How a 4-pad session works
With only 4 pads, you can't weigh both the tow vehicle and caravan at once, so everything happens in two separate passes.
First, the tow vehicle is driven onto the weigh pads while the caravan is dragged up onto blocks that mimic the height of the pads. The idea is to keep the whole setup sitting at roughly its normal towing height so the hitch load is as close as possible to real road conditions. With the rig at that height, the tow vehicle is weighed on the 4 pads while still hitched to the caravan on the blocks. Because there's one pad per wheel, this pass gives you individual front and rear axle weights for the tow vehicle, which is important given the rear axle is one of the most commonly exceeded limits when towing.
Next, the tow vehicle is moved off the pads, the caravan is rolled onto them, the vehicle is unhitched, and a tow ball scale is placed under the hitch point. The tow ball scale height is measured from the actual tow ball height while the caravan was hitched, to try and emulate the tow ball height as it sits while towing. This pass also captures the caravan's axle weight, or individual axle weights if it's a twin-axle van.
Once those two separate measurements are done, you add them together to calculate GCM. That's two weigh events and one calculation, each a separate step with its own potential for variance, adding to the overall probability of a margin of error.
How an 8-pad session works
With 8 pads, one under each wheel of both the tow vehicle and the caravan, the whole rig is weighed simultaneously while hitched. GVM, GTM and GCM are all read in a single pass. GCM is a direct calculation of all 8 weigh pads, or 6 if it's a single-axle caravan; it's a direct reading of the entire system on the scales at once, not derived from two separate measurements. You also get individual front and rear axle weights for the tow vehicle and individual axle weights for the caravan at the same time, all while the rig is sitting in its real towing configuration. The rear axle of the tow vehicle is particularly important here: with tow ball weight pushing down on it and the vehicle loaded for travel, the rear axle is the most common limit to exceed, and 8 pads captures it accurately while everything is hitched and loaded.
- Caravan raised onto blocks to mimic pad height
- Tow vehicle weighed on pads while hitched; front and rear axle weights captured
- Vehicle moved off, caravan placed on pads
- Vehicle unhitched, tow ball scale placed under hitch
- Tow ball scale height measured from actual tow ball height
- GTM, TBM and caravan axle weight(s) read from caravan on pads
- GCM calculated by adding two separate readings
- No cross-check available to confirm tow ball scale height
- Entire rig weighed simultaneously while hitched
- GVM, GTM and GCM read directly, no calculation
- Tow vehicle front and rear axle weights captured while loaded and hitched
- Caravan axle weight(s) captured simultaneously in the same pass
- Caravan unhitched; tow vehicle weighed unhitched on the same scales
- Tow ball scale placed under hitch at measured height
- GTM from tow ball scale checked against initial GTM reading to verify height is correct
- Accurate ATM and TBM confirmed to complete the session
The verification step that makes the difference
After the initial hitched weigh, the caravan is unhitched and the tow vehicle is weighed on its own while it's still on the scales, giving you a clean unloaded vehicle weight. The vehicle then moves and the tow ball scale is placed under the caravan's hitch point.
Here's where 8-pad weighing has a clear advantage: because we already have an accurate GTM from the initial simultaneous weigh, we can use that figure as an extra cross-check once the tow ball scale is in place. The tow ball scale height is measured from the actual tow ball height as it was when hitched, same as in a 4-pad session, but with 8 pads we can then compare the GTM reading from the tow ball scale against the GTM we already recorded. If they match, we know everything is right. It's an additional layer of confidence that a 4-pad session simply can't offer, because that GTM reference never existed.
Once the height is confirmed, we get the ATM (Aggregate Trailer Mass, the total weight of the caravan on its own wheels and jockey wheel) and the confirmed TBM (tow ball mass). The session ends with every figure verified and accurate: GVM, GTM, GCM, ATM and TBM.
In a 4-pad session, the tow ball scale height is measured from the actual tow ball height while hitched. The process is sound, but there's no independent reference to confirm the GTM reading once the caravan is on the scales and the scale is in place. You take the reading and move on.
With 8 pads, the GTM from the initial hitched weigh gives you exactly that reference. Once the tow ball scale is set to the measured height, you can compare its GTM reading against what was recorded in the first pass. It's an extra check that adds confidence to every figure you record, and catches anything that might otherwise go unnoticed.
One session. Every figure. No calculation required.
At the end of an 8-pad TowMetrics session you walk away with GVM, GTM, GCM, ATM, TBM and individual axle weights for both the tow vehicle and caravan, all accurately measured, not estimated or calculated from combined readings. We hand you a written report covering every figure, what your limits are, where you sit against them, and what it means for your setup.
We come to your driveway with the full 8-pad setup, weigh everything loaded the way you actually travel, and give you numbers you can trust.