Not sure if this is already on here but this is crazy, check it out!
http://www.cyclingweekly.co.uk/news/latest-news/man-builds-worlds-tallest-rideable-bike-named-stoopidtaller-video-140480
In the past I have had success mounting a hacksaw blade in my small pneumatic reciprocating saw. Stick the blade into the tube and keep it parallel. It is still a time consuming process but it works. You need to keep the speed low on the saw work on getting the cut started near the top and...