Threaded bolts for rack made of unistrut and tapped into shuttle bus ribs?

chrisstratton

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I'm getting ready to install a roof mounted pv panel rack on my shuttlebus. I recently saw someone (YT) mount brackets by drilling and tapping through the roof into the bus' ribs. I'm considering the same, i.e. to mount 5' segments of strut transversely across the buss, then 10' struts perpendicular top and atop of those. Panels to be mounted using bespoke (hope that's the right word) hardware fittings to the unistrut. Thoughts? I'm a little trepidatious as the last time I shared a roof mount system I learned about and brought here, I was met with sarcasm to the effect that I was causing the insurance industry's abandonment of skoolies...
 

rmaddy

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It's unclear how exactly you are going to mount the Unistrut to the roof. Are you drilling completely through the roof ribs so that you can put a bolt all the way through with a nut on the other side of the rib? That would be the strongest approach if that's what you are going to do. Of course this assumes you have access to the ribs from inside the vehicle.
 

chrisstratton

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On this video, admin at 11:49, you’ll see the guy drilling and tapping.https://youtu.be/IlJ0rzIvhyU
 

rmaddy

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Drilling and tapping is the last option you want to choose. It's only viable if the material you are tapping is thick enough and strong enough to provide sufficient threads. I have no idea what you will be trying to drill and tap so I can't say if it's sufficient.

If you have access to the ribs on the inside of the vehicle then the best/strongest/easiest option is to simply drill completely through the rib from the roof. Then use a bolt long enough that you can add a washer and nut inside the vehicle.

If you can't access the ribs from inside the vehicle and if the material you want to tap is too thin for sufficient threads then you should look at installing rivet nuts (rivnuts). They allow you to get sufficient threads on thin metal.
 

chrisstratton

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look at installing rivet nuts (rivnuts). They allow you to get sufficient threads on thin metal.
That sounds doable and your caution about the 11guage ribbing being too thin is consistent with my referenced article (above).
The interior ceiling is finished and I"d prefer not to pockmark it with bolt heads, so the rivnuts sound good. Anyone else have any thoughts?
 

rmaddy

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There is one challenge using rivnuts in this case. The flange of the rivnut needs to be against the top of the beam. So depending on what other roof material is above the beams you end up having to drill overly large holes in the roof to get the rivnut in place. And the rivnut tool may have trouble reaching depending on how thick the roof materials are.
 

rmaddy

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The rivnut would need to be secured to the beam. So you need to drill a large enough diameter hole in the steel sheath and the plywood for the flange of the rivnut. The 1/4” thickness or so may or may not be an issue for the rivnut tool. You’ll have to look at the tool and see. Maybe make up a mock-up to simulate the situation before you start drilling holes in the roof.
 

corn18

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Rivnuts don't like thick squishy material like wood to grip on. They don't really expand far enough to grip to wood. They do make them long enough for your application, so maybe you can buy some longer ones and see how they work.

I will say that a rivnut tool is one of my favorite tools. I have rivnutted a bunch of stuff to my truck bed and rolling tool chest. And I made my own tilting solar mounts using rivnuts. But it's all metal. Never used them on wood.
 

chrisstratton

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Rivnuts don't like thick squishy material like wood to grip on. They don't really expand far enough to grip to wood. They do make them long enough for your application, so maybe you can buy some longer ones and see how they work.

I will say that a rivnut tool is one of my favorite tools. I have rivnutted a bunch of stuff to my truck bed and rolling tool chest. And I made my own tilting solar mounts using rivnuts. But it's all metal. Never used them on wood.
This is something of a wood sandwich: there’s a steel sheath on the outside and the steel rib on the inside with some plywood in between
 

rmaddy

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Anything else in there such as adhesives, insulation, or even an air gap?

Rivnuts have a “grip” size that determines the thickness of material they can used with. For example you might find a 1/4-20 rivnut in two grip sizes. You might find rivnuts that can handle the full thickness you are dealing with. I’m just not sure how well a rivnut can handle being put on such a “sandwich”. If the wood crushes or, over time, loses integrity the rivnut could work loose. Putting the rivnut directly on only the beam likely had the best final result but it will be harder to install.
 

jasonhc73

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Is drilling and tapping thru roof into ribs of bus a secure way to mount pc panels.
I did it on my cargo trailer build, they are holding quite fine. I have driven with them at 60 mph with zero issues.

I used bolts and fender washers on the inside to spread the squish. I don't think I would chance it by tapping the hole and using the tapped hole for the thread.
 

chrisstratton

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This is something of a wood sandwich: there’s a steel sheets on the outside and the steel rim on the inside with some plywood in between
Anything else in there such as adhesives, insulation, or even an air gap


Other areas are very tight. Perhaps some adhesive I don’t think there’s any airgap. Hope is that if the wood never got wet it probably would never degrade.
 
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