has anyone got experience of these. We have 5 year old on a rack mounted trailer bike which is as solid as a rock and brilliant. 3 year old is too big for rear seat now and so we bought a seat post mounted one but it does seem to wobble and it twists around the post leaving the bike at an angle. I have followed all the instructions and think it is all correct. Has any one else come accross this or am I odd? I have to sort it before holidays as we are off biking again. Sorry I posted in family too but couldnt find leisure section