Tuesday, Apr 14, 2020 at 14:51
Warning - long post.
Here's another one.
"GTM (gross trailer mass)
This is the maximum weight of the loaded caravan that can be supported by its wheels, not counting the portion supported by the car when hitched – or jockey wheel when parked. This rating is set by the manufacturer and cannot be exceeded.
GTM = ATM – tow ball mass"
The definition is right, but the formula at the bottom is wrong.
Towball mass (TBM) is a variable, depending on how the trailer is loaded. If you apply the formula the GTM will vary as the TBM varies. But GTM is a specified limit fixed by the trailer manufacturer, so the formula is an incorrect oversimplification.
Note: If the trailer’s axle is loaded up to GTM, then the difference between GTM and ATM is the ball weight available. If the loaded trailer’s actual axle load is lower than the GTM (lucky you!), then there is a larger margin for towball weight without exceeding the ATM.
On the same page:
"GCM (gross combined mass)
The gross combined mass is a combination of the maximum allowed mass of the trailer and the maximum allowed mass of the vehicle. It’s rare that you’ll need to consider this one.
GCM = GTM + GVM"
This simplified definition is wrong.
Here is the definition of GCM in ADRs. You'll have to scroll down the list.
"GCM is the value specified for the vehicle by the ‘Manufacturer’ as being the MAXIMUM of the sum of the ‘Gross Vehicle Mass’ of the drawing vehicle plus the sum of the ‘Axle Loads’ of any vehicle capable of being drawn as a trailer."
That is, GCM is a maximum sum. It is not the sum of maximums as the Alko definition states - a subtle but significant difference.
It is simpler to think of GCM as simply the maximum permitted mass of the vehicle and trailer combination as specified by the vehicle manufacturer. It is NOT obtained by adding together other weights, though in some cases when you do the result is coincident. (Eg some Landcruisers, where GCM was not specified.)
And the formula doesn't work - simply because it is inappropriate. You cannot calculate GCM with a formula because it is simply a limit stated by the manufacturer.
Lastly, the sentence in the definition, "It’s rare that you’ll need to consider this one." could not be further from the truth. It need to be considered ALL THE TIME if you're setting up for towing!
This is erroneous and confusing information put out by a company that should know better. The trouble is, consumers rely on and use this sort of stuff and are, in turn, mininformed and confused. Then they got on the road overloaded and unbalanced.
If you must have a formula it would be
When coupled, Actual Vehicle mass (not GVM) + actual trailer axle load (not GTM) must be =< GCM
or
When uncoupled, Actual vehicle mass (not GVM) + actual trailer mass (not ATM) must be =< GCM
And within both of those you must not exceed GVM, ATM or GTM.
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Actually, I have a bit of trouble with the ADR definition and how it works with, say, a Ranger/BT50 (GVM 3200) and the max published towed masses for those vehicles of 3500kg.
Using the ADR definition of GCM as being the manufacturer's maximum sum of GVM plus axle load of the trailer, then
GCM 6000 minus GVM 3200 = 2800 as the maximum permitted axle load of the trailer.
Using the Ranger/BT50 max ball weight of 350, that would make the max towable trailer mass (ATM) 3150, not 3500 as advertised.
If you get a GVM upgrade, commonly to 3500 for these vehicles, it's even worse. Using the same calculation, the max towable trailer weight (ATM) would be 2850.
Another argument against the overoptimistic specs for these utes, or am I missing something?
If you've read this far you're maybe as bored as I am with this %^&* isolation stuff LOL
Maybe I should have posted just this:
Combination weights
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907020