System Stability: Keeping Products on Their Feet

Many products spend their lives standing still: a smart speaker on a kitchen counter, a smart display on a nightstand, a phone on a charging stand, a pair of smart glasses folded on a table. They are not supposed to move. But homes and offices are busy places. A sleeve brushes past, a cable gets tugged, someone bumps the table, a cat explores.

When a product tips over, the fall is often the beginning of the real damage: a drop onto a hard floor, a cracked screen, a broken hinge. System stability -- how hard it is to make a product topple -- is therefore a reliability topic in its own right, and one that is decided very early in the design, when the shape and the layout are chosen.

The Basic Physics: Center of Gravity and Footprint

Stability starts with two things every product has:

A center of gravity (CG): the single point where the product's weight can be thought to act.
A base footprint: the area outlined by the points or feet where the product touches the surface.

As long as the vertical line through the CG falls inside the footprint, gravity pulls the product back onto its base. If something tilts the product until that vertical line passes the edge of the footprint, gravity switches sides and the product falls.

This gives the most important single number in stability: the tip angle. For a simple rigid body, the tip angle about an edge is found from basic geometry:

tan(tip angle) = (horizontal distance from CG to the tipping edge) / (height of CG above the surface)

Two consequences follow immediately:

A lower CG makes the product more stable.
A wider base (a larger horizontal distance to the edge) makes it more stable.

As an illustrative example: a product whose CG sits 100 mm above the table and 50 mm from the nearest edge of its footprint has a tip angle of about 27 degrees about that edge. Raise the CG to 150 mm without changing the base, and the tip angle drops to about 18 degrees.

Note "about that edge". A product with a rectangular or oval base has a different tip angle in each direction. The weakest direction -- usually across the narrow side of the base -- is the one that counts.

How Much Push Does It Take?

The tip angle tells you how far you can tilt a product. In daily life, though, products are usually pushed, not tilted. A static balance of moments gives the horizontal force needed to start tipping:

tipping force x (height at which the force is applied) = weight x (horizontal distance from CG to the tipping edge)

So a push applied high on a tall product needs much less force than a push near its base. This is why tall, narrow, top-heavy products -- think of a slim speaker or a display on a stand -- deserve extra attention.

Friction adds a second possible outcome. If the feet do not grip the surface well enough, the product slides before it tips. Whether it slides or tips depends on how the friction coefficient compares with the ratio of base half-width to push height. Sliding is often the safer outcome; tipping is the one that leads to a fall.

Nudge and Incline Tests

Two kinds of tests are commonly used to check stability:

Incline test. The product is placed on a surface that is tilted step by step, in several directions. The angle at which it tips -- or slides -- is recorded. This directly checks the tip angle, including the effects of feet and flexible parts.
Push or nudge test. A horizontal force is applied at a defined height and direction, sometimes slowly, sometimes as a short shove. The test checks whether the product tips, slides or recovers.

A useful public reference is IEC 62368-1 (Audio/video, information and communication technology equipment -- Safety requirements), which includes a stability clause for equipment, with tests such as tilting on an incline and applying a defined force. It is a safety standard and is a good reference for test ideas; the levels a specific consumer product should meet for reliability (not only safety) are set by each product team.

Simple Statics Versus Dynamic Simulation

For many questions, hand calculations based on CG and footprint are enough -- and they should always be done first. But real products are not always simple rigid blocks:

Flexible parts change the geometry. A soft rubber foot compresses on the loaded side, the base effectively shrinks, and the tip angle drops. A stand with a hinge or a flexible neck can lean under load and shift the CG outward.
Dynamic pushes overshoot. A short shove gives the product kinetic energy. Even if the static force would not tip it, the product can rock past its tipping point. Whether it recovers depends on the energy it receives versus the energy needed to lift its CG over the edge.
Cable tugs come from awkward directions. A charging cable pulled sideways or downward at the back of a device acts low, at an angle, and sometimes with a jerk. It can rotate the product about an unexpected edge or drag it toward the edge of the table.
Multi-body products move. A phone in a stand, a speaker on a detachable base, or glasses lying on their hinges are several bodies, not one. The way they shift relative to each other matters.

Dynamic simulation handles these cases. A model of the product -- with its real mass distribution, contact with the surface, friction at the feet and flexibility where it matters -- is pushed or tugged, and the solver tracks whether it settles back, slides, rocks or falls. Engineers can then sweep push height, direction, force level and friction to map out where the product is safe and where it is not.

Rubber Feet and Friction

Feet are small parts with a large effect on stability:

Placement. Feet at the outer corners give the widest effective footprint. Feet set inward, for cosmetic reasons, quietly reduce the tip angle.
Material and hardness. Softer feet grip better, but compress more under the edge load just before tipping, which reduces the effective base.
Friction. High friction stops sliding but makes tipping more likely under a high push. Low friction lets the product slide away from the push. The right choice depends on the product's height and use.
Surfaces vary. Glass, wood, laminate, fabric and a dusty shelf all give different friction. A robust design does not rely on a single friction value.

Why Stability Matters for Reliability

A tip-over is rarely the end of the story. A product that topples off a table becomes a drop from table height, often onto a corner or an edge -- among the more damaging drop orientations. A product that falls over on the table can still hit its screen or a protruding feature.

Stability is therefore the first line of defence: every tip-over that is prevented is a drop that never happens. And stability is cheap to improve early -- once the tooling is cut, moving the battery or widening the base is expensive.

Design Levers

Mass distribution. Place heavy parts -- batteries, magnets, speakers, metal weights -- as low and as central as possible.
Base width. A wider or longer base, especially across the narrow direction, raises the tip angle directly.
Foot placement. Put feet as close to the outer edge of the base as the design allows.
Foot material. Choose hardness and friction for the product's height and likely surfaces, and check the effect of foot compression.
Cable exit. Route the cable low and toward the center, so a tug pulls the product along rather than over.
Stands and hinges. Make sure detents, hinges and flexible necks do not let the CG drift outward under load.
Added ballast. As a last resort, a small weight low in the base can improve stability, at a cost in total mass.

Closing Thoughts

Stability looks simple -- and the first answer really is simple: low center of gravity, wide base. But real products have soft feet, cables, hinges and users who push from every angle. Combining quick hand checks with dynamic simulation lets designers find the weak direction early and fix it while changes are still cheap.

AutoCAE.AI builds AI-assisted simulation workflows that help hardware teams assess stability, drop and other reliability load cases from early concept onward. Visit www.autocae.ai to learn more.