[apalrd]

Let's analyze this from the beginning:

1. Material selection - Alloy and thickness
-I will assume aluminum. Most sheet metal designs in FRC use either a 5052 alloy or 6061 alloy. 5052 is softer than 6061, which makes 6061 harder to bend. This is both good and bad - Many sheet metal shops will only work with 5052 because it is easier to bend. If you use a thin 6061 alloy, it will bend just fine. We use 6061 T6, and bend it on a manual finger brake.
-Common thicknesses are 0.050", 0.063", 0.090", and 0.125". Some designs rely on the materials for strength, but good sheet metal designs rely on the profile of the sections (e.g. making C-channels or triangle beams out of formed sheet pieces) rather than the absolute material strength. We use 0.050" for our chassis and 0.090" for our gearboxes, with a mix of 0.050, 0.063, 0.090, and 0.125 for mechanisms. In 6061, 0.090" is hard to bend, but we do bend the gearbox plates. I've seen most 5052 teams use 0.090 for chassis components, but I have no experience with 5052 in that application.

2. Important forces/loads:
-Most chassis designs I've seen in FRC can handle the standard loads from gravity (weight of robot) and straight line acceleration, although point loads from mechanism attachment can bend thin sheet - it's better to spread out mechanism loads over a larger surface, using rows of rivets. It's also possible to use mechanisms for stiffness, which I'll get to later.
-Good dynamic performance of drop-center drivetrains requires good torsional stiffness. This is the rotation of one driveline relative to the other, and turning forces can pick raise and lower wheels such that the exact wheels you DON'T want to be touching the ground during a turn will be touching the ground, which is bad for dynamic performance.
-Any component which is stressed, or carrying loads which you are considering for chassis performance, should be modeled, anything which is not stressed should be modeled only for packaging at this stage. In some designs, the gearbox is stressed (carrying chassis loads). We (33 2013) use our gearboxes to span the chassis outer and chassis inner, as well as bridge the chassis inner rear and front panels (they are not continuous panels, the outers are). A bellypan as used by west coast drives is often thick plate (0.125" or 0.250") and used for material strength rather than as a section as sheet metal designs primarily use. It will help with racking of the two sides, but not will do nothing in torsion at the thicknesses used for sheet metal.
-Mechanisms can be used to increase stiffness of the chassis, especially if they span the width of the chassis.

3. Sections
-The strength of sheet metal designs primarily comes from the sections. A flange can turn a flat sheet into a L or C channel, greatly increasing it's strength. A box is hard, as it is almost impossible to bend unless it's more than one piece.
-The two chassis rails are almost always C channels. Cross members can be single C channels, but a full box or triangle section is very strong. Our 2012 chassis had two triangle sections crossing the chassis, one of which was also a ball ramp, and was very stiff.

4. Folding sheet metal
-Sheet metal is usually folded on a brake. With a manual finger brake, it's possible to remove fingers to bend tight corners (e.g. your chassis outer panel). Flanges which are bent several times (e.g. a flange off a flange) are usually hard to bend, there are a few exceptions such as the chassis rear in your design, which has two ~45degree bends and should be easy to bend.
-For some pieces which are small, you can bend them by hand with furnace pliers. We made quite a few ~4" square pieces with 4 flanges (one per side), two flanges had rivets and the other two for strength, to bridge chassis inner and chassis outer panels. These were easy to bend on the finger brake. We also once made a piece which connected to itself, this was tricky but we could bend it mostly by hand with furnace pliers. In general, designs which connect back to themselves should be split into two or more panels.

5. Joints
-A lot of stiffness can be lost in joints. Most sheet metal is joined with rows of rivets. There are many options for rivet size (1/8", 5/32", 3/16", 1/4"), spacing (1/2", 3/4", 1", 1.5"...) and material (steel or aluminum). We have used both 3/16" steel and 1/4" aluminum rivets in chassis designs and currently use 1/4" aluminum, but most hand riveters only go up to 3/16". I recommend pneumatic riveters, they're lots of fun.
-If possible, jointing two pieces on two planes is preferred. For example, joining the chassis outer to front panel via the top and front faces is much better than top only, or side face. This is especially important for mid-chassis cross members, as it's easiest to only attach them via the top or side, but both are required for stiffness.
-There are many cases where an axle needs to be supported by sheet metal. For a light-load application like a conveyor roller, you can just make a hole in the sheet metal and insert a bearing or bushing. For drivetrain axles, it's recommended to stiffen the area around the hole with another plate riveted to the chassis. This can also help align the axle and constrain it, depending on design. If you make it slightly beefier to hold the entire axle load, you can also slot the holes and use bolts instead of rivets, allowing tensioning.

6. Generic drivetrain concerns
-Wheel size
-Wheel tread/type
-Gear ratio
-Power input
-Using required drivetrain axles/components to stiffen the chassis
We often use the drive axles to provide stiffness between the chassis inner and chassis outer. Chain tensioners are also a good candidate for this. If designed right, a gearbox in the chassis can also provide a lot of stiffness between the two rails.

7. Manufacturing
Panels are 2d parts and can be cut on a 2d machine, such as but not limited to:
-CNC plasma cutter (not preferred for aluminum, but it does work)
-Waterjet cutter (good tolerance, moderate edge finish)
-Panel laser (excellent, very very expensive tool)
-Turret punch - Limited to specific contours which it has tools for, unless it's integrated with a laser or other cutter. If you use one, it is best to optimize the lightening patterns to what tools it has - e.g. if it has a 2" round punch, it's much faster to use 2" round lightening holes than triangles.
-By hand on bandsaw and drill press, this is sometimes impractical for large parts
Panels are then bend into 3d shapes using a brake. This is either automated or done by hand.
-There are many shapes which can be made in CAD which cannot be formed on a brake. Think about the bend order and how you would bend it for complex parts.

There's no substitute for design experience. No chassis design is perfect, there are many tradeoffs. You really need to build and break it to understand what is wrong with it, but you can catch a lot of possible issues in CAD.