I have 2 bikes. A touring bike and a mountain bike. The touring bike pulls my 40 lb daughter, snacks, and water just fine (with a Burley classic hitch). The mountain bike on the other hand pulls for about a mile then the front of the tire starts to cant to the right, eventually rubbing on the frame. This used to happen on my old univega mountain bike when pulling my son. Anyone have any ideas on how to fix this or why it is happening? Is it because my touring bike is designed to pull a heavier load? Thanks in advance!
Have you ever changed the back wheel for one with a wider axle?
I get a similar effect on my old 15 speed MTB, which was a 5 speed rear. When I fitted a wider wheel with a 7 speed freehub, the dropouts needed spreading by a couple millimetres. I did not parallel the dropouts, so they're very slightly shaped like / \
The upshot is that the Quick Release doesn't bite into enough metal, and sustained hard pedalling allows the wheel to move in the dropouts, eventually giving frame rub on the left chainstay - torque through the chain and cassette on the right-side pulls that side forward.
In your case, I imagine the trailer hitch is on the left side and the overrun pushes the axle forward.
So, the temporary fix is to do the QR up really damn tight, or to use axle nuts done up tight, or even to use softer metal washers which will deform and fill up more space between dropout and axle nut.
The proper fix is to bend the dropouts back into line by cold-setting the steel frame. This takes power and leverage, and patient measuring.