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What is the difference between ATM and GTM Trailer Load?


The maximum weight of a trailer is specified as either its Aggregate Trailer Mass (ATM) or Gross Trailer Mass (GTM).

ATM is the combined weight of the trailer and its full load when it is not coupled to a tow vehicle. GTM is the weight of the fully loaded trailer that is imposed on the trailer’s axle when it is coupled to the tow vehicle. GTM will always be less than ATM as some of the trailer weight is transferred to the tow vehicle when the trailer is coupled to it.

All new trailers built since August 1989 are required to have a plate listing, amongst other things, the trailers Aggregate Trailer Mass, although some trailer plates will also show its Tare Weight (the unladen weight of the trailer) and its GTM. You must ensure the towing vehicle has sufficient capacity to tow the trailer when it is fully laden.

What is Trailer Ball Load?

Ball Load is the amount of weight the fully laden trailer imposes (vertically) on the tow bar of the tow vehicle. Like other towing specifications, it must not be exceeded.

It’s not usually a critical issue with small trailers unless the tow vehicle has a low Ball Load specification, as is the case with many small cars and some European vehicles. However, heavy trailers such as horse floats and caravans often impose quite high Ball Loads on the tow vehicle.

Ball Load is a function of the trailer’s axle position and the manner in which the trailer is loaded.
  • It can be measured at a weighbridge by disconnecting the fully laden trailer from the tow vehicle and resting only the trailer’s draw bar (via the jockey wheel) on the scales, or with special ball mass scales.
  • Ball Load is often around 10% of the trailers ATM however this should not be relied on for every trailer. Where no reliable information is available, the fully laden trailer must be weighed to determine this specification.

What is Tow vehicle capacity?

Towing capacity

A vehicle’s towing capacity is determined by its manufacturer and is based on factors such as the design of the vehicle, the vehicle’s rear axle load, the capacity of its tyres and the effect the laden trailer will have on the vehicle’s attitude and stability.

Safety and vehicle durability are also important factors. The maximum trailer load will be specified to ensure the combination is controllable at all times and that it will not adversely affect or significantly shorten the life of the vehicle’s body and mechanical components.

Vehicle handbooks generally provide the following information:
  • The maximum weight of the trailer, without brakes, that can be towed by the vehicle.
  • The maximum weight of a trailer with brakes that can be towed by the vehicle, the
  • Maximum tow ball load, and
  • Any conditions relating to towing or additional equipment required.

This information is essential to the selection process. The specifications provided represent the absolute limits the vehicle can safely tow.

While towing specifications and recommendations are contained in the vehicle owner’s manual, to make comparisons easier, RACQ Technical Advice can provide this information for a range of popular vehicles.

What is GCVM?

Gross Combination Vehicle Mass (where given) is the maximum allowable weight of the trailer, tow vehicle and the load in the tow vehicle and trailer (including passengers).

What is Tow vehicle Axle Load?

Some manufacturers provide Axle Load specifications for their vehicles. Most commonly this becomes important where the vehicle is capable of carrying a load over the rear axle, in addition to the load imposed by the trailer (the ball load).

In cases where the total of the Ball Load and the load in the vehicle exceeds the allowable Axle Load, it will be necessary to reduce the load carried or rearrange the trailer’s load.

Are there Rules on Tow Bars?

All tow bars made after July 1, 1988, and many made before this date, will have a plate attached that lists the maximum towing weight, the maximum ball load and the make and model of vehicle the bar was designed for.

The vehicle’s specifications will always be the maximum the vehicle can legally tow, even if the tow bar is rated for a higher load. However if any of the tow bar specifications are lower than those given for the vehicle, the tow bar’s specification will override the vehicle’s specifications.

It is common to find tow bar specifications that differ from those given for the vehicle. This usually occurs where the bar is made for a number of different models in the range, or where light and heavy-duty tow bars are offered for the vehicle.

When dealing with older unmarked tow bars, it can be difficult to determine the bar’s capacity. We recommend in this case that you discuss the matter with a reputable tow bar manufacturer who may be able to provide professional advice about its suitability for the application.

What are the rules on Safety Chains?

All States and Territories require the use of safety chains. Safety chains must be strong enough to hold the trailer should the trailer coupling accidentally disconnect, and comply with the appropriate Australian Standard. Trailers up to 2500 kg ATM are required to have one safety chain while trailers from 2,500kg to 3,500kg must be fitted with two safety chains. The “D” shackle used to connect the safety chain to the vehicle’s tow bar must have a load rating equivalent to that of the safety chain.

To prevent the front of the draw-bar hitting the ground if the coupling comes disconnected safety chains must be:-

As short as possible and connected to the tow vehicle.

Be crossed if two chains are fitted.

Do I need Trailer Brakes?

  • Trailers up to and including 750kg GTM do not require brakes*
  • Trailers not over 2,000kg GTM must have brakes that operate on at least one axle
  • Override brakes are acceptable on trailers up to and including 2,000kg GTM
  • Trailers over 2,000kg GTM are required to have brakes that apply automatically if the trailer becomes detached from the towing vehicle
  • Trailers over 2,000kg GTM must have brakes on all wheel
  • Brakes other than override systems must be able to be operated from the driver’s seat

The cost and complexity of trailer brake systems will vary depending on the application, however where required, most light trailers will have either override or electric brakes. Electric brakes require the installation of a control unit in the tow vehicle.

* Note also that some vehicles have a low un-braked trailer weight limit and will therefore require brakes to be fitted to trailers that have GTM of less than 750kg.

Are there Speed limits towing trailers?

Most states and territories don’t have specific references to trailer speeds in their regulations therefore for combinations under 4.5 tonnes the maximum permitted speed will be the posted limit. However Western Australia is slightly different in that it also specifies the posted limit, but for trailers with an Aggregate Trailer Mass of 750kg or greater, the maximum towing speed is limited to 100km/h.

There are a couple of other points to consider however. One is that you have an obligation to always tow at a safe speed, which depending on the combination and conditions, may be lower than the posted speed limit. Also some vehicle manufacturers specify a lower maximum speed when the trailer mass exceeds a certain level. Ultimately, it will be up to you to determine what is a safe and appropriate towing speed having regard to state law, the vehicle manufacturer’s specifications, the road conditions and vehicle stability and safety.

Remember that when towing, your vehicle is heavier and will be harder to start off, accelerate and stop, so you will need to drive accordingly.

Should I do maintenance & repairs on my trailer?

Box trailers and general maintenance.

Probably the biggest killer of box trailers is rust. It’s best to keep your trailer under cover, but if this isn’t possible it’s important to store it so that water can’t pool. If your trailer has drain holes make sure they are kept clear. Avoid leaving grass clippings and other vegetation in it for extended periods, as this is a certain way to cause rust. Regularly inspect for rust and if necessary treat affected areas with a good quality rust converter and then repaint.

Springs, shackles, and U bolts should be regularly checked for damage and security. Tyres deteriorate from standing so they should be inspected every time the trailer is used.

Wheel bearings should ideally be serviced annually as they can deteriorate from standing for long periods. Most small trailers will not have brakes, however where fitted, their operation should be checked every time the trailer is used.

Trailer couplings, safety chains and lights must be maintained in good working condition.

Led lights are more reliable and take less to maintain.

Tilt draw bar are also often forgotten, as the bolts that hold the draw bar on take a lot of wear as the draw bar tilts, the draw bar brackets rotate and wear groves into the bolts until they snap.

Trailer hinges are normally forgotten and freeze up due to the lack of being oiled or greased.

What are Tyre Load & Speed Ratings?

The Load Index is a numerical code associated with the maximum load a tyre can carry at the speed indicated by its Speed Category symbol under specified conditions up to 210km/h. Using a 265/75R16 119N tyre size as an example, the 119N at the end of the size represents the tyre’s service description. A service description identifies the tyre’s load index and speed rating. The first three digits – 119 – represent the tyre’s load index and are followed by a single letter – N – identifying the tyre’s speed rating.

Speed Ratings

Speed ratings are based on laboratory tests where the tyre is pressed against a large diameter metal drum to reflect its appropriate load, and run at ever increasing speeds until the tyre’s required speed has been consistently exceeded.

Australian Vehicle Standards’ rules state that passenger cars must have a tyre speed rating of S (180 km/h) or greater. It is not a legal requirement to match the tyre placard’s speed rating so long as it is not less than S (180 km/h). The speed rating must be stamped on the tyre. It is important to note that speed ratings only apply to tyres that have not been damaged, altered, under-inflated or overloaded. A tyre that has been cut or punctured no longer retains the original speed rating, even after being repaired.

Disclaimer: While a speed symbol is an indication of the speed capability of the tyre, we do not endorse the operation of any vehicle in excess of legal speed limits. tyre
Category Symbol Speed km/h
Category Symbol Speed km/h
E F G J K L M N P Q R S T U H V W Y Z 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 240 270 300 Over 240

265/75R16 119N – The Load Index – 119 – is the tyres assigned numerical value used to compare relative load carrying capabilities. In the case of this example the 119 identifies the tyre’s ability to carry a maximum of 1360 kg (2998 lbs). The higher the tyre’s Load Index number, the greater its load carrying capacity. The Load Index rating also represents the load carrying capacity of the tyres when they are inflated to maximum psi so the load carrying capacity reduces as tyre pressures are reduced.


What is wheel offset and PCD?

Wheel Offset and PCD explained


If you are thinking of purchasing a set of aftermarket alloy wheels, selecting wheels with the correct offset is very important. So what is offset, and why is it so important?

Offset is a measurement of how far the mounting face of the wheel (on the brake side) is away from the center line of the wheel. If the mounting face lines up exactly with the center line it is called “Zero Offset”, if the mounting face is further out towards the show side of the wheel, this is called “Positive Offset”, and if it is further inside the wheel (towards the brakes) then this is called “Negative Offset” (this is much rarer). There are two common ways to measure this though they both provide a different number:

Backspace: Measure from the brake side edge of the rim to mounting surface of the wheel (as shown in the “Straight Edge” section of the diagram below. This will generally be a reasonably high number (at least 80mm or more).

Centerline: Measure the total width of the wheel (X) and then halve this (X/2). Measure the backspace (A) and then subtract “A” from “X/2”. You now have the centreline offset. In Australia this is written as 35P (35mm positive) or 10N (10mm negative). In Europe it is usually written as ET35 etc.

Making sure you have the correct offset is important as it will affect where your wheel sits in relation to your vehicle. A high positive offset means the wheel will sit further inside the wheel and might rub up against the brakes or suspension. Worst case scenario, you will not be able to steer. A high negative offset means the wheel sits further out from the wheel arch and this can cause the tyres/wheel s to rub against the wheel arch. It is illegal for your tyres/wheels to stick out past your wheel arch in Australia.

In addition to this you must also be aware of limitations to changes made to your vehicles Wheel track


Wheel track is the distance between vehicles wheels (side to side). It is measured from the centerline of each wheel to the other. The fitment of wider wheels will often increase the Wheel Track and this can be associated with Offset as well. Increasing the Wheel Track can increase the load on bearings, axles, suspension joints and steering tie rods.

On a passenger vehicle the wheel track must not be increased by more than 25mm beyond the maximum specified by the vehicle manufacturer for that particular model. This means that the offset may not be changed by more than 12.5mm per wheel.

Wheel Track reduction is NOT permitted without the approval of the relevant registration authority.

Off-road and commercial vehicles fitted with front and rear beam axles may increase the Wheel Track up to 50mm in total

All wheels fitted to the front or rear axles must have the same diameter, offset, width and mounting configuration (except the spare).

Where a two-axle vehicle is fitted with different width single tyres, the narrower tyres must not be less than 70%of the width of the wider tyres. As an example: if you were running 10 inch (254mm) wide tyres on the rear, your front tyres can only go as narrow as 7 inches (17.78mm).


PCD stands for Pitch Circle Diameter which basically means the diameter of an imaginary circle running through the center of each bolt hole. The most common PCD values are 100mm and 114.3mm, the difference arising due to manufacturers using metric or imperial measurements (4.5 inches = 114.3mm).

You will need to ensure that your new wheels have the same stud pattern (4 and 5 stud shown above) and same PCD as the wheel hub on your vehicle. In addition to this you need to ensure you have the same center location method and that the center spigot is the same diameter as the original wheel. If this is not the same a metal adapter ring should be used.

What is my PCD?

Here are some PCD’S




75, 90, GTV 5 X 98
145, 146, 155, 164 4 x 98
156, 164 5 X 98
166 5 x 108

A3 5 X 100 
A4, A6, A8, TT 5 X 112
Q7 5 x 130
S3 5 x 100
S4, S6 5 x 112

E30 4 X 100 
E36, E46, E49 5 X 120 

300C 5 x 115
SEBRING 5 x 100
VOYAGER 5 x 114.3

MATIZ & TACUMA 4 X 114.3 
LEGENZA 5 X 114.3 

TERIOS 5 X 114.3 

CAPRI 4 X 114.3 
COUGAR 4 X 108 
FALCON AU & BA 5 X 114.3 
FALCON PRE - AU 5 X 114.3 
FESTIVA 1994 - 2000 4 X 100 
FIESTA 4 X 108 
FOCUS 4 X 108
KA 4 X 108 
LASER 1990 - ON 4 X 100 
LASER SR2 5 X 114.3 
MONDEO 4 X 108 
PROBE 5 X 114.3 
TELSTAR 1988 - ON 5 X 114.3 
TELSTAR PRE - 1987 4 X 114.3 

APOLLO PRE 1993 5 X 100 
ASTRA 1988 - ON 4 X 100 
ASTRA PRE - 1987 4 X 114.3 
BARINA PRE - 1994 4 X 114.3 
BARINA 1994 - ON 4 X 100 
EPICA 5 x 114.3
MONARO 5 X 120 
NOVA 4 X 100 
VECTRA 5 X 110
VIVA 4 x 100

ACCORD PRE - 1989 4 X 100 
ACCORD 1990 - 2001 4 X 114.3 
ACCORD 2001 - ON 5 X 114.3
CITY 4 X 100 
CIVIC 4 X 100
CIVIC TYPE R 5 X 114.3 
CRX 4 X 100 
JAZZ 4 X 100 
LEGEND PRE - 1991 4 X 114.3 
LEGEND 1991 - ON 5 X 114.3 
NSX 5 X 114.3 
ODESSEY 5 X 114.3 
PRELUDE PRE - 1992 4 X 100 
PRELUDE 1992 - ON 4 X 114.3 
S2000 5 X 114.3 

ACCENT 4 X 100 
ELANTRA 4 X 114.3 
EXCEL 4 X 114.3 
GETZ 4 X 100 
GRANDUER 5 X 114.3 
LANTRA 4 X 114.3 
S COUPE 4 X 114.3 
SONATA 4 X 114.3 
TIBURON 5 X 114.3 

S TYPE & X TYPE 5 X 108
XJ, XJS & XK8 5 X 120.65

CARNIVAL 5 X 114.3 

IS200, IS300, ES300, GS300, RX300 5 X 114.3 
LS400, LS430, SC430 5 X 114.3 

121 1990 - ON 4 X 100 
2 4 X 100 
3 5 X 114.3 
323 1990 - ON 4 X 100 
6 5 X 114.3 
626 1988 - ON 5 X 114.3 
929 1987 - ON 5 X 114.3 
ASTINA 4 X 100 
ASTINA V6 5 X 114.3
CX7 5 X 114.3 
MPV 5 X 114.3 
MX5 4 X 100
MX5 2005 - ON 5 X 114.3 
MX6 5 X 114.3 
PROTÉGÉ 4 X 100 
RX7 1987 - ON 5 X 114.3 
RX8 5 X 114.3
TRIBUTE 5 X 114.3 


2002 - ON 4 X 100 

COLT 4 X 114.3
CORDIA 4 X 114.3
EVO MK 1, 2 & 3 4 X 114.3
EVO MK4 ON 5 X 114.3 
GALANT 4 X 114.3
GRANDIS 5 X 114.3 
LANCER 1992 - ON 4 X 100 
LANCER CG - CH VRX 5 X 114.3 
MAGNA 1990 - ON 5 X 114.3 
MAGNA PRE - 1990 4 X 114.3 
MIRAGE 1996 - 2005 4 X 100
OUTLANDER 5 X 114.3 
VERADA 5 X 114.3
3000GT, FTO & GTO 5 X 114.3 

EXA 4 X 100 
MAXIMA 1990 - ON 5 X 114.3 
MAXIMA PRE - 1990 4 X 114.3
MICRA 4 X 100
MURANO 5 X 114.3
NX COUPE 4 X 100 
PULSAR 1988 - 2000 4 X 100 
PULSAR 2000 - ON 4 X 114.3 
SILVIA 4 X 114.3 
SKYLINE 5 X 114.3
TIIDA 4 x 114.3
200SX 5 X 114.3 

205, 206, 306, 307 4 X 108 
405, 406 4 x 108
407, 605, 607 5 x 108


GEN2 4 x 114.3
PERSONA 4 X 100 
SATRIA 4 X 100 
WIRA 4 X 100 

19 4 X 100 
21 4 X 100 
CLIO 4 X 100 
FUEGO 4 X 100 
LAGUNA 4 X 100
LAGUNA 2001 - ON 5 X 108 
MEGANE 4 X 100
SCENIC 4 X 100 

9-3 & 9-5 5 x 110


LEGACY 5 X 100 
SVX 5 X 114.3 
WRX 5 X 100

GRAND VITARA PRE - 2005 5 X 139.7
GRAND VITARA 2005 - ON 5 X 114.3
JIMNY 5 X 139.7
SWIFT PRE - 2004 4 X 114.3
SX4 5 X 114.3
VITARA 5 X 139.7

AVALON 5 X 114.3 
CAMRY PRE - 1992 5 X 100 
CAMRY 1992 - ON 5 X 114.3
CELICA 5 X 100
COROLLA 4 X 100 
CORONA 5 X 100
ECHO 4 X 100 
LEXCEN 5 X 120 
MR2 PRE - 1991 4 X 100 
MR2 1991 - ON 5 X 114.3 
PASEO 4 X 100
PRIUS 4 X 100
RAV4 5 X 114.3 
SECA 4 X 100 
SOARER 5 X 114.3 
SUPRA 5 X 114.3 
TARAGO 5 X 114.3
YARIS 4 X 100 

BEETLE 5 X 100
BORA 5 X 100
GOLF PRE - 1998 4 X 100
GOLF 1998 - ON 5 X100
GOLF VR6 5 X 100
GOLF MK5 5 X 112 
PASSAT PRE - 1997 4 X 100 
PASSAT VR6 (1993 - 1997) 5 X 100
PASSAT 1997 - ON 5 X 112
POLO 4 X 100
TOURAN 5 X 112
VENTO 4 X 100 

S70, S60, S70, S80 & V70 5 X 108
S40 4 X 114.3
V40 4 X 114.3
XC90 5 X 108
360 4 X 100 
440, 460 & 480 4 X 100
850 & 960 5 X 108

What is the wiring color codes

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