V2 Design — Chassis + Stacked Lift Geometry

SECTION 0 / 5

The Architecture

A 4-bar lift with chain bar end-effector. The 4-bar handles vertical reach; the chain bar handles 180° rotation to swing the claw between back-pickup and front-place positions.

The Two-Stage Lift Concept

This design stacks two mechanisms:

4-bar lift (parallel linkage): Pivots at a tower mounted to the chassis. Lifts the chain bar pivot up and forward as it rotates from -45° (down-back) through 0° (horizontal-back) to +90° (vertical-up).
Chain bar arm: Pivots at the END of the 4-bar (the moving end). Rotates 180° from 0° (folded back along the 4-bar) to 180° (extending forward past the chassis).
Pneumatic claw: Mounted at the end of the chain bar. The chain bar's static-sprocket geometry keeps the claw level throughout both rotations.

🎯

Why this combination wins for Override: the 4-bar gives you height (reach the 8.77″ goal); the chain bar gives you 180° sweep (back-to-front cycle). One mechanism does both. The chain bar's static-sprocket design keeps the claw level so the cup never tips during the swing.

Match Cycle (What This Robot Does)

Position behind cup: drive backward toward the cup at midfield or loader.
Pickup mode: 4-bar drops to -30° (down-back), chain bar at 0° (folded back). Claw open, hovering ~3″ above ground at the back of the chassis. Claw closes around cup.
Transit: 4-bar lifts to ~+30° while chain bar swings 0° → 180°. Cup stays level the whole time.
Placement: 4-bar lifts to the goal-specific angle (see § Reach Verification). Chain bar at 180°. Claw extends 7″ past the front edge of the chassis. Claw releases cup.
Reset: chain bar swings back to 0°, 4-bar drops to -30°. Ready for next cup.

Why 3.25″ wheels (vs 4″ or 2.75″)?

3.25″ is the V5RC sweet spot for this application:

Higher ground clearance than 2.75″: rolls over loose pins and cup debris at midfield.
Lower chassis than 4″: chassis sits ~1.6″ above ground (vs ~2.0″ on 4″ wheels), giving 0.4″ more vertical envelope for the lift mechanism.
Compatible with most VEX gear ratios: works with 36T:60T or 36T:72T reduction sets.
Available as omni: 3.25″ omnis are standard part 276-1454, no special ordering.

SECTION 1 / 5

Game Element Math

Override dimensions that drive the lift design. Goal heights, cup/pin sizes, and what your claw must reach.

Authoritative Override Dimensions

Element	Dimension	Source
Cup	6.5″ tall × 3.15″ wide hourglass (164.5 mm × 80 mm)	Manual Glossary, Fig A6
Pin	6.50″ tall, hex prism (3.16″ widest, 1.40″ narrowest)	Manual Glossary, Fig A5
Alliance goal	3.25″ (82.5 mm) tall	Manual Glossary, Fig A7
Short neutral goal	5.77″ (146.5 mm) tall	Fig A7 (Glossary rounds to 5.8″)
Tall center goal	8.77″ (222.7 mm) tall	Fig A7 (Glossary rounds to 8.7″)
Robot start envelope (R3)	18″ × 18″ × 18″	Game Manual R3

Reach Targets

For each scoring scenario, here's what your claw must reach:

Cup placement on goal (single cup, no stack): Goal height + cup half-height (where claw grips) + clearance = 8.77″ + 3.25″ + 1″ = ~13″ claw center height (tall goal) = 5.77″ + 3.25″ + 1″ = ~10″ claw center height (short neutral) = 3.25″ + 3.25″ + 1″ = ~7.5″ claw center height (alliance goal) Cup placement on existing pin (1-stack on tall goal): Goal + pin + cup half-height + clearance = 8.77″ + 6.5″ + 3.25″ + 1″ = ~19.5″ claw center height Cup pickup from floor: Cup is 6.5″ tall; grip mid-cup Claw center height ~3″ above ground

📐

Design target for this guide: reach 13″-14″ claw height. Covers cup placement on all three goal heights without stacking. Stacking on tall goal (19.5″ reach) is a Phase B / advanced upgrade — design now, build later.

Cycle Geometry (Pickup → Place)

The robot needs to:

Pick up cup at the back of the robot (claw at Y ≈ 0″ from back edge, height ≈ 3″)
Carry cup level through the swing (cup orientation matters — opaque/transparent face matters per SC3)
Place cup at the front of the robot on a goal at variable height (Y ≈ 19″ from back, height varies 7.5″ to 13″)

Required claw motion: from (Y=0, H=3″) to (Y=19″, H=13″). That's 19″ horizontal + 10″ vertical travel. Single mechanisms can't do this; that's why we stack the 4-bar + chain bar.

SECTION 2 / 5

Chassis Layout (Top-Down)

Where everything lives on the chassis bottom plate. Chain bar tower at Y = 12″ from back, 4-bar arm length 8″, drivetrain/battery/brain in the back half.

17"

Wheelbase L

Front-back distance between wheel centers. Provides stability margin for forward-extending lift.

14"

Wheelbase W

Left-right between drivetrain wheels. Leaves room for bumpers + 18″ envelope.

12"

Tower Y

From back of chassis to 4-bar pivot. Asymmetric — not center.

4-Bar Length

Parallel link length. Lifts chain bar pivot 8″ up + over.

Chain Bar L

From chain bar pivot to claw center. 180° sweep arc.

Top-Down Chassis Layout (Inches)

↑ Top-down chassis layout. 4-bar tower placed at Y=12″ from the back of the chassis (asymmetric — not center). Battery + brain occupy the back half (Y=2-10″). Optional pneumatic tank fits in the front quadrant. Drivetrain: 4×11W full motors at corners + 2×5.5W half-motors in the middle (push-heavy 55W config from /override-drivetrain-config).

Why Tower at Y=12″ (Not Center)

The geometry constraint: when 4-bar is collapsed (-30°), the chain bar pivot sits behind the tower. When 4-bar is up (+90°), the pivot sits ABOVE the tower. The chain bar pivot's reach forward depends on the 4-bar arm length plus the chain bar length minus the tower's distance from the front edge:

Forward reach (claw at front, 4-bar up + chain bar at 180°): Y_claw = Y_tower + 4-bar_length \times sin(angle_above_horizontal) + chain_bar_length At 4-bar +60°: Y_claw = 12 + 8 \times sin(60°) + 7 = 12 + 6.93 + 7 = ~26″ from back But that's ABOVE the chassis — need horizontal projection Y_claw_horizontal = Y_tower + 4-bar \times cos(angle) + chain_bar \times cos(0°) At 4-bar +60°: Y_horizontal = 12 + 8 \times cos(60°) + 7 = 12 + 4 + 7 = 23″ from back = 5″ past the front edge (front edge is at Y=18″)

So a tower at Y=12″ gives the claw 5″ of forward reach past the chassis edge when at lift angle ~60°. That's enough to score on a goal sitting just in front of the robot. Move the tower further back (Y=10″) and you gain forward reach but lose battery/brain space.

SECTION 3 / 5

Side-View Kinematics

Three positions of the lift overlaid: pickup mode (back-low), transit (mid), placement mode (front-high). Tracks the claw's position through the cycle.

Side-View: 4-Bar + Chain Bar Through Three Lift Positions

↑ Three lift positions overlaid. Pickup (green): 4-bar at -30°, chain bar folded back, claw at H≈1.1″ near back of chassis ready to grab cup. Transit (yellow): 4-bar at +30°, chain bar at 90° (perpendicular, pointing forward), claw at H≈12.6″ carrying cup level. Placement (orange): 4-bar at +60°, chain bar at 180° fully forward, claw at H≈15.5″ above tall goal (8.77″) ready to place. Goals shown at right for reach reference.

Kinematic Equations

Generic equations for any 4-bar angle θ (measured from horizontal-back) and chain bar angle φ (measured from 0° = parallel-back along 4-bar arm):

Chain bar pivot position (end of 4-bar arm): X_pivot = X_tower - L₁ \times cos(θ) H_pivot = H_tower + L₁ \times sin(θ) where L₁ = 4-bar length (8″), X_tower = 12″ from back, H_tower = 8.625″ from ground Claw position (end of chain bar): X_claw = X_pivot + L₂ \times cos(θ + φ - 180°) H_claw = H_pivot + L₂ \times sin(θ + φ - 180°) where L₂ = chain bar length (7″) Note: The chain bar's static-sprocket geometry holds the claw level (no rotation about the claw axis), but the position equations still apply.

SECTION 4 / 5

Reach Verification

For each goal height + scenario, what 4-bar and chain bar angles are needed? Does the design hit every required position?

Required Lift Angles by Scenario

Scenario	Target Claw Position	4-bar Angle θ	Chain Bar Angle φ	Result
Cup pickup at back	Y≈0″, H≈1.1″	−30°	0° (folded)	✓ Reaches
Cup transit (carrying level)	Y≈15″, H≈12.6″	+30°	90°	✓ Above chassis
Place cup on alliance goal (3.25″)	Y≈21″, H≈7.5″	+15°	180°	✓ Reaches
Place cup on short neutral (5.77″)	Y≈21″, H≈10″	+30°	180°	✓ Reaches
Place cup on tall center (8.77″)	Y≈21″, H≈13″	+50°	180°	✓ Reaches
Stack cup on pin (tall goal, +1)	Y≈21″, H≈19.5″	+90° max	180°	~ Just at limit

✓

Design verifies for all three goal heights without stacking. Stacking on the tall goal (cup on top of pin already on goal) reaches H=19.5″, which is at the design's maximum vertical extension (4-bar at 90° vertical + chain bar at 180° forward = 8.625 + 8 + 7 = ~23.6″ theoretical max, but practically limited by mechanical interference and motor torque budget). Stacking is achievable but at the edge — expect difficulty.

Maximum Reach Check

Theoretical max claw height (4-bar at +90° vertical, chain bar at 180° forward but elevation matters): If chain bar swings purely vertically: H_max = 8.625 + 8 + 7 = 23.625" If chain bar swings purely horizontally at +90° 4-bar: H_max = 8.625 + 8 = 16.625" Practical max for cup placement (4-bar at +60-70°, chain bar at 180° horizontal-forward): H \approx 8.625 + 8&sin;(65°) + 0 = 8.625 + 7.25 = ~15.9" Design margin over tall goal (8.77") : 15.9 - 8.77 = 7.1" of vertical clearance — comfortable.

Forward-Reach Check (claw past front edge of robot)

How far past the chassis front does the claw extend? Front edge of chassis: Y = 18″ Claw at Y_claw = X_tower + L₁&cos;(θ)\cdot(forward direction) + L₂ At θ=+60° (placement on tall goal): Note: 4-bar arm direction is up-and-FORWARD only past θ=+90°, otherwise it's up-and-BACK Below θ=90°, the 4-bar is still angled toward the back. Forward reach is mostly via chain bar. Y_claw = 12 - 8\cdotcos(60°) + 7 = 12 - 4 + 7 = 15″ from back = 3″ INSIDE the front edge That means the claw is over the chassis, not past it. To score, the robot must drive forward so the GOAL is under the claw position.

⚠️

Honest correction to my Section 2 example: the math above is more accurate. With this specific 4-bar geometry (8″ arm pivoting back-and-up), the claw at +60° is positioned over the chassis at Y=15″ (3″ inside front edge). To score on a goal, the robot must drive forward until the goal is under the claw — the lift doesn't reach forward past the chassis edge by much. This is a key design tradeoff: if you want the claw to extend forward of the chassis, you need either (a) a longer chain bar, (b) a different 4-bar geometry that points forward when lifted, or (c) accept that the robot drives up to each goal.

Two Design Options to Get Forward Reach

If your strategy needs the claw to extend forward past the chassis edge (so you can score without driving into the goal), pick one of these adjustments:

Option	Change	Effect	Tradeoff
A: Longer chain bar	Chain bar L = 10″	Claw at Y=18″ (at front edge)	Larger sweep diameter (20″) requires careful R3 fold-back; chain alignment more sensitive
B: Forward-leaning 4-bar	Tower closer to back (Y=10″), 4-bar swings to +120° past vertical	Claw can extend 2″ past front edge	4-bar must swing past vertical — requires careful link arrangement to avoid lockup
C: Drive-up strategy (recommended)	No change to mechanism	Robot drives forward to each goal, lift only handles height	Slower cycle time (need to position robot precisely); simpler/more reliable mechanism

✓

Recommendation: Option C (drive-up). The simpler mechanism is more reliable. Override has a 1+1 possession rule (SG6) so cycles are short; you can't speed-cycle your way to victory. Drive precision and lift reliability matter more than forward-reach extension. Save Option A or B for Phase B/refinement after Phase A reliability is proven.

SECTION 5 / 5

Risks & Tradeoffs

Honest assessment of what can go wrong, the CoG impact, and how this design ranks against alternatives.

Center of Gravity Impact

The 4-bar + chain bar adds significant mass at variable height. Estimated component masses:

4-bar tower + structure: ~400 g (fixed, low CoG impact)
4-bar arms (2 parallel) + 1-2 motors: ~600 g (moves with 4-bar angle)
Chain bar arms + sprockets + chain + motor: ~450 g (moves with both 4-bar and chain bar)
Pneumatic claw + cylinder + solenoid: ~280 g (at the very end — biggest dynamic CoG impact)

Worst-case dynamic CoG (placement on tall goal, claw at Y=15″, H=15.5″): Mass at end of arm: 730 g (chain bar + claw, effective center at Y=15″, H=15.5″) Total robot mass (estimate): ~6 kg Static CoG without lift extended: Y\approx8″, H\approx4″ (battery + brain + drivetrain in back) Dynamic shift in Y: (0.73 \times 7) \div 6 = 0.85″ forward Dynamic shift in H: (0.73 \times 11.5) \div 6 = 1.4″ up Dynamic CoG: Y=8.85″, H=5.4″ With 17″ wheelbase length (8.5″ from CoG to front wheels at Y=17″): Forward stability margin: 8.15″ horizontal \div 5.4″ vertical = stability angle 56° ✓

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Verify with the calculator: see /center-of-gravity Section 3 to plug in your specific component masses and arm extension positions. Check that stability angle stays above 50° in all configurations.

Failure Modes

Failure	Likelihood	Consequence	Prevention
Chain bar chain breaks/unhooks	Medium	Claw drops, cup falls, lift unusable until rebuild	Inspect chain weekly; carry master link spare; tension to 3-5mm midpoint deflection
4-bar arm misalignment (parallel bars not parallel)	Medium	Claw tilts during lift, cups dump prematurely	Use precision spacers at all 4 pivots; test by hand-cycling lift through full range before tuning PID
Pneumatic cylinder leak	Low-medium	Claw doesn't grip firmly, cup slips	Use Schedule 80 tubing; replace ball seal every 30 hours of use; bleed system before each match
Tower flex under load	Medium	4-bar pivot wobbles, lift returns to inconsistent positions	Triangulate tower with diagonal bracing; use 2×2× c-channel for tower; bolt to chassis with 8+ #8-32 screws
Tipping during forward extension	Low if CoG verified	Robot falls over on goal placement	Use CoG calculator; widen wheelbase if margin < 50°; weight battery toward back of chassis

Comparison to Alternative Architectures

Design	Reach	Forward Reach	Build Time	Reliability	Verdict
Chain bar alone (no 4-bar)	~9″ height	~7″ past front	~6 hr	High	Simpler, but can't reach 8.77″ goal reliably
4-bar alone (no chain bar)	~10″ height	~3″ past front	~4 hr	Very high	Simpler, but claw doesn't sweep back-to-front
4-bar + chain bar (this)	~16″ height	~3″ inside front (drive up)	~10 hr	Medium	Hits all goal heights + sweep cycle, more complex
DR4B + chain bar	~22″ height	~3″ inside front	~18 hr	Lower	Overkill for Override; specialty for high stacks
Cascade lift	~25″ height	~0″ (vertical only)	~14 hr	Lower	Wrong tool for Override (no horizontal sweep)

V2 Build Plan (Phase B/C Refinement)

This is a Phase B/C upgrade from V1.5 — significant build effort. Suggested rollout once V1.5 is proven and the team decides to upgrade:

V1.5 baseline must be working first. Don't start V2 until V1.5 reliably scores on alliance + short neutral goals in driver practice. Validate the simpler architecture before adding complexity.
Week 1 (V2 chassis + tower): Add 4-bar tower at Y=12″ from back. Keep existing drivetrain unchanged.
Week 2 (4-bar): Build 4-bar arms (8″ or longer — match to your goal-height strategy). Test lift through full range. PID tune.
Week 3 (Chain bar): Add chain bar to end of 4-bar. Static sprocket lockdown. Test 180° rotation.
Week 4 (Pneumatic claw): Replace standard V5 claw with pneumatic version. Add compressor/tank system. Tune cylinder timing.
Week 5 (Integration): Combine all motions in autonomous routine. Pickup → transit → place sequence. Test all 3 goal heights.
Week 6 (Refinement): CoG verification. Reliability testing (50+ cycles). Stability angle measurement.

⚠️

V2 is a major rebuild, not a tweak. Switching from a standard V5 claw to a pneumatic claw alone takes 2-3 days of build + plumbing + tuning. Adding the chain bar on top of a 4-bar adds another week. Only commit to V2 if V1.5 is performing AND your team has the build time to spare. If V1.5 is winning matches and judges award points for reliability, the engineering case for V2 weakens.

💡

V2 partial upgrades that are easier than the full thing:

Just the chain bar end-effector on the existing 4-bar. Skip the pneumatic claw, keep the V5 claw. ~1 week of work.
Just the pneumatic claw replacing the V5 claw. Skip the chain bar. ~3 days of work.
Just longer 4-bar arms (8″ → 10″) to reach the tall goal. Skip chain bar AND pneumatics. ~2 hours of work.

Each gives you partial benefit at much lower cost than the full V2 build.

Cross-References

Decide between lift architectures: /lift-selection
Chain bar deep dive (sprocket math, build steps): /mechanism-chain-bar
4-bar / 6-bar mechanism guide: /mechanism-lifts
Override manipulator (the claw end-effector): /mechanism-claw
Center of gravity calculator: /center-of-gravity
Drivetrain config (the 55W push-heavy base for this build): /override-drivetrain-config
Override game element specs: /override-manual-summary