🚗 CAD · Intermediate · Drivetrain Design

First Drivetrain
in Onshape

Building in CAD before you touch metal means your frame fits the first time, your shafts are the right length, and your motors clear everything they need to clear. This is how you stop rebuilding.

// 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.
Drivetrain plan-first checklist Five planning steps before opening Onshape: field size, wheelbase, motor positions, gear ratio, open Onshape Field size 144" × 144" measure first Wheel base width & length in inches Motor positions count & spacing sketch first Gear ratio RPM target before CAD Open Onshape all answers ready
📝
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

DecisionCommon OptionsWhat to Consider
Drive configuration4-wheel, 6-wheel (WCD), H-drive, X-drive4W is simplest. 6W (2 center traction + 4 omni) resists defense. X-drive is holonomic but mechanically complex.
Wheel size3.25”, 4” omni or traction3.25” is lighter and shorter. 4” is faster at same RPM but higher. Robot height matters for game elements.
Wheel typeOmni, traction, mecannumFull omni = easy to push sideways (bad for defense). Mix traction + omni for balance. Never use mecannum without reason.
Drive width12”–15” typicalWider = more stable. Narrower = easier to fit through game elements. Field passages often constrain max width.
Motor-to-wheel ratioDirect drive, 1:1 through chain, gear reductionDirect drive is simplest. Chain lets you move motors. Gears give ratio options. Decide before building in CAD.
Motor positionInline with wheel, offset via chain/gearInline = 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 / 7
// 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 / 7
// 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.
Shaft stack diagram Drivetrain shaft cross-section: motor, bearing, sprocket, spacers, omni wheel, spacers, bearing, C-channel rail Motor BRG Gear Wheel BRG C-channel rail / mount V5 motor sprocket omni wheel spacers bearing total shaft length — calculate before cutting

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 to find the minimum shaft length needed.

  • Wheel hub width + spacers + bearing block width + motor shaft engagement = minimum shaft length
  • Add 2–3mm of clearance — shafts that are exactly minimum length are hard to remove
  • Use the VEX shaft length table: 1", 1.5", 2", 2.5", 3" standard lengths
  • If your stack requires an unusual length, use shims or adjust spacing rather than cutting shaft
ComponentAxial ThicknessNotes
C-channel wall thickness0.070”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 collar0.250”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 / 7
// 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

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 / 7
// 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

Direct Drive vs Gear Reduction

ConfigurationSpeed (200rpm cart, 3.25”)ProsCons
Direct drive — 200rpm~3.7 ft/sSimplest, fewest parts, most torqueSlow for most game types
Direct drive — 600rpm~11.2 ft/sVery fastLow torque, stalls under defense, heats up
36t → 48t gear (1:1.33)~4.9 ft/sGood middle ground, easy to changeOne more gear to manage
Chain 1:2 (60t on 30t)~7.4 ft/s (200rpm cart)Motor offset, flexible layoutChain 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 / 7
// 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
🔬 Check for Understanding
You finish CADing a drivetrain and the wheel spacing looks correct on screen. The physical build ends up with wheels 3mm too close together, causing binding. What was most likely missed?
The wrong wheel size was used in the model
The CAD model didn't account for the physical thickness of the C-channel walls and hardware — the CAD looks right but didn't model real-world assembly clearances
Onshape's default units are in centimeters, not millimeters
The mate connector was placed on the wrong face
Correct. CAD clearance errors are the most common cause of build-to-CAD discrepancies. Model all hardware (screws, spacers, shaft collars) in the assembly, not just the structural parts. A 3mm error usually means a spacer or hardware thickness wasn't included. Document the correct spacing in your Orange build slide once it's verified physically.
6 / 7
For Coaches & Mentors
Assembly vocabulary, teaching tips, and the official PTC/Onshape drivetrain unit guide.
🛠
Best practice from PTC: Have the actual drivetrain parts physically in the room while students work in Onshape. Even one assembled drivetrain as a manipulative significantly helps students visualize parts in 3D space on screen. The connection between digital and physical is where learning sticks.

Key Vocabulary

These terms are specific to Onshape Assembly work — introduce them before students open the drivetrain document.

Mate
An Onshape feature that positions and connects parts in an Assembly, defining how they relate and move relative to each other.
Fix
Grounds a part in space so it cannot move. Use on the first part you insert to anchor the assembly.
Mate Connector
A local coordinate system used by mates to locate and orient parts with respect to each other. Think of it as a snap point.
Fastened Mate
Mates two parts and removes all movement between them — like bolting them together. No relative motion allowed.
Revolute Mate
Allows only rotational movement about the z-axis — used for wheels, shafts, and anything that spins.
Cylindrical Mate
Allows both rotational and translational movement along the z-axis — used for sliding and spinning components like pneumatic pistons.
Parts Library
A curated collection of pre-built parts for a specific robot platform. VEX parts, motors, and hardware are all available in the VEX Parts Library.
Feature Tree
The left sidebar showing every mate and instance added to the assembly in order. Editing an item higher in the tree cascades updates down automatically.

Instruction Guide

Beginner Students

Start with the Onshape Assembly Basics lesson before the drivetrain. Gather students together and walk through parts as a group. Advanced students or mentors should be available to help.

Advanced Students

Have a lead CAD student walk newer students through this unit. Advanced activity: ask them to model a different drivetrain configuration from scratch using the parts library.

Official PTC/Onshape Resources

📚 Unit Guide — PTC / Onshape
Assemble a Drivetrain
Full instructor guide with vocabulary, preparation notes, instruction tips, and the VEX Onshape document link.
🔗 PTC Resource Center →
🛠 Official VEX Document
Assemble a Drivetrain — VEX
The official PTC Onshape drivetrain assembly document for VEX. Each student needs their own copy to complete the build.
🔗 Open in Onshape →
🎓 Learning Center
Onshape Assemblies
Two self-paced courses: Onshape Assemblies and Introduction to Assembly Design. Hands-on exercises students can complete independently.
🔗 Start Course →
7 / 7
Related Guides
⚙️ Mechanism Sprint → 🔧 CAD to Build → ⚡ Wiring & ESD →
▶ Next Step

Drivetrain modeled. Now CAD your scoring mechanism using the Mechanism Concept Sprint.

⚙️ Mechanism Concept Sprint →
▶ OFFICIAL ONSHAPE TRAINING — LEVEL 3: BUILDING FROM A CAD MODEL
CAD for VEX Using Onshape
Level 3 — Building from a CAD Model
This site is an informational reference. RECF EN4 prohibits AI-generated content in engineering notebooks and programming code. Rewrite everything in your own words. About EN4 →  ·  References & Sources →
← ALL GUIDES