Towing weights explained: GTW, noseweight and D-value

The weight terms, in plain English

• Kerb weight: your car with fuel and standard kit, no passengers or luggage. It is the baseline for the 85% guideline below.
• Gross vehicle weight (GVW) / maximum authorised mass (MAM): the most your fully loaded car may legally weigh.
• Gross train weight (GTW) / gross combination weight: the most the car and trailer may weigh together. This is often the real limit, because a heavily loaded car leaves less allowance for the trailer.
• Braked towing capacity: the heaviest trailer you can tow when the trailer has its own brakes. This is the figure most people need.
• Unbraked towing capacity: for trailers without brakes, capped at 750kg by law and often lower for your car.
• Noseweight: the downward load the trailer puts on the towball (more below).

What is noseweight, and what should it be?

Noseweight is the vertical load the coupling puts on the towball. Get it wrong and the outfit becomes unstable:

• Too little: the trailer can snake or fishtail
• Too much: it overloads the towbar and lightens the steering
• Aim for: around 5 to 7% of the trailer's laden weight, staying within the lower of your car's and your towbar's noseweight limits (commonly 50 to 100kg)

You can measure it with a noseweight gauge before you set off, and our pre-journey checklist covers when to check it.

What is D-value (and S-value)?

D-value is a measure of a towbar's strength, in kilonewtons (kN). It accounts for the dynamic forces of towing, not just the static weight, and the towbar's D-value must meet or exceed your vehicle's requirement. It appears on the towbar's type-approval plate, along with the S-value (the maximum noseweight the bar is rated for).

The formula, with weights in tonnes, is:

D = (T × C) ÷ (T + C) × 9.81

where T is the car's maximum mass and C is the trailer's maximum mass. So for a 2.0-tonne car and a 1.5-tonne trailer: (2.0 × 1.5) ÷ 3.5 × 9.81 = about 8.4 kN. Typical D-values run from around 7 to 10 kN for small cars up to 14 kN and beyond for large SUVs.

Where to find your figures

• Owner's handbook: the full towing specification (the definitive source)
• Weight (VIN) plate: usually inside the driver's door frame, showing GVW and GTW
• V5C: lists the maximum towable mass
• Towbar plate: shows the D-value, S-value and approval number

Our guide on how to check your car's towing capacity walks through reading these.

The 85% guideline

For stable towing, especially for less experienced drivers, the widely used guidance is to keep a loaded trailer to no more than 85% of the car's kerb weight. For a 1,600kg car that is about 1,360kg. It is best-practice advice, not the law, but it gives a useful safety margin.

A worked example

Say your car has a kerb weight of 1,500kg, a GVW of 2,100kg, a GTW of 3,500kg, a braked towing capacity of 2,000kg and a maximum noseweight of 80kg, and your caravan has a MAM of 1,500kg:

1. GTW minus kerb weight = 3,500 − 1,500 = 2,000kg allowance from the GTW
2. Braked towing capacity = 2,000kg
3. 85% of kerb weight = 1,275kg, so a 1,500kg caravan is above the guideline (experienced drivers only)
4. Add 200kg of passengers and luggage to the car and the GTW allowance for the trailer drops to 1,800kg