First CAD of a WCD, how did I do?

I thought about doing that, but then my bumpers would end up being much higher up than I wanted them to be - is there any way to work around that?

When we wanted to adjust bumper height in the season we would change the size of the L bracket that mounts on the rail.

The setup was more or less like this:

Initially, the bumper plywood was flush with the L-bracket and when we wanted to lower them we just took bigger L- brackets and drilled the holes lower in them:


We went one step further along the same idea. We had a sheet metal supplier do up a whole one-piece bumper bracket that extended above the plywood. The nice thing about this design is that the mounting holes ended up bang-on, without relying on the accuracy of wood screws.
CX20-DRIV-P022.PDF (72.6 KB)


You could switch to four inch wheels, which would be a good or bad idea depending on the game.

I will point out that ideally, you want your bumper rails to be over the wheels top height anyway so that during wheel replacement, you can just pull the wheel out sideways rather than having to pull it out and then down out of the bottom of the robot. On my team we have found this very useful for when the robot is just sitting on the ground. If you decide that you’re okay with having to pull the wheel out of the bottom of the robot, at least make sure there is enough space between the edge of the wheel axle and the bumper rail so you can take the wheel off and pull it down between the two. Otherwise, you will have to push the axle towards the inside of the robot to get enough clearance to pull the wheel out from the bottom everytime, which is a serious pain.

This is just something minor I learned from a couple seasons of annoying to maintain drivetrains.


Thank you! This is something I never would have thought of.

This looks great, thank you!

Looks pretty good! CAD methodology critique: your main part studio contains lots of identical parts. I know it can feel weird to have a fraction of a chassis in the part studio, but try to reserve duplicating parts for the assembly.


Another minor suggestion - I like to have our CAD students reserve a spot in the chassis model for handles attached to the bumper rails. Adding handles this early means less chance they’ll interfere with the rest of the robot, and they make for much safer robot handling:
Google Photos

Have you thought about how the various pieces will be fastened to each other? This will have a significant influence on the strength of the chassis. For instance, how will the vertical bumper rail supports be fastened to the side tubes? If you use long bolts that go through both pieces of tube, they will eventually compress the tubes and loosen.

You may also want to use a simple jig to assemble the bumper rail supports to the chassis and the bumper rails onto the supports; i.e. assemble it upside down with the chassis tubes raised off the table by an appropriate amount. Otherwise, it may be difficult to get the bumper rails at the same height and level (front to back) and cause issues when you go to mount the bumpers.

You can use a thin strip of Al (1/8") for the bumper rails on each side since most of the forces will be transferred to the four support pieces anyways. This will allow you to spread the side tubes almost 2" further apart giving you a wider track (more stable chassis while driving) and more room inside.

You may want to delete the corner brackets on the bottom and replace their function with the bellypan. You will have to make cutouts for the VersaBlocks for the front and rear wheels and those corner brackets, leaving the bellypan unsupported for a significant distance (~ 3") at each corner. If the unsupported corners of the bellypan catch something, it may bend back under your robot.

Since your gearbox and drive belts are exposed on the inside of the chassis, it would be a good idea to install a thin wall to separate them from the components you install on the bellypan. Otherwise, a stray wire or pneumatic tube can get caught and be damaged while the robot is moving.

Do you have spacers between the wheels and the gearbox and VersaBlocks? Can you move the wheels closer to the side tubes to reduce how much the wheels are cantilevered (bad memories from 2016)? The VersaBlocks might actually fit inside the wheel hubs. It may be necessary to flip the onmi-wheel so that the screw heads are facing the side tubes.

Edited to add: +1 for the suggestion to add handles made by @nuclearnerd. If you can’t afford the space for a handle, install a large eyebolt into the chassis tubes to allow the use of removable handles.


Yeah you’re right, I’ll make that change.

Thank you for this, this is very valuable advice. There are 1/8" spacers in between the pneumatic wheels and the frame, and the omnis are spaced out so they are on the same centerline as the pneumatic wheels. I’ll definitely incorporate your advice into my design.

If you are able to move the wheels closer to the side tubes, you might be able to go from using 3" tubes for the bumper rail supports to 2" tubes. That will give you another 2" of width between the side tubes. If you do this, make sure your axles are not too long. The outside surface of the bumper rails will define your frame perimeter. The fasteners used on your bumper rails that project beyond the frame perimeter have been acceptable for many years and will most likely be acceptable in future years. Axles projecting past the frame perimeter are not considered fasteners and will not pass inspection.

If you follow the suggestions to get rid of the bumper supports on the front and back, you may want to consider adding a gusset plate from the front and back tubes to the bumper supports at the ends of the side tubes to tie them together. This should add a lot of strength if you are anticipating playing or facing a lot of aggressive defense. There will then be a gap between your bumper and the front and rear tubes. Historically, a gap of less than 1/4" will pass inspection.

If you are going to use thin, pocketed aluminum sheet for your bellypan, it may be advantageous to add an aluminum angle extending from one side tube to the other to stiffen the bellypan to take the weight of your battery. If you install the battery up against the front or rear tube, the aluminum angle can also help retain your battery.

If you are installing a large scoring mechanisms on top of the chassis, it may be better to install your control system parts somewhere other than on your bellypan i.e on some vertical surface(s) around the mechanism. Otherwise, the mechanism may block access to your control system parts making it difficult to install them and troubleshoot them. You may then consider installing the bellypan on the top surface and installing the control system parts on the bottom side of the bellypan. You would want to also install a polycarb sheet on the bottom to protect your control system. This type of installation means you will have to flip the robot on one side or on the front/back to get access to the control system. You should only do this if your team can do wiring to a high standard so you don’t need to access the control system very much.


I have made a bunch of changes based on everybody’s suggestions: Onshape

How does it look?

Ideally, I think you’d want 2 handles per side, so that your 2 drive team members who lift it can use both hands, rather than balancing it with 1 hand at the center of each side. Other than that, looking pretty good!

Edit: Looks like you only have 1 L-gusset in the assembly - need to add those to the other 3 corners as well :ok_hand:

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With the bumper side supports like that, is this something that can happen?


Yes, that is something I was concerned about - how do teams normally go about solving this?

If your bumpers are all connected (1 solid bumper around the full robot), this adds a good bit of rigidity. This idea also works if you have 2-piece bumpers (1 left, 1 right) that are C-shaped and are supported on the front and back chassis rails. A common hybrid of this method is a solid bumper with a gap only on the front for the intake.


4488 ran similar bumper supports to this except 1/16" and no side bar, albeit less of a lever arm than that. But being 1/8" I personally wouldn’t worry about the aluminum giving up. We had some problems with the bottom fasteners but once we switched the bottom rivets to bolts it worked fine.
the torsion could also be slightly mitigated with some spacers coming from the 2x1 into the bumper:
im a bit confused as to what advantage comes from the bumper mounts being placed above the wheels, could you explain what its doing?
Edit: also looks like you are missing some gussets in the corners on the top of the chassis.


Wrapping the bumper rail around the front and back of the robot helps (like your original sketch). That, and/or making C-shaped bumpers pretty much takes care of the torsion issue.


What are your thoughts behind using both a drop center and omnis? To have both is redundant.