Torsion frames are a complicated topic – especially when you are married to an engineer. He loves all the details.  

Steel vs Aluminum

Make your frame out of steel. End of story. Still not convinced? Steel has three times the stiffness of aluminum and three times the density. Important note: stiffness and strength are not the same thing! This is a stiffness problem, not a strength problem, for a torsion frame, you need stiffness. Stiffness refers to how much the material will deflect under load. For any grade aluminum, or any grade steel – doesn’t matter – the stiffness of steel is three times as great. Strength is how much load the material can handle before failing, and there are aluminum alloys that beat steel in this regard, but for torsion frames, what you are really concerned with is stiffness. You really don’t want your torsion frame to deflect and cause any kind of twisting in your habitat box (habitat boxes are expensive!). You would need three times the aluminum to have an equivalently stiff frame, so unless you plan to have your box up so high that it would be silly, go with steel. Plus, if you did that you would have negated any weight savings because you would need three times as much for an equivalent stiffness. Basically, you want to have as stiff of a frame as possible in the space constraints that you have, so steel is the only choice. By their very nature, torsion frames will always be thicker than you want them to be and heavier than you want them to be, if they are done correctly (stiff enough). If you make them as thin and as light as you want, you will have problems. The torsion frame is the foundation for your box, the weight that the box carries needs to be supported by the frame, instead of counting on the box strength and stiffness to carry the load. Habitat boxes are generally pretty stiff, but they are composites and it is a hard thing to analyze. You wouldn’t know until it is too late that your box isn’t stiff enough, and that would be a very unhappy day. It all comes down to analyzing the cross-sectional stiffness and strength of the frame and the torsional stiffness of it, and that’s why Jake did a bunch of FEA (Finite Element Analysis) on our frame. We have put a lot into this project, and finding out that we didn’t make our frame stiff enough would be heart-breaking (and bank breaking!).

Cross Section

Tubes are stiffer than an equivalent amount of material in a solid form. For example, a 5”x5” tube ¼” thick is going to have much greater stiffness than a 3”x3” tube with a much thicker wall. In fact, even if there is more steel in the 3”x3” tube, the stiffness will still be greater in the 5”x5”. There is not a linear relationship for stiffness to weight ratio if the form factor is different – bigger tube out of the same material is generally stiffer. There are charts out there with cross-sections and wall thicknesses that show the different stiffnesses and strengths (?). We used rectangles as large as possible for our frame. We actually wanted to go bigger, but ran into a sourcing issue – not uncommon these days. The only way to get really good torsional stiffness with weight efficiency is to make your tubes as big as possible.

Another thing to understand about tubes is that in a 3”x5” tube, for example, there is a stiffness in the 3” dimension and in the 5” dimension. The 5” dimension will have more stiffness, so you will want to orient it so the the 5” dimension becomes your height. The torsional stiffness is a bit different, torsional stiffness is the two stiffnesses added together. Some people make their torsion frame with a tube that has a 6” height and 2” wide because they only think about the vertical load, but the torsional stiffness is not sufficient. You would be better off going with a thinner wall and a wider tube which would give you more stiffness in all directions.

Types of torsion frames

Rail on Rail

The tried and true version of a torsion frame is rail on rail construction. It has been used for a long time and is an 80/20 type of solution. It is pretty simple and is used on delivery trucks and box trucks and all kinds of things. This is the frame type that we chose, it isn’t perfect, but we believe that it is the best solution for us, given the kind of chassis frame that we have and in combination with Liquid Spring. I did another post on why we chose the Ford F550 and Liquid Spring suspension. In the rail on rail, your torsion frame is on top of the chassis frame. The idea is that when the wheels on one side go into a pothole, the “rail” of the chassis drops away from the “rail” of the torsion frame, so the torsion frame “rail” on the opposite side is now carrying all of the load and the rest of the torsion frame needs to be stiff enough to continue to support the rest of the box without deflecting.

Three Point

Conceptually a three point is ideally what you want because three points make a plane, however, the big downside is that you are concentrating all of the load on three points on your chassis frame. Because of this, you must be very careful not to introduce bending moments to the chassis frame where they are not supposed to be. In other words, typically you would have two of the mounting points right up behind the cab, a pivot point at the center at the very back. That takes half of your box weight and puts it at the very end of the chassis frame where is is cantilevered off – and very likely on a frame extension that you put on. This introduces too much stress back there, your chassis frame is not designed for that kind of loading. Some people have combined a three point and rail on rail which is pretty creative, but that does add quite a bit of height. Load distribution is the biggest downside by far to the three point frames. However, most of the three point frames that have been done have been on super heavy duty frames like a Unimog where they have a C section frame that can handle that kind of loading, much more so than a Ford F550 frame which is a pretty optimized frame that is not meant for that kind of loading at all.

In both types of torsion frame design, you have body sway issues. Think about going around a sweeping corner and how the load is going to be carried. With a three point, the box will pivot on one of the sides of the triangle, and with rail on rail it will pivot on one of the rails, which reduces body sway. We have added a Liquid Spring suspension to reduce the sway even further and it works beautifully.

Some people with three point torsion frames have added rubber bushings between the torsion and chassis frames to try and reduce body roll and help with weight distribution, but that pretty much defeats the purpose because you are introducing a connection between the frames when the whole point of the three point was to not have a connection between them.

Mounting Points

There are two places to mount your torsion frame to your chassis frame in the rail on rail construction. Behind the cab, or over the rear axle. We chose to mount on the perpendicular axis behind the cab. There are a couple of reasons for that choice. Mounting it over the rear axle means that there will be more relative motion between your cab and your box. This also means that you need more clearance between the cabover and the cab which reduces the height in the cabover (we wanted to stay under a certain height overall). We also have a pass through between the box and the cab. So, less motion between the cab and box at that point is better.

With the front mounting (behind the cab) it shares the load between the front and rear axle as well when the chassis frame is twisting. We added a horizontal pivot axis to try and create a point constraint on two points behind the cab to allow the frame to twist at a very small scale. If you just mounted the box to the front with a flat plate with a bunch of bolts for example and then twisted the chassis frame from front to back, you are increasing the stiffness in the middle and as the frame twists you are introducing stress concentrations, which is bad. The chassis frame is engineered to flex from the front to the back, so you want it to be able to have that freedom and if you constrain it, you are changing the stiffness at that point and will create a crack.

One nice thing about the rear mounting option is that it is a bit simpler to mount. In the back mounting position, you can mount it with plates because the twisting is mostly taking place in front of that point.

With the behind the cab mounting position, we have to have something at the back to prevent our box from skidding off when going around corners and from flying off when going over a large bump. Neither of those things are great for your box. However, the issue again is not to connect your chassis frame to your torsion frame in a way that tries to twist your torsion frame. You want to allow the chassis frame to drop away when needed. The brackets that we put on the rear limit side to side and front to back movement.

As far as preventing the box from flying off the rear: typically, people use an “opposing springs” concept for rail on rail torsion frames, and that is not what we are doing. In this concept, there are brackets on the torsion frame and brackets on the chassis frame and there is a bolt between them, but instead of bolting the brackets together directly the bolt has springs on the top and the bottom. It looks complicated and it is called “opposing springs”, but it doesn’t really mean anything. It is literally just two springs – as far as the mechanics work out – both are being compressed the same amount when there is a separation between them (like when going over a big bump). The problem is, if your frame drops away a certain amount (like a big pothole), the load of the chassis frame is now on your torsion frame because the separation that those springs allow is not very much, depending on their compression rate. We have seen systems out there that only have an inch of separation allowed which is not nearly enough. The spring compression rate determines how much your chassis frame can drop away from the torsion frame, and if you bottom out on it, you are in the situation where your chassis frame is basically hanging from your torsion frame. 

Your goal is to keep wheels in contact with the ground; if you are going down the highway or normal roads, this isn’t hard to do. However, if you are going over terrain, obstacles, potholes, etc, and your torsion frame is tied too closely to your chassis frame, there are two things that could happen, both bad  - either your wheels will lose contact with the ground, meaning that your chassis is hanging from your torsion frame, or the chassis frame will twist your torsion frame putting lots of stress into your box. There is enough travel in the suspension on normal consumer cars that allow the wheel to drop down 4-5 inches. The springs that come with the F550 only have a travel of about 1” from 0-12,000 pounds. That is not enough travel to keep that wheel in contact with the ground. That is why the chassis frame on a F550 is flexible, to compensate for the stiff suspension and keep the wheels in contact with the ground.