adambumpus

I'm having a little trouble with our Alleycat trailer bike and my wife's Specialized hybrid. The arrangement works great with my Gian hybrid because I have a straight top tube. My wife's bike have a sloped top tube so the seatpost quick release is slightly below the top of the rear wheel. This makes it so that the front of the trailer bike rubs on the back tire whenever they hit a bump.

Any ideas for getting this arrangement to work better?

Can I get an extra seat post clamp to keep the mount from sliding down too far?

Fibber

A similar geometry issue prevented my wife from pulling our Gary Fisher (Trek) Mountaintrain 206. In her case, I could not do much about it as she didn't have the seatpost far enough out to slide the mount up high enough to reliably clear the rear wheel. It was compounded by it being a suspension seatpost.

I do not see any reason why your basic idea would not work. A simple piece of metal tube & clamp slipped over the seatpost, with the trailerbike mount riding above that would probably work. Just don't raise it up much, and make sure that there is still plenty of seatpost inserted into the bike frame to distribute the stress. The higher mount point will create a cantilever effect that will put greater forces on the seatpost and frame. You don't want anything to break!