Another Calculator to Assist in Understanding Your Rig’s Compliance
The (very) basic limits and loads you need to be aware of and understand when you start your trip are:
- weight of the car;
- weight of the trailer; and
- combined weight of the two.
You also need to know
But, to dig a little deeper, you need to also know the:
- loads on all axles;
- ball weight of the trailer on the car’s hitch; as well as the
- percentage of the trailer’s weight that is imparted on the ball weight. (Often this isn’t a make or break figure but it can be a good indicator).
Ignoring the percentage of ball weight on the hitch for a moment, your car and trailer has a stated manufacturer’s maximum limit for each of these figures.
These maximum figures have been determined with the engineers’ design in mind, the manufactures’ build considered and the individual components themselves. It’s not up to us mere mortals to say we can exceed these figures and drive off with disregard to, or blind uncertainty of these numbers.
Rest assured, if you and your rig are involved in any accident on the road, the police and the insurance companies will most likely be looking at these figures.
How Do I Find My Maximum Permissible Loads
Have a look in your glove box – under the tissues and broken pens you’ll probably find your Vehicle Owner’s Manual. Here, perhaps somewhere towards the back, you’ll find the specifications and maybe even a section on towing.
Otherwise, check on the Interweb or other reliable sources such as your automobile club.
Look for the modifications
Have a look too around the engine bay or inside the door panels to see if your car has had any vehicle modifications that will change your maximum loads. (eg Gross Vehicle Mass – GVM and Gross Combined Mass – GCM). Refer to https://campandtravel.com.au/wp-content/uploads/2019/10/Guide-to-Tow-Vehicle-and-Trailer-Load-Measurements-and-Requirements-and-Their-Effect-on-Campers-and-Travellers.pdf for an explanation of these terms – and more.
Check the car’s tow bar
You will also need to check any stamps or plaques on the car’s tow bar, the actual tow ball hitch and the ball.
Your trailer should have a compliance plate fixed to the draw bar or inside the door or one of the outside cupboards. This plate will provide the trailer’s Allowable Trailer Mass (ATM), Gross Trailer Mass (GTM) or load on the trailer axles. For the purposes of this exercise, we don’t need to know the Tare as this is an empty load (and will never be seen again).
The next step is to get weighed. There are perhaps two major ways that you can get your car and trailer weighed.
How to Weigh my Car and Trailer – Professionally
The first, is to go to a commercial weigh station (or have a mobile service come to you) who do these measurements for a fee. These services would usually present you with a detailed report at the end of the service that outlines how compliant your rig is in terms of the maximum capacities and capabilities of your car, trailer and setup.
For most mobile weigh station services, you’ll find that they will have numerous scale pads to put down on a level surface and have you drive your car onto them so that there is one under each wheel of the car and trailer and one under the hitch (when disconnected).
They’ll use these figures to provide you with the report and any recommendations.
How to Weigh my Car and Trailer – Do It Yourself
The second method is where you do this yourself (and assume all responsibility).
Unless you have access to several scales and a level surface, you will need to front up to a weigh bridge (usually public) and progressively drive over the fixed scales to get reading that you can use to derive the necessary figures that will give a guide to your rig’s compliance.
The process I outline here is about as quick as you will get – but, if you’re using the services of a council tip or scrap metal yard etc you will need to sweet talk the operator and ask nicely and perhaps too make prior arrangements.
To make this easier, many weigh bridges advertise their CB channel on a board near the office that will allow you to open these discussions.
To do this, drive over the one set of scales as follows:
While the car and trailer are hitched:
- Weigh the car’s front axle – and move forward;
- Weigh the car (front and rear axles) (GVM) – and move forward;
- Weigh the car (front and rear axles) and trailer (GCM) – and move forward; and
- Weigh the trailer (axle/s) (GTM) – and drive off.
With the trailer unhitched:
- Weigh the car’s front axle – and move forward;
- Weigh the car (front and rear axles) (GVM) – and drive off.
What Do I Do With These Figures?
Whether you have derived the figures yourself or through a professional weigh bridge you can now plug these figures into Camp and Travel’s latest Calculator to determine how compliant your loads are.
Maximums – Here you enter the maximum permissible loads found from your car’s Owner’s Manual and compliance plates in the ochre coloured fields.
Actuals – This is where you enter the actual loads that you have weighed at the scales. Again enter these in the ochre fields for both the car and trailer; and the car by itself.
Calculated Fields – As not all fields can be weighed on a roll-on-roll-off scales, calculated fields are provided here and will self-populate once you have populated all necessary fields.
Over / Under – On the right-hand side of the Calculator, a figure will be displayed which tells you how much tolerance you have between the maximum permissible figures you’ve entered and the actual loads recorded. If the value is negative (ie the maximum has been exceeded) it will be displayed as a red highlight.
What Do I Do if I have Exceeded My Maximums
First things first – check your figures and be sure that you have understood the numbers and readings correctly.
Next – talk to your mechanic and trailer manufacturer to see how they have considered these figures in their design and build; and see what can be done to minimise your issues.
It may be too that a GVM upgrade on the car is your solution; or perhaps a GCM upgrade or a ATM / GTM upgrade to the trailer.
Or it could just be that you leave three cartons or beer behind and buy stuff when you get there.
Look too at how and where you pack your travel gear.
Camp and Travel’s Load Calculators (refer https://campandtravel.com.au/four-calculators-that-will-help-you-with-towing-and-travel-loads/) can help here as you plug in what items you want to move where and see their potential effects.
The results shown in these calculators are purely mathematically derived and do not take into account the many variances that can occur in the real world of cars, trailers, travel speeds and road conditions. As such, these calculators should be used only to determine what-if scenarios in the broadest sense and should always be followed up with actual weights taken at recognised weigh bridges. You are responsible for the loads you carry and for understanding the capacities and capabilities of your vehicle and trailer – and yourself. If in any doubt ask an expert in the relevant field.
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