While devastating fires on trucks and trailers can start in several ways, some 65% are caused by issues related to the wheel and brake assembly area, according to TT Club’s claims analysis data for 2019.

How do these trailer fires start?

There are numerous moving components to a truck or trailer, particularly around the wheel and brake assembly. From a practical perspective ignition will be initiated through a build-up of heat and the presence of oxygen and a fuel source. While fires on trucks can originate in a number of areas, the primary areas of concern are around the wheel and brake assembly and have been identified as:

Hot brakes, wheel bearings and hub failures

The way a vehicle is driven, particularly when loaded with heavy cargo can give rise to extreme heat build-up under heavy braking conditions, resulting in excessive brake temperatures. This can lead to bearing grease igniting and fires spreading to involve the tyres. Heavy braking or dragging brakes can be of particular concern on brake drum systems as the heat is more easily transferred through the drum to the tyre.

Brakes can also remain partially applied, even with no pressure on the brake pedal. This condition is typically referred to as ‘dragging brakes’, often associated with the last trailer as a consequence of low air pressure, which may be slow to replenish. This condition also results in excessive brake temperatures.

Maintenance of the brake assembly is also crucial in mitigating the risk of fire. An incorrectly positioned or locked component of the mechanism can lead to increased friction and heat build-up. It has also been reported that, poor brake balance can cause disc brakes to overheat with subsequent ignition of bearing grease and or detritus. Hydraulic components that through poor maintenance develop even the smallest of leaks can be particularly problematic. Hydraulic oil being expelled under pressure from even a pin-sized hole will create a mist – which can be particularly easy to ignite where a sufficient heat source is present.  

Wheel bearings are an essential part of the wheel assembly being the point where the wheel and the axle are connected. These lubricated steel ball bearings allow the wheel to rotate smoothly with minimal friction. Wheel bearing failure can also result in dragging brakes and be a source of heat if not properly maintained. This failure can be exacerbated or instigated due to overtightening of  poorly lubricated wheel bearings. An overheated bearing, for example due to excessive wear, can create heat which may ignite bearing grease or other combustible material.

Wheel hubs low on oil can reach an elevated operating temperature. This temperature can be sufficiently high to ignite adjacent combustible material, for example the hub oil and tyres. A hub or wheel bearing failure combined with braking can induce additional load on a brake components. The surface temperature  around the braking system and wheel hub can increase significantly transferring heat to the tyre which can then ignite.

Flat or underinflated tyres

Tyres can be difficult to ignite but once burning they are difficult to extinguish. Tyre fires can occur due to under inflated or flat tyres (dual or single) causing the surface area of the tyre to deform and come into contact with nearby surfaces of the vehicle, such as the chassis. Friction can then cause the tyre to get hot enough for ignition to occur.

The deformation of a flat or poorly inflated tyre in contact with the road surface can also increase the temperature of the tyre this deformation brings the deflated tyre into contact with an adjacent tyre which in turn heats as the tyres rotate. While melting of the tyre may occur before ignition, a fire may initiate when the vehicle comes to a stop.

Road debris

There are occasions when road debris can become lodged under front or rear axles or wedged within wheel arches. If the debris contains a metal component, which comes into contact with a moving surface, sparks can be produced. This in turn can cause combustible debris to ignite.

Pre-trip/first use inspections

While undertaking first use checks at the start of each shift, it is prudent to visually check the condition of mud guards/spray flaps as well as the inflation of the tyres and fluid leaks, so far as is reasonably practicable. Identifying potential issues and having a defect procedure to ensure that the issue is rectified will assist in mitigating truck fire risks.

Such inspections should also serve to identify deficient repair or maintenance undertaken by a repair facility. A component not installed or replaced correctly, an incorrect or incompatible part or a critical protection device not re-installed could each give rise to a fire incident that could be avoided if identified early.

What actually catches fire in truck fires?

At the point of ignition, it is useful to consider what is actually catching fire in such incidents. In the very early stages of development, the cargo is typically some distance away from the wheel assembly and protected by the trailer curtain/sides/floor or container.

Rubber tyres are composed of numerous combustible materials such as carbon, oil, benzene, toluene and sulphur as well as rubber. Sufficient heat build-up will cause tyres to combust. This has the potential to give rise to tyre blow out. While the blow out event might not itself accelerate the progression of the fire, it is likely to assist the spreading of the fire from the tyre as burning fragments may be propelled outwards.

Mud guards/spray flaps are designed to capture and prevent spray from the road surface, a surface that can contain many different contaminants including detritus and oils. Where sufficient heat builds, it is possible for mud guards/spray flaps to combust and spread to other combustibles. Keeping these vehicle components clean, free of detritus and in sound condition will mitigate the risk of combustion.

Lubricants or hydraulic oils leaking from poorly maintained, worn or damaged seals or hose can bring a viable fuel into contact with hot surfaces, debris, and detritus. Given the right conditions these leaking fluids can ignite or contribute to the ignition of other combustible material.

TT Club advice for truck/trailer fires

• Safety is the paramount concern, personnel should not place themselves at risk of injury. The emergency services should be called to attend and safely extinguish the fire. Depending on the circumstances, it might be prudent to establish a cordon around the immediate area to ensure that third parties are not able to expose themselves to injury.
• Preservation of the scene is vitally important to facilitate an investigation, photographs of the vehicle/scene as the circumstances develop can be of great assistance in determining the seat of the fire and likely cause. Contact your insurer at the earliest opportunity, since this stage of the investigation is not only vital to establish liability, but also to assist mitigate future incidents. It is important to understand that vital evidence could start to deteriorate as time passes and become irretrievable.
• Consideration should be given to the early appointment of experts to investigate (often under the direction of your insurer). Fire experts deploying forensic techniques will in most instances be able to identify the seat of the fire. Even where the fire is catastrophic, fire patterns, in combination with vehicle maintenance and defect history, can lead to the identification of the point of ignition.

Fire incidents involving trucks are a frequent occurrence globally. If identified early and fire-fighting equipment and trained personnel are present, it can be possible to contain such fires causing minimal damage. Where these attributes do not come together, however, fire can quickly take hold, giving rise to devastating damage to equipment, cargo, property and of course endangering life. There exists no certainty as to how effectively a fire can be fought; factors such as weather, combustible materials, training, equipment and how long the fire has been burning will all affect the ability to fight the fire. With so many unknowns, clearly, prevention is better than cure.

We gratefully acknowledge the assistance in the preparation of this article of John Gow, Senior Investigator, IFIC.

We hope that you have found the above interesting. If you would like further information, or have any comments, please email us, or take this opportunity to forward to any colleagues who you may feel would be interested.

We look forward to hearing from you.

Peregrine Storrs-Fox

Risk Management Director, TT Club