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Cobra Trailer Spare Tire mount failure



 
 
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  #21  
Old July 18th 15, 04:32 PM posted to rec.aviation.soaring
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Default 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
  #22  
Old July 18th 15, 04:58 PM posted to rec.aviation.soaring
BobW
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Default Cobra Trailer Spare Tire mount failure

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.
  #23  
Old July 19th 15, 01:48 AM posted to rec.aviation.soaring
Mark628CA
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Default Cobra Trailer Spare Tire mount failure

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.
  #24  
Old July 21st 15, 07:11 PM posted to rec.aviation.soaring
2G
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Default Cobra Trailer Spare Tire mount failure

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
  #25  
Old July 22nd 15, 12:17 AM posted to rec.aviation.soaring
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Default Cobra Trailer Spare Tire mount failure

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

Mark

  #26  
Old July 22nd 15, 01:54 AM posted to rec.aviation.soaring
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Default Cobra Trailer Spare Tire mount failure

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
  #27  
Old July 22nd 15, 02:57 AM posted to rec.aviation.soaring
Papa3[_2_]
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Default Cobra Trailer Spare Tire mount failure

I did the under-side mod this weekend as well. Made up a backing plate to help spread the load on the inside of the trailer. I mounted mine just behind the axle to avoid interfering with the surge brake mechanism. 4 grade 8 bolts poking down through the floor locked in place with jam nuts and fender washers on the bottom. Rim goes through the lug holes and held in place with 4 more nylock nuts with washers. Bought an aftermarket spare tire cover to keep everything from getting too messy - we'll see how/if that hold up. While I don't love the idea of crawling under the trailer, I figure that the flat tire can temporarily sit in the front of the trailer until we get to a convenient location to repair and put everything back where it belongs. In 25 years of trailering gliders I've only had 2 flats while on the road (maybe I've been lucky?), so hopefully it's a rare event.
  #28  
Old July 22nd 15, 11:48 PM posted to rec.aviation.soaring
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Default Cobra Trailer Spare Tire mount failure

Just to close the [recent] loop, I mounted my spare flat on the floor under the nose of the glider. BTW, I was able to "reuse" the hole in the trailer floor where a bolt securing a bracket under the floor was already secured. The bracket is a simple strut that supports the brake rod about half way back from the coupler to the axle and keeps it from flapping around. I noticed that the bracket had been bent back by a previous collision with something under the trailer, not seriously but enough that it would have given me pause to locate the spare tire there. There's not much load on the bracket or the spare tire.

I have 13" wheels but went up one size in tire the last time I bought them to get more ground clearance (don't recall the size). As a result, the tire is JUST small enough to lie flat between the wings and not interfere with the spar dollies rolling in and out. Conceptually, the factory mounting the tire above the nose cone gives a lot more flexibility because there's more width there (due to the wing root shape) and the dollies are irrelevant. But obviously the load on the mount is a lot higher.

Chip Bearden
ASW 24 "JB"
U.S.A.
  #29  
Old July 22nd 15, 11:49 PM posted to rec.aviation.soaring
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Default Cobra Trailer Spare Tire mount failure

And, yes, getting at the spare will require pulling out the fuselage on the side of the road. But that's a reasonable price to pay for me even though I've had more than a few trailer flats over the years.

Chip Bearden
ASW 24 "JB"
U.S.A.
  #30  
Old July 23rd 15, 12:52 AM posted to rec.aviation.soaring
6PK
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Default Cobra Trailer Spare Tire mount failure

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.
 




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