Gross Combined Mass (GCM)
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Understanding Gross Combined Mass (GCM) for Safe Towing and Vehicle Management
What is Gross Combined Mass (GCM)?
Gross Combined Mass (GCM) is a crucial term used in the automotive and towing industries, referring to the maximum allowable combined weight of a tow vehicle, its trailer, and all their contents. This weight includes the vehicle itself, the trailer, passengers, fuel, cargo, and any additional accessories or equipment.
Understanding and adhering to Gross Combined Mass is vital for ensuring safety, legality, and optimal performance when towing.
For the purpose of this article, “Trailer” refers to a caravan, horse float, or any other trailer.
Why is Gross Combined Mass (GCM) Important?
- Safety:
Exceeding the Gross Combined Mass can significantly impact the stability and handling of both the tow vehicle and the trailer, increasing the risk of accidents and vehicle sway. It can also reduce braking performance and vehicle dynamic stability, and cause premature wear and tear of vehicle components. - Legal Compliance:
Driving a vehicle that exceeds its GCM can result in fines, penalties, and even the invalidation of insurance coverage. - Vehicle Longevity:
Operating within the recommended GCM helps maintain the longevity of your vehicle. Excessive weight can cause undue stress on the engine, transmission, suspension, and brakes, leading to premature wear and costly repairs. - Fuel Efficiency:
Staying within the GCM can also help in optimising fuel consumption. Overloading can cause the engine to work harder, leading to higher fuel usage.
How is Gross Combined Mass (GCM) Calculated?
GCM is specified by the vehicle manufacturer and can be found in the owner’s manual or on the vehicle’s compliance plate. It is the sum of:
- Gross Vehicle Mass (GVM):
The loaded weight of the towing vehicle, including accessories, fuel, cargo, passengers, and Tow Ball Mass (TBM) when hitched. - Gross Trailer Mass (GTM):
The loaded axle group weight of the trailer, including fluids, cargo and accessories when hitched.
GCM = Gross Vehicle Mass (GVM) + Gross Trailer Mass (GTM)
Factors Affecting Gross Combined Mass (GCM)
- Vehicle Design and Specifications:
All vehicles have a Gross Combined Mass specified by the manufacturer, based on its design and intended use. - Type of Trailer:
The type and size of the trailer can significantly affect the GCM. Larger trailers, or those designed to carry heavy loads, will contribute more to the overall mass, leaving less GVM for your tow vehicle. - Cargo and Passengers:
The weight of the cargo, passengers, fluids and any additional equipment can quickly add up, pushing the total weight towards or over the GCM limit. It’s important to remain within payload limits. - Accessories and Modifications:
Additional accessories to trailer such as additional solar panels, tool boxes, bike racks etc or to the tow vehicle such as roof racks, bull bars, winches, driving lights, drawers, secondary batteries, long range fuel tank, larger wheels etc add to the vehicle’s weight, reducing the available payload and impacting the GCM. Always consider the weight of these additions when calculating the total mass.
Choosing the Right Equipment
Selecting the right vehicle and trailer combination is essential for staying within the Gross Combined Mass GCM. When choosing a towing setup, consider:
- Vehicle Towing Capacity:
Ensure the tow vehicle has a high enough towing capacity to accommodate the trailer and its load, as well the tow vehicle payload. If packing a considerable amount in your tow vehicle whilst towing, it is likely you will exceed the rear axle load of your tow vehicle. Leave weight to spare rather than pushing to the limits. - Vehicle Maximum Allowable TBM:
Ensure the maximum allowable TBM of the tow vehicle can accommodate at least ~10% of the trailer’s ATM i.e. a 3T caravan will have an estimated 300 kg TBM. Note the TBM is calculated as part of your tow vehicle’s payload. In most cases, this assumes ~half the available payload. - Trailer Design:
Choose a trailer that suits your specific needs (i.e. onroad/offroad) and is well designed to achieve optimum balance and TBM. Ideally with water tanks front and rear, or close to the axle, with heavy items evenly placed side to side, and front to rear (i.e fridge, batteries etc).
Conclusion
Understanding Gross Combined Mass is vital for anyone involved in towing, whether for recreational purposes or professional use. By adhering to the specified GCM, you ensure the safety, legality, and efficiency of your towing setup. At Weigh My Ride, we provide professional towing assessment and weighing services to help you accurately measure and manage your vehicle and trailer’s weight, ensuring you stay within legal and safe towing limits.
Remember, always check your vehicle’s manual for specific GCM details and seek professional advice if you are unsure about your towing setup. Safe towing is not just about having the right equipment but also about understanding and respecting the weight limits to ensure a safe and comfortable journey.