Originally Posted by robobike316

I did this with a large pipe a number of years ago on an old steel racing frame. Simply hammered the pipe in from the rear toward the bb and then made sure the rear end was still aligned to the desired spacing. I got enough space to run cyclocross type knobbies on a fairly tight road frame with no problems.Canti studs were also brazed on. I used this bike off road for a few years- it was the fastest mountain bike on the trails!
Obviously this could compromise the chain stay brace joints (or whatever that is called down there) or even the bb to stay joint (if there is no brace), I didn't have any problems, and was more than happy with my off-road rocket.