Interesting. They told me something similar but different. Didn't say a word about attaching it to something other than the axle. And said don't do it, but if you do, check with the online forums and a mechanic. But we don't recommend it.

"Unfortunately, Rad does not recommend this modification or add on to our bikes due to the extra strain it can add to the motor, motor mounts, and wiring.

As for adding a trailer to your bike, or any third-party component, I would encourage you to seek the expertise of a professional bicycle mechanic.  Generally, we cannot recommend any parts that we haven't tested for safety and compatibility with our bikes (and determined to be compatible and safe).  You may also be able to check online forums for Rad Power Bikes to see if any other customers have had success adding these kinds of parts to their bikes, though we still recommend consulting with a professional mechanic."

Here's what I ended up doing: I used a small piece of pipe to attach to the coupler and to the bike trailer. It works fine.

I'm having problems attaching a Schwinn child/pet trailer to my RadRunner after using it for several years on a conventional bike.

The coupler (photo 1) won't sit flat because of a bolt located just above the axle. Another coupler (photo 2) fit the axle ok, but didn't have enough space for my trailer arm. A third coupler that misses the bolt above the axle (photo 3) is recessed, so there isn't enough room for the stock axle nut to attach.

I tried to find a conventional nut to attach to the axle and the threads on the ones that were the right size weren't compatible.

My next thought may be to take some metal tubing to connect to another coupler that is too small to attach directly to the trailer hitch. I looked exhaustively fo r another coupler that's slightly larger and fits the axle but to no avail. My thought was to get a smaller-sized tube, attach it to the coupler and attach the other end to the trailer arm and use washers to account for the difference in size between the tube and the trailer arm.

Does anyone have a better solution? Thanks.