Ballast weight is how the industry has historically answered the PCMS stability question. Add enough weight to the base, and the unit stays put. The logic is intuitive, and for decades it has been accepted as sufficient.
The problem becomes apparent the moment you deploy on sloped terrain, wet grass, or in a sustained wind event. Weight creates a downward force. It does not create resistance to lateral movement. Those are different physical problems, and solving one does not solve the other.
Two Failure Modes, Two Design Responses
Stability under lateral load has two distinct failure modes.
The first is overturning — the unit rotating around its base edge until it tips. Ballast addresses this. Mass distributed low in the frame lowers the center of gravity and increases the restoring moment that resists tipping. More weight, properly placed, resists overturning.
The second is sliding — the unit translating laterally across the ground surface without rotating. This occurs when lateral force exceeds the frictional resistance between the unit’s base and the ground. Friction depends on both the weight pressing down and the characteristics of the contact surfaces. On sloped, wet, or vegetated ground, the friction coefficient drops substantially regardless of how much the unit weighs.
A heavily ballasted PCMS on a wet grass foreslope has substantial weight pressing down. It also has a low-friction interface with the ground. The weight helps resist tipping, but the unit can still slide. Those are two separate equations, and ballast only solves one of them.
What Sandbags Actually Do and Do Not Do
Sandbags add weight, which increases frictional resistance to sliding. That logic is correct as far as it goes. What it does not account for:
Placement is not standardized. One operator places bags tightly against the frame on all sides. Another stacks them differently. A third uses whatever is available. The stability contribution varies across deployments because the process varies.
On wet grass, the bags themselves can slide. Their friction contribution on soft or sloped terrain is substantially lower than it appears. On a cross-slope, bags add weight but do not reposition the center of gravity or level the unit — the geometric destabilization the slope creates is still there before any external force is applied.
Over a multi-day deployment, sandbags shift. Wind moves them. Maintenance visits disturb them. Soil compresses unevenly underneath. A unit adequately ballasted at setup may not be 72 hours later.
Texas DOT has approved sandbag ballasting designs for temporary sign supports with a maximum of 12 square feet of sign face for wind loading purposes. A full-size PCMS display panel presents substantially more wind-exposed surface area than that. The wind load on a full-size PCMS panel at highway wind speeds is not a problem sandbag ballast around a trailer base was designed to solve.
What Cleated Engagement Does Instead
A cleated footplate changes the ground contact from a friction-dependent interface to a mechanical engagement. Metal teeth penetrate the soil surface and resist lateral movement through the shear strength of the soil against the teeth — not through friction between two surfaces.
On a firm grass foreslope, that shear resistance is substantially higher than the frictional resistance a flat pad or rubber foot provides under the same load. On soft soil, the teeth penetrate deeper and engage more volume. On a wet surface where a flat pad would slide, the teeth hold because they are mechanically locked into the ground.
This matters most at the three terrain conditions most common in real PCMS deployments. On a sloped unpaved foreslope at 15 to 25 percent grade, cleated teeth engage the soil perpendicular to the slope direction, providing resistance to downhill movement that ballast cannot generate. On wet grass, the teeth penetrate through the surface layer into firmer soil beneath, maintaining engagement independent of moisture. On extended deployments over soft ground, teeth distribute load across more contact points and resist the uneven settling that causes gradual positional drift.
Why Both Systems Work Together
Weight and mechanical ground engagement solve different problems. A unit that resists overturning but can slide will drift laterally until it contacts something or slides close enough to the travel lane to become a hazard. A unit that grips the ground but lacks adequate mass may resist sliding but remain vulnerable to overturning in severe wind.
The Spyder Platform addresses both failure modes through its structural design. The frame carries enough integrated mass to keep the center of gravity low and resist overturning, while the cleated footplates on each leg engage the soil mechanically to resist sliding. The footprint in deployed configuration is wide enough that both systems — weight distribution and ground engagement — work together across the range of terrain conditions a highway foreslope actually presents.
The result is consistent across operators and conditions. Cleated engagement is not a variable process — the teeth set into the soil as the leg loads, the same way every time. Units that hold position mechanically require fewer return visits to adjust or reposition after weather events, reducing crew time on the shoulder in proximity to live traffic.
Ballast resists overturning. Cleated ground engagement resists sliding. On flat, dry pavement, weight alone may be sufficient. On sloped, wet, or soft terrain — where far-offset PCMS placement actually requires the unit to sit — it is not.
Sandbags have been the industry’s answer to stability on challenging terrain for decades. They work well enough often enough to have persisted. They also introduce placement variability, maintenance dependencies, and failure modes on exactly the conditions that require the most reliable stability.
Cleated footplate engagement removes those dependencies. The unit holds position because its legs are mechanically locked to the ground, not because someone placed sandbags correctly on a wet foreslope before driving away.
For specifications on the Tarsus Cleated Footplates, Spyder Leg design, and the Spyder Platform’s stability system, contact INEX or request a demo at inex.net.
