Drivetrain: For the Crater Good

For the past couple days we’ve been working on designing a drivetrain for the MCC. The one thing to keep in mind when designing a system is to always refer back to your list of robot features to make sure you’re still on the right path. You can find all our robot features in the “FTC Minimum Competitive Concept (MCC) Robot” post we made earlier today. For reference, the features for the drivetrain are:


  • 4-6 wheel drive
  • Geared between 16:1 and 20:1
  • Can easily drive into and out of Crater


We started off by designing a simple drivetrain in CAD. This one is based on the VersaChassis Mini System:

We already know from yesterday that our new Flex Wheels can work as a drive wheel.

To help test out some of the concepts like wheel spacing and positioning we made a version of this drivetrain out of VEX EDR structure:

For the sake of speed, we moved the control system from the test platform we tested yesterday to this robot. However, we quickly found that this robot struggled with becoming high centered on game objects.

After this, we decided to try and add an additional set of wheels in the middle of the drivetrain. These wheels were idler wheels that were not driven.

Here’s a video of some testing we did with this configuration:


We also did some testing to see the pushing power of the Flex Wheels against other robots. So we took some of the robots sitting around the VEX office and tried to push them sideways:


We still weren’t happy with this drivetrain setup, as we still struggled getting into the Crater and still had trouble with becoming high-centered on Minerals. To help with this, we decided to try a larger wheel. We don’t have a 5” or 6” Flex Wheel, so this meant we would need to use a 6” VEXpro traction, omni-direction or mecanum wheel. We first tried all omni-directional wheels:


This worked a little better, but we found we needed a lot of speed to get into the Crater. This was due to the omni-directional wheels not having enough traction against the perimeter of the Crater. We then switched two of the omni-directional wheels out for traction wheels:


This worked somewhat better, but we still had issues getting high centered on the balls. We went back to the drawing board and decided to try doing something a little less conventional by creating an A-Frame chassis that created a ton of clearance between the wheels:

This design allowed us to go back to using Flex Wheels, which we believe were the best wheels we tested when it comes climbing into the crater. Here are some videos:


There is still some iteration testing we need to do, such as increasing the angle of the A-frame from 45 to 60 degrees, and shortening the overall robot so it can fit under the Lander to avoid defense.

Another change we want to make is putting a VersaPlanetary 180 Degree Drive on the drive motors. This will give us more space between the drive motors and reduce the amount of game objects we ‘rake’ out of the Crater when we drive in and out.

Since the 180 Degree Drive has a 2:1 reduction between the motor and the first gear stage, we can also drop a gear set from our robot, which will decrease weight even more. However, this will limit our gear reductions to 18:1 (2:1 -> 9:1) or  20:1 (2:1 -> 10:1). All the videos you’ve seen in this post use a 20:1 reduction, so we’re fine with being inside this range. If we wanted to go slower we could use a 180 Degree Drive into a 2 stage VersaPlanetary and get 24:1 (2:1 -> 4:1 -> 3:1) or 30:1 (2:1 -> 5:1 -> 3:1)

If we review our list of drivetrain features we feel like this drivetrain setup does everything we want our drivetrain to do:


  • 4-6 wheel drive (Check)
  • Geared between 16:1 and 20:1 (Check)
  • Can easily drive into and out of Crater (Check)



P.S. Part of us really wanted to stick with using omni-directional wheels. There’s something oddly satisfying about jumping into the Crater at full speed. We guess we’ll have to find a way to do the same with Flex Wheels.