I have a 17ft Fibreglass Boat on a tandem trailer. At the weekend I noticed at certain speeds (80-90kph and also at approx 105kph) the sides of the trailer seem to rapidly shake up and down by approx 7cm. The shaking seems to be cntered just infrom of the wheels at the point where the trailer starts to angle inwards. I am concerned that the trailer or boat may become damaged. (although the boat does no move at all with the shaking).
Could it be that the wheels need balanced (I can only see the from pair from my driving position) as they almost seem to bounce on the road with the shaking? Or tyre pressures? Any advice would be welcomed.