Cobra Trailer Square Tongue bolt breakage fix

[Scott Fletcher]

See attached.  I'm retired now, so I spent more time on this than necessary, that's what we retired guys do. Well that and the fact that I still have a license to practice engineering,  and while I like helping people, you have to anticipate that the lawyers might just show up.

If you have a square tongue Cobra Trailer, I would advise doing this modification, or some thing else that accomplishes the same mission.  This isn't the only possible way of doing this.  Failure of the original  bolts isn't an if question, it's a when question.  At a minimum change out the existing (2) bolts with longer shanked ones, and inspect them for wear annually.  With the three bolts properly installed, the annual inspection is probably not needed.

If you don't have experience drilling through thick steel, don't be afraid to engage someone who is, to drill that third bolt hole for you.  Breaking that first small drill bit off inside the tube would turn minutes of work into days of work pretty quick.  That didn't happen to me, but I worried about it the entire time.   Taking off the emergency brake handle and using a mag drill would be better than the way I did it.  

I would like to thank UH for nudging me to look at those bolts.  I had one headed towards failure, and there are no external indications of a problem.

SF

[Craig Funston]

Scott,

Thanks for the post. I’d be more inclined to ream the holes slightly and replace the bolts with ASTM A325 structural bolts + ASTM A563 DH nuts. They’re both available in hot dip galvanized through Portland Bolt. 

On Nov 18, 2025, at 9:24 AM, Scott Fletcher <211fl...@gmail.com> wrote:



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<Cobra Trailer Tongue bolt fix.pdf>

[Erik Mann]

It's posts like this that restore my faith in the value of user groups.   Thanks Scott!! 

[Ryszard Krolikowski]

What am I'm missing?

Aren't those 2 bolts working as shear pins?

I see Al Ko bolts are too short making threaded part working as a pin.

Both of you suggested smaller dia 1/2 inch bolts(12.7 mm). 

Wouldn't pin last longer if it's tight fit, almost pressed ,(read : hammered) without scratching, to keep matching surface chemistry so no future corrosion?

Also I don't understand torque.

Scott suggest, without installing internal sleeves , torquing new bolts to solid metal torque specs?

Is your square pipe with high bolt torque will have same torque next time you check?

Isn't collapsing to inside wall of the pipe, by torquing your bolt, determinating the hardest question for collapsing square pipe: which way we brake?

Is drilling extra hole in the boom for attaching non structural accessories will make your boom stronger?

Lock nut yes if you understand, they are one time use, but why lock tight?

Isn't that we have to choose here and not double it?

Last question: how often you plan to remove these bolts and using neverseize?

Ryszard

To view this discussion visit https://groups.google.com/d/msgid/rasprime/A31851D7-5B4E-469C-8DAF-EE00412501B8%40gmail.com.

[Scott Fletcher]

Craig 1/2" A325 Bolts have a clamping load of 12,000-14,000 LBS,  (only slightly above the M12 Class 10.9's 11,792lbs)

 so those would work

If, You add a third 1/2" bolt.

The existing bolts are located where most of the bolt tension is used up bending metal prior to the start of clamping

Especially the front one, where there are some added structural elements the bolt is fighting.

Without the third bolt's leverage to get metal to metal contact started, 1/2" bolts won't get it done.

With the original two bolts installed, I over torqued the front one down to see what it took to start clamping.

I was tossing that one  anyway so I didn't care if I streched or broke it.

17,626 Lbs is what it took to start clamping, so a 1/2" bolt isn't going to get you where you want to be, if you want to stick with two bolts.

I left that out of the write up so no one would consider over torquing as a possible solution

The Al-Ko  Formed C-section fits too loose on the square tube,

The square tube has some sleeving inside it to prevent collapsing the side wall with the bolts, so it's good to go.

Formed (thick) metal tolerances aren't all that great without a secondary machining process which didn't happen here.

That's where this party got started

[Scott Fletcher]

Ryszard

Q: What am I'm missing?

Aren't those 2 bolts working as shear pins?

A: No they are not shear pins, not precision holes, the drilled hole - bolt clearance  gives the pieces room to pivot on one of the bolts, and slam into the other one.

Q: I see Al Ko bolts are too short making threaded part working as a pin.

A: The threaded end just makes it easier to damage via the shearing action, it also reduces the bolts shear strength.

But the bolts are not failing in the shear from pulling the trailer they are failing from the impact shear.  The impact shear will eventually fail a bolt with shank all the way through.

Q: Both of you suggested smaller dia 1/2 inch bolts(12.7 mm).

A: I didn't say that, (3) M12 bolts are fine, and the additional clamping force from switching to (2) 1/2" bolts is insufficient to the task required because of the geometry of the parts.

If the formed C-channel fit the Tube better the 12MM bolts have plenty of shear strength, and if they didn't use up most of their available clamping force bending the sides of the C-channel back down to the tube, they would also have plenty of clamping force.

Q: Wouldn't pin last longer if it's tight fit, almost pressed ,(read : hammered) without scratching, to keep matching surface chemistry so no future corrosion?

A: Sure, take the square tube and the hitch assembly off of the trailer, fixture it in a drill press, drill a hole, ream it to size, move it to a press and press in the pins

through.   I only like over complicated solutions when I not the one doing the work.  

Q: Also I don't understand torque.

Scott suggest, without installing internal sleeves , torquing new bolts to solid metal torque specs?

Is your square pipe with high bolt torque will have same torque next time you check?

Isn't collapsing to inside wall of the pipe, by torquing your bolt, determinating the hardest question for collapsing square pipe: which way we brake?

A: The existing holes through the square tube already have sleeves inside the tube to prevent collapsing the side wall of the tube with the bolts.

Cobra got that part right, since they are roughly on the centerline of the tube.  The bolt I added is close enough to the bottom of the tube that it's not going to collapse the sidewall.  

There is a lot of bearing surface there between the two parts not enough PSI to collapse the tube.    

Q: Is drilling extra hole in the boom for attaching non structural accessories will make your boom stronger?

A: I would consider the lose of the tube's strength due to a 13mm hole through the tube negligible

and by clamping the C-section to it properly it is now acting as a doubbler and the total strength in that area has increased as a result. 

Q:Lock nut yes if you understand, they are one time use, but why lock tight?

Isn't that we have to choose here and not double it?

A: With two bolts in the original location, 1/2" or M12, I don't think there is enough clamping force there to prevent movement

so the bolts need to be inspected.  One time use nylon insert lock nuts, or locktight, you choice either would work. 

Q: Last question: how often you plan to remove these bolts and using neverseize?

A: with three bolts I'm never planning to take them off. With the third bolt bending the formed channel down to the sides of the tube

the first two bolts can do their intended job.  I didn't measure them but I would estimate the the two 90's on the formed channel are more like 93's

with the square tube under it having 90 Deg. corners. 

I used The Locktite Anti-Seize primarily as a bolt lubricant, to get consistent bolt tension and clamping force.

I'm uncertain as to the bolt lubrication properties of locktight, and the timing of when it switches from fluid to glue.

And even though I never plan on taking these bolts out, the minute I glue something together, I find a reason to 

take it apart.   There's also Pat Fults, he was the grouchy old millwright that worked for me for 25 years.

He put never-seize on every single bolt.  His shit never came loose unless you made it come loose.

He ruined more of my shirts with that stuff than I want to talk about.  He also was able to out cuss 

any other three men I ever worked with.

The problem here is the designers CAD system had 90 degree corners on the square tube, and 90 Degree bends on the C-Channel that fits over the top of it.

It's possible and more than likely that the square tube is sufficently close to 90 degrees.  The formed C-channel, probably not going to have perfect 90 Degree bends

especially after the dies have a little wear on them.  With the lag time between shipment, and failure, a lot of stuff goes out the door before the designer 

realizes that life isn't as perfect as their cad system says it is, and makes adjustments.  I would make the third bolt on the bottom of the channel legs into a design revision, once i knew I needed to do it.

Good questions

SF

[Craig Funston]

Hi Scott,

I think the solution you came up with is something that can be accomplished by the majority of the gliding community. Thanks for putting that together. 

I wasn't suggesting reducing to two bolts. My thoughts around going to A325 are that they are available HDG which will help with long term corrosion. They also have longer shanks with reduced thread length. The 1/2" bolt is slightly larger than the 12mm they would replace which offers some latitude to ream to a closer tolerance hole.

Getting tolerances on bent parts to match the rolled tubes is always going to be problematic and applying more tension on more bolts is a fairly simple way to get closer to a slip-critical connection. Shimming is another option but takes some skill and patience to get everything right. The two bushings through the HSS allow for a true slip-critical bolted connection and the addition of the third bolt provides additional grunt to get the legs of the bracket closer to the tube. Not a code compliant SC bolted connection, but close enough to get the strength necessary and way better than the original connection.

Thanks again for your work on this and sharing your results.

Best regards,

Craig

[Scott Gross]

Hi,

I put two vertical bolts in to the clamp the two parts together, so they won’t rock.  I also putting new longer 1/2 bolt in horizontally. This has worked well on several trailers.

Scott 

Sent from my iPhone

On Nov 18, 2025, at 6:21 PM, Craig Funston <nimbu...@gmail.com> wrote:

Hi Scott,

To view this discussion visit https://groups.google.com/d/msgid/rasprime/4cf5d849-af28-4f27-8d5c-1466bb346ba8n%40googlegroups.com.

[Mark Mocho]

I agree with Scott Gross. A vertical bolt through the "saddle" and the internal tube (preferably with a sleeve to prevent squeezing the square tube) will prevent the "rocking" that is probably causing most of the shear damage to the existing lateral bolts. You will have to remove the saddle to install an internal sleeve, but you are already removing the bolts so it shouldn't be too difficult to move the saddle out of the way enough to get the sleeve into the square tube. I have disassembled several AL-Ko hitches (both square and round tube styles) so it isn't a big deal for me. Other than that, thanks for the parts list- I already have ordered the nuts, bolts washers and spacers from McMaster-Carr.

[Scott Fletcher]

No design survives contact with the customer.  Some concerns were raised about the original scheme using metric fasteners,and putting another hole in the tube.  I know the original scheme will work, this one should work just as well, and it addresses the following customer concerns. 

1.      I don’t want to drill another hole through the square tube.

2.      I want to use zinc plated fasteners to match the zinc plated trailer tongue assembly's metallurgy.

3.      I want to use the ½” bolts I’ve already put through the top two holes.

4.    I don't like those nylon lock nuts.  Since lock tight probably isn't going to work with a properly lubricated bolt, this is the best I could come up with .

 

This scheme does involve drilling two holes through the bottom of the C-channel that goes on the outside of the square tube, adding 2 additional 3/8" bolts under the square tube.  The purpose of these bolts is to help bend the est. 93 Deg bends of the C-channel down to where the sides of the C-Channel touch the outside of the square tube, so that when the ½” bolts on top are tightened they are using most of their available bolt tension to clamp the two pieces together rather than spending most of their available bolt tension, bending metal, with very little left for clamping.  The intent of this modification is to clamp the two pieces together firmly enough to stop the “rocking” and eliminate the bolt breakage that many have been experiencing.  Based on my tests, it is my opinion that just switching out the top two bolts from 12mm to ½” is not enough to achieve the clamping force necessary to keep the two pieces from moving separately, and that the ½” bolts, by themselves, will eventually fail.

If you are not happy with either of my 2 schemes, you on your own, cause I'm done.  If you drill through the top, you are drilling through the manufacturer's stamped data plate which is a sin, This scheme already violates the commandment "Thou shall not mix metric and US standard fasteners on the same contraption".  Both of these can be found in Compressions 10:1 right after "He who is without oil pressure shall throw the first rod", and before "It is easier to get a camel through the eye of a needle than it is to get Never-Seize off of everything in sight, after putting it on only one bolt".

Stacking multiple sins up on one project, leads to it getting snake-bit which is bad, very very bad.  Invoices don't get paid, lawyers are engaged, and everybody leaves angry at everybody else, months or years after it was expected to be finished.

SF

[Mark Mocho]

And another rule:

No Engineer will ever take suggestions from another Engineer."

[Scott Fletcher]

Ah  but you better listen to the welders, sometimes those guys can make you look really stupid if you don't.

Scott Fletcher

120 Ashmore RD

Greer, SC 29650-2926

C: 864-420-1749

E: 211fl...@gmail.com

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From: rasp...@googlegroups.com <rasp...@googlegroups.com> on behalf of Mark Mocho <markm...@gmail.com>
Sent: Friday, November 21, 2025 7:21 PM
To: RAS_Prime <rasp...@googlegroups.com>
Subject: Re: [RAS_Prime] Cobra Trailer Square Tongue bolt breakage fix

 

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[Mark Mocho]

I have had so many arguments with Engineers because they seem to think welders and mechanics have three elbows in each arm.

[ka...@uplink.net]

Here's a pic of an easier/better fix for the problem of sheared square tongue bolts. 15 minutes - problem solved. 

To view this discussion visit https://groups.google.com/d/msgid/rasprime/LV8PR16MB61507D61E7A33C4D04C36700F9D2A%40LV8PR16MB6150.namprd16.prod.outlook.com.

 

[Scott Fletcher]

That bolt through the top will work, as long as you don't put the original bolts in shear with the new one through the top, or crush in the bottom of the square tube with the nut  without a sleeve in the tube, or a hefty backer plate on the bottom of the tube to transfer the load to the parallel side walls, and that bolt's shear strength is enough to carry the entire load in single shear. So, is easier/better your final answer? 

Scott Fletcher

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From: rasp...@googlegroups.com <rasp...@googlegroups.com> on behalf of ka...@uplink.net <ka...@uplink.net>
Sent: Friday, November 21, 2025 9:36 PM
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[Karl Striedieck]

Original tubes are nested so new bolt simply pulls the two tubes back to that position. The new bolt is only about an inch long. It doesn't go through the square of the tube. Get under and look up at the two tongue pieces, you'll see.  No sleeve necessary (or possible). The bolt is #8 strength and it's not loaded in shear. Definately stronger (no more shear), easier (10 minute job if you have the bolt and a drill and bit.) The bolt is the gold one in the pic.

[Scott Fletcher]

Ah, yes, the top bolt is a bump stop, not a through bolt.  That should work.

But I just like the idea of getting the original two bolts back to being bolts.

Instead of poorly executed shear pins.

More than likely, we are just moving the point of failure to the next weakest

Point that we have not discovered yet.

I pretty much did that with a friend's tow behind dolly for

transporting his Jeep behind his pickup.

We fixed the first four weak points that failed, 

Most of those occurred in inconvenient and remote locations. 

He quit towing the Jeep before we discovered #5

Scott Fletcher

120 Ashmore RD

Greer, SC 29650-2926

C: 864-420-1749

E: 211fl...@gmail.com

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From: rasp...@googlegroups.com <rasp...@googlegroups.com> on behalf of Karl Striedieck <ka...@uplink.net>
Sent: Saturday, November 22, 2025 9:16 AM

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[Karl Striedieck]

Not sure what a "bump stop" is. There's no way to avoid the two original horizontal bolts from being in shear with veritcal loads due to rough roads. The new bolt eliminates all those shear loads.

[Karl Striedieck]

Putting three bolts in shear may solve the problem but why not just one #8 in tension? Zero problems with Duo and single seater trailers over 1000's of miles. It's also a couple minute job if you don't count going to Lowes for the bolt. 

[Tom Watson]

i don't see any pic?

To view this discussion visit https://groups.google.com/d/msgid/rasprime/507842ae-a926-4699-b2f0-ad2901d684dfn%40googlegroups.com.

[Charles Mampe]

OK, related question.

How many tongue issues when towed behind cars or SUV's with short distance from rear axle to ball?

How many issues behind RV's or long bed pickups?

My understanding is that tongue issues (broken round tube, broken bolts on square tubes) are on vehicles with long distance from rear axle and ball, usually with a heavy tow vehicle.

Bad roads, poor shocks, higher tow speeds all make things worse.

[Karl Striedieck]

The pic is on this thread Nov 22. Email me (karls at uplink.net) if you can't find it. 

[Scott Gross]

I had two Cobra trailers working towards failure towed behind my Honda pilot. I replaced the two horizontal bolts and put in 2 vertical bolts on both trailers.

Scott Gross 

Sent from my iPhone

On Nov 23, 2025, at 11:39 AM, Karl Striedieck <ka...@uplink.net> wrote:

The pic is on this thread Nov 22. Email me (karls at uplink.net) if you can't find it. 

To view this discussion visit https://groups.google.com/d/msgid/rasprime/a21aab35-2781-4a0a-a4eb-2d2a79bd2f8an%40googlegroups.com.

[Tango Eight]

Nice write up Scott, and an important subject.  I've fixed a couple of Cobra Trailer tongues.  

My ASW-27 aluminum top trailer weighed roughly1600# loaded.  The bolts got worn but the holes were okay, I replaced with longer grade 10.9 12mm bolts so that both sides of the tongue assembly were supported by the bolt shank, problem solved.

My new to me in July '24 JS-1 came home in an ASH-31 glass top trailer.  This beast is closer to 2400# and its a couple feet longer.  I'd spotted the worn bolts at inspection time, asked the seller if he'd had any issues.  He had (including one bolt failing completely) and he'd replaced with original length bolts.  When I got into this, I discovered that the replacement bolts were thrashed and the holes elongated.

I reamed the holes with a "bridge reamer" to 9/16 (14mm). This style reamer is tapered at one end, straight on the back end, perfect for this job and less than $30 at Amazon.  A 1/2" drill will do nicely.

New bolts from McMaster Carr, showing the right length of shank.  Use high strength, corrosion resistant bolts.  I selected fine threads and used elastic stop nuts.

Stack washers to establish proper grip like so.

Cut off the excess and dress the ends if you like.  I've got 8 or 10K miles on this repair, it's all good.

Evan Ludeman "T8"

[John Johnson]

FWIW, the original bolts on my well-travelled 2015 trailer carrying an ASH31 show no signs of shear movement or thread damage.  I'll still replace them with longer bolts but mine don't seem to have any damage or wear.

JJ

[Andy Blackburn]

Hey Scott,

Looking at your 2nd concept in particular, it seems like the issue with the existing bolts is that you just can't get the threads to turn enough to get the C-channel to bend enough to get much clamping force - presumably because the bending plus clamping force is just too much to be able to keep twisting the nut tighter before something gives way. I'm not sure if the issue is that you strip the threads or something else - because there's always a bigger torque wrench. 

It occurred to me that instead of installing two helpers bolts at the bottom of the C-Clamp you could just use an external clamp to do this work while you are tightening the two factory nuts to get the C-channel deflection you need to achieve clamping. Maybe that's too much force for a normal clamp and maybe there's enough spring-back in the C-channel that it would over-stress the nuts if it's an operating load as opposed to the load from torquing the nut down - but if the strength of the bolt/nut interface is the issue maybe a deeper nut would help with this?  Also aren't their higher strength alloys that would allow you to keep torquing the nut until you get the desired clamping?

Thanks for posting this. Super helpful.

Andy Blackburn

9B

[Scott Fletcher]

Think of the bolt like a spring, you want it to stretch in the elastic zone, but not under enough tension to permanenty deform.  So for a given size an grade of bolt, there is a happy zone for bolt tension.  The bolt grades and the torque i gave you are in the happy zone.  Deviate from that and you might get suboptimal results.  Using a clamp and then removing it would over stress the bolt.  The bolts I listed are common high strength bolts.  Not unobtanium.

Sf

To view this discussion visit https://groups.google.com/d/msgid/rasprime/84091a33-5e3a-4676-91ed-23374df4acean%40googlegroups.com.

[Dave Nadler]

So Scott, you doing a repair clinic at Perry?
From Down Under,
Best Wishes for 2026, and Best Regards, Dave YO

[Tom Seim]

Has anybody run this by Cobra for their comments?

[Scott Fletcher]

I didn't run this by Cobra.  The Entire Cobra trailer tongue assembly is a buy out item from AL-Ko.

My experience in getting good answers for corporrations as large as AL-Ko is poor.  

I suspect that Al-Ko's legal team would have them deny that this problem actually exists.

 

Their original design is a good one if the C-Shape has been formed correctly. Given the 

Nature of the pressing operation, the amount of welding on the C-shape and the temperature

Effects of the galvanizing process, this would be a pretty elusive problem for them to solve.

Bolt threads located in a shear plane is sloppy work, but I bet that they have the stress calculations

 To back up accepting the reduction in shear strength vs the additional hardware cost for fixing that issue.

However they didn't factor in the impact load effect for the two pieces moving relative to each other at speed, 

Which is the result of an unanticipated manufacturing tolerance issue with the C-Shape.   I'm not sure how 

You would calculate that with out doing enough guessing to invalidate the results.

After a doing a few trailers now, I've reached the following conclusions.

1.  This is a QA problem with the press that forms the C-shape that goes over the square tongue.  

Not all trailers have this problem.  

2. My recommended approach would be to replace the original top two bolts per my first

Instruction set.  If you do not observe any evidence of wear on either of those bolts stop there.

These bolts help each other quite a bit.  Steel is really flexible at low deflections and bolts

Give you more clamping force than you think.  So repeat the final torque pattern until everything quits

Moving.  This is where the tread lubrication really pays off, so don't skip that step.  Don't over

Torque them, a little bit Mo, ain't better here.

( I wouldn't expect any objection from Al-Ko at this point all we are doing is replacing the 

Original bolts with longer versions of the same bolt and adding some spacing hardware to 

get the threads out of the shear plane of the two pieces being bolted together.  This step

Increases the shear strength of the bolts by approximately 30%.  The assembly instructions

And torque values I used are generally acceptable good practices.) 

3.  IF  you see wear on the original two bolts - THEN add the additional bolting under that goes

 under the square tubing that I outlined in the second instruction set. 

 

(bolt wear indicates what I consider to be an out of tolerance pressed C-Shape.  Adding

The two bolts under the square tubing fixes that by forcing the C-Shape back into tolerance far enough

To allow the original bolts to do the clamping job they were intended to do, and stop the impact loading on the bolts.  I wouldn't expect any

Pushback from Al-Ko on this step.  Adding these bolts does not weaken anything else in the assembly, 

and requires minimal skill to execute).

Scott Fletcher, P.E.

120 Ashmore RD

Greer, SC 29650-2926

C: 864-420-1749

E: 211fl...@gmail.com

---

From: rasp...@googlegroups.com <rasp...@googlegroups.com> on behalf of Tom Seim <toms...@gmail.com>
Sent: Sunday, December 28, 2025 11:46 PM

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[Scott Fletcher]

Dave, 

I have no plans to give a trailer maintenance class at Perry.  There are a few others out there that could do a much better job than I could.

I fixed some pre-existing (to me) issues with my trailer after receiving advice from people actually qualified to give advice on this subject, and I'm comfortable with how I did the work, but it is always dangerous to make assumptions on a data set of one.

Since I'm not getting into the trailer repair business, it is going to stay a dataset of one.

The chaos of this type of forum is interesting, but I'm kinda-sorta starting to regret getting this started.

I'm a firm believer in collaboration creating a better outcome, and I think that has occurred here, but at a certain point you have to quit collaborating, and just go get it done.

A Plan....Is something that never happens.

Theoretically and Actually are two completely different things.

Scott Fletcher

120 Ashmore RD

Greer, SC 29650-2926

C: 864-420-1749

E: 211fl...@gmail.com

---

From: Scott Fletcher <211fl...@gmail.com>
Sent: Monday, December 29, 2025 8:35 AM
To: rasp...@googlegroups.com <rasp...@googlegroups.com>

[Eric Greenwell]

Publishing on RAS-prime is more like "peer review" than collaboration, which may happen after the "peer review" taper off.

Eric

[ka...@uplink.net]

A repeat of an earlier suggestion I don't see anymore.

 

This issue has a very simple and quick fix: a new bolt installed vertically at the front of the lower channel that holds both pieces together. This removes all shear forces. Of course the two originals stay in place. See pic. It's the gold colored bolt.

 

Karl Striedieck 

 

karls at uplink.net 

814 237 7996

To view this discussion visit https://groups.google.com/d/msgid/rasprime/2facd846-a09a-4a31-be5c-b0488c5a8607n%40googlegroups.com.

 

[Tom Seim]

Then I would contact AL-Ko. Maybe you get an answer, maybe you don't, but it's the price of an email. Oh, that's right: emails don't cost anything.

Tom

[Mark Mocho]

Al-Ko in Elkhart Indiana has closed permanently, so your only alternative is to contact Al-Ko Kober in Germany. Note that Al-Ko products are subject to the tariffs, so products have dramatically increased in price.

[Mike Koerner]

My apologies if you were hoping not to continue this discussion into the new year.

Al-Ko has made millions of trailer tow bars and hitches. They work great.  Why does the glider community have problems with them? (My wife, for example, is the undisputed world champion trailer thrower.)

I think the answer may lie in the unusual length of our trailers and, as a result, their relatively huge rotational inertia about the axle (the moment of inertia goes up as the square of length). When either a trailer or tow vehicle’s wheels go down in a dip, or up over a bump, the trailer simply rotates about its axle to adjust to the change in relative height of the tow ball. Absent rotational inertia, or with the minor rotational inertia of the shorter trailers most Al-Co products are mounted on, there is no significant load associated with this rotation. However, our longer trailers resist rotation mightily. Every bump in the road, taken at high speed, puts huge vertical loads on the tow bar and hitch. It’s not the weight of the trailer that matters. It’s the rotational inertia.

Our trailers are also relatively low slung to keep the rear ramp angles low to make it easier to roll the fuselage in. This, combined with their length, means we are often “high-centering” our trailers (dragging the tail) on bumps and grades. This too puts enormous vertical loads on our tow bars and hitches.

With that in mind, I would hesitate to drill additional holes in the top and bottom sections of the square draw bars. Under vertical loads these areas act as flanges where extreme fibers are subject to maximum loads, whereas drilling through the sides of the bar, the web, is relatively benign.

Otherwise, on the face of it, Karl’s solution would seem to add another shear pin to the collection. Since the existing two bolts have two shear planes each, the top pin would increase the shear area by 25% (assuming his bolt is the same size as the existing horizontal bolts). Since determining whether this vertical pin would carry more or less of the vertical impact loads than the other pins is beyond the scope of my current contract, I have to assume that this solution increases the mechanism’s life by 25%. A row of bolts across the top would further increase its life, except, as mentioned previously, I don’t think we should be drilling through the top or bottom of the draw bar at all.

As Scott said, clamping forces should hold these parts together. The bolts should be in tension not in shear. That will alleviate the hammering wear clearly visible in the photo of his bolts, which undoubtedly resulted from bumps in the road acting against the trailer’s rotational inertia. Shear loads may be unavoidable during high-centering events, but his solutions accommodate this by moving the threads out of the shear plane.

I would adopt his second solution myself, but my cobra trailer has one of the older round draw bars, with multiple welds and reinforcements holding it together.

Mike Koerner

[Mark Mocho]

Mike-

Good analysis. One thing that also contributes to drawbar failure is that American tow vehicles with longer dimensions between the vehicle axle and the hitch ball exaggerate the rotational inertia. Extreme axle to hitch overhangs like those associated with motorhomes are even more likely to increase the drawbar loads. I remember that several years ago, round drawbars were noted for cracking and failure when towing behind some Class A motorhomes. European tow vehicles are more likely to be passenger cars, where axle to hitch dimensions are much shorter. Also, many European countries limit trailer tow speeds to 100 kph or even less.

Karl's suggestion of a bolt through the two drawbar components to eliminate the slippage between the square rear section and the formed saddle that comprises the forward section with the brake mechanism is definitely an excellent solution. It essentially locks the two sections together vertically, greatly reducing the shear loads on the horizontal bolts. However, Scott's suggestion of using longer bolts and spacers to move the shear loads off the threads and on to the bolt shank where it belongs is also an excellent suggestion. The third bolt through the drawbar horizontally may not be necessary with the Karl's vertical bolt, but I will leave that to the Finite Element Analysis afficionados out there. 

The third horizontal bolt is certainly more difficult to install, as drilling the holes through the drawbar components isn't a trivial matter when it has to be done with hand tools and with the drawbar still on the trailer. A little "slop" in the hole will allow for movement around the bolt and shear loads on the other bolts will not be reduced as much as expected.

Personally, I am replacing the two lateral bolts with longer ones, using spacers to get the shear load off the threads as Scott recommends. But I am also adding the vertical bolt as Karl suggests. I work on Cobra trailers quite a bit and can see merit in both ideas.

[Darren Braun]

mmm...Nastran and the like...back in the day was often compute limited.  However, making a detailed model isn't much use if we don't have the loading detailed in some form of shear/moment/loading diagrams.  But I think the use cases/setups are varied so I'll offer up this paper courtesy Univ of Queesland with the idea that some additional conditions be considered in the form of structural resonance of the system.  It might answer why some have never had an issue while others have had serious wear, creep and fracturing.  Overall a good discussion to highlight we should all inspect those tongues for any sign of impending failure.

https://espace.library.uq.edu.au/view/UQ:d12effa/Duckworth_Matthew_thesis.pdf?dsi_version=c936e1db4bbd39171eef70f9d8f07c2b

Darren

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