So I figured that the spare tire is on the back of the trailer, quite securely -- maybe there is a way to piggyback off the spare somehow?
My first job was to check the bolts mounting the spare, which is on the back wall of the trailer. I discovered the bolts were loose (!), and the mount was supported by little tee nuts inside the trailer, which were all stripped. I think that the original design of this trailer was for light duty, which is not the way we treat this trailer -- we take it off-road (slowly) and on long trips at highway speeds on California's roughest freeways. (I'm lookin' right at you, I-5.)
So I upgraded the bolts mounting the spare -- instead of quarter inch ordinary steel bolts into tee nuts with no washers, they are now three-eighths stainless steel with wide fender washers inside and out, into massive lock nuts, all anchored with quarter inch steel plates on the inside of the trailer, in order to spread the load over as many square inches as possible. Now, instead of having the spare fall off, the whole back wall will give way at once. (Joke.)
This is what the upgraded beefy steel plates look like, on the inside of the back wall:
OK. The spare is secure. Next, how to mount the gas can on top of the spare? The goals are safety, lack of vibration, and ease of use, in that order. Even with the reinforced bolts, the spare still vibrates in response to an impact:
Over time, vibration can lead to materials fatigue and failure. So the gas can mount, ideally, should dampen that vibration.
Here is what I came up with, in summary (with details to follow below): using the bolts from the spare tire mount as an anchor, I extended two half inch threaded rods up to the level of the rim of the tire. I then mounted a horizontal platform on those two vertical rods. A heavy milk crate is bolted to that platform. The gas can goes in the milk crate.
Here is what the finished product looks like:
And now, the component steps. This is the cross-bar anchored to the bolts supporting the spare tire mount -- I drilled holes in the angle bracket with my drill press to fit the bolts, and you can see the holes in the ends that will accept the vertical rods:
Here are the vertical rods rising from the cross-bar:
The platform will slide over those rods. Note that there are pairs of nuts, tightly jammed together with lock washers, that will support one side of the platform.
Here is the spare tire on its mounting bracket, with the rods extending upward:
Here is the platform, mounted on the rods and resting on the top of the spare tire -- notice that on top of the platform there are nuts and washers holding the platform rigidly in place:
Here is a closer shot of the platform:
In that shot, at the very back of the platform, you can see the edge of an angle bracket extending across the back side. The angle bracket is bolted to the platform -- the heads of the bolts are recessed:
This is a shot from underneath the platform, showing the angle bracket:
(By the way, if the components look distinctly un-glamorous, that's because this was all made from spare parts and pieces stashed in the corners of my workshop.)
Once I had the platform assembled, I put a piece of thin plywood inside the crate to sandwich the bottom of the crate, using countersunk wood screws -- the screws go through the crate and into the platform:
This milk crate is very sturdy -- no chance of materials failure. Inside the crate, I built a foam "nest" to hold the gas can securely on all sides, using (of course) scraps of foam, including old purple "pool noodles:"
I knew from past painful experience that if a plastic gas can is subject to chafing for a long time, it will wear through. So no hard surfaces are in contact with the gas can. This is a shot of the gas can in its little nest:
Note, by the way, that the nozzle of the gas can is pointed inward. Obviously, in order to add fuel, I loosen the collar and rotate the nozzle 180 degrees.
This is a side view of the crate and the gas can on top of the tire:
Notice the straps coming down from the crate and around the tire -- that is a ratchet strap. By strapping the crate to the tire and anchoring the crate (on the platform) to the upright rods, the whole assembly vibrates much less than it did with just the tire on the mount. Although it is hard to see in this shot, the straps are twisted to cut down on vibration due to the wind at freeway speeds.
This is a closer shot of the gas can in the crate:
You can see a piece of thick vinyl tubing extending across the crate and through the handle of the gas can, preventing it from bumping up and down while on the road. Also, notice the bungee cord around the perimeter -- it is also designed to reduce the vibration of the ratchet straps.
If we feel the need to lock the gas can in place, that will be easy to do with a combination bike cable lock.
That's it -- a lot of detail for a fairly small project. But the design issues were more interesting than the fabrication process, which was pretty straightforward.