TowSafe by Mister Hitches - the original and still the best.
Ensures that your ball weight (TW) is correct is essential for safe towing. If your ball weight is too light or too heavy you can experience sway, loss of traction and loss of steering control - all of which is extremely dangerous.
Your ball weight should be approximately 10% of your gross trailer weight (GTW).
When loading your trailer or caravan you can use the low cost TowSafe ball weight scales to check tongue/ball weight!
Knowing your ball weight is essential for safe towing!
To use, simply position the trailer in a safe spot and attach it to the soft ball head of the trailer weight scale as you would to the tow ball of your car. The scales will then provide a weight rating.