Small 5th Wheelers / 5th Wheels / Fifth Wheel RVs for Australian Utes by Venture Fifth Wheelers Australia

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How to Calculate 5th Wheel Towing Capacity
Most Australian vehicles don't have a specific "5th wheel towing capacity" rating. So how do you determine the size 5th wheeler you can tow?

Online 5th Wheel Towing Capacity Calculator

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Quick and Easy

Gross Combination Mass - all vehicles have a rated GCM. This is the maximum total weight of the vehicle and its load, plus the trailer and its load.

Example:
A vehicle with a GCM of 4,750 kg, and a tare weight of 1,760 kg. Assume two people (total 170 kg), luggage and load in vehicle (350 kg). The vehicle's laden mass is (1,760 + 170 + 350 =) 2,280 kg. This leaves (4,750 - 2,280 = ) 2,470 kg which you can tow. Assume you're going to load the 5th wheeler with 500 kgs (water, food, clothes etc). You therefore need to start with a 5th wheeler that has a maximum tare weight of (2,470 - 500 = ) 1,970 kg

You should also check that this weight does not exceed the vehicle's other weight limitations such as towing capacity, gross vehicle mass, payload etc.

 

Detailed

This example uses a 2005 Toyota Hilux SR5 Dual Cab V6 Petrol and a Venture 630 (21') fifth wheeler.


Tare/Kerb Mass (Vehicle)

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  • This refers to the mass of the empty vehicle or trailer.
  • In the case of a vehicles - it usually includes oil in the engine and tranmission, and coolant in the radiator. However, it exclude occupants, fuel, luggage and goods.
  • The kerb mass of the example Toyota Hilux is 1760 kg

Tare Mass (Trailer)

  • This refers to the mass of the empty vehicle or trailer.
  • In the case of a trailer such as a 5th wheeler - tare refers to the weight of the unit when it leaves the factory. It generally excludes water in tanks, gas in bottles and batteries.
  • The tare mass of a Venture 630 is 1800 kg.

Gross Vehicle Mass - GVM

  • This is the maximum allowable laden mass of a motor vehicle, and includes the vehicle itself, plus its contents like passengers and goods. It includes the pin weight of the fifth wheeler that is resting on the tow vehicle. This is usually the sum of the Tare Weight and the Payload .
  • The GVM of the example Toyota Hilux is 2810 kg.
    • This means that the Toyota can carry (GVM - Kerb Weight = 2810 - 1760 = ) 1050 kg

Payload

  • The maximum permissable weight that the vehicle is able to carry. This usually includes fuel, passengers and any load.
  • It genearlly equates to the difference between GVM and the Tare Weight
    • The Toyota Hilux does not have a payload listed in its specs. Therefore we assume it is (GVM - Tare Weight = 2810 - 1760 = ) 1050 kg

Towing Capacity

  • The maximum permissable weight of a trailer that can be towed by the vehicle. Some older vehicles do not have a towing capacity listed.
  • The Toyota Hilux has a towing capacit of 2,250 kg
  • 5th Wheel Towing Capacity
    • Some vehicles such as the Ford F250 have this additional tow rating for fifth wheelers specifically. It is usually higher than the towing capacity.

Aggregate Trailer Mass - ATM

  • The ATM is maximum allowable laden mass of the fifth wheeler, and includes the fifth wheeler itself, plus its contents like water, gas, food, clothes etc. (Much like the GVM for a vehicle.)
  • The ATM is the total of the GTM plus the pin weight.
  • A Venture 630 has an ATM of 2500 kg.
    • This means that the you can carry (ATM - Tare Weight = 2500 - 1800 = ) 700 kg.
    • The fresh water tank holds 170L which equates to 170 kg.
    • The grey tank holds over 100L, and you would not generally travel with this tank full, although it may contain some weight.

Road Ready Weight / Wet weight / Laden weight

  • The weight of the trailer when ready to travel.
  • Includes anything that you have put in or on the trailer.
  • Needs to be less than the ATM

Ball Weight

  • The weight of the trailer resting on the tow ball of the vehicle. Normally used when referring to conventional trailers.
  • Refer to "Pin Weight" for 5th wheelers.


Pin Weight

  • Refers to the amount of vertical load that the fifth wheeler imposes on the rear of the towing vehicle.
  • This weight must be included when calculating the weight being carried by the vehicle.
  • With a Venture Fifth Wheeler, the pin weight is approximately 20% of the weight of the fifth wheeler.
    • A Venture 630 has a tare pin weight of (20% of tare weight = 20% of 1800 kg = ) 360kg (if the fifth wheeler is unladen).
    • A Venture 630 has a ATM pin weight of (20% of ATM = 20% of 2500 kg = ) 500 kg (when the fifth wheeler is at its maximum capacity).

 


Gross Trailer Mass - GTM

  • The GTM is the maximum that can be supported by the fifth wheeler's road wheels when the trailer is coupled to the towing vehicle.


Gross Cominbation Mass - GCM

  • The maximum permissable weight of the combined vehicle and trailer together. This is usually the sum of the GVM and Towing Capacity.
  • If a vehicle does not have a specified GCM you can normally calculate it using the vehicles GVM + Tow capacity (however, it is not always the case).
  • The example Toyota Hilux has a GCM of 4750 kg
    • This means that it can carry and tow a total of (GCM - Kerb Weight = 4750 - 1760 = ) 2990 kg.


Example - Determining maximum weight to carry

  • Vehicle laden weight (excluding pin weight) (X - D) = 2280 kg
    • Tare weight (A) + B (passengers) + C (goods carried in/on ute*)
    • 1760 kg + 170 kg + 350 kg
    • 2280 kg
  • Maximum fifth wheeler weight (W) = 2,470 kg
    • GCM - Vehicle laden weight (X - D)
    • 4750 kg - 2280 kg
    • 2470 kg
  • Maximum Venture 630 load weight = 670 kg
    • Tare weight (E) = 1800 kg
    • Maximum load (F) = Maximum fifth wheeler weight - Tare weight
    • 2470 kg - 1800 kg
    • 670 kg
    • You are able to load 670 kg of goods (including water, gas, clothes, food etc) into the Venture 630.
  • Pin weight (D) = 494 kg
    • 20% of fifth wheeler weight
    • 20% x 2470 kg
    • 494 kg
  • Weight being towed (Y) = 1,976 kg
    • Fifth wheeler loaded weight (W) - Pin weight (D)
    • 2470 kg - 494 kg
    • 1976 kg
  • Weight carried on vehicle (B + C + D) = 1,014 kg
    • B (passengers) + C (goods carried in/on ute*) + D (pin weight)
    • 170 kg + 350 kg + 494 kg
    • 1014 kg
  • Weight of vehicle (X) = 2,774 kg
    • A (kerb weight) + B (passengers) + C (goods carried in/on ute*) + D (pin weight)
    • 1760 kg + 170 kg + 350 kg + 494 kg
    • 2774 kg
    Total weight of the rig = 4,750 kg
    • Weight of vehicle (including pin weight) (X) + Weight being towed (Y)
    • 2,774 kg + 1,976 kg
    • 4,750 kg
  • Check against our recommendations and manufacturer specifications
    1. The total weight of all items being carried in/on the vehicle, including persons, fuel, items in the cabin, items on the tray, the weight of the hitch, and the pin weight should not exceed the payload capacity of the vehicle.
      1. B + C + D <= Payload
      2. 170 + 350 + 494 <= 1,050
      3. 1,014 <= 1,050
      4. OK
    2. The total weight of all items being carried in/on the vehicle, including persons, fuel, items in the cabin, items on the tray, the weight of the hitch, the pin weight , and the tare weight of the vehicle should not exceed the vehicle's GVM .
      1. A + B + C + D <= GVM
      2. 1760 + 170 + 350 + 494 <= 2,810
      3. 2,774 <= 2,810
      4. OK
    3. The sum of the road ready weight of the trailer and the total weight of all items being carried in/on the vehicle, plus the tare weight of the vehicle should not exceed the vehicle's GCM
      1. X + Y <= GCM
      2. 2,774 + 1,976 <= 4,750
      3. 4750 <= 4,750
      4. OK
    4. The weight being carried on the axles of the trailer should not exceed the vehicle's towing capacity .
      1. Y <= Towing Capacity
      2. 1,976 <= 2,250
      3. OK

Note - this example uses the maximum possible load in the fifth wheeler..

Keep in mind, that just because your vehicle's specifications allow you to tow a certain amount of weight, doesn't mean that you should. More weight means more fuel cost, and less performance from your vehicle.

* note that the good carried in/on ute includes the weight of the hitch itself (the turntable that is fitted to the ute) and weighs approximately 85 kg.

Some of the definitions contained on this page have been sourced from the Caravan Clinic article in Caravan World November 2005, p125, by Tom Olthoff. Others have been modified specific to fifth wheelers.

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