Four-Bar Lift
Sizes a four-bar: height, the arc it sweeps, the torque it needs, and the angle at which the two bars collide — which is usually what limits the travel rather than anything you chose.
Inputs
The linkage
Pivot to pivot along one bar: tower bolt to end-effector bolt. Both bars are this length on a parallelogram, which is the only kind that keeps the intake level.
Between the two pivots up the tower. This is the lever for how far the linkage can travel before the bars meet, and it is the one dimension teams routinely set by accident.
From horizontal. Where the bars sit tucked at the bottom.
Where they stop at the top. If this is past the collision angle, the collision wins.
What it carries
Every bar that swings, added together. A four-bar built on both sides has four.
The moving plate at the far end that the bars pin into and the intake bolts to.
Drive
Results
Bars collide at
—°
Set by tower spacing and member width alone.
What limits the travel
—
Vertical travel
—in
Height at the top
—in
Furthest forward reach
—in
Effective weight
—lb
Peak torque needed
—lb·in
Same in newton-meters
—N·m
Does it hold?
—
Spacing for your wanted angle
—in
What the tower would need to reach the angle you asked for.
Stall is a holding figure, not a lifting one. Past the first mark it holds but barely moves; past the second it is running hot for nothing.
How much of the available swing your chosen angles actually use.
Watch it move
Drag the slider to swing the linkage. The bars turn red where they collide, and the dashed line shows how far forward the coupler reaches — notice it is furthest out with the bars level, not at the top.
How this is calculated
A four-bar is the tower, two bars of equal length, and the coupler they carry. Because the bars stay parallel, the coupler translates instead of rotating and the intake holds its orientation for the whole stroke. That is the entire reason to build one.
height h = pivot height + L sin θ reach x = L cos θ peaks at θ = 0 W_eff = 0.5 × bars + 1.0 × (coupler + end effector + payload) torque τ = W_eff × L × cos θ collide θ = arccos(member width ÷ tower spacing)With your numbers
The collision angle is usually the real limit
The two bars are offset along the tower by the pivot spacing. As they rise, the perpendicular gap between them closes asspacing × cos θ, and they meet when that equals the width of the metal. Nothing about the bar length or the payload enters into it.
Two useful things follow. Tower spacing is the lever for angular range: with 1 inch members, going from 2 inches of spacing to 4 buys 15 degrees of travel for the cost of a taller tower. And because arccos of anything is under 90 degrees, a correctly built four-barphysically cannot reach the position where the linkage would flip into the crossed configuration and the intake would end up permanently askew. The collision protects you from the singularity.
It swings out, then back
Reach is L cos θ, so it peaks with the bars level. An arm that starts tucked below horizontal moves forward as it rises to level, then back as it continues up. Peak overhang, peak torque and peak tipping moment all land at the same instant, which is why a lift can feel stable at the bottom and at the top and try to tip you over halfway.
One hole out and it is not a parallelogram
The intake only stays level if both bars are the same length and the coupler matches the tower spacing. Build one bar a single hole longer, half an inch, and the coupler rotates through the stroke instead of translating. This is the most common four-bar build error and it is invisible until the intake is visibly crooked at full height.
Where the coefficients come from
Each bar is pinned at one end, so its center of mass rides at half the radius and it counts 0.5. The coupler and everything on it rides the full radius and counts 1.0. The same accounting runs through every arm lift on this site, which is why they can all be compared on one number.
Sources & assumptions
Cartridge stall torques are published. The collision angle and the coefficient model are derived, both checked numerically: the collision identity against its arcsine form, and the torque against a finite-difference derivative of potential energy computed from real link positions.
Weights, spacing and efficiency are yours. This page models the parallelogram case only. A deliberately non-parallel four-bar needs the coupler angle solved through the stroke, which is a different tool.
- Purdue SIGBots Wiki: VEX Motors — checked August 2026
- VEX Robotics: V5 Smart Motor & Gear Cartridges (276-4840) — checked August 2026
- VEX Robotics: C-Channels (8 sizes) — checked August 2026
- V5RC Override Game Manual v1.1 (2026-27) — checked August 2026
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Where this goes next
Explore further on the full mapWhere the answers on this page can be used, and which tools could have given you the numbers it asks for.
Comes after
Two-Bar Liftfills in your arm length hereYou are here
Four-Bar Lift
Where your answer goes
Rubber Band Assist Calculatorsize the bands that span those same two barsbrings your pivot spacing with itLift Speed & Gear Ratiofind the reduction that raises that weight fastest without stallingbrings your reach with itShaft & Structure Load Checkcheck the pivot shaft survives the peak twistbrings your motor count with itSix-Bar Liftget more height and reach once the bars start collidingbrings your lift load with ita number carries across the next question it raises