DR4B Geometry Calculator
A double reverse four bar reaches roughly twice as high as its bars are long. This works out how high, how low it folds, and whether it starts legal.
Inputs
Pivot to pivot on a bar of the stage that pivots off the tower.
Pivot to pivot on a bar of the upper stage. Measure both: a DR4B is meant to have them equal, and when they are not, the payload swings instead of going straight up.
Angle of the bars from horizontal at rest. A DR4B that packs flat often rests a little below horizontal, so this can be negative.
How far the bars swing up. 90 is straight vertical and rarely reachable.
Floor to stage 1's bottom pivot, on the fixed tower.
Up the middle carriage: from stage 1's top pivot to stage 2's bottom pivot. Measure it on your lift. It is the room the two stages fold into, so it is rarely zero, and it adds to every height.
Whatever the lift carries: intake, claw, tray. Negative if the top of it sits below stage 2's top pivot, which is where a hanging intake usually is.
Results
Vertical travel
—in
Height raised
—in
Top of what it carries, floor to the top.
Height folded
—in
Top of what it carries, at rest.
Height at your chosen angle
—in
Ceiling if bars went vertical
—in
Room the stages fold into
—in
The middle carriage gap, which is the gap the two stages pass through.
Payload swings forward by
—in
Zero while the two stages are the same length.
Reaches forward, folded
—in
The middle carriage, out in front of the tower.
Folded inside the starting cube?
—
Watch it fold into itself
Stage 1 swings forward from the tower, the middle carriage rides up with it, and stage 2 swings back the other way, which is what keeps the payload over the base. Drag it down to nothing and watch where the two sets of bars go: they lie in each other's space, a middle carriage gap apart. That gap is why they can.
How this is calculated
A four bar swings its payload up along an arc, which means the payload also swings outward. A double reverse four bar stacks two of them with the second mirrored, so the second stage's outward swing cancels the first's. The payload goes straight up, and the two stages add their lift together.
rise per stage = bar length × sin(angle) total rise = 2 × bar length × sin(angle) height = first pivot + middle gap + total rise + carriageWith your numbers
Hence the rule of thumb: a DR4B travels about twice its bar length. Twelve inch bars give roughly two feet of travel, from a mechanism that folds down to almost nothing.
The middle carriage is in every height
Travel is the easy half, and it is the half the rule of thumb covers. Where a paper answer misses a built lift is the middle: stage 2 does not pivot off stage 1's top pivot, it pivots off the middle carriage, some way above it. That gap sits under everything the lift carries, at every angle, so it is in the folded height, the raised height and the inspection answer, and it changes the travel not at all.
It is not a number you can leave out and be nearly right, because the gap is doing a job. Walk the linkage folded flat: stage 1 lies forward from the tower, stage 2 lies back over it, and the two sets of bars want the same space. They pass each other with the middle gap between them, and it works out to that same gap at every angle, not only when folded. A DR4B with no middle carriage could not collapse into itself at all. So that gap has to clear a bar and the hardware on it, and whatever it clears by is height the robot carries around all match.
Measure it on your own lift, from stage 1's top pivot to stage 2's bottom pivot, and the drawing above will fold the way yours does.
Why the last few degrees are not worth chasing
Because rise depends on the sine of the angle, the gain flattens badly near the top. Going from 45° to 60° adds a useful amount. Going from 80° to 90° adds almost nothing, and buys it with the worst mechanical disadvantage the linkage ever sees, since the torque arm is longest when the bars are near horizontal but the leverage is worst near vertical.
Most working DR4Bs stop somewhere around 80°. The height you lose is small, and the mechanism is far better behaved.
Folding is the other half
The folded angle sets how flat the lift packs at the start of a match, and it is the number that decides whether the robot passes inspection. A lower folded angle costs you nothing in maximum height, since travel depends on the difference between the two angles, and it buys room for everything else.
The catch is that a nearly flat four bar has almost no mechanical advantage at the moment it starts to lift. That is why so many DR4Bs use rubber bands tuned to help hardest at the bottom of the stroke. TheRubber Band Assist calculator sizes and places them.
Sources & assumptions
The 18" starting envelope is from the Override2026-27 game manual. Everything else on this page is geometry from your own measurements, so there are no VEX figures to go stale.
Assumes two equal stages, equal angles, perfect pivots and rigid bars. A real DR4B flexes under load, and the two stages rarely track each other exactly, so measured height usually falls a little short of this.
The legality check covers folded height only. It says nothing about width, length, or any other rule an inspector applies.
- 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
18" Footprint CheckerYou are here
DR4B Geometry Calculator
Where your answer goes
Required Lift Torquecheck the motors can hold that bar length up at its worst angleRubber Band Assist Calculatorwork out the bands that hold both stages up so the motors do lessbrings your arm length with itTipping & Wheelie Thresholdsee whether raising that much weight that high tips the robot overuses your lift height again18" Footprint Checkermake sure the folded stack still fits inside the starting cubea number carries across the next question it raises