Mars Caravan Weights Guide: ATM vs GTM vs Ball Weight (Explained Simply)

Weights 101: ATM, GTM & Ball Weight — What They Mean and Why They Matter

Caravan weights aren’t just paperwork — they directly affect safety, handling, braking, and whether your setup is legal. This guide explains ATM, GTM and ball weight in plain language for Mars owners.

Note: Always refer to your compliance plate and vehicle limits. If you’re unsure, use a weighbridge.

ATM (Aggregate Trailer Mass)

ATM is the maximum total weight of the caravan when fully loaded, including everything in/on it.

  • Includes water, gear, food, clothing, accessories, and added extras
  • Includes the load on the tow ball
  • Measured when the caravan is not attached to the tow vehicle (supported by its jockey wheel)

GTM (Gross Trailer Mass)

GTM is the weight carried by the caravan’s wheels when it’s attached to the tow vehicle.

  • It does not include the tow ball load (because that load is on the car)
  • It’s essentially: caravan weight on its own axles/wheels

Ball Weight (Tow Ball Mass)

Ball weight is the downward force the caravan applies to your tow ball. It affects stability more than almost anything else.

  • Too light ball weight can contribute to sway
  • Too heavy can overload the tow vehicle’s limits and affect handling
  • Ball weight changes depending on how and where you load your gear

Why these numbers matter

  • Legal compliance (vehicle and caravan limits)
  • Safe braking and stability
  • Reducing sway risk
  • Protecting suspension, chassis, tyres, and tow vehicle components
  • Avoiding insurance and warranty complications linked to overloading

The easiest way to get real numbers: use a weighbridge

Best practice is to weigh your setup in real-world loaded condition:

  • Caravan loaded for travel (including water and gear)
  • Tow vehicle loaded as you actually travel (people, fuel, luggage)
  • Measure ball weight if possible (ball weight scale or relevant method)

If you’re not sure how to interpret your weighbridge results, include them in a support ticket and we can point you in the right direction.

Common loading mistakes (that change ball weight)

  • Heavy items stored at the very rear (often increases sway risk)
  • Too much weight in the front storage (can overload ball weight)
  • Water tank fill level changing ball weight unexpectedly
  • Adding accessories (toolboxes, racks, extra batteries) without re-checking weights

What to include in a support ticket (if you want help)

  • Mars model + year
  • Tow vehicle make/model
  • Your weighbridge results (if available)
  • Any accessories added (batteries, toolboxes, racks, etc.)
  • How you typically travel (water full/empty, bikes on rear, etc.)

Next in this series: How to Load Your Van for Safe Towing (Simple Rules That Work)

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