Just wondering if there’s any way a near layman with access to basic tools in home shop (welding power supply (scarcely used this one), drill, angle grinder, etc.) can replace rear fork dropouts on a 26” retro MTB (chrome-moly frame setup). I want semi-horizontal dropouts instead of vertical ones like the ones in the pictures below. The semi-horizontal dropouts are to be used with a Rohloff IGH (rim brake version) and I really don’t want a chain tensioner or an eccentric bottom-bracket. There are many ads with nice retro MTBs where I’m based, but almost none of them have semi-horizontal dropouts and if they have them the bikes don’t fit the bill for other reasons. So that’s why I started to think about buying an MTB I like and DIY-converting it to semi-horizontal dropouts. I don’t have an easy access to a frame builder. The best option to me would be semi-horizontal dropouts with a hanger for a derailleur to be able to use a wheel with either an IGH or a derailleur drive train.

I don’t consider myself an absolute layman when it comes to tinkering with bicycles but here I’m a bit sceptical, as it seems some expertise, knowledge and experience are needed to do it, but maybe I’m not privy to something .

A file could likely solve this.

What Canaboo suggested is a good idea if there is room for a big enough slot while still leaving enough metal behind it.

Replacing the dropouts is not really doable without proper framebuilding skills and equipment which means torch brazing or TIG.

As a hack you could try extending the existing dropout backwards and then you can cut a bigger slot. The actual dropout is about 5mm thick so could be welded with MIG, flux-core or even stick. Grind down the back of the existing dropout, cut a curved shape out of a bit of 5mm plate (or whatever the thickness you have is), bevel both parts, and get a good weld in there. Grind it all flat again (so never mind a bit of spatter and boogers) then extend the dropout backwards with a grinder, dremel and/or file depending on your patience level. Finally trim off any of the new bit of metal you just welded on that you don't need and tidy it up. You want to leave a bigger piece on there while actually welding it to act as a bit of a heat sink. ​​​There's no reason why this couldn't work perfectly well if you make a nice job of it. But practice on a scrap Walmart frame before mutilating the nice cromoly retro MTB frame you just spent ages hunting down.

You could just use a Rohloff chain tensioner.

I think you would botch it up. Use a chain tensioner

The axle slots will need to be greater then a half inch long to insure chain tensioning ability with any cog/ring combos. I don't think the existing drop outs have that much meat.

Sue you could try but as Eric said I doubt it would turn out well. There's more then just removing old and welding on new. For someone who has such an expensive goal (Rohloff hubs are not inexpensive) I'm surprised that such a kludge attempt is being considered. Andy