So, you’ve weighed your loaded 4x4, you’re under your GVM and now you think you’re good to hit the road? Sorry to be the bearer of bad news, but it may not be that simple.
Your vehicle’s Gross Vehicle Mass may be a lump sum number, but underneath this big number are two smaller numbers: your front axle and rear axle load ratings. Each of these three numbers is prescribed by the vehicle manufacturer or, if you have a GVM upgrade, by an aftermarket suspension manufacturer.
Essentially, your GVM tells you how much your vehicle can weigh when it’s fully loaded, and your axle ratings tell you how much weight the front and back of your vehicle can safely handle. Unfortunately, this means you can’t simply pile gear into your rig without a second glance, because you need to take into account how much weight is on the rear axle and how much weight is on the front axle (as well as the overall GVM rating). A quick but important note: the sum of your front and rear axle load ratings is a bigger number than your rig’s GVM, meaning you have some flexibility when it comes to spreading weight throughout your vehicle. To put things simply though, so long as you’re under your GVM, your front axle rating and your rear axle rating, your setup is safe, legal, and ready to shoot for the horizon.
Now, since the rear of your vehicle is your biggest cargo area and where your tow ball lives, it makes sense that your rear axle rating is the number you need to pay closest attention to. A rig that is under its GVM can still be well over its rear axle load rating; in fact, this is pretty common. However, just because it’s common doesn’t mean it’s something you should accept for your own setup. Aside from the potential legal or insurance consequences, an overloaded rear axle restricts your vehicle’s handling and its ability to perform evasive maneuvers, while it also creates wear and tear that can cost a pretty penny down the line. Some expensive examples include a bent chassis or broken coil towers, both of which will cost you monetarily but can also put a quick end to your adventure.
We’ve established that overloaded axles are a problem and that they’re more likely to happen on the rear axle, but the tough question to answer is the next one: how do you figure out how much weight is over each axle?
This is such a complicated question because where the weight sits in, on or around the vehicle actually changes the maths of how much weight it’s loading onto each respective axle. Plus, there’s no defined line in the middle of a vehicle where weight on one side only contributes to the load on one axle, and weight on the other side of the line only contributes to the other axle. An easy example of this are front seat passengers. You would logically think that the weight of the driver only contributes to the front axle load, but it’s actually split between the front and rear – the front might take 70% of the weight of a driver, which leaves 30% adding to the rear axle loading. The same goes for roof-loaded cargo and fitted 4x4 accessories like side steps, awnings, fuel tanks and the rest. How each item’s weight is split between the front and rear axle depends on where it’s located, as well as the individual type of vehicle you have.
If this wasn’t complex enough, let’s throw another spanner in the works – tow ball download weights. If you’re towing a 2000kg trailer, the weight the tow ball feels is around 200kg. However, because the tow ball is situated around a metre behind the rear axle, this creates added leverage. What this means is that, even though the tow ball download weight from the trailer is 200kg, the distance from the rear axle creates extra leverage, which means that 200kg rear axle load can increase by around 30% - meaning that 200kg tow ball weight can actually add around 270kg to your rear axle load (and detract 270kg from your overall payload).
Now, before you throw your hands up and put this in the Too Hard Basket, don’t fret – the above maths examples are there just to paint a picture of how weight distribution works in theory. In fact, finding out how much weight is over your axles (epic maths sums aside) is a simple process of loading up your rig and heading to a weighbridge. To measure the front axle load you can simply drive your front wheels onto the weighbridge and get a reading, then do the same for the rear wheels to get the rear axle load.
Aside from getting a front and rear axle mass measurement, you can also measure the weight your trailer is placing on your tow ball by driving your vehicle onto a weighbridge (trailer attached but not on the weighbridge), getting a reading and then detaching the trailer and measuring the weight of the vehicle on its own. The first reading will tell you how close you are to your GVM, while the difference between the two readings is your tow ball download weight. Finally, you can measure how close you are to your GCM (Gross Combination Mass) by weighing your full vehicle setup and trailer together.
If you get a nasty surprise when it comes to your GVM or axle loads, there is a potential solution: a certified GVM upgrade. A GVM upgrade is a special kind of suspension upgrade that’s been tested and certified to increase the safe and legal load-carrying capability of a vehicle. By increasing your vehicle’s GVM, you’re increasing your overall payload and how much weight you can distribute over each axle – not to mention that better suspension gives you a better driving experience and better off-road capability.
Aftermarket suspension experts like TJM have sales staff, fitters and kits that can give you the right GVM upgrade for your vehicle. This allows you to get equipped, load up your rig and hit the road (or track) without a backward glance.