Driver Knowledge Tests

What determines a vehicle’s towing capacity

A light vehicle will be rated to tow a certain weight by its manufacturer. This figure is influenced by several factors.

The manufacturer’s rating

The maximum gross combination vehicle mass is the maximum permitted weight of the towing vehicle, the load (including passengers) and the weight of the trailer itself and it is defined by the manufacturer. If the maximum gross combination mass is 5000kg and your vehicle weighs 2500kg including its load, that leaves 2500kg for the trailer. Even though you might not be carrying that amount, it’s the theoretical maximum that counts.

The mechanical capability of the vehicle is important:

  • Axle loading: the rear axle will bear roughly 10% of the weight of the trailer
  • Brakes: the brakes must be capable of bringing the extra trailer weight to a stop, if the trailer isn’t braked
  • Tyres: the rear tyres must be able to support the additional weight without delaminating or overheating
  • Drivetrain: the clutch or torque converter must be able to cope with the additional strain due to adding extra weight
  • Suspension: the rear suspension must be able to take the trailer tongue load. When optimally loaded, a 2500kg trailer will exert 250kg of downwards pressure on the tow hitch.

Type of trailer

Braked or unbraked: an unbraked trailer (i.e. one with no brakes) cannot have a capacity exceeding 750kg. It’s recommended that the laden weight of an unbraked trailer should not exceed 75% of the unladen weight of the towing vehicle and then only if the towing vehicle’s brakes and tyres are in excellent condition. For a braked trailer, the trailer must not exceed 1.5 times the unladen weight of the towing vehicle.

For example:

If the towing vehicle weighs 1500kg, an unbraked trailer must not exceed 1125kg (this is a moot point, as legally the unbraked trailer cannot exceed 750kg). A braked trailer must not exceed 2250kg.

Tow bar rating

The tow ball and tow bar will have a rating to a certain capacity. This could be more or less than the vehicle’s rating. In either case, the maximum weight is based on the lesser of the two.

Note that the tow ball may have a different rating to the tow bar. The maximum rating is based on the lesser of the two.

The tow ball might be rated to 3500kg, but the tow bar might not be

Practical limits

There are rough guidelines for how heavy a trailer can be in relation to the towing vehicle. A small, light, two-wheel-drive vehicle will not be able to tow as much as a large four-wheel-drive SUV as the trailer will have too much influence on the towing vehicle. This could cause the trailer to jackknife or the towing vehicle to not have enough weight to bring the trailer to a halt in a short enough distance.

If you are new to towing a trailer, read our guide to towing a trailer safely.

If you don’t know how much your vehicle and trailer weigh, you can weigh them at a public weighbridge.

Darren is an expert on driving and transport, and is a member of the Institute of Advanced Motorists

Posted in Advice
Read previous post:
How to increase your chances of passing your driving test

Learning how to drive is an essential skill that needs to be taught as early as possible. Driving encompasses road...