Introduction
Our team has been developing a simple drivebase design in preparation for the 2022 FRC season, in order to preemptively save ourselves a little time when it comes to our drivebase platform week 1. The result is a fairly traditional West coast drive style versachassis, derivative of our recent drivebase designs. A few things have been tweaked and changed, but it was largely designed to be a simple and bombproof mechanism.
Starting point
The foundation for this mechanism was a traditional versachassis. The choices our team has made in the past acted as a starting point for this mechanism iteration, and are listed below:
- 29”x29” frame, for size uniformity and machinability (using only 2 sticks of versaframe per chassis)
- Six wheel drive platform using andymark hi-grips and vex omnis, with all wheels driven.
- Wheel choice has consistently been 6” for us since 2017, but we are likely to choose 4” wheels if the field proves flat enough to allow it.
- We’ve been using 4 neos to drive the robot since 2020 and have found these ideal for the design. We’ve also habitually been using WCP’s SS flipped gearbox for the size and serviceability it offers.
- We’ve historically used a polycarbonate or polypropylene bellypan (usually perforated), and have supported it with an aluminum cross-member.
Changes this year
We had several issues with our online robot drivebase last year, and wanted to address each one in this new design iteration. The issues and their corresponding changes will be bulleted below:
- Frame dimensions: to improve wheel accessibility and ease of machinability, the drivebase dimensions have been changed to 30”x29”, granting an extra half-inch of width on either side (mostly for wheel accessibility)
- Wheel accessibility: in the past our drivebase designs featured a spanning 1”x1” cross member to mount bumpers, which blocked our ability to remove our wheels easily in the pit. This has been addressed by shifting to a tetrapod-style bumper mounting design, using 2x1 VF stock as standoffs. The lateral strength of these standoffs is increased by tying them into the front and rear frame rails of the robot through the bumpers themselves, although these can be bracketed additionally, if needed.
- Chain tension: we’ve had previous issues with cantilever on our drive sprockets, and shifted these as close to the chassis rail as possible to mitigate these. The same inboard shift was done for the wheels to minimize bend forces on the drive axles.
- Bumpers: we transitioned to a 5” bumper board height (from 4.5”) to increase the consistency of the uppermost bumper surface. As an example: our 2020 robot’s intake would sometimes strike soft foam, and sometimes strike solid wood as it deployed, leading to insecurity regarding floor spacing. The bumpers also now clear the robot frame by ⅛”, to simplify mounting.
- Bumper mounting: as mentioned above our bumper mounting method was changed to accommodate serviceability, but has also changed in overall ethos. The bumpers are now held by a series of 8-32 studs, going through mostly unmodified COTS brackets. This shift towards unmodified bracketry was intended to improve consistency and tolerance with bumper mounting, as well as ensure clearance and easy access to bumper wing nuts.
- Electronics bellypan: a new polycarbonate bellypan was designed, able to be machined quickly with hand tools. It featured an inverted shelf for PDH mounting and room for ballast if needed (particularly on our practice/mule drivebase).
Design for adaptability
The design aims to be driven by a small number of root dimensions, everything else being relative to these master variables. This means the design can be changed quickly as robot strategy shifts and updates. The design also allows for a multitude of wheel and drive gearing combinations, mostly because of the inherently customizable nature of a west coast drive style chassis. A list of variables has been drafted with the aim of being answered on day 1 or 2, allowing for a speedy start to machining. This is, of course, barring a radical design overhaul being needed, a-la 2016. The list of design variables is pasted below:
- wheel size/gearing (for 12 fps free speed)
- drop center (y/n)
- wheel type (pneumatic or standard hi-grip, # of omnis)
- drivebase dimensions (30”x29” as a default, should be pretty obvious if we need a small bot)
- wheel placement (likely won’t be changed, but it should be pretty obvious if it does need to be)
- cutout (Y/N, size) - biggest question (most likely to change + most impactful)
- gearbox placement (rear or middle)
- battery placement and mounting/securing style
- pneumatics requirement (y/n, number of tanks, tank placement)
Design for rapid manufacturing
Having an assembled mule chassis and competition chassis by the end of week 1 would be a very nice target to hit, and would allow for a rapid start to prototyping and mechanism testing, as well as provide a feeling of confidence in software and drive team members. A few things have been done to make this drivebase easy to machine, namely making the frame dimensions such that a chassis requires only one cut per 59” versaframe stick, using the integrated versaframe holes wherever possible, including for bumper mounting, designing holes to be centered on versaframe grooves, and designing a 3d printed drilling jig for hole placements where this is not the case (namely the gearbox mounting holes).
Conclusion
The result of this is a quite simple and robust drivebase design, that should be able to be rapidly applied to whatever game FIRST announces come kickoff weekend. Barring any extraordinary games (where the drivebase will have to be considered and developed as a dedicated mechanism), this drivebase design should save us a time and allow for a more rapid start to mechanism development.
CAD can be found here, and BoM here.
Questions and feedback are welcome!