How does the calculator work?
The calculator takes the inputs you give it and calculates a range of results which help you determine whether or not you are within various towing weight limits. It is only as good as its inputs. The calculator cannot however replace a professional trailer weighing operation as that will give you the exact real-world weights as opposed to what the specification sheets say. Here’s a list of trailer weighing operators.
Entering data into the calculator
Yes, it’s unfortunately complex, lots of numbers to go find and enter.
- Actual Trailer Weight – the weight of the trailer, ready to tow. This should be more than the trailer’s tare/kerb weight (which the calculator doesn’t use at present), and less than the trailer’s ATM. You’ll need to measure this on a weighbridge, or estimate.
- ATM – Aggregrate Trailer Mass. The maximum the trailer can weigh, as stamped on its weight placard. The equivalent for the towcar is GVM. Found on the trailer placard (see image below).
- TBM – towball mass. Downforce on the towcar’s towhitch. You will find a guide on the trailer placard, and a maximum on the towcar specification sheet. Also see TBM note below.
- GTM – maximum load on the trailer’s axles. ATM – TBM = GTM. Not used in the calculator.
- Towcar unladen weight (tare or kerb) – the “stock”, unladen weight as on the showroom floor. The definition varies from manufacturer to manufacturer, and tare is slightly different to kerb but there’s no consistent definition of each. Both the trailer and towcar have a tare and/or kerb weight. Consider both weights unladen, but be sure that when comparing vehicles/trailers the same definition is used. Found on carmaker websites, or in owner’s manuals.
- Actual Towcar Weight – the weight of the towcar, ready to tow. This should be more than the towcar’s unladen weight, and hopefully, less than the towcar’s GVM. You’ll need to measure this on a weighbridge, or estimate.
- GVM – Gross Vehicle Mass. The maximum the towcar can legally weigh. ound on carmaker websites, or in owner’s manuals.
- Towcar TBM limit – the maximum TBM the towcar cna handle (see note below). The trailer TBM is the TBM created by the trailer on the towcar, the towcar TBM limit is the maximum TBM the towcar can handle.
- GCM – Gross Combination Mass. The maximum the towcar and trailer can weigh. May be less than GVM + ATM. Found on carmaker websites, or in owner’s manuals.
- Wheelbase – distance between the front and rear axles. Found on carmaker websites, or in owner’s manuals.
- Overhang – distance between the rear axle and towhitch. You’ll need to measure this for yourself.
- Axle loads (front/rear) – maximum load permitted on the front and rear axles, which combined should be greater than, or equal to the GVM. Found on carmaker websites, or in owner’s manuals. This information is sadly not always available.
- % of weight on rear axle – used to calculate the axle loads. Usually 50% for a stock, unladen car, work on 60% for towing, and exclude the effect of TBM as that’s calculated.
Two other terms used:
- Payload (towcar) – difference between tare weight and ATM, amount the towcar can carry. Not required to be measured, this is calculated by the calculator.
- Payload (trailer) – difference between tare weight and ATM, amount the trailer can carry. Not required to the measured, and not currently calculated by the calculator.
Here’s a visual summary:
Here’s an example trailer placard, used for determining ATM, and to give an idea of TBM:
Here’s a vehicle (towcar) placard show maximum towing capacity and towball mass:
Also read this.
TBM Note – 10%
There is an unfortunate fixation with TBM (towball mass) needing to be 10% of the trailer ATM. This figure is given out as some sort of gospel when in fact there is no science or logic behind it. Many trailer tow perfectly well with 5% TBM, and conversely, some may require 15%. There’s no easy way to determine the optimal TBM, but one way that doesn’t work is to take an abitrary figure and assume it’ll sort all your towing stability problems. I have even seen pro weigh operators suggest to their clients their TBM should be increased from 8 to 10% by putting extra weight on the drawbar…this would increase TBM but very likely reduce stability and certainly increase rear axle load. Now 10% may work for your rig, but don’t make the assumption it’s all you need and a target to aim for. Centralising weight is a much better idea.
Also note that the towcar TBM limit will be the lower of; the car’s TBM rating, the towbar’s TBM rating, and the tow tongue’s TBM rating. So for example a 2012 Ranger PX may have a 335kg TBM maximum, but be fitted with a towbar good for towing 3500kg and 350kg TBM. The maximum TBM remains at 335kg. You always take the lowest figure.
- Combined weight towcar and trailer – simply the actual trailer weight plus actual towcar weight.
- Margin to GCM – the difference between the combined weight and GCM. So if you have a 2500kg car, 2500kg trailer, and GCM of 5500kg that would be 2500+2500=5000 – 5500 = 500kg.
- Margin trailer weight to ATM – your trailer weight should be less than the ATM. Example; ATM is 3000kg, your trailer weighs 2700kg, so 300kg difference. The TBM is included in the trailer weight.
- Margin to max TBM – this is the difference between the towcar’s TBM limit and the trailer’s actual TBM. So for example the towcar has a limit of 250kg, your TBM is 275kg, you’re 25kg over. Watch for European cars which often have TBM limits too low for Australian trailers.
- Rear axle load – the load on the rear axle, and whether it has exceeded the limit or not. Every kilogram of TBM adds around 1.5kg of weight to the rear axle…sounds wrong, but it’s true. Watch this.
- Front axle load – as above, but adding TBM reduces the front axle load, so it’s not normally a limit that’s reached.
- Margin to GVM – how far away you are from your GVM, including effect of TBM.
- Margin to max tow – how far away you are from your maximum tow; for example 2500kg trailer, max tow 3000kg, you’re 500kg away.
- Payload left before reaching a weight limit – how much extra weight you can put in your car when you’re set up to tow. This is a complex calculation; there’s actually three to do; GVM, GCM and rear axle load. Whichever is reached first sets your limit.
- Heaviest trailer you can tow – as above, but heaviest trailer you could tow.
At the bottom of the results are some extra calculations. These are:
- TBM percentage – the towball mass as a percentage of trailer weight. For more on TBM, see above.
- Percentage of trailer weight to towcar weight – towcar weight as a percentage of trailer weight. The heavier the towcar relative to the trailer, the more stable the tow will be. Watch this.
- GCM to GVM + max tow – ideally, the GCM is the same as the sum of the GVM and max tow, so for example GCM 6000, GVM 3000, max tow 3000. If it isn’t, and it is often less, then you’ll often find yourself limited by the GCM rather than the max tow.
- Axle sum limit to GVM – you want the sum of the front and rear axle loads to be more than the GVM, so you have front/rear load flexbility. For example, GVM may be 3000kg, and front axle 1500, rear 1700 total 3200. This is especially important for towers as the rear axle is often the first limitation you run into when determining the heaviest trailer you can tow.
The results are wrong!
Possibly, but before you comment please read this:
- TBM counts towards payload, so if you have say a 3000kg GVM, car weighs 2750kg ready to tow, and TBM is 250kg, then you’re up to 3000kg GVM.
- The heaviest trailer you can tow might be limited by GVM, GCM, maximum braked tow or the rear axle load limit…whichever comes first! So there are multiple calculations done to see which limit is reached first.
- There’s no units marked (kg or lb) as the calculations are the same.
If you do have an improvement suggestion or find an error, please let me know in the comments.
Why can’t I just select my car and trailer and it works it all our for me?
Because every towcar and trailer combination is different. In particular, there’s many different limitations to specific vehicles which one calculator cannot cover without becoming overly complex. For example:
- Some vehicles like Pajeros reduce TBM limits as trailer mass increases.
- Nissan have guidance to reduce GVM by more than the TBM.
- Sometimes the max tow rating changes depending on gearbox and engine.
- Some vehicles require a weight distribution hitch to tow their maximum, or are speed-limited, or require special towing brackets.
What you’ll need to do is get your vehicle and trailer weighed, figure out your load limits, then plug them in.
It is possible to link to a datasource where I can look up much of the data such as GVM and GCM, but that costs significant $ and there’s only so much I can give away for free.
What else do I need to worry about when setting up to tow?
- Your trailer should be as light as possible relative to the towcar. The heavier the trailer, the more the trailer can boss the towcar around.
- More towball mass is not necessarily good – it can prevent trailer sway, but too much brings too much straight-line stability and cornering becomes problematic.
- Trailer sway…a complex subject, explained here.
- Run tyre pressures commensuate with the load. Usually that means a bit more in the rear tyres as those are heavily loaded – watch this to understand rear axle loads. There is actually a reduction in weight on the front tyres.
- Weight distribution hitches? Watch this.
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