Cobra Top Hinge Bolt Failure - Revisit

[Whiskey Delta]

Any further information or new developments regarding the bolt failure
discussed last year? Discussion found here:
http://groups.google.com/group/rec.aviation.soaring/browse_thread/thread/5ab46ee572ef6bb6

Thanks,

WD

[Steve Koerner]

WD: The writeup I did on my solution seems to have passed muster at
the factory as I have learned that some folks have been referred to it
by Cobra. Here's the link again: http://www.box.net/shared/9dvnt7qgs8

Steve Koerner
www.wingrigger.com

[Andrzej Kobus]

Are you making these backing plates for sale or is the factory going
to create a modification kit, asking just in case?

[Dave Nadler]

The backing plate does not really help, as there are reinforcement
inserts in the top where the bolts go through, but not in the broader
area under the handle.

Distortion of the corners of the trailer is a separate problem from
the bolts. I sketched a solution that Alfred said he would build;
this reminded me I hadn't received the parts and I just sent Alfred
and Ralf an email. I've been a wee bit busy, sorry this hasn't
gotten completely sorted yet...

Hope this helps,
Best Regards, Dave "YO electric"

[Steve Koerner]

On Apr 23, 7:52 pm, Dave Nadler <d...@nadler.com> wrote:
> On Monday, April 23, 2012 2:09:27 PM UTC-4, Steve Koerner wrote:
> > WD:  The writeup I did on my solution seems to have passed muster at
> > the factory as I have learned that some folks have been referred to it
> > by Cobra.  Here's the link again:http://www.box.net/shared/9dvnt7qgs8
>
> > Steve Koerner
> >www.wingrigger.com
>
> The backing plate does not really help, as there are reinforcement
> inserts in the top where the bolts go through, but not in the broader
> area under the handle.

So a backing plate that distributes the load over an area that is
around ten times greater than the area under the original handle does
not really help. That's a pretty interesting conclusion, Dave. I'm
glad you have this all sorted out for them now.

[Dave Nadler]

The area where the bolts goes through has a solid material between
the inner and outer laminates; other than that its just the expanding
core material they use (no compressive strength). Spindelberger
told me it does not help to add a backing plate because of this...

Hope that is clear,
Best Regards, Dave

[Andy]

On Apr 24, 4:25 am, Dave Nadler <d...@nadler.com> wrote:

> The area where the bolts goes through has a solid material between
> the inner and outer laminates; other than that its just the expanding
> core material they use (no compressive strength). Spindelberger
> told me it does not help to add a backing plate because of this...
>
> Hope that is clear,
> Best Regards, Dave

Dave,

My trailer has not such reinforcing inserts. In fact I proposed
introducing such inserts as the permanent fix. Only the handles
themselves have hard inserts.

If there were inserts the outer shell would not have collapsed toward
the inner shell causing the bolt tension to be eliminated. I have
photos of the severe deformation of the outer shell under the handles
of my trailer.

In my opinion the insert fix would be far superior to the backing
plate solution but it fell on stony ground.

Andy

[Steve Koerner]

The structure immediately under the handles had clearly collapsed.
There is no such solid material between the inner and outer laminate
on mine.

The backing plate has resolved the problem quite well.

[payn...@gmail.com]

I've looked at my hinge bolts a number of times, but can't detect any problems. Are there any symptoms that precede bolt failure, i.e. loose handles?

TIA,

Frank (TA)

[Ian Reekie]

We checked all the Glass fibre top (twin skin) Cobra trailers at our site.
(over 10 older trailers ) None had so far suffered a hinge failure.
The most obvious sign of a potential problem is loose bolts which are
easily tightened. One older trailer had a couple of mm slack that could be
tightened out.
I changed the bolts on mine for longer stronger bolts (12.9 spec) that do
not have the thread section across a load bearing junction.
I noticed a lot of corrosion present on the bolts removed from my 13 year
old trailer stored in a English climate. It might be prudent to replace
them after about 10 years depending on local conditions.
I refitted with a aluminium backing plate for good measure but it probably
is not necessary as all the compression between the twin skins seems to
have taken place.

[Ian Reekie]

At 22:21 25 April 2012, payn...@gmail.com wrote:

[John Sinclair]

At 22:21 25 April 2012, payn...@gmail.com wrote:

Check the hold-down clamps at the aft end of your trailer, If the top latch
doesn't match up with the lower attach bracket like it did when
new.....................that means the top has moved forward and may be
setting up the failure mode with the hinge bolts. I found this condition on
both aluminum and fiberglass tops and then found sheared off rivets in the
hinge bolt bracket.

Another item to remember; avoid opening the trailer when a 10+ knot
cross-wind is blowing. An open top acts like a sail and can place
tremendous loads on the hinge bolts.
JJ

[John Galloway]

As a contributory preventative measure how about doing
something at the aft end to prevent forward movement of the
trailer top during towing? For example, angle the aft trailer-top
clamps at 45 degrees as viewed from the side with the lower
fitment being to the rear.

[JohnDeRosa]

It would seem that this problem is related to using the trailer on the
road. Steve Koerner's link above states "...lonitudinal loads on the
trailer top hinge plate occur as the trailer and its tow vehicle pass
over bumps on the roadway."

This implies that if the trailer sits at a gliderport most or all of
the time, you may never experience this failure at all. Correct?

- John

[Steve Koerner]

John -

Subsequent to that writing, I think I really concluded that the
biggest issue may be the open trailer top in a crosswind as JJ
mentions here and as was discussed in the year ago thread on this
topic. Lots of theories were put forth on the matter.

As JJ points out, the crosswind issue is an avoidable situation
although thermal winds and dust devils can present a surprise at times.

[John Sinclair]

>As a contributory preventative measure how about doing
>something at the aft end to prevent forward movement of the
>trailer top during towing? For example, angle the aft trailer-top
>clamps at 45 degrees as viewed from the side with the lower
>fitment being to the rear.

There are some pretty strong forces at work here. Two struts that are
strong enough to raise the top, are trying to move the top forward when it
is down. Don't think the latches would last long tyring to fight the
struts. If top mated over steel posts in the lower trailer sides at the
rear, it might be strong enough to resist the forward forces. Spindelburger
is always improving his product and I'm betting we will see a fix on his
new product..........If anyone has enough money to buy one! I was told a
new Cobra cost $40K. Can this be right?
JJ

[payn...@gmail.com]

JJ,

According to my quick perusal of the 2011 price list on W&W, a 15-meter aluminum-topped trailer with some options was around $17K (abt 13-14,000 euros).

TA

[dbra...@gmail.com]

Hey Guys, I am noticing signs the top of my fiberglass Cobra is moving forward. I am going to replace the bolts as recommended by many, and may do the backing plate solution as well. The deformation of the fiberglass is worrisome. Can the clamping of the hinge plate and removal of bolts be done with the glider in the trailer? or is it recommended to empty the trailer first?.. for those who have done it.

BTW, I also see what JJ identified and that's the 4 rivets on one side are popped and gone. I wonder if I can realign the frame and get either rivets or small fasteners back in there?
TIA,
Darren

[Dave Nadler]

On Wednesday, May 15, 2013 1:06:10 PM UTC-4, dbra...@gmail.com wrote:
> Hey Guys, I am noticing signs the top of my fiberglass Cobra is moving
> forward. I am going to replace the bolts as recommended by many, and
> may do the backing plate solution as well.

Which will not help with the top migrating forward.

> The deformation of the fiberglass is worrisome.
> Can the clamping of the hinge plate and
> removal of bolts be done with the glider in the trailer?

Yes, but BE CAREFUL. See below.


I drafted a note for Spindelberger but didn't hear back,
need to chase them...

http://www.nadler.com/Antares/Cobra_top_front_corner_reinforcement_01.pdf

Hope that helps,
Best Regards, Dave

PS: The part they made for me won't fit many trailers;
there are different extrusions used at the lower edge
of the top.

[dbra...@gmail.com]

Wow, that seems like a good solution for me. Hmmm. I called one dealer yesterday and they did not mention this so I doubt they are aware of it. Let us know what you find. Anybody else able to get this part?
Thanks,
Darren

[Dave Nadler]

Um, I designed it with Alfred, its not yet an official product.
I'll try get in touch with them next week and see if they will
make these generally available. Key is the ability to jack the
front back into position and hold it there...

You could weld up your own, not at all high-tech here !

[dbra...@gmail.com]

Ah, I see, well its a good idea. Was just thinking how I could possibly fabricate something similar. Mine is not nearly as bad as the one shown in blue, but I could stand to move it 1/4".

[dbra...@gmail.com]

On Wednesday, May 15, 2013 4:50:45 PM UTC-7, dbra...@gmail.com wrote:
> Ah, I see, well its a good idea. Was just thinking how I could possibly fabricate something similar. Mine is not nearly as bad as the one shown in blue, but I could stand to move it 1/4".

Dave, on my trailer the hinge plate is at an angle w.r.t the side rails. How did you deal with that in terms of jack bolt alignment? Is the jack bolt in your design perpendicular to the hinge plate?

Thanks,

PS. I am glad I am looking at this because my hinge plate bolts, while not broken, were certainly bent. and corroded. :o

[dbra...@gmail.com]

This solution is close but not quite complete. I've studied this a fair bit and the side rails of the cobra trailer, at least mine is not quite as stout as one would think. You can actually flex it with just your hand. Its only 1/16th inch material and unclear how well mounted to the lid. I am concerned about buckling the side rails for trailers with large displacement issues once you start cranking on the jack. If it buckles, you're screwed. Interestingly the trailer top struts are not mounted to the side rails. They actually mount to a cross member that is glassed to the lid. I think something like this cross member should be added to the location of this jacking mechanism to help distribute the load to more than just the rail. This will also be synergistic in taking vertical loads associated with the jack for those that have an angle to deal with (rail w.r.t to the hinge plate of about 5 degrees) instead of torsionally loading the side rail.

[Dave Nadler]

On Wednesday, May 22, 2013 1:09:29 AM UTC-4, dbra...@gmail.com wrote:
> This solution is close but not quite complete...

This solution mirrors current Cobra production, which adds a
block screwed to the side rail and pressing against the hinge
plate. Thus compression loads (not torsional) are carried
just as with new trailers...

The side rails are not enormously stiff and vary depending
on the top (different models have different extrusions).
However, they don't seem to buckle...

[dbra...@gmail.com]

Interesting. Okay, well, mine has both a wimpy side rail and an angle to deal with so I have the worst of all worlds. Correct there is a compressive load component but because of the angle I have vertical component. The resultant is a vector load that is not aligned with the side rail, hence the torsional component I am having to deal with that twists the rail. I have no choice but to add a cross member. What can you do, Spindelberger had a design flaw that exposes itself over time.

[Bill D]

I have to deal with a ~2000 vintage Cobra for a 2-seater. Examination of the top and hinge plates showed no problem - yet. However, the explanations here helped visualize the problem - and, maybe, a potential alternative solution. Basically, I'm thinking of a system that forces the top to close in perfect alignment with the bottom.

If the main problem is a constant forward force on the hinges with the top closed, then why not use "something like" the already present alignment tabs near the front of the trailer to take up the load instead of the hinges? If a top had already moved forward, wedged tabs would force the top back aft as it was closed. All it would take is a pair of ball bearing studs on the insides of the bottom half of the trailer. This way, there would be no horizontal load at all on the hinges. If I'm imagining this right, the ball bearing studs and wedged tabs would be far enough forward the glider wings couldn't contact them.

It should also be possible to implement a wedge and roller on the outside of the trailer back near the lift struts. This would force the latches into alignment while relieving pressure on the hinge plates.

Of course, this wouldn't help with the top open.

[Dave Nadler]

On Wednesday, May 22, 2013 2:21:07 PM UTC-4, Bill D wrote:
> why not use "something like" the already present alignment tabs
> near the front of the trailer to take up the load instead of the
> hinges? If a top had already moved forward, wedged tabs would
> force the top back aft as it was closed. All it would take is
> a pair of ball bearing studs on the insides of the bottom half
> of the trailer. This way, there would be no horizontal load
> at all on the hinges. If I'm imagining this right, the ball
> bearing studs and wedged tabs would be far enough forward the
> glider wings couldn't contact them.
>
> It should also be possible to implement a wedge and roller
> on the outside of the trailer back near the lift struts.
> This would force the latches into alignment while relieving
> pressure on the hinge plates.
>
> Of course, this wouldn't help with the top open.

Wedge/roller behind the lift struts would take the side
load in tension which would be good. Probably could be
done on the inside. Needs a gradual taper otherwise
you won't be able to close the trailer.

I simply created an adjustable version of what
Spindleberger currently builds in new trailers...

[Dave Nadler]

On Wednesday, May 22, 2013 1:09:18 PM UTC-4, dbra...@gmail.com wrote:
> Interesting. Okay, well, mine has both a wimpy side rail and
> an angle to deal with so I have the worst of all worlds.
> Correct there is a compressive load component but because of the
> angle I have vertical component. The resultant is a vector
> load that is not aligned with the side rail, hence the torsio

> component I am having to deal with that twists the rail.

Aarrggg - I see. The bits line up better in my trailer,
consequently only minor torque.

> I have no choice but to add a cross member. What can you do,
> Spindelberger had a design flaw that exposes itself over time.

Right. The best solution would be remove the top and reinforce
the corner - not so easy ! Which is how I arrived at the solution
for my trailer - I could implement this without an overhead hoist.

[dbra...@gmail.com]

Just thought I would report - I was able to fabricate some parts out of aluminum channel and steel channel based on Dave's design and document. Mine was done without welding since I am not a welder, but riveted to the side rail. An additional piece of aluminum sq tubing was added to the shell behind the side rail as piece of mind should the side rail want to twist. I am happy to report that after gradually torquing the jacking bolt, and shifting the top maybe 1/4 inch, the shell closes a lot better. I needed it more on one side than the other and this design helps addresses it well. I highly recommend this approach if you find the closing the trailer top is difficult on your fiberglass Cobra and see clear evidence of the top shifting fwd. It took me one week, couple hours each evening to fabricate the pieces and I'm not a machinist.

Darren

[Dave Nadler]

On Tuesday, June 18, 2013 4:14:30 PM UTC-4, dbra...@gmail.com wrote:
> Just thought I would report - I was able to fabricate some parts out of aluminum channel and steel channel based on Dave's design and document. Mine was done without welding since I am not a welder, but riveted to the side rail. An additional piece of aluminum sq tubing was added to the shell behind the side rail as piece of mind should the side rail want to twist. I am happy to report that after gradually torquing the jacking bolt, and shifting the top maybe 1/4 inch, the shell closes a lot better. I needed it more on one side than the other and this design helps addresses it well. I highly recommend this approach if you find the closing the trailer top is difficult on your fiberglass Cobra and see clear evidence of the top shifting fwd. It took me one week, couple hours each evening to fabricate the pieces and I'm not a machinist.
>
>
>
> Darren

Very Nice ! Send me some pictures, lets get it on a web site somewhere...
Onwards to Hobbs !
See ya, Dave "YO electric"

[Eric Greenwell]

Yes, indeed. I want to see those pictures, too.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm
http://tinyurl.com/yb3xywl

[dbra...@gmail.com]

I'll try to get some pics. One thing to add to the procedure, if it is not already mentioned, after shifting the trailer top, one must re-adjust the wing dolly "keepers" or restraints. You'll want to snug those up again so the wings remain constrained during transport.