Bungee
Latex or rubber elastic cord acting as an extension spring. Force follows Hooke's law, so it rises as the lift extends; pretension and routing set the operating window.
Best for
Teams that want meaningful gravity assistance with COTS parts, minimal machining, and easy trackside replacement.
Poor fit for
Lifts that need near-constant counterspring force across the full stroke, or precise position holding at every height.
ComplexityLowComplexity: Low, level 1 of 5. Cord, anchor points, and routing pulleys. No custom manufacturing is strictly required.
Cord, anchor points, and routing pulleys. No custom manufacturing is strictly required.
Force accuracyApproximateForce accuracy: Approximate, level 2 of 5. Hooke's-law force grows linearly with stretch while the gravity load is roughly stepped/constant, so exact matching is only possible at one or two positions.
Hooke's-law force grows linearly with stretch while the gravity load is roughly stepped/constant, so exact matching is only possible at one or two positions.
PackagingLightPackaging: Light, level 2 of 5. Cord runs alongside existing structure; the main demand is a clean routing path and anchor locations.
Cord runs alongside existing structure; the main demand is a clean routing path and anchor locations.
Tuning effortest.IterativeTuning effort: Iterative, level 3 of 5. Pretension and cord count are adjusted by trial. Easy to change, but expect several iterations, and re-checks as cord relaxes.
Pretension and cord count are adjusted by trial. Easy to change, but expect several iterations, and re-checks as cord relaxes.
In-season riskest.Low–moderateIn-season risk: Low–moderate, level 2 of 5. Failures are usually gradual (cord creep) and cheap to fix. Keep spare cord and inspect between matches.
Failures are usually gradual (cord creep) and cheap to fix. Keep spare cord and inspect between matches.
Click a metric to see why it’s rated that way. Ratings marked est. are editorial estimates awaiting test data.
Quick recommendation
Use this when
- You want a counterspring working within days, not weeks
- Your team can only source COTS or hardware-store parts
- Partial gravity compensation is enough (reducing current draw, not perfectly holding position)
- You can accept re-tensioning and periodic cord replacement as routine maintenance
Avoid this when
- You need the lift to float at any position without brake or motor power
- Cord routing would rub against slides or stringing and add friction
- The force mismatch at the ends of travel would overpower your motor or slam the lift
- You cannot inspect and replace cord regularly (latex fatigues and creeps)
Mechanism overview
Bungee counterspringing is elastic cord — latex tubing, shock cord, or "bungee" — anchored to the robot's structure and to the lift, stretched so its restoring force pulls the lift upward. It is the counterspringing method with the fewest parts: cord, two anchors, and optionally a pulley or two for routing.
The catch is built into the physics: elastic force grows with stretch (Hooke's law), while the gravity load on the lift is constant or stepped. A straight line can't follow a staircase — so bungee is always a compromise between over-assisting somewhere and under-assisting somewhere else. Pretension is the knob that shifts where the compromise lands.
Terminology used on this page
- Pretension — the stretch (and therefore force) already in the cord at zero lift extension.
- Stiffness (k) — added force per unit of additional stretch; depends on cord material, diameter, and length (longer cord = softer).
- Working stretch — the cord's stretch range across lift travel. Latex cords typically live happily up to ~100% elongation; check your cord's rating.
Continuous and cascade implementations
On a continuous lift the gravity load steps upward as stages engage (why) — the hardest target for a linear force. Common routings:
- Direct to first stage: cord from the base structure to the first moving stage. Simple; the cord's force grows just as the later stages (more weight) engage, which roughly points the right way, but matching is coarse.
- Continuous-style cord run: cord routed over a pulley at the top of the base rail down to the carriage, so cord stretch tracks total extension. Longer stretch range → softer cord possible → flatter force.
- What moves / attaches: anchor on static structure; moving end on the stage or carriage; pulleys on static structure only (a pulley on a moving stage halves the stretch per extension and doubles the force — sometimes useful, usually just confusing).
- Force profile: linearly increasing with extension.
- Packaging: nearly free — cord runs along the slide envelope.
- Advantages: minutes to install, trivially adjustable (change pretension or add a strand), cheap to iterate.
- Limitations: exact match at only one or two extensions; routing friction adds hysteresis; cord relaxes over weeks.
Cascade's near-constant load is a friendlier target for bungee than the continuous staircase: a long, soft, heavily pretensioned cord approximates a constant force over the stroke.
- Where it attaches: between static structure and the first moving stage (load there is the full reflected weight), or along the carriage run if the cord can survive the longer stretch.
- Force profile: still linear — but if pretension dominates (high initial stretch, soft cord), the relative force change across travel is small.
- Packaging: the long pre-stretched cord run needs a path; doubling back over pulleys folds it into the slide length.
- Advantages: the "pretension-dominant" trick makes bungee surprisingly competitive on cascade lifts.
- Limitations: high standing tension all the time — anchors, string, and slides carry it even at rest; more stored energy to respect.
Physics and force matching
In plain terms: Cord force equals stiffness times total stretch — the lift's extension x plus the pretension stretch x₀ you built in at installation. At x = 0 the cord already pulls with force k·x₀.
Assumes: Cord behaves linearly in its working range · Stretch equals lift extension (1:1 routing)
In plain terms: The load the cord is trying to cancel: the currently-moving mass times gravity, reduced by lift angle. On a continuous lift this steps upward; on cascade it's flat.
In plain terms: Positive error means the cord overpowers gravity there (lift wants to drift up, motor must pull down); negative means the motor still lifts the difference. A linear F_cord crossing a stepped F_gravity gives zero error at most at a few crossing points.
| Symbol | Quantity | Unit | Notes |
|---|---|---|---|
| Effective cord stiffness | N/m | Measure it — varies by brand, diameter, length, age | |
| Lift extension | m | — | |
| Pretension stretch | m | Set at installation | |
| Matching error | N | RMS of e(x) is the single-number match quality |
Where should the line cross the staircase? (choosing k and x₀)
You have two knobs (k, x₀) and one target curve. Two reasonable strategies:
- Match the endpoints' average: choose k and x₀ so the cord force equals the gravity load at the middle of the first stage and the middle of the last stage. The error is then bounded by half the total step height, split evenly between under- and over-assist.
- Never over-assist: keep F_cord ≤ F_gravity everywhere (cross at the top of travel). The lift never creeps up and always settles down — predictable behavior at the cost of less average assistance.
Minimizing RMS error e(x) lands near strategy 1. The force graph below computes RMS and max error live while you drag the sliders — faster than algebra, and it matches how you'll actually tune it on the robot.
Worked example: Sizing cord for a 90 cm continuous lift
Total moving mass 2.5 kg (≈24.5 N vertical load at full engagement), 3 stages × 30 cm, target ≈80% average assist.
Target force at mid-travel: ~0.8 × mean load ≈ 16 N. Taking 60% of that from pretension: x₀ chosen so k·x₀ ≈ 9.6 N, and stiffness supplying the rest across half the 0.9 m travel: k ≈ (16 − 9.6) / 0.45 ≈ 14 N/m. With a cord of stiffness 28 N/m per meter of free length, that means ~2 m of free cord (doubled over a pulley) pre-stretched ~0.7 m.
These numbers are a starting point — real cord stiffness must be measured. The calculator below runs this arithmetic for your numbers.
Interactive force graph
Watch how stiffness and pretension trade against each other: stiffness sets the slope, pretension lifts the whole line. Try making the error small at the top of travel versus the bottom — you can't have both on a continuous lift.
Force vs. extension — Bungee
Illustrative model, not measured data
View chart data as a table
| Extension (cm) | Gravity (N) | Counterspring (N) | Motor (N) |
|---|---|---|---|
| 0.0 | 11.8 | 8.9 | 2.9 |
| 7.5 | 11.8 | 10.1 | 1.7 |
| 15.0 | 11.8 | 11.3 | 0.5 |
| 22.5 | 11.8 | 12.5 | -0.7 |
| 30.0 | 11.8 | 13.7 | -1.9 |
| 37.5 | 16.2 | 14.9 | 1.3 |
| 45.0 | 16.2 | 16.1 | 0.1 |
| 52.5 | 16.2 | 17.3 | -1.1 |
| 60.0 | 16.2 | 18.5 | -2.3 |
| 67.5 | 20.6 | 19.7 | 0.9 |
| 75.0 | 20.6 | 20.9 | -0.3 |
| 82.5 | 20.6 | 22.2 | -1.6 |
| 90.0 | 20.6 | 23.4 | -2.8 |
Design and sizing
Bungee starting-point estimator
Pretension force
11.8 N
Effective stiffness
17.4 N/m
Force at bottom
11.8 N
Force at top
27.5 N
This gives a starting point only. Real cord stiffness varies by brand, diameter, and age — measure your cord's force at known stretches with a luggage scale, then iterate pretension on the robot.
Choosing cord
- Measure stiffness yourself: hang a known weight (or pull with a luggage scale) at several stretches and plot force vs. stretch. Catalog numbers for elastic cord are unreliable, and stiffness drifts with age.
- Length is a design variable: stiffness is inversely proportional to free length. If the force change across travel is too steep, use a longer cord doubled over a pulley rather than hunting for a magic cord.
- Strands multiply force: two parallel strands double both stiffness and pretension force. Adding/removing strands is the coarse adjustment; pretension is the fine one.
Pretension and anchoring
- Anchor with knots around smooth features or purpose-made cord anchors — screw threads cut latex.
- Make pretension adjustable: a cleat, a row of anchor holes, or a knot you can re-tie. You will re-tension during the season as the cord relaxes.
- Leave service loop: enough tail to re-tie the knot a few times before the cord is too short.
Routing and friction
Route the cord so it touches nothing between anchors except intended pulleys. Cord rubbing on slide edges or string adds friction that shows up as hysteresis — different effective assist going up versus coming down — and chews through the cord.
Symmetric vs. one-sided
One cord on one side is usually fine for light assists — the slides carry the small off-axis moment. For heavier assists, mirror cords on both sides to keep the lift from racking, exactly as with symmetric constant-force mounting.
CAD, manufacturing, and assembly
There is little to manufacture — that's the method's selling point. The CAD work is routing, not part design:
- Model the cord path at minimum and maximum extension; check nothing intersects it (cord at tension will find every sharp edge).
- Place anchor features early: an anchor you can reach with the robot assembled is the difference between a 30-second re-tension and pulling the lift apart.
- If using pulleys, standard COTS pulleys with a bearing are fine; the loads are small.
Serviceability
Plan for replacement, not just installation: cord is a consumable. Keep pre-cut, pre-measured spare cord in the pit with the knot positions marked. A cord swap should take minutes and restore the exact previous tune.
Testing and validation
- Unpowered force check: with the motor disconnected, pull the lift through its travel with a luggage scale, up and down. The difference between the up and down readings is your friction + hysteresis; the average is your net residual load.
- Release test: move the lift to several heights and let go. Note where it drifts up (over-assisted) and settles down (under-assisted) — this maps your error curve e(x) with zero instruments.
- Repeat measurements 2–3 times and after a day of rest — fresh latex relaxes measurably in the first sessions.
- Powered tests: log current and time full cycles before and after installing the cord; verify retraction time is still acceptable (the motor now works against the cord downward).
- Cycle + wear test: run 100+ cycles, then inspect every contact point on the cord for wear and re-check pretension.
Real-world examples
Bungee assists appear on many competition lifts and intakes, but this site only lists examples that are documented and verified — and none have been collected yet.
Your team here
Bungee— · architecture unrecorded
A documented bungee-countersprung lift with routing photos, measured before/after current, and honest lessons learned belongs here.
Placeholder card — a real, verified team example belongs here. Contributions welcome.
Your team here
Bungee— · architecture unrecorded
Especially wanted: a cascade lift using the pretension-dominant approach described above.
Placeholder card — a real, verified team example belongs here. Contributions welcome.
Common failure modes
Cord relaxes; assist fades over weeksMedium severity
- Cause
- Latex creep under standing tension, accelerated by heat and UV.
- Symptoms
- Lift current slowly rises back toward pre-bungee levels; release test settles faster than it used to.
- Prevention
- Adjustable anchors; log the release-test behavior so drift is visible; de-tension for long storage.
- Fix
- Re-tension at the anchor; replace the cord when re-tensioning stops holding.
Cord snapsHigh severity
- Cause
- Wear at a sharp edge or hole, aged latex, or working stretch beyond the cord's rating.
- Symptoms
- Bang; lift instantly heavier; whip marks near the routing path.
- Prevention
- Smooth-radius routing only; inspect before events; stay inside rated elongation.
- Fix
- Replace with pre-measured spare; fix the routing that caused the wear.
Lift won't fully retract / slams at topMedium severity
- Cause
- Over-tensioned cord: assist exceeds gravity at the extremes of travel.
- Symptoms
- Motor stalls or draws heavily holding the lift down; lift jumps upward when released low.
- Prevention
- Bias tune toward under-assist; verify with the release test at both ends of travel.
- Fix
- Reduce pretension or remove a strand.
Hysteresis from routing frictionLow severity
- Cause
- Cord dragging on structure or string between anchors.
- Symptoms
- Different behavior up vs. down; visible cord wear at the rub point; squeaking.
- Prevention
- Route clear of everything; pulleys at every direction change.
- Fix
- Re-route; replace the worn cord section.
How this compares
Constant-force springs hold any position and don't relax over time, but need spools, printed parts, and per-stage sizing. Bungee wins on install time and cost; loses on accuracy and maintenance.
GuideConstant torque gives steel-spring consistency at the winch but demands drum engineering. Bungee is the opposite trade: zero engineering, continuous upkeep.
GuideA sprung linkage can shape force to match gravity almost exactly — bungee makes no such attempt. If a rough assist is enough, bungee gets you 80% of the benefit for 5% of the effort.
GuideFor the full matrix, force-curve overlay, and decision summary, see Compare Methods.
References
- counterspringing.com planning notesEngineering source
Framing source: methods list, stringing trade-off, simplicity-first design philosophy.
- Elastic cord manufacturer data (to be added)Manufacturer data
Rated elongation and force tables for common latex tubing and shock cord sizes.
- Verified bungee implementations (to be added)Robot example
Placeholder — submit documented examples with photos and measurements.