First Drivetrain in Onshape

// Section 01

Plan Before You Open Onshape

The most common CAD mistake is opening the software and starting to drag parts around. Spend 10 minutes with paper first. Your CAD will be cleaner and faster.

📝

Paper first, CAD second. Sketch your drivetrain top-down on graph paper. Mark: overall footprint, wheel positions, motor positions, where the electronics will go. One square = one VEX hole (0.5”). This sketch becomes your first notebook entry.

Drivetrain Decisions to Make Before CAD

Decision	Common Options	What to Consider
Drive configuration	4-wheel, 6-wheel (WCD), H-drive, X-drive	4W is simplest. 6W (2 center traction + 4 omni) resists defense. X-drive is holonomic but mechanically complex.
Wheel size	3.25”, 4” omni or traction	3.25” is lighter and shorter. 4” is faster at same RPM but higher. Robot height matters for game elements.
Wheel type	Omni, traction, mecannum	Full omni = easy to push sideways (bad for defense). Mix traction + omni for balance. Never use mecannum without reason.
Drive width	12”–15” typical	Wider = more stable. Narrower = easier to fit through game elements. Field passages often constrain max width.
Motor-to-wheel ratio	Direct drive, 1:1 through chain, gear reduction	Direct drive is simplest. Chain lets you move motors. Gears give ratio options. Decide before building in CAD.
Motor position	Inline with wheel, offset via chain/gear	Inline = compact. Chain offset = more motor placement flexibility, longer robot or more internal space.

VRC Drivetrain Sizing Guidelines

Max robot size: 18” × 18” at the start of the match (check current game manual — may vary)
Target drivetrain footprint: 12”–15” wide × 13”–16” long — leaves room for mechanisms
Wheel spacing: Space wheels evenly so the robot pivots cleanly around its center. For 4-wheel: equal distance front/back from center. For 6-wheel: center wheel 1/16” lower to rock slightly.
Motor count: 4 drive motors maximum recommended. Leaves 4 for mechanisms.
Gear ratio guidance: 200 RPM motors direct-drive with 3.25” wheels = ~3.7 ft/s. Most VRC drives run 300–600 RPM equivalent at the wheel — that is 4–8 ft/s.

Before opening Onshape to design your drivetrain, what should you do first?

Open Onshape and start inserting C-channels to see what fits

Sketch the drivetrain on paper first — decide wheel configuration, size, motor count, and overall footprint before touching CAD. This sketch is also your first notebook entry.

Look up another team’s design online and copy their dimensions

Build the physical frame first and then model it in CAD afterward

1 / 6

// Section 02

Building the Frame

The drivetrain frame is two C-channel rails connected by crossmembers. In CAD, start with the rails — everything else is positioned relative to them.

Assembly Sequence — Frame First

1

Insert both side rails (C-channels)

Insert two identical C-channels from the VEX library. Common choice: 1×2×35 C-channel (35 holes long, 1 hole wide, 2 holes tall). Fix the first one to the origin — this is the ground reference. Position the second one parallel, spaced to your target drive width.

Fasten mate tip: Use the Fasten mate to lock the first C-channel to the origin (0,0,0). Every other part in the assembly is then positioned relative to it. Never leave parts floating in the assembly.

2

Set the track width

Track width = distance between the outside faces of the two side rails. Use a Parallel mate between the two rail faces, then a Distance mate to set the exact spacing. For a 13” wide drive using 3.25” wheels: rails would be roughly 12” apart face-to-face — always verify against your actual wheel assembly width in CAD.

3

Add crossmembers front and rear

Two crossmembers (shorter C-channels or angle bars) connect the rails front and rear. These are what make the frame a rigid rectangle instead of two rails that can rack. Mate them using the VEX hole pattern — align holes on half-inch increments.

4

Verify the footprint

Use Onshape’s Measure tool to confirm the outer dimensions match your paper sketch. Check both the side-to-side (track width) and front-to-back (wheelbase) measurements. Fix discrepancies before adding anything else.

Take a screenshot here. Label it “Drivetrain Frame — Top View” with dimensions marked. This is notebook evidence for the “Build” EDP phase.

⚙️

VEX hole alignment in Onshape: All mates between VEX parts should snap to VEX hole centers — 0.5” increments. If you find yourself entering decimal values like 0.73” in a distance mate, you are off the grid and something is misaligned. Use Onshape’s “Snap to geometry” feature to click exactly on hole centers.

2 / 6

// Section 03

Shaft Planning

Wrong shaft length is one of the most common physical build errors. CAD gives you exact shaft lengths before you cut anything. Every shaft needs support at both ends and at least one bearing per side.

The Shaft Stack — What Goes on Every Drive Shaft

Every drive wheel shaft follows the same pattern from left rail to right rail. Model this in CAD before you touch metal and you will never cut a shaft too short.

Standard Drive Shaft Stack — left to right (not to scale)

C-Rail

RAIL L

Bearing

BRG

Spacer

SP

Wheel

WHEEL

Spacer

SP

Gear/Motor

GEAR

Spacer

SP

Bearing

BRG

C-Rail

RAIL R

Collar

COL

SP = spacer · BRG = bearing flat · COL = shaft collar · Total shaft length = sum of all components

Shaft Length Calculation

Add up every component’s axial thickness along the shaft. This gives you the minimum shaft length. Round up to the next available VEX shaft length (available in 0.5” increments from 0.5” to 4”).

Component	Axial Thickness	Notes
C-channel wall thickness	0.070”	Same for all VEX structural metal
Bearing flat (each)	0.125”	Two per shaft (one per side)
Spacer — 0.5”	0.500”	Most common size
Spacer — 0.375”	0.375”	Fine-tune shaft stack height
Wheel hub (3.25” omni)	1.125”	Verify in VEX CAD model
Gear (large, 1” wide)	0.500”	Check actual model — varies by size
Shaft collar	0.250”	Always on the outboard end

💡

The CAD workflow: In Onshape, insert the shaft into your assembly and use Distance mates to place each component at its correct position along the shaft. The shaft will visually extend past the collar or be too short if your calculation was wrong — you catch this before cutting anything.

⚠️

Bearing placement rule: Every shaft must have a bearing within 1 inch of any significant load (wheel, gear, motor). Unsupported shaft span + load = bent shaft at competition. In CAD, any span greater than 1.5” without a bearing should be flagged for review.

Your shaft stack calculation gives you a total axial width of 3.38”. What shaft length do you order?

3” — the closest standard length below the measurement

3.5” — round up to the next available VEX shaft length. A shaft that is too short cannot be used; one that is slightly long can be collared short.

3.38” — cut a 4” shaft to exactly this length

4” — always order the next full-inch length

3 / 6

// Section 04

Wheel Placement

Where you place wheels determines how your robot turns, how it resists being pushed, and how much traction it has. These decisions are permanent once you drill the frame.

Wheel Placement Rules

4-wheel drive: Place two wheels symmetrically front/back of center. The turn radius is approximately half the wheelbase. Shorter wheelbase = tighter turns.
6-wheel drive (WCD): Front and rear omni, center traction. Drop the center wheel 1/16” lower than the others — this causes the robot to rock slightly on the center wheel, giving full traction engagement without all 6 wheels on the ground simultaneously. Model this 0.0625” drop explicitly in CAD.
Wheel spacing from frame edge: Wheel face should be 0.25”–0.5” inside the outer frame boundary. No part of the wheel should extend beyond the robot footprint.
Chain/belt clearance: If using chain between motor and wheel, verify there is at least 0.125” clearance between the chain and any structural component along the entire run.

Omni vs Traction — Placement Strategy

🚗 Omni Wheels

› No side resistance — slides freely laterally
› Turns cleanly — no scrubbing on carpet
› Opponent can push you sideways
› Use on front/rear of 6WD, or all 4 on 4WD

🡵 Traction Wheels

› Full side resistance — resists being pushed
› More friction in turns — scrubs carpet
› Use in the center of 6WD
› Worth the turning penalty for defense resistance

📊

CAD check: After placing all wheels, use the top-down view and measure the contact patch center of each wheel. They should be symmetric about the robot’s centerline. An asymmetric wheel layout will cause the robot to pull left or right under autonomous — this is one of the most common causes of auton drift.

4 / 6

// Section 05

Motor Placement & Gear Ratio

Where motors go affects weight distribution, mechanism packaging, and wiring length. Model them in CAD before drilling motor mount holes.

Motor Placement Guidelines

Center of gravity: Keep heavy components (motors, battery, brain) as low and as centered as possible. High CG = tipping risk when pushing or being pushed.
Serviceability: Every motor should be removable without disassembling the entire robot. Model it in CAD and visualize the removal path. If you have to remove 3 other parts to get to a motor, that motor will cause pit problems at competition.
Wiring clearance: Motor cables exit from the back. Leave at least 1” of clearance behind each motor for the cable to route without sharp bends.
Motor temp: Motors need airflow. Fully enclosed motors in tight cavities overheat. Leave at least 0.5” around the motor body.

Direct Drive vs Gear Reduction

Configuration	Speed (200rpm cart, 3.25”)	Pros	Cons
Direct drive — 200rpm	~3.7 ft/s	Simplest, fewest parts, most torque	Slow for most game types
Direct drive — 600rpm	~11.2 ft/s	Very fast	Low torque, stalls under defense, heats up
36t → 48t gear (1:1.33)	~4.9 ft/s	Good middle ground, easy to change	One more gear to manage
Chain 1:2 (60t on 30t)	~7.4 ft/s (200rpm cart)	Motor offset, flexible layout	Chain tension, more parts

⚡

The rule of thumb for Push Back 2025–26: 4–6 ft/s is the competitive range for most game types. Fast enough to reach scoring positions first, slow enough to actually score accurately. Design your gear ratio to land in this range and you can always swap motor cartridges later to fine-tune.

⚙ STEM HighlightPhysics: Torque, Speed, and Gear Ratio

Motor power = torque × angular velocity (P = τ × ω). Gear ratios trade speed for torque: multiply the input RPM by the gear ratio to get output RPM, and divide the torque by the same ratio. A 200RPM motor driving a 36t gear into a 60t output gear produces: 200 × (36/60) = 120 RPM at the wheel, with 60/36 = 1.67x more torque. You cannot increase both speed and torque at the same time from the same motor — this is the fundamental tradeoff every mechanical engineer works with.

🎤 Interview line: “We chose our drivetrain gear ratio by calculating the torque-speed tradeoff. We needed enough speed to reach scoring positions first but enough torque to resist defense robots. We modeled three options in Onshape and chose the one that best balanced both requirements.”

5 / 6

// Section 06

Verification Checklist

Before you hand the CAD off to the physical build, run through every item. Each check prevents one specific type of build error.

CAD Verification — Before You Build

🔬 Drivetrain CAD Review

Screenshot for Your Notebook

Take these four screenshots and attach them to your engineering notebook under the “Build” phase. Label each one clearly.

📷

Top-Down View

Shows overall footprint dimensions. Mark the track width and wheelbase with Onshape’s measure annotations.

📷

Isometric View

Shows the full 3D assembly. Good for confirming motor clearances and overall shape.

📷

Side View

Shows robot height and wheel-to-ground contact. Confirm 6WD center wheel drop if applicable.

📷

Exploded / Section

Shows how the shaft stack goes together. Valuable for the build team and for notebook decision documentation.

What’s Next

⚙️

Mechanism Concept Sprint

Design intake/arm before you build

🔧

CAD to Build Handoff

BOM, screw plan, fabrication notes

🔪

Odometry Pod Build

Drivetrain accessory you can also design in CAD

6 / 6

First Drivetrainin Onshape

Drivetrain Decisions to Make Before CAD

VRC Drivetrain Sizing Guidelines

Assembly Sequence — Frame First

The Shaft Stack — What Goes on Every Drive Shaft

Shaft Length Calculation

Wheel Placement Rules

Omni vs Traction — Placement Strategy

Motor Placement Guidelines

Direct Drive vs Gear Reduction

CAD Verification — Before You Build

Screenshot for Your Notebook

What’s Next

First Drivetrain
in Onshape