TOWING A TRAILER

There is some “confusion” regarding the possible weight of trailer that can be towed behind motorhomes, predominantly those that are fitted with “Stage” rating plates. I will use the Stage rating plates below from the same vehicle to illustrate the misconception.

Stage 1 Rating Plate, as it left the Fiat factory

The above plate informs us that Fiat has rated the GVM (Gross Vehicle Mass) at 4400 kg and the GCM (Gross Combination Mass) as 6500 kg.

Stage 2 Rating plate. Fitted once the ALKO chassis has been fitted to the Fiat Cab

However, at the next stage in the build of this vehicle, ie fitting of the ALKO chassis, ALKO has determined that it is suitable for a GVM of 5000 kg with a GCM of 6000 kg. Note, each subsequent stage plate overrides the previous rating plate.

Stage 3 Rating Plate. Fitted by the builder of the motorhome body. (Would be a Stage “2” plate if the chassis was a “Fiat” Chassis)

When Dethleffs fitted the motorhome body to this vehicle, they decided that the Stage 2 ratings were to their satisfaction, so didn’t change them. However this plate still overrides the previous rating plates. Also note the ratings can go up or down at each stage.

So while Fiat determined that this vehicle was suitable for a GCM of 6150 kg, it can only operate at 6000 kg. Not a big difference in this case.

To illustrate a little further, the AutoTrail motorhome the rating plate below is from, started off with Fiat factory rating similar to the above. However AutoTrail’s Stage 2 rating plate has reduced the GCM (GTW) to 4620 kg, nearly a difference of 1500 kg.

AutoTrail Stage 2 Rating Plate

Sure you can still tow a trailer behind it, but don’t forget that the trailer can only weigh 4620 kg less whatever the towing vehicle actually weighs. If the towing vehicle is fully loaded to 3500kg, then the most the trailer can weigh is 4620 – 3500 = 1120 kg. ie a garden trailer, not a big boat or a car!

The next weight to consider is the weight the towbar is designed to tow or MTM (Maximum Towed Mass), which will be printed / stamped on the towbar identification sticker / plate. Whatever the last Stage rating plate might say, you can’t exceed the towbar rating, and vise versa. Using the AutoTrail above as example, if the towbar had a rating of 1500kg, and you connected a 1500kg trailer behind it, the weight of the towing vehicle can’t exceed 4620-1500 = 3120kg to be legally under the Gross Combined Weight.

A few other issues to consider when thinking about towing a trailer behind a motorhome. 1, the weight of the towbar itself, and 2, the imposed weight on the towbar from the trailer. Why might you ask? As a lot of motorhomes have large rear overhangs, any weight placed at the rear is going to apply a greater load to the rear axle than the applied weight, often around 50% greater. So you fit a 50 kg towbar and associated chassis reinforcement, the rear axle now weighs 75 kg greater. Then connect up say a 1500 kg boat that applies 10% of its weight onto the coupling (150 kg), that adds a further 230 kg to the rear axle. Many motorhomes are already close to or exceeding their allowable rear axle weights without a towbar. However, in many cases LoadSafe NZ can uprate the rear axle allowable weight limit and GVM. Which will also provide a more stable and comfortable ride.

There are 7 weights your combination must be under to be legal. Being over on any one of them makes your combination illegal. It is assumed all ratings are determined by the vehicle manufacturer, rather than maximum weights as determined by NZTA for heavy vehicles. Towing vehicle is a motorhome, ute, car etc. Towed vehicle is a caravan, trailer, or A frame towed vehicle etc.

  1. The towing and towed vehicle combined weight has to be under the GCM of the towing vehicle. Note, If this is over 6000 kg a Class 2 license is required.
  2. The towing vehicle needs to be under its GVM rating with the trailer connected to the towball.
  3. Each axle of the towing vehicle needs to be under its axle rating. Remember the weight applied to the rear axle will be greater than the vertical weight applied to the towball by the trailer.
  4. The trailer coupling weight (ie the weight applied on the towball) needs to be under the allowable tongue weight of the towing vehicle.
  5. The trailer GVM has to be under the towbar maximum towed mass (MTM) rating (braked or unbraked) of the towing vehicle.
  6. The trailer total weight (ie axles plus towball) has to be under the trailer GVM.
  7. The trailer axle(s) weight must be under the allowable axle as determined by the trailer manufacturer.

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