pic: WCD Sheet Metal Hybrid

So here is a pretty long description for this. This particular design was one that Adam and I discussed a LONG time ago. I want to say about a year and a half and I finally got around to getting this guy done. (Not easy when your desktop is in California over the summer)

Regardless, this was an awesome learning process for me individually. I havent been inclined to design while Adam is much faster than me and we do our best to give the majority to the kids. Im really there voicing design considerations, doing some light analysis, prototyping with the kids, and managing a bit of fabrication. This was definitely something that helped me gain more skill with design and I am pretty proud of it.

As for a quick background, Adam and I were discussing alternatives to drives with sheet sponsors. We quickly narrowed down that some teams dont use sheet sponsors to the utmost advantage. The frame members that they have are prone to buckling and major deflection with moderate force because of how they are fabricated and joined. This discussion lead us to this drive which utilizes the bellypan as a structural member allowing us to negate the front and back 2x1. This contiguous sheet piece makes the drive strong since you need to have high shear forces at each of the rivets attaching the structure but also the reactions from impact also traveling through the 2x1 as well. Your entire frame is 3 pieces and extremely rigid while losing some weight with the two 2x1s.

There have been others on Chief that have done the following design (Alec and others…sorry I cant remember) and if I had a sheet sponsor I would probably consider heading down this route as well. Obviously for me though, more concrete analysis to solidify that choice would be 100% necessary.

The transmissions are dual speed with both the high gear stage and the low gear stage at 6:1 and 13.63:1 respectively. The respective velocities for high gear and low gear are both ~15 and ~6.5 FPS. I do believe this is a well balanced drive for FRC with the high gear not being too unreasonable and with a relatively low gear while also looking at not tripping the breakers.

Attached is a .STEP file of the following CAD model. What Im looking for is any sort of comments, concerns, or criticism that can be made. Adam has stepped in to help with a few things but I would love to hear others thoughts as well.

WCD Sheet Hybrid STEP

I like the looks of this! We’ve done a WCD/Sheet metal hybrid the past two years that leans more towards the sheet metal side of things. It uses nested C-channels to form a box structure in place of the long 1x2s, with belts captured between them. This looks to be a good way to squeeze out some of the generous horizontal space this takes up for teams with more milling capabilities than us.

What function do the small flanged pieces near the sprockets serve?

My gut tells me you want more meat on both the bumper supports and battery mount. I’d be worried about the bumper mounts paralellogramming upwards (or worse, downwards) under load, and the battery mount’s overhung corners getting bent by a poorly aimed battery, but the overall chassis structure looks great!

If you’re talking about the 4 flanged pieces near the outside wheels, those are bearing blocks that fit in a slot cut into the 2x1. There’s a little cam that is used to change the center-to-center distance between the sprockets in order to tension the chain.

Other question, what do you have going on for bumper mounts there? What kind of hardware/system will they attach to on the bumper, and are they’d integrated into the bellypan or are they separate parts?

What thickness aluminum are you picking for you belly pan? I’m assuming that you are picking 5052 for the alloy, since sponsors don’t like bending 6061.

I’m kind of worried about your front and back rails crumpling under impact. Our 2013 drive base is pretty smashed in, and that was with at least a 2" flange and a bunch of other reinforcement.

I’m torn on the bumper plate strength. It does look a bit small, but your designs have always had flat plates there, and those seem to hold up. Adding in a bend which is perpendicular to the bumper would help enormously. A different design would be a flat plate on the top of the frame with 2 1/2" bends along the edges which are perpendicular to the frame.

Thanks for sharing!

I really like the elegance of this design. A lot.

My only two comments:

-I think that you should use a ‘truss-like’ pattern in the bellypan, i.e. triangular cut-outs. I would bet that using triangles instead of quadrilaterals will make the belly-pan stiffer and stronger.

-My personal preference for joining would be riv-nuts instead of rivets. I like the idea of being able to easily remove and service parts. It looks like you could remove and replace a whole drive pod with just a few bolts, which is pretty cool.

Initially, we had thin flat plates for bumper supports to bridge the WCD gap. After our second event, we iterated to a bended support. We had a bend perpendicular to the bumper that went between top and bottom plates, so the bumper support was riveted on the top and bottom of the WCD rail. This worked out very well for us, lasting through many Aerial Assault matches. Would definitely recommend using a similar approach in place of your current solution.

Overall, very cool/creative idea, I like it.


I won’t comment on the rest of the design, as this is Cory’s, not mine, but the bellypan is a fairly similar to what we and others run.

The goal with the diamonds is to create a single web of material from corner to corner. In theory the belly pan could just be the two corner to corner strips without issue. The bellypan really isn’t loaded in any other direction (aside from component weight), and additional material doesn’t really contribute. It’s worth noting our superstructure generally places a crossmember somewhere along the frame which helps the front/rear crossmembers a lot.

Also, we consider the bellypan integral to the frame, so rivets are a much lighter and faster mate.

Do you think that the frame is really strong enough for using 1/16" wall 2x1? Also, what is your source for that 2x1?

If the extra strength isn’t needed then please ignore my suggestion :slight_smile: If the frame rails need more support to resist bending, then it may be worth considering.

No doubt rivets are lighter and work well. Rivnuts are simply my general preference.

I don’t think Cory specified thickness, I assume he did .125" but do not know.

Both thicknesses we get from Coast Aluminum, who is a full metal supplier. We buy 20’ lengths and pay by the pound. IMO this is the best way to buy metal for FRC.

Thanks for the supplier. I downloaded the STEP file and it is 0.0625" wall.

Since you’re in MA, I doubt they would shop that for (nor would it be logical).

However, I’m certain there is someone filling an equivalent role out there. Figure out where machine shops buy their aluminum and ask them.

We inquired about an order through Coast Aluminum last year for that material specifically and it didn’t work out because shipping to New York was prohibitive for them. However, we were able to find 0.080" wall 2 x 1 6061 aluminum rectangular tube through Alro Metals. It arrived with a kink in the middle, likely from shipping, but we were able to use the material on either side of the kink anyway with little waste. We used this for our rear rail and bumper supports on our WCD. We used 0.125" wall on our front rail due to concerns about the stresses on the frame in that area from our 4 large torsion springs, but I would venture to say we would use 0.080" on that front rail in most years moving forward.

I looked into Alro a while ago for .062" wall, and they had it. 1251 showed it to me as I think it’s still where they order from.

Check out page 10-52 Metal and Plastic Distributor | Industrial Supplies | Alro Metals

If you’re talking about the 4 flanged pieces near the outside wheels, those are bearing blocks that fit in a slot cut into the 2x1. There’s a little cam that is used to change the center-to-center distance between the sprockets in order to tension the chain.[/quote]

I meant the angle pieces highlighted in blue below, not the 973-style WCD bearing blocks.


They’re labeled “Chain Guard”.

You might check out Admiral Metals. That was the place we would go to for metal while I was out at school near Boston. If I remember correctly, they had a pretty expansive area of ends that they would sell by the pound for a good price.

Oh hey, this looks familiar. :wink:

Honestly though, this is a much more thought out version of the “hybrid construction” WCD than anything I was capable of doing when I was a student.
I like this version better. Please build it one of these days and post pictures!

Thanks for the responses everyone. I am going to go through one by one with the posts.


If you didn’t take a look already, it is 5052 for the aluminum alloy with all current components being .090 thick. I know you know that sponsors don’t like bending 6061 just as much as the material itself. (If anyone doesn’t know, 5052 is a more pliable material than 6061 as well as less likely to fracture. It’s commonly used in FRC and other applications)

As for the front and back edges of the drive, this has crossed my mind numerous amounts of times since the beginning of this drive with how our 1/16 2x1 front cross members get destroyed every year. I do think, with some thought, I will make some bent member in there to support the loading seen at the corners. Honestly though, inserting a 2.5x2 inch piece of wood wouldn’t be the worst option! I’ll be taking a larger look at this now that the drive is done.


As Adam previously mentioned, the added support of the bellypan doesn’t come from different methods of webbing. It’s directly correlated to the cross-sectional area of the material that is there taking the load. With what Adam referenced, the cross-sectional area taking the load is directly related to the roughly 1" side of the 2x1 on the bellypan and mostly through the 2x1’s themselves. For reference, many teams have used much more flexible materials for bellypans in the past. I know we used Garolite a few times as well.

Also, riv-nuts would be a solid option as to taking apart the base. I know its your preference but drilling out a rivet might be faster in some cases then unthreading a bolt.


My mistake on the 2x1. They should be .125 thick. A quick fix in Solidworks changed that and the step file in both the topic as well as the picture have been updated.

Bryce & Joe

Typically we have an extensive amount of wires that need to be ran all along our bellypan. The angles are 3/4 x 3/4 angle that protect those wires from the chain at all costs. Also they were used for guiding and organization purposes for the wires. I honestly could have left them out but I decided to throw them in last minute.

Joe, the battery plate could possibly use more support but right now I haven’t taken a closer look and also haven’t went through some “light” FEA to see stresses. I can certainly take a better look and adjust where necessary. Ideally, it would only be two more bends on each side with the same thickness or smaller to help support.

Michael & Austin

From what I saw by your posts, perpendicular definitely sounds the way to go since you’re not acting on only the rivets and actually on the 2x1. I have a picture below that shows off the rendition I believe you both were trying to explain.

If this is exactly what you were looking at suggesting please let me know that way I can add it in and get this guy updated.


Thanks for the words buddy. Unfortunately, there is no major sponsor that we have for sheet currently. My old team did help us out in 2012 with some sheet parts but as of now the only resource we have used was one or two teams willing to help us out. If there was ever a sponsor that showed up with sheet capabilities, I would be certainly inclined to make this!

Just also wanted to point out that the bearing blocks are not exactly what 973 is accustomed to using. The picture below shows the blocks and that they are slightly similar to the VersaBlocks by WCP/Vex. I wanted to see what other geometries could be used in this situation to support the drive shafts.

However this isn’t the best solution to drive bearing block design. I can only preach to the 973 standard because of the large advantages they provide. There are great benefits to it and I can’t stress that enough.