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We tow a boat on an unbraked single axle trailer. It has with the trailer a total weight of around 750kg. When going on holiday with the car loaded the rear suspension is sagging a little but tows fine. Inflatable air bags inside the rear springs have been suggested to raise the rear of the car a little in these situations. I am keen to do this but hear that fitting these requires certification. Would suspension air bags be the best option? Is certification needed? If so likely certification cost?

Hi there,
It might be best to speak with an off-road specialist who could let you know what is available and the pro's and cons of such systems. Information of this kind would be more likely available from an enthusiast.

Thank you. The Vanguard is not really an off roader. Its a slightly stretched Rav4 with a bigger engine. Are you able to advise re the certification, particularly whether or not it would be required?

The type of modifications you are speaking of would be quite popular with off-road shops which is why they would be good to talk to. The WoF rules are outlined in this link-https://vehicleinspection.nzta.govt.nz/virms/in-service-wof-and-cof/general/steering-and-suspension/steering-and-suspension-systems
Any modifications will have to fall inside the modification threshold or certification is required.