be something to look at, don't know if it is appropriate for this material... $\endgroup$ – Pete W Feb 18 at 0:04

There are a few alternatives.

You can use blind rivets to attach the pieces together. This has the downside that its not easy to separate the pieces later. But is generrally faster as you can skip threading phase

Another alternative is a blind rivnut (many rivnuts need acces to both sides). This has the advantage of being able to disasemble the structure.

An alternative to a rivnut is a expansion bolt. It can be entirely removed afterwards if need be.

In addition there is a whole category of other properitary onesided single sided structural fasteners.

Or use a friction drill as it makes the structure about 3 times as thick at the drill location. This can then be tapped. Be sure to choose a bit that keeps the drill side flush. Main benefit of this method is that it makes the hole and the thickening in one operation. Also it does not require acess of the opposite side of the hole. But may be cost prohibitive and not exactly easily field installable. Best solution structurally. Not sure what it does to your paintjob though.

Remember to protect the area again to prevent rusting.

  • $\begingroup$ The use of rivnuts will be the best solution for me. I had heard of them before but I never looked into them and realized what their use case is. $\endgroup$ – rmaddy Feb 19 at 17:39

Rivnuts are great, until they aren't. Get one that doesn't have a good grip and spins when you want to remove the bolt and it's all over.

Your idea of using 1/4-28 threads is a good one, especially in steel and especially with the backup of VHB tape. With sufficient VHB tape, you could almost get away with tape only, but the risk factor is high.

One-eighth of 28 is 3.5, which is the number of threads you'll have engaged in the steel using that tap. With a quarter-twenty tap, it's only 2.5, but it is steel and if you don't lay into the wrench and strip the threads.

Thread locker will be good for additional reliability.

  • $\begingroup$ Three full thread contacts should be adequate. But then again there may be considerable windload to account for. $\endgroup$ – joojaa Feb 17 at 21:14
  • $\begingroup$ One can expect windload to be in shear, so the bolts have to take the force in that direction. There will be a lever arm on anything higher than roof level, of course. The quantity of bolts will matter and the VHB tape is helpful as well. $\endgroup$ – fred_dot_u Feb 17 at 21:16
  • $\begingroup$ Well one can expect most of it to be but it can alo pull upward quite effectively. Atleast civil engineers have to account for wind pulling roof upwards in their codes. $\endgroup$ – joojaa Feb 17 at 21:19
  • $\begingroup$ But could be interesting to calculate if this could be done with the tape alone. Would you beed to strip the paint to get best possible grip. $\endgroup$ – joojaa Feb 17 at 21:21
  • $\begingroup$ Good to know my thought of using finer threads was in the right direction. But I really like the idea of using rivnuts since it will give the bolt 3x the number of engaged threads. I just need to make sure they are installed well. $\endgroup$ – rmaddy Feb 19 at 17:41

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