Cobra Trailer Spare Tire mount failure

Thanks,

Did you do a flat plate or anything inside to reinforce instead of metal pieces like in on the harbor freight design?

Kevin

On 7/18/2015 9:15 AM, Mark628CA wrote:

It may be obvious to most people, but do
not drill through the trailer side without taking your wing out first.

Ha ha ha! Years ago I was drilling out rivets on both sides of an HP-14
aileron rib. All went as planned until on the second side near the trailing
edge. When the drill broke through it happened to be perfectly aligned with
the rivet hole on the (nearby) opposite side, a possibility I hadn't
considered. I still have the scar in the palm of my other hand from the chunk
of muscle and tissue that was attached to the drill bit upon extraction. At
least the sheet metal holes ended up round and not oversize...

Bob W.

Kevin-

I didn't reinforce the inside, as the extruded aluminum sidewall of the trailer is pretty stiff and the round head bolts have a pretty large bearing surface, but then the design I used is more like the second example from Etrailer.com. It spreads the load pretty well from top to bottom. I think even the Harbor Freight model would work, but a lot depends on how heavy the spare is and whether it sticks way out, increasing the lever arm. Rough roads will also exaggerate the load, which is why I would prefer the Etrailer model.

The problem with this design has 2 major problems:

1. The stress generated by the motion of the spare fore and aft is directly transmitted to these 2 8mm bolts.

2. The 8mm bolts are welded to the mount. Welding changes the metallurgical nature of the bolts, embrittling them at the weld (https://en.wikipedia.org/wiki/Hydrogen_embrittlement).

Your fix solves the first problem by adding a 3rd mounting point, but does nothing about the 2nd problem. At least if the weld breaks again you have redundancy. I would prefer to see a flat plate welded to the upright pieces (wand gussets for more lateral strength) with the bolts attached by lock nuts (no welds). Then you know you will have their full strength. A loose tire banging around the inside of the trailer can do an incredible amount of damage, so a design overkill is clearly warranted.

My Cobra trailer has this same inadequate spare tire mount, but it is not used. Instead, the tire is sitting on the floor upright, secured to the storage bin by a single large threaded rod.

Tom

Good suggestions all around. Has anyone tried mounting the spare on top of the trailer?

Mark

On Tuesday, July 21, 2015 at 7:17:16 PM UTC-4, wrote:
Good suggestions all around. Has anyone tried mounting the spare on top of the trailer?

Mark

How about putting the spare in the storage box under the trailer, will it fit?
Glen

On Tuesday, July 21, 2015 at 5:57:59 PM UTC-7, wrote:
On Tuesday, July 21, 2015 at 7:17:16 PM UTC-4, wrote:
Good suggestions all around. Has anyone tried mounting the spare on top of the trailer?

Mark

How about putting the spare in the storage box under the trailer, will it fit?
Glen

That is exactly where I keep mine. With the jack and all the other tools needed.
P.S. I find this box cumbersome for regular use. The rollers tend to not work too well after a while, but is a perfect place for the spare.