On June 6, 1944, the largest military invasion ever attempted took place along the beaches of Normandy, France. Thousands of men landed on the coast, along with tanks, jeeps, and artillery and fought their way inland against relentless German resistance. One of those pieces of artillery was a Swedish designed portable cannon called a Bofors. It was a 40 mm anti-aircraft gun that was used in many different applications and mounted on many different vehicles, including ships. It also came mounted on a four-wheeled carriage which gave it the capability of being setup and ready to fire in as little as two minutes. While the gun was interesting, it was this carriage that caught my attention when I saw it in a museum in France.

One of the main aspects of the design that allowed for such a rapid deployment was an extremely innovative suspension system that I have never seen before. The cannon has independent suspension at each corner consisting of equal length upper and lower control links attached to a rotating front and rear cross shafts that, when turned up to 140 degrees, lower the carriage onto pre-deployed support pads. The whole procedure looks like it could be accomplished in a matter of seconds. The suspensions at each corner are identical except that the front allows for steering.

To fully understand how the system worked, I’ve built a computer model to show it in action. Here you can see the basic setup where the big fore/aft tube represents the body that the gun would be mounted on:

or normal driving (or towing in this case) you can see how the double link suspensions allow the body to move up and down like any suspension would.

Once the gun reaches its desired position, the suspensions are rotated so the gun can be lowered onto its support pads. For the front suspension, this happens as follows. At first, the front suspension, which is attached to the tow bar, is in the driving position, meaning the body of the carriage is raised and ready for transport. The tow bar is horizontal and can be attached to a tow vehicle. Once the tow bar is un-hooked from the tow vehicle, it is then rotated to a vertical position. Since the tow bar is attached to the front suspension, rotating it rotates the entire front suspension anywhere up to 140 degrees depending on how much is needed to set the gun down on the pads.

Due to the offset between the cross shaft and the wheel center, this rotation causes the carriage to drop. The rear suspension has the same mechanism but instead of a tow bar, it uses the gun support as its lever.

Here you can see how this works in the model:

Once both front and rear suspensions have been rotated, and assuming the support pads have been deployed, the lowering of the carriage onto the pads means it is now supported in a stable position ready for firing. Super simple, super quick.

When the gun needs to be moved, pulling down on the tow bar and gun support reverses the whole procedure and the front and rear suspensions rotate back to their driving positions, thereby raising the carriage off the support pads which can then be returned to their transport positions. The gun is then ready to be moved to another location. Of course, a gun like this is very heavy and it wouldn’t be possible for a person or even a few soldiers to pull the tow bar down to raise the gun off it’s support pads. But the makers of the carriage have thought of that and provided a mechanism at the front and rear ends made up of springs housed inside the main carriage body actuated by levers and rods attached to the front and rear cross bars.

Image via Bulletpicker

Rotating each suspension compresses the corresponding spring inside the carriage body which helps carry the weight of the entire gun, making it much easier to raise when needed.

With the front and rear suspensions in their driving positions, they become fairly simple though effective double wishbone designs with equal length links top and bottom. Parallel and equal length links mean the wheels always stay upright relative to the carriage without any change in camber and keeps things simple. Some of you may have noticed that neither my model nor the photos of the suspension show a spring. This is because the springs are housed inside the rotating tubes.

Image via Bulletpicker

As you can see from this cutaway, a coil spring is housed in the cross bar and is actuated by a lever attached to the upper links very similar to the mechanism that aids in lowering and raising the unit. As the suspension compresses, the upper links rotate and the lever pulls on the spring rod that extends through the spring and pulls on the spring plate, compressing the spring. Such a setup would make for a well controlled ride and allow the gun to be easily towed over rough terrain.

The last part of the mechanism are two steering tie rods connecting the front suspension to the tow bar. These rods allow for the front wheels to be steered around corners.

As far as brakes go, there seem to have been two versions of this carriage: one with air brakes (like the one I saw) and others, which I presume came later, with electric brakes similar to what we have on trailers today. All used drum brakes on all 4 wheels.

From what I’ve been able to learn, the carriages for these guns were made by Firestone, although I don’t know if they were responsible for the design. If anyone knows anything about these, I would be very interested in learning more about the origins of this suspension.

All in all, a very simple but effective and innovative design that made an already outstanding anti-aircraft gun even more versatile.