Two-Bar Lift
Sizes a single pivoting arm: how high it reaches, how much torque it needs, and by how much the intake tilts on the way up, which is the thing that decides whether a two-bar is the right choice at all.
Inputs
The arm
Along the arm, from the middle of the pivot bolt to wherever the intake bolts on. VEX holes are half an inch apart, so this lands on a half inch.
Everything the arm reaches is measured up from here.
Balance the bare arm across a finger. A plain C-channel balances at its middle, so half the arm length is the usual answer. A motor bolted partway out drags it further.
From horizontal, negative pointing downhill. Where the arm rests at the bottom of its travel.
Where it stops at the top. 90 is straight up.
What it carries
The bare arm on its own, weighed off the robot. Both sides together if it is a double arm.
The claw, intake or hook bolted to the end, empty.
Kitchen scale. The heaviest thing you intend to pick up, not the average.
Drive
How many motor turns per arm turn. A 12 tooth driving an 84 tooth is 7.
Losses through the gears and bearings. Around 95% per clean mesh, so a two stage reduction lands near 90. Drop it if anything binds.
Results
The intake tilts by
—°
One for one with the arm. This is the whole story of a two-bar.
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
Your drive gives
—lb·in
Does it hold?
—
Reduction needed
—:1
Stall is a holding figure. Past the first mark the arm holds but barely moves, and past the second it is cooking the motor to stand still.
Watch the intake tilt
Drag the slider. The cup on the end is bolted rigidly to the arm, so it turns exactly as much as the arm does — that is the whole character of a two-bar. Watch what happens to anything sitting in it.
How this is calculated
A two-bar is one moving link on one pivot. It is the lightest and simplest lift there is, and the only one on this site whose end effector does not hold its orientation.
height h = pivot height + L sin θ reach x = L cos θ peaks at θ = 0 tilt φ = θ + start rotates one for one W_eff = 0.5 × arm + 1.0 × (end effector + payload) torque τ = W_eff × L × cos θWith your numbers
The tilt is the decision
Sweeping from −20° to 70° rotates the intake by exactly 90°. Whatever was level at the bottom is vertical at the top. That single fact is why four bars and chain bars exist, and it is also why a two-bar is still the right answer when the payload does not care: a hook, a puncher, a flipper, or anything where the tilt is the point.
If the intake has to stay level, no amount of torque fixes this. Therubber band assist page will help with the load, but the geometry needs a different linkage.
Why the arm counts half
The arm's own weight acts at its balance point, roughly halfway out, so it rises half as far as the tip does and costs half as much torque per pound. Everything bolted to the end rides the full radius and counts once. That same idea sets the coefficients on every arm lift, and it is why a heavy arm costs less than a heavy intake.
Everything peaks together at level
Reach is L cos θ and torque is proportional to cos θ, so both are largest with the arm horizontal. An arm that starts tucked below level swings forward as it rises to horizontal, then back as it continues up. Peak overhang, peak torque and peak tipping moment all happen at the same moment, which surprises teams whose lift is stable at the bottom and at the top.
What this leaves out
Stall torque is a holding figure. A mechanism sitting at 90% of stall will hold the load and barely move it, and it will get hot doing so. Treat anything above about 60% as needing more reduction rather than more optimism. Friction in the pivot is not modelled either, which is part of what the efficiency input is standing in for.
Sources & assumptions
Cartridge stall torques are published: 2.1 N·m at 100 RPM, with the 200 and 600 RPM figures derived from the same motor through their gear ratios. Shaft yield figures come from the shared VEX data used across this site.
The coefficient model is derived. It was checked against a finite-difference derivative of the potential energy computed from real link positions, agreeing to under 1e-9 lb·in.
Arm, end effector and payload weights are yours. So is the transmission efficiency, which is a stand-in for losses this page does not model individually.
- Purdue SIGBots Wiki — VEX Motors — checked 2026-08-17
- VEX Robotics — V5 Smart Motor & Gear Cartridges (276-4840) — checked 2026-08-17
- VEX Robotics — C-Channels (8 sizes) — checked 2026-08-17
- V5RC Override Game Manual v1.1 (2026-27) — checked 2026-08-17
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