Hi all,
I have no idea if such beasts exist so this may be just the warped dream of a twisted noob.

I have an olds cool hybrid with 27" wheels. I have another old bicycle that is basically stripped that also has 27 inchers on it. I'm not planning on doing any extreme MTB-ing, but I thought it would be cool to throw some knobbies on the extra set of wheels. That way I could easily switch out depending on where I'm riding.

Obviously, they would be pretty narrow knobbies, but it would be an improvement over taking street wheels onto the dirt and weeds.

Any ideas? (besides "get a second bike")