I'm thinking about trying to change the spacing of the rear triangle of a 3-speed so it's wide enough to fit the SRAM dual-drive system. That would require a change from the current 110mm spacing up to 135mm. That's a big jump. Is that a stupid idea? Does anyone have experience with attempting something like this?

well, if you have a steel frame you could be able to pull it off, or you might be better off to ask somebody who works with steel a lot more.

It's a steel frame for sure - I just didn't know if this idea was just plain bad. I'd be willing to give it a try if someone said they had sucess with such a project.

Never tried that big of a jump, but it is still only 1/2" per side. If you make sure the dropouts are parallel after, it probably will be OK. How much do you like the frame? A rear triangle failure is not very dangerous.

That wide I'd go to a welder\ frame builder.
Play detective, somebody who does this won't charge you much, but they may be hard to find.
The rear strut tri braces might have to be altered i'm thinking, otherwise you'll be widening only the tube past the struts.
It has to be aligned very well. That's bigger than i've read about.

I once did a 126 to 135 spread and at one point had over spread them to 145. Everything was still solid. Be careful.

Well, there's a couple of different ways to look at it.

The first is that you are going to be putting quite a lot of stress on the brake and chainstay bridges. If you have a problem, that's probably where it will show up.

The second way of looking at it is that you are messing with an old 3-speed frame. It's value is next to nothing so, even if you ruin it, you won't have lost much.

The only question that I have is the sanity of spending as much money as a SRAM dual-drive system is going to cost on this frame. But then again, it's not my bike.

Thanks for the replys and advice. I'm just going to give it a shot since it seems possible. I'll post the results one way or the other. As to the question of sanity, I agree it's not the most normal idea - I just get a kick out of thinking about it. I love 3 speeds, and I have a dual drive on another bike that I really love. So I got a new dual drive, and I'm going to combine two things I like to make an old worn in bike shift like a champion.