Two stacked four-bars rising nearly vertically with the load centered over the chassis. The most powerful and most-respected lift in VRC history. Read Lift Mechanisms first if you have not chosen a lift type yet.
Prerequisite: Build a simple four-bar first. The DR4B is two four-bars stacked, and trying to learn both stages at once is how teams burn three weeks. Build a four-bar to floor goal height in a weekend, then come back here.
Why the DR4B Exists
A single four-bar swings forward as it rises — the end effector traces an arc, not a line. For low scoring (under ~24 inches) that is fine. But when a game asks for vertical stacks, tall posts, or precise placement at maximum height, swinging forward fights you: the robot gets longer at full extension, the center of mass moves outside the wheelbase, and the lift wants to tip the chassis.
The DR4B answers this by stacking a second four-bar in mirror orientation on top of the first. As the bottom stage swings forward and up, the top stage swings backward and up. The forward and backward motions cancel each other — what is left is nearly pure vertical rise, with the load staying centered above the robot base.
Where the DR4B Wins
Skyrise (2014–15) — teams stacked seven-section towers; vertical reach with level placement was the entire game. DR4B-equipped robots dominated.
In The Zone (2017–18) — cones stacked on mobile goals up to 18+ tall. The DR4B + passive cone intake combo was the meta. This guide's primary historical reference.
Modern descendants — whenever a game asks for vertical stacking or scoring at maximum height, the DR4B archetype reappears. Override may or may not call for it — do not commit until the manual confirms.
Where It Loses
Mid-height games (24"–36") — a six-bar or chain bar is faster to build, easier to tune, and reaches enough.
Mobile-goal pickup games (Tipping Point) — a four-bar mogo lift is more reliable. DR4B was largely overkill that season.
Speed-of-cycle games — if scoring depends on rate (cycles per minute) more than maximum height, the DR4B's mass and acceleration profile are working against you.
⚠
Override caveat: Override is being revealed Friday April 24 with the manual on Monday April 27. Do not start a DR4B build for Override before reading the manual. Build one as a learning project against In The Zone's requirements — the geometry skills transfer to any vertical lift you build later.
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// Section 02
Linkage Geometry 📈
The math behind why a DR4B rises vertically — and what causes it to drift sideways when you build it wrong.
The Parallelogram Principle
Every four-bar in VRC is a parallelogram linkage. Four pivots form a parallelogram, with the two side bars equal in length and the two horizontal bars equal in length. As the side bars rotate, the top horizontal bar always stays parallel to the bottom (chassis). That is what keeps your end effector level.
For a DR4B, you stack two of these parallelograms. The bottom one swings forward and up, the top one swings backward and up. If the geometry is correct, the horizontal motions cancel exactly and the payload rises straight up.
The Drift Math
Each four-bar contributes some forward (or backward) translation as it rises. Call the bottom stage forward-drift d₁ and the top stage backward-drift d₂. The net horizontal motion is:
Net drift = d₁ − d₂
If both stages have identical bar lengths and matching drive angles, then d₁ = d₂ and the net drift is zero. The lift rises perfectly vertically. Most DR4B problems on the field trace back to this equation being violated.
What Breaks Vertical Motion
Bar length mismatch. Even 2 mm of difference between top and bottom bars produces visible lean at full extension. Cut your C-channels to identical lengths — measure with calipers, not eyeballs.
Stage-rate mismatch. If one stage moves faster than the other (gear ratio difference, slipping chain, motor power asymmetry), the lift drifts mid-rise. Both stages must rotate at the same angular rate.
Pivot misalignment. If the four pivots of either stage do not form a true parallelogram, the bar that was supposed to stay horizontal will tilt as the lift rises. You will see this as the payload rotating instead of staying level.
Slop multiplied through stages. 0.5° of pivot slop in stage 1 plus 0.5° in stage 2 stacks to 1° total at the payload. Over a 30-inch lift, 1° is about 0.5 inches of error at the top — enough to miss a stack.
Pre-CAD Decisions
Before opening Onshape, decide three things:
Bar length per stage. Total lift height ≈ 2 × (bar length × sinθmax), where θmax is the maximum upward angle of each stage (typically 70–80°). For a 30-inch lift, target 16–18-inch bars per stage.
Stage-mid spacing. The mid-section (where the two stages meet) must be wide enough to accommodate gears, screw joints, and motor mounts. 4–5 inches is typical.
Mounting footprint. The bottom stage's ground link sits on the chassis. Make sure your chassis rails have hole alignment for this.
🧠
CAD-first methodology: Animate the full DR4B in Onshape before cutting any metal. Step through the motion in 10° increments and verify the payload stays vertical at every position. If it drifts in CAD, it will drift on the field. The Mechanism Sprint guide walks through the linkage modeling workflow.
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// Section 03
Build Order 🧰
Step-by-step assembly. Build stage 1 alone first, verify motion, only then add stage 2. Skipping the verification step is the most common build mistake.
Parts List
Part
Quantity
Notes
2-wide aluminum C-channel (matched lengths)
4
2 for bottom stage bars, 2 for top stage bars. Cut to identical length.
2-wide C-channel (mid-section)
2
Connects stage 1 top to stage 2 bottom on each side. ~5".
1-wide C-channel or angle
2
Top platform that holds the end effector.
High-strength shafts (axles)
4–8
Pivots at each of the 8 four-bar joints. HS recommended for lift rigidity.
Standard or HS gears (60T or 84T)
4
Two on each side of the mid-section. Gear ratio determined separately (see next page).
Bearing flats
16
Two per pivot — one inside, one outside. Do not skip.
Shoulder screws or 2" #8-32 screws
8
The pivot axles. Screw joints > standard axles for stability.
Nylock nuts (#8-32)
16+
Used at every pivot. Never use keps nuts.
V5 motors (11 W)
2–4
Mounted to the mid-section, driving the four mid-section gears.
Rubber bands #32 or #64
6–14
Tensioning. Quantity tuned for lift weight (see Rubber Bands page).
Tools Required
Star drive (T15) screwdriver and 5/16" socket / nut driver
Measuring tape and digital calipers (for bar length matching)
Hacksaw or VEX cutting tool (for trimming C-channels)
Allen keys for set screws
Reusable zip ties or masking tape for temporary holds while assembling
Build Sequence
1
Cut all four lift bars to identical length. Lay them next to each other before cutting and verify they match within 1 mm. A single mismatched bar will produce drift you cannot fix later. Cut, deburr, and label each bar with masking tape (BOTTOM-LEFT, BOTTOM-RIGHT, TOP-LEFT, TOP-RIGHT).
2
Build the bottom stage (stage 1) only. Assemble the lower four-bar — two parallel bars connecting the chassis to the mid-section. Insert bearing flats at all four pivots. Use shoulder screws or 2" screws as the pivot axles. Add nylock nuts but do not over-tighten — the joint must rotate freely.
3
Mount stage 1 to the chassis temporarily. Use clamps or zip ties — not permanent bolts yet. The lift will move when you swing it; you need easy adjustment. The two bottom pivots should sit flush against the chassis with bearing flats on both sides.
4
Verify stage 1 moves freely and stays parallel. Swing the lift through its full range by hand. The mid-section bar should stay perfectly horizontal at every angle. If it tilts, your pivots are not forming a parallelogram — check pivot spacing on the chassis side first. Stop here until stage 1 is perfect. Do not add stage 2 to a broken stage 1.
5
Build stage 2 on the mid-section. Stage 2 is the inverse of stage 1 — the two parallel bars now connect the mid-section up to the top platform. Same construction: bearing flats at all four pivots, shoulder screws or 2" screws as axles, nylock nuts.
6
Install the gear coupling at the mid-section. Each side of the mid-section needs two gears that mesh together — one driven by stage 1's rotation, the other by stage 2's rotation. They must turn at opposite rates so when stage 1 rotates up by θ, stage 2 rotates up by θ as well (mirror motion). Mesh the gears so this happens.
7
Mount motors to the mid-section. Motors drive the mid-section gears, which then transmit power to both stages. Mounting to the mid-section (rather than the chassis) means the motors do not need long chains or external shafts — less weight, less slop. Verify motor direction: pressing the lift-up button should drive both stages upward, not one up and one down.
8
Add rubber band tensioning. See the Rubber Bands page for placement and count. Start with fewer bands than you think you need — you can always add more. Too much tension will cause the lift to fly upward and slam into the chassis on the way down.
9
Drive the lift through full range under power. Verify both stages move at the same rate, end effector stays level, and motion is smooth. Listen for grinding (gear mismatch), watch for sag (stage rate mismatch), and check for binding (over-tight nylock nuts).
10
Now permanently bolt to chassis. Once stages 1 and 2 are verified working together, replace the temporary clamps with permanent bolts. Add cross-bracing between the two sides of stage 1 (a single high-strength shaft running across is ideal — see Screw Joints page).
⚠
The biggest mistake: Building stages 1 and 2 simultaneously. If anything is wrong, you cannot tell which stage is causing it. Build sequentially, verify each stage in isolation, and the integration step becomes trivial. Teams who skip stage-1 verification spend 3–5 days debugging mystery drift.
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// Section 04
Gear Ratios & Motor Sizing ⚙
How much torque you need, what cartridge to choose, and how to fit a DR4B under modern wattage caps.
The Standard Ratios
Almost every successful DR4B in VRC history used one of these four reductions. They are good starting points — tune from here based on your specific lift weight and game piece mass.
1:5
Fast, low-load. Lightweight DR4Bs with empty intakes or single light objects. Used on featherweight builds. Speed-prioritized.
1:7
Most common. All-round balance for V5 cone-stacker-style builds. 491A In The Zone reference. Good default.
1:9
Torque-prioritized. Heavier lifts or heavier game pieces. Slightly slower but holds position better under load.
1:11.7
Very heavy lifts. Compound ratio (3:5 then 1:7). Used by 17C in Skyrise. For when 1:9 still strains.
Cartridge Selection
Cartridge
Output RPM
Torque (in·lb)
Best For
🔴 Red (100 RPM)
100
2.10
Heavy lifts, paired with shorter reductions (e.g. 1:5)
🟢 Green (200 RPM)
200
1.05
Most common DR4B choice. Balanced with 1:7 or 1:9 reduction.
🔵 Blue (600 RPM)
600
0.36
Speed-priority, paired with very heavy reduction (1:11.7+)
The intuition: cartridge selection trades off with gear reduction. A green cartridge with 1:7 reduction has the same final wheel-output speed as a blue cartridge with 1:21 reduction — but the torque profiles are different. Most DR4Bs run green + 1:7 because it is the cleanest mechanical implementation.
Torque Calculation
To verify your motor count is sufficient, estimate the torque required at the motor:
For a typical V5 DR4B carrying a passive cone intake (~0.5 lb of cones at full stack):
Lift mechanical weight: ~3 lb
Effective lever arm at full extension: ~10 inches
Required torque at output: ~30 in·lb
With 1:7 reduction: ~4.3 in·lb required at motor
One green V5 motor produces 1.05 in·lb — 4 motors needed minimum — or 2 motors with rubber band assist.
Wattage Caps and Motor Count
Modern V5RC games may impose wattage caps on subsystems. As of the upcoming Override season, the drivetrain is capped at 55 W (5 motors equivalent). Most caps apply to the drivetrain only, but always read the manual carefully — if a cap exists on the lift, design accordingly.
📊
Practical motor budget for a DR4B: 2 motors with strong rubber band assist is the lightweight-build minimum. 4 motors with moderate rubber band assist is the typical all-round build. 6 motors is overkill in most cases — that fourth pair is rarely worth giving up an intake or mogo lift motor for. For Override 2026-27: the 55W drivetrain power cap means you have to budget motor wattage between drive and lift carefully. See the Override 55W Drivetrain Decision guide for how to allocate.
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// Section 05
Rubber Band Tensioning 🍺
Elastic potential energy is how a 2-motor DR4B can lift loads that would otherwise need 4 motors. Geometry of band placement matters as much as count.
Why Bands Matter
A DR4B at full extension has its largest torque demand at the bottom (lift weight is fully supported through the longest moment arm). Without rubber bands, motors must produce maximum holding torque at full extension — which means oversizing the drive ratio for the worst case.
Rubber bands store elastic potential energy when the lift is down (bands stretched), and release it as the lift rises (bands contract). The result: motors are doing less work at the bottom of the stroke and the band provides the lift force you would otherwise need a bigger motor for.
Band Placement Geometry
Where you anchor the bands affects how much energy they store. Two principles:
Maximum stretch when lift is down. The band must be near its rest length (or stretched slightly past it) when the lift is at full extension, and stretched significantly when the lift is at the floor. If the band stretch does not change much through the lift motion, you are not getting useful energy storage.
Anchor across the pivot. The band must be anchored on opposite sides of a pivot — one end on the rotating bar, one end on the static frame. This way the rotation of the bar changes the band length.
Band Count Tuning
Start with fewer bands. A 2-motor DR4B carrying a light intake typically needs 6–10 bands #32. Heavier lifts may require 12–14. The right count is the one where you can hold the lift at any height with motors disabled. If the lift falls, you need more bands. If it floats up on its own, you need fewer.
📋 Band Tuning Procedure
Start with 6 bands #32 distributed evenly between left and right sides
Power off the motors and lift to mid-height by hand
Release the lift — if it falls, add 2 more bands; if it rises, remove 2
Repeat until lift holds steady at mid-height with motors off
Verify motors can drive lift up and down smoothly under load
Test holding at full extension under maximum payload — bands plus motors should not stall
Common Band Mistakes
All bands on stage 1 only. If you only band stage 1, stage 2 still needs full motor torque to rise. Distribute bands across both stages.
Anchoring on the wrong side of the pivot. If the band length does not change as the lift rises, you are getting zero useful work from it.
Too many bands. Lift slams up uncontrollably. Motors fight the bands on the way down. Energy is wasted heating up motors. Less is more until the lift can be held with motors off.
Crossed bands. If you cross bands over a pivot or another moving part, they will eventually fray and snap mid-match. Route them clean.
🎗
Replacement schedule: Rubber bands degrade with stretch cycles and UV. Change them at every event — budget for $2 of bands per event. A snapped band mid-match costs more than every band you will replace in a season.
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// Section 06
Screw Joints — The Stability Secret 🔒
The single biggest upgrade you can make to a DR4B. Replaces standard axles with screws-as-axles to eliminate slop and rigidly couple both sides of the lift.
What Is a Screw Joint?
A screw joint uses a partially-threaded screw (typically 2" long, #8-32) as the pivot axle instead of a standard VEX shaft. The screw has two key properties standard axles do not:
Threaded portion bites into nylock nuts on both ends — eliminates the side-to-side slop that standard axles develop.
Smooth shoulder portion provides the rotation surface — bearing flats grip the smooth shoulder, exactly like a real shoulder screw.
The result: zero radial slop, zero axial slop, and the joint maintains its tightness over hundreds of cycles. On a DR4B with 8+ pivots, this matters enormously.
Why It Works on DR4Bs Specifically
A standard DR4B has 8 pivots (4 per stage). Each pivot contributes some slop — even 0.2 mm per pivot adds up. Through the kinematic chain of the lift, that slop multiplies. By the time you reach the payload at full extension, you may have 5–10 mm of total wobble. That wobble is the difference between landing a cone on a stack and knocking three cones off.
Screw joints reduce per-pivot slop to near-zero. The DR4B becomes a precision mechanism instead of a flexible one. Veteran teams report that switching to screw joints alone, with no other changes, has been the most impactful single modification on lift accuracy.
Implementation
1
Choose 2" #8-32 screws with smooth shoulder portion. Typical hardware-store star drive screws work. The shoulder length (smooth portion) must be longer than the C-channel + bearing flat thickness on each side — about 1 inch of smooth shoulder is plenty.
2
Insert through bearing flats on both sides. The bearing flat sits inside the C-channel. The smooth shoulder rides in the bearing — this is the actual rotation surface. Verify smooth rotation at this stage before adding the nut.
3
Thread on a nylock nut, finger-tight then quarter-turn. Tighten only until the bars cannot slip side-to-side. Over-tightening compresses the bearing flat into the C-channel, causing binding. Test rotation after each adjustment.
4
Use a screw long enough to span both sides. A single 2" screw passing through both bars (left side + right side of the lift) creates a rigid cross-member that prevents lateral lean. This is the "mid-section cross-shaft" technique — arguably the second-biggest stability upgrade.
🏆
The 333A reference: Team 333A's DR4B Tutorial on the VEX Forum is the community-standard reference for screw joints. Read it before your first DR4B build — the photos and step-by-step process clarify what text alone cannot.
Some teams use VEX shoulder screws (sold by VEX) at pivots. These are stronger and cleaner than DIY screw joints, but typically shorter (3/4" or 1") — they cannot span both sides of the lift in one piece. Use shoulder screws at single-side pivots and 2" #8-32 screws at cross-spanning pivots. Mixing both is fine; the screw type at each pivot depends on what role that pivot plays.
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// Section 07
Integrating End-Effectors 🔗
A DR4B is a delivery vehicle for an intake, claw, or chain bar. Mounting the end-effector incorrectly invalidates everything you tuned in stages 1–6.
The Top Platform
The top platform of stage 2 is where you mount whatever your DR4B is going to carry. Three rules:
Keep it light. Every gram on the top platform multiplies through the lift — it sits at the longest moment arm, so it requires the most motor torque to lift. Use polycarb where possible, half-cut C-channels where structural needs allow.
Center it. Mount the end-effector so its center of mass is directly above the lift's vertical axis. Off-center loads cause the DR4B to lean — even with perfect linkage geometry.
Plan cable routing first. If your end-effector has motors or sensors, the smart cables must run down the lift. Pre-plan their path through the bars or alongside them. Cables that pull tight at full extension will break.
Common Top-Platform Mechanisms
Passive Intake (cones, rings)
Funnel-shaped catcher with no moving parts. Game piece self-aligns by gravity into the funnel as the lift descends over it. The In The Zone meta. Lightest possible end-effector.
Chain Bar (extended reach)
Add a chain bar mechanism on top of the DR4B for additional horizontal reach at maximum height. The classic In The Zone late-season build. See Chain Bar Deep Dive.
Active Claw / Roller Intake
Motorized grabber for game pieces that do not self-align. Adds weight and a motor port but enables more reliable pickup. Used when passive intake will not work.
Pneumatic Latch
Solenoid-driven grabber for fast lock/release. Very fast actuation but limited by pneumatic budget. Useful for endgame mechanisms.
Avoiding Mechanism Interference
If your robot also has a mogo lift (mobile goal lift), it sits on the opposite end of the chassis from the DR4B — one mechanism in front, one in back. Verify in CAD that:
The DR4B at full extension does not contact the mogo lift in any position
The mogo lift in any position does not contact the DR4B's ground stage
Both mechanisms can operate simultaneously without colliding
Cables, pneumatic lines, or chains do not interfere across mechanisms
📊
The In The Zone two-mechanism layout: RD4B (DR4B) on one end carrying a passive cone intake on top; 4-bar mogo lift on the opposite end. 4-motor turbo drive in between. This three-subsystem layout was the meta — 90%+ of finals-level robots used some variant of it.
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// Section 08
Failure Modes & Tuning 🔧
Diagnosing the most common DR4B problems. Symptom → cause → fix.
Common Failure Modes
Lift leans forward at full extension
Cause: Top stage bars longer than bottom, or top stage rotates faster than bottom.
Fix: Verify bar lengths with calipers. Check gear meshing and ratio at the mid-section.
Lift leans backward at full extension
Cause: Bottom stage bars longer than top, or bottom rotates faster than top.
Fix: Same as above — check geometry and gear ratios.
Lift twists left or right (not symmetric)
Cause: One side moving faster than the other — usually motor mismatch or chain slip on one side.
Fix: Add cross-shafts (HS shafts spanning both sides) at the mid-section to lock the two sides together.
Gears skip teeth under load
Cause: Center-to-center spacing too far, or gears not fully meshed.
Fix: Use VEX's standard gear spacing (2 holes between center-to-center for 60T-to-60T). Check for bent shafts.
Lift sags slowly when stopped at any height
Cause: Insufficient holding torque — motors are not enough, or rubber bands not assisting enough.
Fix: Add bands (preferred) or change to a higher gear reduction. Use motor brake mode in software.
Lift moves but binds at certain angles
Cause: Pivot is over-tightened, or two pivots are slightly misaligned.
Fix: Loosen all nylock nuts a quarter-turn. If binding persists, check that all four pivots of each stage form a true parallelogram.
Lift bounces uncontrollably when reaching top
Cause: Too many rubber bands — motors cannot resist the upward acceleration.
Fix: Remove 2 bands at a time until the lift reaches top smoothly. Or add a software speed cap when nearing extension.
Smart cable on intake disconnects mid-match
Cause: Cable is pulling tight at full extension.
Fix: Add slack with a cable loop at the mid-section pivot. Reroute cable along the bar instead of across the pivot if possible.
PID Tuning for Lift Position
Once your DR4B is mechanically sound, you will likely want to drive it to specific heights via code. The recommended path:
Use a rotation sensor on a lift pivot (preferred) or use a motor encoder. Rotation sensors are more accurate but require an extra V5 port.
Map heights to encoder values empirically. Drive the lift to known heights (floor, mid, max) and record the sensor value. Hard-code those values; do not try to compute them from geometry.
Use simple PID — start with kP only, add kD when the lift overshoots, add kI only if it has steady-state error. Most well-built DR4Bs need only kP and kD.
Slew the output — ramping motor commands smoothly prevents the lift from slamming when called. Slew rate of ~0.2 (0–1 over 5 ticks) is a good default.
EZ-Template has built-in PID for lift subsystems. See the PID Diagnostics guide for tuning steps.
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// Section 09
CAD References & Verification 📂
Public CAD files to study, verification checklist before competing, and historical reference robots.
Public CAD to Study
No publicly-shared Onshape file specifically from the In The Zone era exists for the canonical 5225A or 8068E robots — that season predated widespread Onshape adoption. The closest modern references:
A four-bar parallelogram linkage is a kinematic chain with one degree of freedom — once you fix any one bar's angle, every other bar's position is determined. By Grubler's Equation: degrees of freedom = 3(n−1) − 2(j) where n = number of links (4) and j = number of pivot joints (4), giving 3(3) − 2(4) = 9 − 8 = 1 DoF. Stacking two parallelograms in a DR4B keeps DoF at 1, which is why a single motor can control the whole lift.
Interview line: "Our DR4B is a serial kinematic chain with one degree of freedom. The two parallelogram stages are coupled through a gear train at the mid-section, so a single motor input drives both stages simultaneously while preserving the parallel-output property of each stage."
Check for Understanding
Your DR4B leans forward at full extension. The bars and gear ratios on both sides are identical. What is the most likely cause?
Rubber band tension is too high
Top stage bars are longer than bottom stage bars
Motors are running at different speeds
PID gains are not tuned correctly
Why: If the top stage has longer bars than the bottom stage, its forward translation per degree of rotation is larger. The two stages no longer cancel, and the lift drifts in the direction of the longer-bar stage. Same-length-bars is the most common diagnostic check, and even small differences (~3 mm) are visible at full extension.