I have the Trek version that attaches to the seatpost of the adult's bike. As with all of these units, it came with a stated maximum weight limit. I use it with a child who weighs 60 pounds, and is a very good bike rider. But, we had problems:

- the first time we used it was with an adult bike with traditional geometry that put the attachment up about 34 inches from the ground. The "fit kit" that came with it allowed about 1/8th inch of "free play" at the seatpost, but that tiny bit of free play magnified into very noticeable sway back under the child's seat. He did not like the side to side sway, so we went back to the drawing board.

- the second time, we attached it to a mountain bike with a compact frame. That lowered the attachment point to about 29 inches off the ground. I bought a second "fit kit" from Trek that had some smaller shims than the first kit. The smaller shims provided an attachment that allowed the bracket to pivot smoothly, but with zero freeplay. The trailer was much more stable, and the side to side wobbling was eliminated.

The problem now is the kid. If we are going on a short ride of two or three miles, he would much rather ride one of his own bikes than be on a trailer bike. I am only able to persuade him to ride the trailer bike when we are riding ten or fifteen miles, or riding on a busy road where he normally is not allowed to ride his own bike.

By all reports, the lower the attachment to the adult bike, the better. The lighter the child, the better. Regardless of the weight "warning" in the owner's manual, I don't think the Trek trailer would be easy to control with a child of 70 pounds or 75 pounds, unless the adult rider is exceptionally heavy and exceptionally strong. If someone wants to ride tandem with a child weighing 75 pounds, they really ought to be looking for a true tandem bike.