Just about all vehicles have a "noseweight" limit of 50 - 75 KG especially for towing
With any more so far behind the rear axle you could affect the stability of the vehicle and it's ability to run in a straight line as you could be tending to lift the front wheels off the ground. Extra stiff rear springs will only increase this effect like a "seesaw" as the rear suspension will not be able to deflect to absorb any of the weight
You might also find that if VOSA were to weigh the car/caravan combination at a roadside checkand there was more than 50KG nose weight that it could be illegal. I can't confirm this for certain at present but it would seem sensible
I didnt give enough thought to the balance effect. 75kg is actually miserably low when towing caravans. Any Ford bigger than a C-Max has a higher tow ball limit. Most caravans have a tow ball weight of above that with some reaching over100kg. I have just purchased a small 2 berth van with aball weight of 85kg. although I can balance it to give me 75kg.
I did some more digging on the technical aspects of this.
It comes down to the type approval process for each particular variant of a vehicle and how the extra weight on the towball then affects the front and rear axle loading and more especially how the change in weight may change the effectiveness of the brakes (there are a very complicated set of rules on braking force utilisation- I've done the Annex 10 calculations before!!!)let alone the steering
There apparently should be a figure quoted on the VIN plate to say what the nose weight limit is
Hi Brian most caravaners i think look to about 7% of the max weight on the ball on mine this equates to 1232kg *7% = 86kg. Obviously this is to high for the max and i move things in the van to give me about 65kg on the nose. I also use the rubber spring assistors when towing. I have found the Max to be a great tow car. I believe the 75kg weight comes from various factors not just the springs. Hope this helps