Whether you’re towing a caravan, trailer, or camper, knowing your vehicle’s weight limits is key to towing safely, legally, and with confidence.
One weight you must know is your D-value.
Here is a simple guide to what the D-value is and how to ensure you are not overloaded.
What is D-Value
What “D-Value” Stands For
In the towing industry, “D-Value” (Downward Value) is a force that refers to a towing component’s “dynamic load capacity“.
So what does dynamic load capacity mean? Dynamic load capacity is the maximum force that a towing part can safely support when the vehicle is in motion, such as while driving.
This is different to a static load capacity, which is the maximum recommended force that can be applied while a vehicle is stationary.
D-Value Simplified
Basically, your D-value indicates how much force can be transferred to the towbar before the trailer becomes unstable.
Once the downward force is higher than your towing part’s recommended D-Value, there can be some serious consequences.
The Consequences of Going Over Your D-Value
Loss of Control and Handling
When there is too much force going down on your towball, the back of your car and the front of your towing vehicle tilt toward the road.
With the front of your car in the air, you can no longer steer.
Wheel/Tyre Overload and Tyre Blowouts
Too much downward force on your towball can lead to excessive force on your tyres and increase the risk of a tyre blowout.
Not a good thing to experience when on a nice holiday.
What Towing Parts Have a D-Value?
D-values apply once the towing component application exceeds 3,500 kilograms ATM.
A key point is that if a company only displays the static load capacity of its part, it is likely that they have not tested the product to an official standard to calculate the D-Value.
How to Calculate Your D-Value
In Transport, a D-value is a rating in kilo-newtons typically assigned to mechanical couplings and reflects the dynamic loading limits between a towing vehicle and a trailer.
The full equation that calculates your D-Value is:
D-Value = 9.81 x (Tow Vehicle GVM x Trailer ATM) divided by (Tow Vehicle GVM + Trailer ATM).
So, for example, a 12-tonne truck towing an 8-tonne trailer has a D-Value of 47Kn.
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