Chapter 2: Stops, Bumpers & Hard‑Lock States

Created by Sarah Choi (prompt writer using ChatGPT)

Joints & Range-of-Motion Libraries — Stops, Bumpers & Hard-Lock States

Stops, bumpers, and hard-lock states are the small mechanical truths that make a mech feel engineered instead of “posed like a doll.” A joint isn’t only defined by how far it can move; it’s defined by how it refuses to move past its limits. When you depict limits clearly—through physical stops, compressible bumpers, and lockable states—you give the audience confidence that the machine has boundaries, that it can brace under load, and that it won’t magically fold through armor. You also give production teams a gift: predictable ranges, consistent collision behavior, and clear reasons for why a pose is legal or not.

This chapter treats limits as a visual language you can design intentionally. For concept artists on the concepting side, it’s about making poses believable and designing joints that “read” their range-of-motion. For concept artists on the production side, it’s about preventing rig surprises, reducing clipping, supporting gameplay states, and creating repeatable ROM sheets that map to real constraints.

1) Limits are a feature, not a restriction

Designers sometimes fear that showing limits will make a mech feel less capable. In practice, limits make a mech feel more capable because they imply structure, strength, and bracing. A joint that has a visible stop looks like it can push against something without tearing itself apart. A joint that can “hard-lock” looks like it can carry heavy loads, absorb recoil, or hold a crouch without constantly burning energy.

Concepting-side: limits are a posing tool. They define your iconic silhouettes—deep crouches, braced firing stances, kneeling stability, ladder climbs.

Production-side: limits are a pipeline tool. They define safe IK ranges, reduce foot skating, and help animation avoid unintentional hyperextension.

2) Stops, bumpers, and locks: three different behaviors

These terms often get lumped together, but they communicate different mechanical behaviors.

A stop is a hard contact between rigid parts. It creates a clear maximum angle or travel limit.

A bumper is a compressible element that cushions the end of travel. It reduces shock and noise and makes the stop feel less violent.

A hard-lock state is a deliberate mechanism that holds a joint in a position so it resists movement under load. It’s a state change, not just a passive limit.

Concepting: a stop is a shape; a bumper is a material; a lock is a mechanism with a “latched” read.

Production: stops help define rig constraints; bumpers help sell weight and comfort; locks help define gameplay states (brace mode, siege mode, recoil lock).

3) Stops by joint class: where limits live

Different joint classes want different kinds of stops.

A hinge naturally supports visible stops: tabs, brackets, knuckle housings that collide at a known angle. Hinge stops can be made very readable.

A ball joint often hides its limits inside a collar or socket. Its stop is a rim contact or an internal geometry constraint. To depict it, you usually show a collar that clearly restricts angle.

A universal joint tends to have tighter allowable angles and can collide quickly. Depict that with compact housings and limited clearance.

A planar/sliding joint uses end-of-rail caps, bump stops, and travel collars. Its stop is where the slider bottoms out.

A compliant joint uses material limits more than hard stops. It may still have a “hard stop” to prevent over-flex, but its normal limit is a gradual resistance.

If you choose a joint class but depict stops that don’t match it (for example, a ball joint that shows no collar and somehow rotates 180 degrees), your ROM language becomes inconsistent.

4) The anatomy of a good stop: tab, seat, and load path

A believable stop usually has three visual ideas.

A tab (or feature) that makes contact.

A seat (a surface designed to receive the tab).

A load path that carries force into thicker structure.

Concepting: draw stops as designed interfaces, not random collisions. The tab should look intentional, and the seat should look reinforced.

Production: this makes modeling easier and avoids ugly intersections. When the stop is designed as “tab into seat,” you can build clean contact surfaces that hold up under close-up camera.

A stop that looks like two armor plates accidentally smashing together reads like a mistake, not a design.

5) Bumpers: how to show “cushion” in a hard-surface world

Bumpers are what make heavy machines feel survivable. They suggest noise control, impact management, and maintenance sensibility.

Concepting: bumpers are a material change. A rubberized block, an elastomer pad, a spring stack, or a hydraulic end-of-travel cushion can be depicted with softer edges, layered forms, and a different surface finish.

Production: bumpers are also animation cues. When a joint hits a bumper, the motion can ease in and settle rather than snapping to a hard stop. VFX and audio can sell it: a muted thunk instead of a sharp clang.

Bumpers are especially useful for feet, knees, and elbows—places where impacts are frequent.

6) Hard-lock states: why mechs “brace” and how to depict it

Hard-lock states are when the joint becomes temporarily rigid or constrained by a mechanism. This is common in real machinery (outriggers, stabilizers, locking pins) because it allows the structure to carry load without constant actuator effort.

Concepting-side: hard-lock states are story states. A mech might lock its knees in siege mode, lock ankles for recoil control, or lock a shoulder for a heavy weapon mount.

Production-side: hard-lock states are gameplay and animation states. They explain why a mech can hold a firing pose without jitter, or why it can safely carry a heavy object.

Depiction cues for hard-lock states

A locking pin that slides into place is a strong, readable cue.

A collar clamp that tightens around a ring suggests a locked axis.

A ratchet or tooth ring suggests discrete lock positions.

A strut brace (like a fold-out support bar) suggests a temporary structural reinforcement.

When the lock engages, the silhouette should change a little. If nothing changes visually, the audience won’t register the state change.

7) Soft limits vs hard limits: designing “feel” in the ROM

Not all limits should feel like a brick wall. Soft limits are gradual resistance as you approach the end of range.

Concepting: soft limits can be implied by compliant elements, nested sleeves, or tapered clearances. They create a sense of controlled motion.

Production: soft limits help animation because motion can ease into the limit naturally. Hard limits can look abrupt unless the animation is carefully timed.

A good rule is: hard limits for bracing and heavy load; soft limits for expressive, agile motion.

8) Depicting clearance: the simplest way to make limits believable

Clearance is the space that allows motion. Without clearance, limits feel accidental.

Concepting: in your ROM drawings, show the gap that closes as the joint flexes. Show armor plates that slide past each other. Show recessed sockets that allow rotation.

Production: clearance is collision prevention. If the design has no clearance, rigs will either clip or artificially restrict motion. Both look bad.

A simple depiction trick is to design the joint region as a “hollow zone” with nested armor, rather than a solid stack of plates.

9) Visual hierarchy: stops should be readable, not noisy

Stops live in joint zones, which are already detail-heavy. The stop must be readable without adding clutter.

Concepting: make the stop one bold shape. A single tab. A single collar ring. A single brace strut. Avoid multiple competing stop features.

Production: keep stop geometry clean so it can be modeled and baked well. Overly intricate stop shapes will shimmer and alias in motion.

If the stop is important for gameplay (siege lock, recoil brace), give it a distinct silhouette and possibly a distinct material or color accent.

10) Stops in motion: how to show them in concept art

Stops and bumpers are easiest to communicate with a small motion sequence.

Concepting: show neutral, near-limit, and at-limit poses. Add one small callout: “tab seats here,” “bumper compresses,” “lock pin engages.” You don’t need a blueprint—just a clear visual statement.

Production: these sequences become reference for riggers and animators. They can also inform VFX: sparks if a hard stop slams, dust if a foot bumper compresses.

If you’re designing a lock state, show both unlocked and locked silhouettes. The difference should be readable.

11) Hard-lock states across the body (common use cases)

Knees: lock for siege firing or heavy lifting.

Ankles: lock for recoil control or stability on slopes.

Hips: lock for braced stance or to prevent twist under load.

Shoulders: lock for heavy weapon mounts or tool operations.

Spines: lock for “carry mode” or to stabilize sensors.

Tracks/wheels: lock suspension height or lock track tension for specific terrain.

Concepting: decide where your mech needs to become a “structure” instead of an “actuator-driven pose.” That’s where lock states make sense.

Production: lock states can reduce animation complexity. If a joint is locked, it stops needing micro motion and can feel stable.

12) ROM libraries: how to encode stops and locks in your deliverables

A ROM library should not only show maximum angles. It should show how limits behave.

Concepting: include a small legend: hard stop, soft stop, bumper zone, lockable positions. Even a simple icon system can help.

Production: these cues can map to rig constraints and state machines. A locked knee might have a different animation set, different IK stiffness, and different audio.

When ROM sheets include lock states, you reduce later redesign. Teams can build around the intended behavior from the start.

13) A compact checklist for stops, bumpers, and hard-lock depiction

Can you clearly identify where the limit is and what makes contact?

Does the stop look intentional (tab into seat) rather than accidental collision?

Is there an implied load path into thicker structure?

If the joint needs cushioning, is there a bumper material or shock cue?

If the joint needs bracing, is there a readable lock mechanism and a clear locked silhouette?

Do clearance and armor overlaps support the intended range without constant clipping?

Do your ROM drawings show neutral, near-limit, and at-limit poses (and lock/unlock if applicable)?

If yes, your joints will feel real in motion and your production pipeline will thank you.

14) Quick exercises to build limit literacy

Take a simple biped knee and design three limit solutions: a pure hard stop, a bumper-cushioned stop, and a lockable knee for siege mode. Draw each in neutral and max flex. Notice how the silhouette and the storytelling change.

Then take a ball-joint shoulder and design the collar limits. Draw max raise, max cross-body, and max back swing. Add one visible stop cue that explains each limit. This will teach you how to make multi-axis joints feel believable.

When you design stops, bumpers, and locks as a coherent part of your joint language, you turn ROM from an abstract idea into a visible system. That’s how mechs feel engineered, how animation feels grounded, and how the audience believes the machine could exist.