pic: Drivetrain WCD

Hey CD
This is a drivetrain that we used for a recent offseason challenge last week, and I am working on updating it. I was wondering if anyone had any ideas on how to keep the robot legal, as it is not a perfect rectangle (the axles are sticking out).

Some Specs:

  • 5.52-1 Geardown
  • 4 inch AM performance wheels
  • Dead axle
  • Center wheel is dropped 1/8 inch
  • Transmission: CIMple Box (Andymark)

Thanks, and Happy Holidays!

You really ought to have more chain wrap on that gearbox sprocket. Either use idlers or two separate chains to go to the middle and rear wheels. You will also probably want another point of support for the CIMple boxes - currently, they are just attached by that single flange, and will want to toe downwards due to chain tension.

What are you using for axles? (Is it a shoulder bolt?)

Looks like a great start!

Jared beat me to it.

Just a little side note that’s not a big deal, but in your title it says WCD. A West Coast Drive has the center wheels dropped (which you have) and directly driven by the transmission. Also, the 4 outer wheels would be directly driven by live axles, either hex shafts or keyways (most do hex shafts.) with bearing blocks integrated inside the chassis rail. The chain would also be run on the inside of the inner chassis rail.

Pic of directly driving the center wheel off the gearbox.

Here is a pic of a WCD. Notice how the front wheels are driven.

Great job on your chassis.

While this drivetrain isn’t a WCD, it doesn’t really matter.

How are you affixing the tube corners together? Welding? The diagonal cuts might not hold the shape as square as you might like.

You’re going to need a lot more chain wrap on that gearbox.

With the chain wrap, try separating that single run into two runs, one to the middle wheel and one to the back. That single run makes me nervous because one chain breaks or falls off or whatever, and the entire side of the drivetrain is useless… which means that your robot cannot do what it is designed to do, namely move around and score points.

@Jared341: It took a while to upload this pic. Since then I have added chain tensioners that have added to the wrap, but I may want to add even more wrap. What do you suggest? The axles are just 1/2in shafts with snap rings on either side

@O’Sancheski: Yeah I realized that after I posted it :ahh: Do you think having the center wheel driven is useful?

@Chrisisme: Yeah we have a lot of experience welding in our shop. As long as you have a good welding jig the chassis will stay square.

@EricH: Good point I will work on a redesign

Thanks all for the excellent input!


An easy way to get a lot of wrap is to add a chain run. Keep the one between the front two wheels, have one just between the back two, and then only drive one wheel (usually the center makes the most sense) with a chain run off the gearbox. You can also do as Eric described. Then all sprockets have ~180 degrees of wrap (plenty if you don’t have too much slack) and tensioning gets easier.

As Chris said, the diagonal cuts in the corners probably won’t hold the frame square as easily. They will also be a bit harder to cut exactly how you want, so I would suggest making the interface square, butting them together something like this:

| or _|

Also, be careful with how low the chains between the wheels hangs. It looks pretty close to ground, and this leads to more friction, more wear, and a stray bolt on the ground could cause the chain to jump. Either bigger wheels or smaller sprockets on the wheels would help.

You could maybe replace the 2x1 tube with 1x1 to make it lighter since you don’t need the space for bearing blocks, but 2x1 will certainly be rigid enough. I assume it’s 1/8" wall.

And ditto on the gearbox support.

Looks good overall!

I would highly recommend directly driving the center wheel. The biggest benefit of directly driving the center wheel is that if you pop chain, you will always have a wheel being driven on that side of the robot. It also saves a little weight (very little) because you will save a length of chain on both sides.

Hope this helps

It’s a tradeoff, though, because a direct drive option won’t let you adjust your speed as easily through the season. For less experienced designers especially, being able to change wheel speed on the fly can be invaluable.

Another drawback with direct drive is that you have less flexible manupulator mounting… usually its not an issue but things like ball intake chutes like seen in 09 may get in the way of direct drive. of course, in that case, you could try direct driving the rear wheels, but that is a whole other discussion.


I updated the drive train with your suggestions, and fixed the shaft problem. What do you guy think?

@O’Sancheski: I would like to work on a drive train that utilizes a direct drive center wheel, but I think I will keep this design chained just because that’s what I am used to working with. Thanks for the input!

Thanks again,


Awesome. I like the cam tensioners too, very neat design.

The two idlers on the gearbox. Are they on bearings? What size bolt are they?

All the tension on your chain will cause a force on your idlers upward, wanting to bend the bolts.

Also, if your idlers are not on bearings, there will be a lot of friction between the chain and those bushings, due to the same tension from the chain.

If it was me, I would use the two chain loops.

I’d agree that two chain loops would be a good idea. Those idlers will probably add a fair bit of friction, and may be failure prone, depending on what size bolt you have in there. And if an idler fails, or a chain breaks on that run, you’ll loose that entire side of the drive. With two loops, you can fail a chain, and still have a wheel or two on that side.

The idlers are just HDPE rollers. I plan to add a plate connecting the two bolts so that they cannot move away from each other.