The maximum nose weight of the coupling head is shown on the type plate of the coupling itself. If the nose weight is too high you will feel the load on the back of the towing vehicle and this can make the ride quite harsh. If the nose weight is too low then the trailer will be unstable and can cause it to swerve at speed.
If you are towing the same cars regularly then there is a very simple process to ensure you always tow with the correct nose weight:
1) Couple your unloaded trailer to your car. (The unloaded nose weight is usually around 60kg). Measure the height of the coupling from the ground.
2) Load your trailer as usual making sure you have a positive nose weight.
3) Using a set of scales, lower the jockey wheel onto the scales and uncouple the trailer from the towing vehicle.
4) Move the towing vehicle away then lower the nose of the trailer using the jockey wheel until the coupling is the same height from the ground (not the scales) as it was previously when unloaded on the towing vehicle.
5) Re-position the load inside the trailer until you have the desired nose weight
6) Position the telescopic wheel stop bar inside the trailer so it is touching the front wheels of the car, you can mark its position. This way when you next load it, you can simply drive up to the bar.
7) Re-couple to the towing vehicle and unload your trailer. You can do this for each vehicle you transport ensuring safe towing on each journey. If you find that you often transport other equipment, spear wheels etc then this must be taken into account.