But even when your equipment is calibrated and your process is consistent, unexpected discrepancies can still appear—especially when comparing calculated towball weight (based on axle weights) with the actual measured towball weight from a scale.

One of the most overlooked but significant causes of these mismatches is a phenomenon known as slope-induced bias, caused by gravitational load transfer.

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What Is Gravitational Load Transfer?

From a physics standpoint, every object on an incline is subject to gravitational force, which can be broken into two components:

Normal Force: The vertical component acting perpendicular to the surface

Parallel Force: The component acting along the slope

It’s this parallel component that causes the object—in this case, a caravan—to shift its weight downhill on a slope.

That’s gravitational load transfer in action.

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How It Affects Weighing

In mobile weighing, especially on driveways, streets, or uneven surface, we often don’t have the luxury of a perfectly level surface.

Even a slight incline or decline can cause a shift in how weight is distributed between the caravan and the tow vehicle.

1. When Hitched on an Incline (Nose-Up):

Gravity pulls the caravan rearward.

Additional force is applied downward through the towball, increasing the vehicle’s rear axle load.

This results in a calculated towball weight (via axle difference) that’s higher than the actual vertical weight measured with a towball scale.

2. When Hitched on a Decline (Nose-Down):

The caravan tends to roll forward.

The tow vehicle resists this with its brakes or transmission, absorbing compressive force.

This also increases the load on the rear axle, again causing the calculated TBW to be higher than measured.

In both cases, the tow vehicle ends up “feeling” more weight than is actually pressing vertically down through the coupling—an effect of slope-induced bias.

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Measuring Unhitched? Watch Where the Force Goes

When the caravan is unhitched on a slope and chocked to prevent movement, another subtle mistake can occur:

If the chocks aren’t resting on the weigh pads, the gravitational pull is still resisted by the tow vehicle or ground friction outside the weighing system.

But if you place the chocks on the weigh pads, or weigh the axle group while fully supported, the entire gravitational load is accounted for in the caravan’s own weight—isolating it from the tow vehicle and preventing slope-induced error.

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How to Minimise Slope-Induced Bias

To maintain weighing accuracy in real-world environments, here are best practices to follow:

1. Prioritise Level Ground

If possible, always weigh on flat, level surfaces.

Repeatable, accurate results almost always come from controlled, level conditions.

2. Use a Towball Scale at the Coupling

Measure actual vertical towball weight directly with a calibrated scale.

This value is unaffected by load transfer and is your best reference point.

3. Check for Mismatches

If calculated TBW (based on hitched/unhitched GVM) significantly exceeds the measured TBW, suspect a gradient or gravitational effect.

Consider adjusting the caravan axle weights by the difference, especially if using that data for compliance.

4. Weigh Uncoupled with Chocks on Pads

If weighing the caravan separately on a slope, ensure chocks are used and rest on the same surface or weigh pads as the axle group to absorb any parallel gravitational force.

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Final Thoughts

Understanding and accounting for gravitational load transfer and slope-induced bias is part of what separates a professional mobile weighing operator from someone just collecting numbers.

By recognising the effects of even slight gradients and compensating appropriately, you improve not only the accuracy of your measurements, but also the safety and confidence of your clients.