At the end of last season I started to develop some serious vibration in the trailer at 20mph.

With the boat on the trailer, doesn't matter if I'm speeding up or slowing down, right when I hit 20mph the boat starts to rock side to side pretty hard then it clears up within a few seconds, right about the time I get to 25mph if I'm speeding up or 15mph on the brakes.

The last time I took the boat out last summer, my girlfriend parked the trailer when we unloaded and she said the trailer was bouncing. I personally haven't been able to pull it without the boat on the trailer, but guess I'll have to in a couple of weeks to see if I can diagnose the issue.

Any idea what could be causing this or what I should look for? There's not much than can cause it. Bearings, brakes, bent wheel. I'm capable of fixing the issue on my own, just uncertain on where to start.

It's a single axle trailer by the way.