welding a steer tube, am I asking for trouble?
 

I want to convert my old steel MTB to threadless, and I want to do it cheaply. I know how to weld and my father has the equipment. Will an extended steer tube be safe to use? (assuming that everything is square) Or am I looking for a catastrophic failure here? the bike will be used for street use only, I have other bikes for trail use.

-mike-

Welding on an extension is asking for trouble. Cheapest way is to just get one of those conversion stem inserts. That way you can even keep the old headset.

Actually, this is a not so uncommon method of extending a fork steerer. But normally it is done by cutting the steerer, and brazing a sleeve inside along with a piece for extending it the desired length. This reinforces it adequately. I don't see why the steerer couldn't be cut, and a threadless piece brazed on as long as a sleeve is used to reinforce the area cut. Don't know how you would do this with welding though.

You could weld it if there was an internal tubing used as a slug.

My 1.125" stearing tube is 1.02" internal, You could insert a 1" tubes braze, weld, even epoxy it in there (at least 3" overlap for that), and then just use 1" threadless stem on the extended tube. remember this needs to be the good stuff like 4130 aircraft tubing. You have to decide where to make this joint. One option would be inside the head tube and use 1.125 down and 1" headset parts, or you could do it so it transitions underneath any spacers above the headset.

I'm not sure why you would want to do this, both systems are fine as they are.

def. need a slug in there. maybe leave it to a professional frame builder. This has been done before..