Questions about WCD for offseason

I’m thinking about designing and building a West Coast drivetrain in the off-season to be able to implement it in the 2020 game. I’ve got a few questions about going about doing that.

  • Where does the chain go and how is it tensioned?
  • How do you mount gearboxes to VersaTube?
  • How can we construct it with as many COTS components as possible so we can reuse the parts to conserve funds?
  • Is a belly pan necessary?
  • If anyone has cad of a simple design could they share it please.

We are planning on using Vex bearing blocks to mount the wheels, use VersaTube for the structure, and use a 2 CIM single speed gearbox on either side, with a six wheel center drop.
To machine this we have access to a very good drill press, a bandsaw, chopsaw, and hand tools.

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There’s some example CAD models on Thisbe page that should answer most of your questions.

As far as bellypan, it’s not strictly necessary but you’ll be hard-pressed to find an equally straightforward method of mounting electronics.


Here is a video series that takes you through the entire process of building our WCD. We’ve been using this general design since 2015.


I’ll echo that. If you want a COTS WCD (cantilevered axle) chassis, you can’t do better than the VEX versa chassis (with versa blocks and cams to tighten the chain). The total cost is comparable to the KOP chassis. It’s a little harder to assemble (some drilling and riveting required) but not hard that a rookie team couldn’t do it. The Versa Chassis is a little weaker than the KOP chassis, but so much easier to change wheels, which is the killer feature IMO.

Is there a way to mount a 2 CIM single speed gearbox instead of a 3 CIM two speed?

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If you use a WCP SS or the Vex SSDR, I think the mounting holes are the same as the ball shifter.

  • chain goes in between the gearbox and the wall that supports bearing nearest to the wheel (i know, kinda confusing, check out some of the cads mentioned above)
  • box channel/versatube is probably the most COTS way i can think of.
    use a WCP gearbox, and just bolt it to one wall of the box(drill a hole for access)
  • bellypans are not necessary, but you usually end up with them anyway if you want to mount stuff on the bottom. A drivetrain only bot for offseason probably needs a bellypan, cuz you won’t really have anywhere else to mount your electronics.
    scroll down to drive train in 254’s 2018 technical binder, they used a box channel wcd last year

The center axle doesn’t have to be mounted to a versa block. The four # 8 mounting holes are so small they’re basically garbage anyway - my one gripe about the versa chassis system. Instead, if you’re able to make an accurate 1 1/8 hole in the center of the drive rail, you can use that hole to pilot whatever gearbox and match drill the mounting holes through the tube.

(If you do mount to the versa block though, drill out and tap those holes for #10-32, and use loctite. You’ll thank me)

The special thing about WCD gearboxes is that they are a little thicker to allow two loops of chain to pass through. Most of VEX and WCP offerings accommodate the chain already.

Nothing is going to be 100% bolt together kit bot with this, but this is the #1 reason why to dive into a project like this in the off-season. It’s great to research a bunch and solve as many conflicts before ordering. With the WCD there are so many combinations of gearboxes, chain, belts, in tube or out of tube.

We built our first WCD chassis last fall using the 2018 MCC parts list Located on the West Coast Products page. The CADD and BOM are all listed. We did deviate some on the final mounts for the Gearbox but that was for our own needs. We did end up using what we learned and built a WCD for a 2019 robot and also shifted to Chain in tube. That was a blind adventure that we did trip up on trying to fit #35 chain and sprockets. We knew and planed only one section would possibly fit but when that did work out the way we wanted we quickly punted and installed #25H in tube. Ended up being happy with the results.

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I’m in the process of designing 1777’s offseason robot, which features a VersaFrame WCD chassis using the VEX dual-speed ball shifters. The wheel axles are using screws and washers for retention, so it does require a lathe unless you make the axles slightly longer to accommodate shaft collars. The interesting thing about the CAD is that the length, width, wheel spacing, etc. can easily be adjusted by simply changing a number or two on an Excel spreadsheet. I can PM you the design if you’d like, although the Excel parametric part won’t work if you aren’t using Autodesk Inventor.

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The reason we want it to be as COTS as possible is for us to be able to take the parts that we bought during offseason and then build something a bit more specialized for the competition.

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The Versa chassis stuff linked in the BOM of the MCC is all COTS.

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Well thunderhex already has hole that can be tapped to 1/4-20.



Another question. Is it possible to run 25 gauge chain in tube through a piece of VersaTube?

Yes, especially with the AM double sprocket now.

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Just throwing this out there: If you are going to go through the effort of building your own WCD, I would suggest using 2 NEOs per side or 3 minicims per side. Otherwise, the KOP chassis is probably just as good and allows you to focus your efforts elsewhere.

Also, don’t forget to include encoders on your drivetrain. You want one per side and it should spin at the same speed as the wheels if possible.

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  1. Chain goes in one of three places: (1) on the inside of the rail to which your gearbox is mounted. A number of the West Coast Gearboxes actually have space in the gearbox for pulleys or sprockets; (2) inside the tube itself (generally, a vex VersaFrame 1x2 tube); (3) on the outside of the rail to which your gearbox is mounted, behind the wheels.

For the past two years, we’ve gone with option (3). A lot easier to swap out belts.

  1. Depends a bit on the gearbox. In general, you use a bearing block and drill a hole through the VersaTube. For hole-cutting, we used an Annular Cutter (links here on CD someplace).

  2. As others have pointed out, look to vex. You won’t be able to reuse your 1x2 tubing, but everything else (gearbox, bearing blocks, wheels) should be fine.

  3. It’s a good idea, but not because of the WCD.

regarding the single speed gearbox, you need to be careful about your shaft length. Last year, we used the plastic SSDR 2-cim Vex gearbox – the shaft sticks out a bit over 3 inches from the gearbox. Then, you add 1" of Versaframe, and you’re down to a bit over 2". This year, we used the West Coast “Flipped” gearbox and just replaced the shaft with a longer piece of hex. (Also had to replace a bearing in the gearbox.) If you do that, you need to have some other support for the shaft along the way, so it’s not just supported by the gearbox.

Three tools that will be helpful to you:

(1) An Arbor press. The 1 ton press from Harbor Freight is cheap and more than sufficient.
(2) A 14x20 tap for Vex thunderhex – tap a hole, insert a screw with a washer and now you have something that holds your wheel onto your shaft. (Use locktite on the screw, but keep it off the wheel!)
(3) Hougen Annular cutter. Makes very nice 1 1/8 inch holes on a drill press. Depending on your gearbox, the 1 1/2 in cutter might be helpful.


Nice. Is there a disadvantage to chain in tube? I want to run it so that the chain doesn’t interfere with the insides of the bot.

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The disadvantages is that it is (often) harder to design with COTS parts and that if the chain needs to be serviced it is not easily accessible.

You mentioned wanting to use versablocks. I would suggest using the cams that VEX sells as well and just running the chain outside the tube.