Laminated Top Bows Broke Again.

[BE_ZERO_BE]

When I restored my '16 Touring in 2008 I installed a new top, new sockets and new laminated top bows.
In about 2012 one side of the rear bow broke right at the socket.
At that time I repaired it by inserting a 1/4" stainless steel rod between the bow and socket.
The rod penetrated about 3" - 4" into each side.
It has held up well.

While doing work this last summer I put the top down.
Some time later I noticed that the second bow from the rear was broken at the socket on both sides.
I did not want to go through the hassle of replacing the bow not knowing the availability.
I decided to use the same method to repair this bow.

I made a fixture to match drill the bow and socket.
I drilled the bow to about 3" and the socket to about 4".
The rod was inserted into the bow and the socket.
JB Weld was used as an interface filler between the bow and socket.
I happen to have some large diameter thick wall heavy duty adhesive shrink tube.
Once shrunk, it is about 1/4" thick and very rigid.
The internal adhesive melts to form the the bow and socket.
This should help reduce the flexing between the bow and socket.

Since 2008, three bow to socket interfaces have sheared.
I am beginning to think that laminated bows may not be as rugged as steam bent bows at the bow/socket interface.
I know these repairs will last a long time.
I hope I don't have to do this again.

[BE_ZERO_BE]

[Scott_Conger]

Never in my life have I seen cross-grain laminations in anything other than plywood. I would NEVER have expected a cross-grain lamination to last in this use, ever.

That design is a complete travesty.

I feel for you (and anyone else who has a car sporting these things!)

[Oldav8tor]

I have laminated bows in my 1917 but they don't look anything like the ones pictured. They have held up very well.

[Rich P. Bingham]

I am beginning to think that laminated bows may not be as rugged as steam bent bows . . .

I think you have proved it.

[Scott_Conger]

comparing those laminated bows to ALL laminated bows is not exactly a fair comparison, and is why it is easy to find exceptions to Bob's experience.

Bob, you were done dirty when you got those things.

[Rich P. Bingham]

. . . I would NEVER have expected a cross-grain lamination to last in this application.

That design is a complete travesty.

Another example of reproduction parts not made true to type. Substitutions of material and / or method made (in ignorance ?) for reasons of convenience for the purveyor, or for economy that end in failure and are ultimately more expensive for the end user than if they had been done correctly per original.

Further, the Model T era may well have been the high point of technology in producing articles made of wood. Departing from the choice of species and fabrication methods used originally may result in unexpected failure modes.

[otrcman]

Just to be clear, I believe Scott and Rich are saying that the fault in Bob's bow is that the grain of the laminations is running the wrong way.

Think of it like a laminated archery bow. The grain in a laminated archery bow always runs from TIP TO TIP, and in that orientation a bow (archery AND Model T top alike) are very strong. But having the grain run across the long axis is very weak.

[Allan]

Scott has nailed it. The bows are laminated strips of plywood, so much of the grain is across the bow rather than along it. True laminated items are in fact stronger than steam bent solid timber, as today's adhesives used in their construction is of superior strength to the timber they are gluing together.
In such laminations, ALL the strips used have the grain running along the length of the timber.

Allan from down under.

[Humblej]

Bob,
Your repair will be temporary, it will break again, just in a different spot. In my opinion all the plywood bows need to be replaced with either steam bent wood or laminated wood with the grain running the length of the bow. What you have is essentially bows that are at best only 1/2 the thickness of the original Ford design. They will break again.

[BE_ZERO_BE]

I now have a much better understanding of why my bows failed.
The weak point (other than the design of the bows themselves) is the interface between bow and the socket.
I believe the repair is solid and will last.
The remaining bows will be vulnerable to the same failure.
I will be more vigilant when folding the top for transport.

Thank you to all for your comments - They are enlightening.

[Rich Eagle]

That certainly is the weak point, Bob. My steam bent bow broke there from allowing too much snow to accumulate.

viewtopic.php?f=2&t=12830&p=96070&hilit=grain#p96033
Oak will fracture a certain amount in the process of bending. I exceeded the limit for this one. Only one bend in 8 failed.
These problems with the laminated as well as the steam bent ones seem to be very rare. The Forum is a great place to report them.
Rich

[BE_ZERO_BE]

I think that the transition from the rectangular cross section of the bow to the oval cross section of the socket creates a natural fracture line.
When I did mine, I stair-stepped the the transition rather then taper the bow to the socket.
In hindsight, that may have been another contributing factor to my problems.

In any event, the car is back on the road.

[Rich P. Bingham]

I think that the transition from the rectangular cross section of the bow to the oval cross section of the socket creates a natural fracture line.

I think you're right Bob. Whatever flexibility the bow requires from stresses of movement and vibration stops right at the socket. The tubular metal socket bolstered by the wood filling it would be much more rigid than the rest of the bow. That this was not an uncommon failure is borne out by the change to the design of the irons,?when the irons were made to form the radius, and "bows" were a straight wooden piece.

Very glad you're back on the road Bob !!

[TRDxB2]

If the bow didn't brake there at the socket from stresses, would something else brake or bend?

[Allan]

Rich, that's an interesting take on the reasons for the change in the way hood sockets/bows were made. It makes some sense, but I wonder how much the change was made to simplify/cheapen their manufacture. Either way, working with the later bows is much less demanding for the hobbyist, and requires little in the way of special skills/techniques.

Allan from down under.

[Rich P. Bingham]

Allan, I'm sure you are right, that design change would have been driven by Ford's implacable pressure to produce ever cheaper and more efficiently. As fewer and fewer components of the car were purchased from "outside", more parts were formed from sheet metal stamping. Unlike most similar changes in latter days, the redesigned parts were equal to or better than previous ones.

The developments in top construction are interesting, as initially, designs were no different from what had been used for decades on horse-drawn vehicles. Higher speeds and wind load, vibration and the dynamics of those stresses soon required a different approach.

[BE_ZERO_BE]

Frank (TRDxb2),

For the most part I agree with you about stress on the bows.
When the top is up and in position, the bows are in axial compression with the sockets.
It is unlikely this is when the bows broke.
I only found broken bows after I put the top down.
Putting the top down by myself is awkward at best.
The touring tops do not "fold up" nicely.
I would bet a shinny new nickel this is when It occurred.
It probably didn't take much force to break the bows at the socket.

I will be putting my T into a toy hauler which will require putting the top down.
I will be especially vigilant during that operation.

[BE_ZERO_BE]

While cleaning up the shop I ran across the pieces of the top bows that I trimmed off.
As you can see they were "laminated" from nine layers of very thin plywood.