Originally Posted by kubcat
the weight on the tow bar should be 5%-10%. 5% is what we use on the bigger trailers say 3500kg and closer to 10% on the very light trailers.
I am pretty sure these figures come from the Australian registration authorities but can't find a link right now.
Pretty standard rule of thumb worldwide I think.
Too little tongue weight may cause trailer sway (fishtailing of the trailer - disconcerting at best, dangerous at worst.)
Too much tongue weight causes excessive squat on the tow vehicle, and may lighten up the front wheels enough to affect steering.