A single arm that keeps its end-effector level by coupling it to a static sprocket via chain. Lighter than a four-bar, longer reach, and the partner mechanism on top of the In The Zone DR4B.
Prerequisite: Read Lift Mechanisms Overview first. Chain bars are most useful as a partner to another lift — especially the DR4B. Build at least one simple lift mechanism before attempting this.
What Is a Fixed Chain Bar?
A chain bar is a single rotating arm with an end-effector at the tip. By itself, that arm would tilt the end-effector as it swings up — useless for stacking. The trick: a chain runs from a fixed sprocket at the arm's pivot to a matching sprocket at the end-effector. As the arm rotates, the chain forces the end sprocket to rotate opposite to the arm. The two rotations cancel, and the end-effector stays at constant orientation.
It is called a "fixed" chain bar because the base sprocket is locked to the chassis — it does not rotate with the arm. The community also calls this a virtual four-bar because it produces the same level-output effect as a four-bar linkage with much less hardware.
Where Chain Bars Won
Sack Attack (2012–13) — AURA (New Zealand VRC) is widely credited with first applying the chain bar in VRC.
Skyrise (2014–15) — chain bars used as Skyrise placers paired with a DR4B for cube stacking.
In The Zone (2017–18) — many teams stacked a chain bar on top of the DR4B to extend reach onto elevated stationary goals. This guide's primary historical reference.
Tipping Point (2021–22) — chain bars served as alliance goal latchers; AURA-style designs reappeared.
Where Chain Bars Lose
Heavy loads. The single arm flexes more than a rigid four-bar. A chain bar carrying a heavy intake at full extension will sag.
Maximum height. A chain bar reaches farther than a four-bar but not as high as a DR4B. Choose chain bar for reach; DR4B for height.
Beginner builds. The static-sprocket trick is non-obvious. Many teams build "chain bars" whose base sprocket is not actually fixed and never figure out why the end tilts.
⚠
Override hook: If Override has a roller-driven point swing similar to In The Zone's mobile goals, a chain-bar-on-DR4B becomes immediately relevant for picking up and placing the swing-able element. Do not commit until Monday's manual confirms scoring geometry.
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// Section 02
Sprocket Math ⚙
Why a chain bar works at all — and how the sprocket ratio determines whether the end-effector stays level or rotates at a known angle.
The Static Sprocket Insight
A normal sprocket on a normal axle rotates with the axle. A static sprocket is locked to the chassis and does not rotate when the axle through it spins. Imagine drilling out the bore of a sprocket, sliding it onto an axle, and bolting the sprocket body to the chassis — the axle spins inside the bore, but the sprocket itself stays put.
Connect that static sprocket to a free sprocket via chain. As the arm holding the free sprocket swings around the static sprocket, the chain wraps differently — this changes how the free sprocket is forced to turn. With a 1:1 ratio, the free sprocket counter-rotates exactly as much as the arm rotates, keeping the end-effector level.
The 1:1 Ratio (Most Common)
For a level-output chain bar:
static sprocket teeth = end sprocket teeth
VEX chain works with 6T, 12T, 18T, 24T, 30T sprockets. Most chain bars use matching 24T sprockets — large enough to mesh chain reliably, small enough not to hog space.
Other Ratios (Custom Output Angles)
Static : End
End-Effector Behavior
Use Case
1:1 (24T:24T)
End stays level (counter-rotates equal to arm)
Standard cone stacker, ring placer, claw mount
2:1 (24T:12T)
End rotates the same direction as arm at half rate
Specialized: arm angle − end angle = constant
1:2 (12T:24T)
End rotates opposite arm at twice rate
Niche — intake that aggressively tilts as it rises
For 99% of competitive applications, use 1:1. Custom ratios are clever but rarely justified.
Chain Length Calculation
The chain forms a loop with two parallel runs — both contributing to total length:
For a 12-inch chain bar with 24T sprockets: chain length ≈ 2(12) + π(1.5) ≈ 28.7 inches. Round up to the nearest chain link count and add a master link for easy tensioning.
HS chain over standard: high-strength chain stretches less under load and resists skip events much better. The cost difference is trivial; always use HS for chain bars.
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// Section 03
Chain Bar vs Four-Bar ⚔
Both produce a level output. Both lift things. Choose based on load, reach, and packaging — not aesthetic preference.
Property
Four-Bar
Fixed Chain Bar
Load capacity
✓ Higher — rigid linkage shares load
✗ Lower — single arm flexes; chain stretches
Reach (horizontal)
✗ Limited by arm length
✓ Can be longer for same envelope
Build complexity
✓ Lots of pivots but conceptually simple
✗ Static-sprocket trick is non-obvious
Weight
✗ Heavier — two parallel arms + bracing
✓ Lighter — single arm + chain
End-effector mounting
✓ Two mounting points
✗ Single mounting point at arm end
Slop / precision
✓ Low slop with screw joints
✗ Chain has inherent backlash
Failure modes
✓ Few — bent bars or stripped pivots
✗ Chain skip, sprocket loosening, stretch
Motors needed
1–2
1 (typically)
Best for
Heavy loads, precision placement
Light loads, longer reach, packaging-constrained
When Chain Bar Beats Four-Bar
Long reach with limited chassis depth. A 14-inch four-bar takes 14 inches of robot length when folded; a chain bar with a 14-inch arm folds to 1–2 inches of arm thickness.
Light end-effector. Passive intakes, single-cone claws, hooks under 0.5 lb. Chain bar handles these without sag.
Folding into starting size cube. Four-bars cannot fold below their arm length; chain bars can pivot independently.
Mounting on top of a DR4B. The DR4B provides lift; you need a swing-arm at the top for horizontal reach. The In The Zone classic combo.
When Four-Bar Beats Chain Bar
Heavy game pieces. Mobile goals, stacked rings, anything over 1 lb — chain stretch becomes visible.
Precision at maximum reach. A four-bar with screw joints has near-zero slop. Chain bar drifts.
Defense impacts. Knocks knock chain off sprockets much more easily than they bend a four-bar.
🧠
Hybrid pattern: Some advanced teams use a four-bar for the primary lift and a chain bar for end-effector tilt. Four-bar handles the load; chain bar provides last-inch precision rotation. Best of both, at complexity cost.
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// Section 04
Build Order 🧰
Step-by-step assembly. The static-sprocket step is critical — if your base sprocket rotates with the arm, you have built a regular arm, not a chain bar.
Parts List
Part
Quantity
Notes
2-wide aluminum C-channel (arm)
1–2
Two stacked for rigidity, one for lightweight. Length = reach requirement.
HS sprocket 24T
2
Static base + free end. Match tooth count.
HS chain #25
30–40 links
HS strongly preferred over standard.
HS shafts (axles)
2
Arm pivot + end-effector axis.
Bearing flats
4
Two at each pivot.
Standoffs (1.5")
2–4
Lock the static sprocket to chassis.
Master link or chain breaker
1
Master link preferred for tensioning.
V5 motor (11 W)
1
Drives arm rotation.
Reduction gears
2
Typical 1:5 to 1:7 ratio.
Rubber bands #32 (optional)
2–4
Assist arm raise; reduces motor load.
Build Sequence
1
Mount the static sprocket to chassis. Use 2–4 standoffs to bolt the sprocket directly to the structural frame — the sprocket must not rotate with anything. Drill 4 holes through the sprocket body for screws. Verify by hand: only the axle through the bore can rotate.
2
Insert arm pivot axle through the static sprocket bore. The static sprocket has been drilled out so its bore is free. Add bearing flats on both sides. Test rotation by hand — smooth, no binding.
3
Build the arm. Cut C-channel(s) to your reach length. Mount one end to the pivot axle from step 2 — the arm now rotates around the static sprocket.
4
Mount the end sprocket to its own axle at the arm tip. This sprocket does rotate with its axle. Add bearing flats and shaft collars. The end sprocket axle is what your end-effector mounts to.
5
Run the chain. Route from static sprocket along top of arm, around end sprocket, back along bottom of arm to static sprocket. Close with master link. Chain should run parallel to arm with minimal slack.
6
Mount end-effector to end sprocket axle. Whatever the chain bar carries bolts directly to this axle. As arm rotates and chain forces axle to counter-rotate, end-effector stays at starting orientation.
7
Verify level output by hand. Manually rotate arm through full range. End-effector should remain at exactly the same orientation. If it tilts, your static sprocket is rotating with the arm — go back to step 1.
8
Mount drive motor. Attach 60T or 84T gear to arm-pivot axle, smaller gear to motor shaft. Typical reduction 1:5 to 1:7. Run cables, bolt down, test under power.
9
Tension the chain. Press chain at midpoint between sprockets — should deflect 3–5 mm and snap back. See next page for full tensioning procedure.
⚠
Most common mistake: Building a "chain bar" whose base sprocket spins with the arm. Chain does nothing — end-effector swings like a regular arm. Verify in step 7 before powering up. Hold the arm and watch the sprocket: it should not move relative to the chassis when you rotate the arm.
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// Section 05
Chain Tensioning & Maintenance ⚙
Chain skip is the #1 chain bar failure mode at competition. Proper tension and consistent maintenance prevents it.
The Tension Sweet Spot
VEX #25 chain has a usable tension range of about 3–5 mm of midpoint deflection. Press chain at midpoint between sprockets — should give a few millimeters and snap back. Too tight or too loose both cause failures:
Too tight
Arm rotation feels stiff. Motor draws extra current. Sprocket bushings wear faster. Eventually sprocket teeth deform under continuous high-tension load.
Just right
3–5 mm midpoint deflection. Chain runs smoothly. End-effector tracks arm rotation precisely. No skip events. Motor current reasonable.
Too loose
Chain visibly sags. Skips teeth under sudden loads. Once a skip happens, end-effector orientation is permanently offset until you reset the chain.
Tensioning Methods
1
Add or remove chain links. Most direct method — remove if too loose, add if too tight. Master link makes this a 30-second adjustment. Without master link, you need a chain breaker tool.
2
Slot-mount the end sprocket axle. If your arm has slotted mounting holes for the end-sprocket axle, you can adjust center-to-center distance directly. Continuous adjustment, finer than adding/removing links.
3
Add an idler sprocket. A free-spinning small sprocket (12T) on a tensioning slot pushes against one chain run. Used on advanced builds where chain length is critical.
Maintenance Schedule
Every practice session: press-test chain tension. Adjust if it has stretched.
Every event: inspect chain for elongated links (sign of wear). Replace if any link looks visibly stretched compared to fresh chain.
Every season: replace HS chain entirely. Chains have a finite useful life under repeated load.
Never lubricate. VEX chain is designed to run dry. Oil attracts dust which becomes abrasive paste. Keep the chain clean.
🏭
Match-day kit must include: spare master link, 6 inches of replacement chain, both sprocket sizes you use, and a chain breaker tool. A snapped chain mid-elimination round without a spare ends your tournament early.
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// Section 06
The DR4B + Chain Bar Combo 🧩
Putting a chain bar on top of a DR4B was the late-season meta in In The Zone. Maximum height + horizontal reach in one mechanism stack.
Why Stack Them
A DR4B alone gives you maximum vertical height with the load centered over the chassis. But scoring on elevated stationary goals (In The Zone's 10-point goals were elevated and set back from the field perimeter) required reaching up AND out. Neither mechanism alone solves this. Together:
DR4B handles vertical lift — raise the chain bar assembly to scoring height while keeping it level.
Chain bar handles horizontal reach — once at height, swing out to drop the cone onto the elevated stationary goal.
Mounting the Chain Bar to the DR4B Top Platform
The DR4B's top platform (top of stage 2) becomes the "chassis" for your chain bar. The static sprocket bolts to the DR4B's top platform — everything else (arm, end sprocket, end-effector) sits above.
Two complications to watch for:
Cable routing. The chain bar motor needs power, so a smart cable must run from the brain up the DR4B. Route it through the lift bars or alongside, with slack at every pivot. Pre-test by manually moving the lift through full range with cable attached.
Center of mass shift. Adding a chain bar at the top of the DR4B raises and offsets the center of mass. The DR4B's rubber band tension may need adjustment after integration. Re-tune.
The In The Zone Build Pattern
The canonical late-season ITZ robot architecture — assembled on roughly 90% of finals-level robots:
Front of chassis: 4-bar mobile goal (mogo) lift — grabs and places mobile goals into scoring zones.
Center of chassis: 4-motor turbo drive (sometimes 6-motor for late season). Note: under Override 2026-27's 55W rule, 6-motor turbo drives are no longer legal — see Override Drivetrain Decision for current options.
Back of chassis: DR4B with chain bar on top, passive cone intake on the chain bar end-effector.
Two mechanisms (mogo lift and DR4B+chain bar) on opposite ends of a tank drive. Each does one job well. The intake is passive — cones self-align by gravity into a funnel as the lift descends over them.
📊
Reference robot: Several In The Zone World Championship-level teams used this exact architecture. 5225A "The Pilons" (2018 HS World Champions) and 8068E (Singapore reveal) are the most thoroughly documented. See In The Zone Meta Guide for full team-by-team breakdown.
Trade-offs
✓ Maximum scoring versatility — mobile goals AND elevated stationary goals.
✓ Single drive can fit both mechanisms (mogo on front, DR4B-chain on back).
✗ Both mechanisms eat motors — budget carefully.
✗ Stage-2 weight is significant — DR4B motor sizing must account for chain bar mass.
✗ Two simultaneous failure modes — either mechanism can disable the other if cables interfere.
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// Section 07
Failure Modes & Tuning 🔧
Symptom → cause → fix. Most chain bar failures trace back to chain tension or sprocket mounting issues.
Common Failure Modes
End-effector tilts as arm rises
Cause: Static sprocket is rotating with the arm (most common build error).
Fix: Verify static sprocket is rigidly bolted to chassis. The axle through it must spin freely; the sprocket body must not move.
End-effector tilts at random heights
Cause: Chain skipped a tooth on one of the sprockets.
Fix: Reset chain to correct alignment. Re-tension chain. Check for damaged sprocket teeth.
Chain comes off sprocket
Cause: Tension too loose, or impact from defense, or sprocket teeth worn.
Fix: Re-tension. Add chain guides (small standoffs near sprockets) to prevent derailment under impact.
Arm sags at full extension
Cause: Single C-channel arm flexing under load.
Fix: Add second C-channel parallel to first (stacked arm). Or add tension cable from arm tip back to base.
Motor stalls trying to lift arm
Cause: Chain too tight (excessive friction), or insufficient gear reduction.
Fix: Re-tension chain. If still stalling, increase reduction (1:7 → 1:9). Add rubber band assist.
End-effector lags behind arm rotation
Cause: Chain stretch (worn chain) or backlash in sprocket bores.
Fix: Replace chain with new HS chain. Tighten sprocket-to-axle connection (set screws or shaft collars).
Sprocket spins on axle (not rotating arm)
Cause: End sprocket loosened from its axle — set screw backed out.
Fix: Loctite (Blue) the set screw and re-tighten. Add a shaft collar pinning sprocket to a flat on the axle.
PID Tuning for Arm Position
Drive the chain bar to specific angles via code:
Use a rotation sensor on the arm pivot axle (preferred) or motor encoder.
Map angles to encoder values empirically — do not compute from geometry. Drive to known positions (folded, horizontal, max-extension) and record sensor values.
Simple PID — start with kP, add kD when overshooting, kI rarely needed.
Slew the output to prevent slamming. Slew rate ~0.2 (0–1 over 5 ticks) is a good default.
A chain bar is a chain-coupled rotational constraint. The static sprocket creates a fixed reference frame; the chain transmits angular displacement to the end sprocket with a transfer function determined by sprocket ratio. For a 1:1 ratio: φend = −φarm. The negative sign produces the level-output behavior. This is a kinematic application of relative motion — the end-effector is held stationary in the world frame even though it is rotating in the arm frame.
Interview line: "Our chain bar uses a static sprocket as a fixed reference, with chain transmission to a free sprocket at the arm tip. Because the sprocket ratio is 1:1, the end-effector rotates equal-and-opposite to the arm in the arm's frame — which means it stays level in the world frame."
Check for Understanding
You build a chain bar but the end-effector tilts as the arm rises. Both sprockets are 24T. What is the most likely cause?
Chain is stretched
The base sprocket is rotating with the arm instead of being locked to chassis
Sprocket sizes are wrong
Motor reduction ratio is wrong
Why: The static sprocket must be rigidly mounted to the chassis. If it rotates with the arm, the chain is doing nothing useful — the end sprocket has no fixed reference to counter-rotate against, so the end-effector simply swings with the arm like a regular non-leveling arm. Sprocket sizes being wrong or chain being stretched would cause different symptoms (gradual tilt or skip events, respectively), not consistent tilting.