Tips, Tech & Tinker Time: Know Your Weights
Not sure what all the lingo is around vehicle weights? Well, it’s time to break it all down.
If you’ve ever browsed the specs of a car or trailer, you’ve likely come across a jumble of acronyms that look like they were cooked up in a government agency’s alphabet soup kitchen. GVM, GCM, ATM, GTM… it’s enough to make your head spin! But don’t worry; we’re here to untangle the mess and make sense of these weighty matters.
GVM: GROSS VEHICLE MASS
First up is GVM, or Gross Vehicle Mass. This one’s pretty straightforward—it’s the maximum weight your vehicle can legally weigh when fully loaded. This includes the vehicle itself, all your passengers, fuel, luggage, and that extra-large coffee you grabbed for the road. The key word here is “maximum.” If you exceed your GVM, not only are you breaking the law, but you’re also putting extra strain on your vehicle’s suspension, brakes, and other critical components. It’s important to remember that your trailers Tow Ball Mass is also included in your vehicle’s weight when it’s hitched up, too.
GCM: GROSS COMBINATION MASS
Next, we’ve got GCM, which stands for Gross Combination Mass. This is the big daddy of weights, as it represents the total mass of your fully loaded vehicle plus the fully loaded trailer. It’s essentially the maximum weight of the total combination. Now here’s the kicker, you would think that your GCM is simply the combination of your vehicles maximum weight (GVM), and its braked towing capacity, right? Well, not always! Many vehicle manufacturers, usually for 4WD utilities, set the GCM lower than the two weights combined. So, if your vehicle is fully loaded to its GVM, it may not be able to tow a trailer as heavy as the towing capacity it claims.
For example, a 2018 Ford Ranger has a GCM of 6,000 kg. If your fully loaded to the vehicle’s GVM of 3,200 kg, that only leaves you with 2800 kg for your trailer. If your trailer weighs more than that, it’s time to either shed some pounds or upgrade your setup.
GALR/GAWR: GROSS AXLE LOAD RATING/GROSS AXLE WEIGHT RATING
Now, let’s talk axles. GALR (Gross Axle Load Rating) or GAWR (Gross Axle Weight Rating) refers to the maximum weight each axle of your vehicle can bear. This is crucial because an overloaded axle can lead to tire blowouts, suspension failure, and a rather uncomfortable conversation with your insurance company.
For example, if your front axle has a GAWR of 1,600 kg and the rear axle has a GAWR of 1,800 kg, you need to distribute your load so that neither axle is carrying more than its rated capacity. It’s all about balance—both in life and in loading your vehicle.
BTC: BRAKED TOWING CAPACITY
When it comes to towing, BTC, or Braked Towing Capacity, is a number you don’t want to ignore. It represents the maximum weight your vehicle can tow with a trailer that has its own braking system. This is where the “braked” part comes in—trailers with their own brakes can safely carry more weight because they’re not relying solely on your vehicle’s brakes.
If your car’s BTC is 2,500 kg, this means it can tow a trailer weighing up to 2,500 kg, as long as the trailer is equipped with brakes and no other weight limits are breached in the process.
ATM: AGGREGATE TRAILER MASS
ATM, or Aggregate Trailer Mass, is the total weight of your trailer un-hitched when fully loaded. This includes the trailer itself, all your gear, and whatever else you’ve managed to squeeze in. It’s the number you need to keep in mind when you’re hooking up your trailer to your vehicle.
For example, if your camper trailer has an ATM of 2,200 kg, that means fully loaded, it shouldn’t weigh more than 2,200 kg.
GTM: GROSS TRAILER MASS
Similar to ATM, GTM (Gross Trailer Mass) is the weight of the fully loaded trailer but measured when the trailer is hitched up to your vehicle and your vehicle’s tow ball is taking some of the weight. It’s essentially the weight of the trailer’s axle when it’s hitched up. Naturally, it’s usually a bit less than ATM because part of the trailer’s weight is supported by the tow vehicle.
If your trailer’s GTM is 2,000 kg, it means that when it’s hitched up and ready to go, the combined weight of the trailer and its load should not exceed that number.
TBM: TOW BALL MASS
Finally, we have TBM, or Tow Ball Mass. This is the downward force the trailer exerts on the tow ball of your vehicle. It’s a critical factor because too much weight on the tow ball can negatively affect your vehicle’s handling, while too little can make your trailer prone to swaying.
For instance, if your trailer’s TBM is 200 kg, your vehicle needs to be able to safely handle that weight on the tow ball. Too much TBM, and you might find your steering a bit wobbly—too little, and your trailer could start dancing behind you. As a general rule, most trailers are designed to have around 10% of the trailers total weight on the tow ball, but this really is a loose figure and should be defined by the manufacturer of your trailer.
PAYLOAD
This refers to how much more weight you can add to your vehicle or trailer before you reach its maximum weight limit. To calculate the remaining payload, you need to know what your vehicle weighs, and subtract that figure from the maximum weight rating.
TIPS FOR STAYING UNDER WEIGHT LIMITS
1. First, do your homework before you buy. Remember, payload is king! You need to know exactly how much weight you can add to the setup.
2. As a general rule, you want to ensure the vehicle’s braked towing capacity is at least equal to the trailer’s ATM, but it’s always wise to allow plenty of wriggle room.
3. Second, weigh your fully loaded setup! This is the only surefire way to know where you stand.
4. Lastly, distribute your load evenly, keep an eye on your axles, and don’t be afraid to leave a few non-essential items behind. Your future self, and your rig, will thank you.
Understanding these acronyms might not make you a towing genius overnight, but it will definitely keep you on the right side of the law and make your towing experience a whole lot smoother!
