After more than a week of "fine-tuning", my Trek Mountain Trail 24 Trail-A-Bike is finally running smoothly with NO side to side wobble, and with smooth pivoting around turns and over bumps. My nephew and I had two long, enjoyable rides on Saturday.
The original set-up, on a tall road-bike, had considerable side to side wobble until a shim was placed under the "sizing" insert that fits inside the metal mounting clamp. But, there was still a tiny bit of wobble, even with a rubber shim.
Saturday, I bought a second mounting kit to fit the Mt. Trail on a mountain bike. The second kit including a slender, smooth plastic insert to fit inside the sizing insert. That black plastic pivot insert was NOT included in the first mounting kit. I discovered that the CORRECT way to mount the trailer is to place the black plastic "pivot" insert on the seatpost, select one of three grey plastic sizing inserts (whichever fits tightest on that seatpost, place the sizing insert over the pivot insert, and attach the clamp.
Correctly attaching the second "kit" to the mountain bike took less than five minutes. On the moutain bike, the Mt. Trail rides smoothly, with No side to side wobble. My nephew got so confident (over-confident) in the stability of the Mt. Trail that when I turned my head to see how he was doing, I discovered he was riding with his feet up on his handlebars. Eight year olds.
Mounting the Mt. Trail on a compact frame, instead of a tall road bike frame, lowered the bottom of the mounting bracket from being 34 inches off the ground, to just 28 inches of the ground. That brings the "pull" of the trailer six inches closer to the center of balance of the "lead" bike, and makes the lead bike less sensitive to the kid moving around on his bike. Also, the "fat" tires of the mountain bike provide a good deal of extra stability, compared with the 25mm tires on the road bike.
So, bottom line: if the Trek Mt. Trail is installed on a mountain bike, using all THREE pieces of mounting gear: the pivot insert, the sizing insert, and the metal clamp, the result is a smooth stable ride, with no side to side wobble, and smooth pivoting around corners.
All of the "wobble" was caused by the black pivot insert being omitted from the first mounting kit. And, neither the first, nor the second mounting kit including instructions (although a label on the attaching arm warns "Don't ride until you have read and followed the instructions".
So, if you buy the Trek unit, insist the dealer provide the written mounting instructions, and make sure the mounting kit includes the black plastic pivot insert, plus the three grey sizing inserts. The slight hassle of getting the trailer attached correctly will result in excellent performance, at a reasonable price.