Does GCM need to be the sum of GVM and max braked tow?
Yes, and no.
The biggest factor in towing stability is a heavier towcar than the trailer. So, ideally from a specification perspective, the max tow should be less than the towcar’s unladen weight, certainly less than its GVM. And from an operating perspective, the towcar should weigh more than the trailer, as a general rule.
Say we have a towcar with GVM of 3000kg, max tow 3500kg, towing 3500kg. The towcar will always be lighter than the trailer. But then we add in the GCM of say 6000kg.
We take 6000 GCM – 3500 max tow = 2500kg, max weight of the towcar. Now the towcar’s maximum weight is reduced, as we’re hitting the GCM first, and the one time you want to be at GVM is when you’re towing heavy. Would be better if the GCM was 6500kg, so GVM 3000 plus max tow of 3500kg.
But what about TBM? The GCM calculation is tricky, because the TBM becomes part of the vehicle’s weight, counting towards its GVM, and is still part of the trailer’s weight, counting towards the max tow. However, for the GCM you only count it once, despite the fact it is now part of both towcar and trailer weight. Let’s assume max TBM is 10%, or 350kg.
So, one way to look at a good maximum is that GCM should equal GVM plus max tow minus TBM, so that’d be 3000 + 3500 – 350 max TBM, 6150kg, no need for it to be 6500kg.
However, this overlooks two points:
- Assumes you’ll use max TBM when towing to the max. This may not be the case, maybe instead of 350kg you may well run 300kg, maybe 280kg. Then you’d miss out on payload, not by much, but by 50, 70kg and that’s critical these days. So, the ideal should be more like 6250kg in this case.
- Doesn’t allow for dog trailers which have a turntable and wheel at each corner, and no significant towball mass. You don’t see these recreationally, but they are common for commercial use and none are light.
So what’s the answer? No, GCM doesn’t need to equal braked tow plus GVM, but if it does, you’re got TBM flexibility. If it’s under by say 50kg, like the LC300, then no worries. If it’s more than that, approaching any reasonable TBM, you may be GCM-limited before any other limit is hit. That’s why I just prefer a simple GCM = max braked + GVM equation. The reason is safety, when towing heavy trailers, you want the towcar as heavy as it can be.
