Thursday, April 27, 2017

Boxes of Torque

If you are weary traveler, searching the corners of the internet trying to find some information on Ford Falcon torque boxes, you are in the right place. Welcome to Ironhydroxide, a place where we tell stories about building an old econobox Falcon into the kind of high performance car it was never intended to be. If you are a regular, you already know this and you are probably just here for the gifs and youtube links.

What is a torque box? This is a question many a classic Ford owner has asked himself and most have not received a satisfactory answer. Today that is about to change.

In the late 1950s, The Big Three realized that they were about to get their butts whooped by the same group of schnitzel-eaters that they had whooped only 15 years earlier. Volkswagen, the brainchild of everyone's favorite fascist had grown powerful and was going for round two in world domination. The Beetle, known to the Germans as the Käfer, was steadily marching across the globe and even into the driveways of red-blooded Americans. Robert McNamara, who went on to become John F Kennedy's Secretary of Defense, recognized this creeping threat to the security of the good people of America Ford Motor Company. In response, he did something drastic and led Ford into battle with what was the smallest car they had built since the 1930s. This car was the Falcon and it was a massive success. It was cheap to build because it was a unibody construction, meaning that they no longer had to manufacture a frame and body.   After the first two years of production Ford had sold one million Falcons. For perspective, Toyota has sold about 400,000 Camrys each year for the past decade or so.

Now that the Americans were on the offensive they could now start to do what Americans do, that is, shove V8s into everything. This was 1963 and America couldn't wait for the Mustang to give them a little V8 car. America needed it now, and the good engineers at Ford obliged by shoving a V8 into the Falcon. These same engineers soon found that the unibody of the Falcon had the structural rigidity of mom's spaghetti and that became a real problem once had to deal with more than 100 horsepower. The solution to this problem was the Torque Box.

So this brings us full circle. What exactly is this miracle of engineering, the torque box? In the diagram below, you can see some of the main load bearing components of a unibody structure. The engine and supsension mount to the front subframe. Forces applied to by the engine or suspension on the body go initially into the front subframe. The red arrows shows the forces that would be applied by driving over a bump. A torque box, shown in pink, creates resistance to the rotation of the front subframe by tying it together with the rocker panel.

Ford unibody torque box diagram
Mustang passenger side torque box
While the torque box found its way into Falcons, Mustangs, Fairlanes, and just about every other unibody Ford, that doesn't mean they all had them. My car was definitely in the "no torque box" camp. (WARNING: ENGINEERING JARGON AHEAD) This means that all the twisting forces  and vibration at the back of the front subframe are being transmitted through the floor and firewall of the car. Long time readers may remember my vibration issues, but long story short, there is a nasty vibration that permeates the structure of the car when the engine is near 3800 RPM. I suspect that the resonant frequency of the interface of the front subframes and floor/firewall is around 63 Hz or some multiple thereof, and the engine at 3800 RPM (63 Hz) is exciting that interface. I theorize that if I can stiffen this interface, it will move the resonant frequency out of the range at which the engine can excite it.
Translation: I think adding torque boxes to Grace may eliminate a stubborn vibration issue in the body structure. 

So naturally I wanted to know how to install Falcon torque boxes. I spent hours using the google machine, trying to learn all I could. Boiled down, I learned this: Nobody knows crap about Falcon torque boxes. A few people know a few things about Mustang torque boxes. The misinformation is deep. Mustangs and Falcons are similar, so Mustang torque boxes should work... right? Wrong. 

I figured the only way I was going to learn what I needed to know was to spend some sweat and dollars, so I started by removing the fenders and ordering a set of torque boxes. 

Not having any clue what might fit in my Falcon, I ordered a set of torque boxes for a 1965-68 convertible mustang. I knew that convertible Falcons had torque boxes, so maybe these would be the same. I soon learned that all unibody Fords in those days had a drastically different rocker panel than hardtop cars, so I ate the massive cost of return shipping on those parts. Lesson learned. Convertible torque boxes DO NOT fit on hardtops.

Mustang convertible driver's side torque box side view
Mustang convertible driver's side torque box top view

The other thing I learned with the torque boxes in hand and fenders off was that the early Falcons had much shorter rocker panels than Mustangs, and that they were not designed to have torque boxes at all. In the picture below, the blue line represents where a torque box would go, and the red line shows how long the rocker panel would need to be to effectively transfer torques and forces from the front frame rail and torque box into the rocker panel.

At this point I was not sure it would work, but I went ahead and ordered torque boxes and rocker panel extensions for a 1965-70 Mustang Coupe or Fastback. When they arrived, I mocked them up and found that these parts were not interchangeable with Falcon.

Because the Falcon is the Mustang's progenitor, you'll often hear that they have interchangeable parts. This is a bunch of crap. There are not enough directly interchangeable parts for this to even be a useful myth. I found myself in the familiar position of building my own performance parts for the Falcon out of cardboard. From here, the pictures do most of the talking.

If you've taken the time to look carefully at my design you may notice a few things. First, this torque box is quite a bit smaller than the Mustang boxes. This is on purpose, the front end of a Falcon is shorter than a Mustang, so Mustang sized torque boxes would interfere with the wheel if they could be installed on a Falcon. To compensate for this and make fabrication easier, I built them out of thicker steel than the Mustang boxes. Second, the rocker panel does not extend over the outside of the torque box. I do have concerns about this. Without that extra length in the rocker panel, stiffness will not be as high as it could be. If I feel the stiffness is lacking, I will find a way to lengthen the rocker panel about 4 inches.

(Disclaimer: this information applies only to round body Falcons from 1960-63. I have no idea if it applies to 64-70 Falcons. I believe minor changes were made to the chassis in 64)

Now that I've done one side, I'm hoping the other side will go quickly. I am anxious to see if my engineering theory was correct, and a stiffer body will eliminate the vibration issues. In the meantime, enjoy some cars I found in my neigborhood while on a Sunday walk. It seems I'm not the only one in town with a Falcon...


  1. I'm sure we're all holding our collective breath for "part 2" of this thesis, Jesse. Curious minds wanna know if your theory about the resonant frequency - hopefully cured/changed by these torque boxes - is/was the root cause of the vibration/noise you've been experiencing.....I know I am! Ray

    1. Fingers crossed! If it turns out to be right, I'll feel really smart! If not, it's business as usual.

  2. Have you considered subframe connectors?

    1. I actually installed SFCs before I had the engine running. Ironically enough, it is possible that they have contributed to the vibration issue by changing the stiffness of the structure around the firewall/subframe interface.