Why Foam-Filled Tires Can Appear Flat and What to Do About It

[MikePhua]

Foam-Filled Tires Are Not Immune to Compression
Foam-filled tires are often used on aerial lifts, telehandlers, and heavy equipment to eliminate the risk of punctures and provide consistent support under load. These tires are injected with polyurethane foam that hardens into a dense, rubber-like core. While they eliminate air pressure concerns, they are not immune to long-term compression, deformation, or structural fatigue. A common issue arises when foam-filled tires begin to look underinflated or sag, especially after years of use or prolonged static loading.
In one example, a 2004 Snorkel TB60 manlift weighing 22,000 lbs showed visible sagging in two of its factory foam-filled tires. The sidewalls appeared partially collapsed, prompting questions about whether the foam had deteriorated or compressed internally.
Terminology Clarification

• Foam-Filled Tire: A pneumatic tire injected with polyurethane foam to replace air, creating a solid core that resists punctures.
  
• Sidewall Sag: The visual flattening of a tire’s sidewall, often mistaken for low pressure in foam-filled tires.
  
• Valve Stem Goo: Residue from foam injection that can clog valve stems, making it difficult to inspect or modify the tire.
  
• Counterweight Ballast: The weight of foam-filled tires is often factored into the machine’s stability calculations, especially for aerial lifts.
  

Why Foam-Filled Tires Sag Over Time
The foam inside the tire can compress under sustained load, especially when parked for long periods. Unlike air, foam does not rebound quickly. Over time, the foam may crush or settle, causing the tire to appear flat even though it remains structurally intact. This is particularly noticeable when machines are parked on uneven terrain, such as a 15-degree slope, where gravity exaggerates the sag on the uphill side.
Can Foam Be Refilled or Rejuvenated
Unfortunately, once foam has compressed or deteriorated, it cannot be re-inflated or injected with water or additional foam. The foam hardens permanently after curing, and any attempt to add material will not restore its original density or shape. If the tire is severely deformed, the only solution is to replace the entire tire and foam assembly.
Preventive Measures for Long-Term Storage
To reduce foam compression during storage:

• Move the machine periodically to redistribute weight and relieve pressure points. A monthly repositioning is recommended.
  
• Park on level ground to avoid uneven loading across tires.
  
• Use cribbing blocks or jacks to lift the machine slightly and reduce direct pressure on the tires.
  
• Inspect tires annually for signs of cracking, delamination, or excessive sag.
  

When to Replace Foam-Filled Tires
Replacement is necessary when:

• The tire sidewall collapses to the point of affecting machine stability.
  
• The foam core separates from the casing, causing wobble or vibration.
  
• The machine’s specifications require foam-filled tires for ballast, and the current tires no longer meet weight requirements.
  

Always consult the manufacturer’s guidelines before switching to pneumatic tires. In aerial lifts, foam-filled tires often serve as part of the counterweight system. Replacing them with air-filled tires may compromise safety and violate load ratings.
Conclusion
Foam-filled tires offer durability and puncture resistance, but they are not immune to long-term compression and deformation. Visual sagging does not always indicate failure, but it can affect performance and safety. Regular movement, proper storage, and timely replacement are essential to maintaining the integrity of foam-filled tires on heavy equipment.