One of the guys at my church who is a welder and I took it upon ourselves to redo the hitch system on my old instep child carrying trailer. We took a sway control arm kit that contained an 1 1/4" ball, and mounted that on a plate that sits between the wheel and the seat post (higher than the wheel of course). It attaches to the bike on the four points that you would attach a rack to. This part works great! We then took the tongue that used to attach to the rear axle, turned that up, and welded in the inner part of the sway control arm that contains the hitch. The hitch attaches to the ball using a cotter pin. I took it for a maiden voyage and it was wonderful. I put the kids in it and honestly could barely tell they were back there. The bike could lean and not be affected by the weight I thought it was perfect until my wife informed me that it was jerking the kids around horrible every time I peddled. We have tried cutting the tongue where it leaves the trailer (straight up and down) and inserting the old stock spring that was on the end of the tongue, but now the trailer won't track right, it touches the ground in the front at times, and it pulls the tongue down onto the wheel. How can I get some back and forth give, and still keep the sturdiness? Any ideas from you all who have tried seat post hitch designs?

Thanks