thook

So, I did bunch of searching and reading. I'm impressed! The lot of you FB'ing guys are quite informed. Within all the great info, though, I still have the question....

Intentions:

I've toyed with the idea of building a bike frame, someday. After all the reading of everything involved......tools, materials, skill required.....I'll probably never do it. It's just not that important. Seemed like it'd be a fun adventure for a while, but then reality of it all sets in. It's a lot of cost just to be a backyard hobbyist. But, I've atleast learned how to make a homemade jig...hehehe.

That in mind, I've long deliberated over modifying a mtn bike frame I've had that's a little too big. Then, I thought.....given the paragraph above....I'd just sell or trade it. But, within this time, I just haven't found something I was really looking for....though, it may be out there. But, nothing seems to be getting any cheaper...lol!

So, with this mtn bike frame, I imagine I could have it mod'd and wind up with exactly what I've wanted and still cost less than another frame. (Plus, I've just gotten tired of acquiring crap). The frame is a DB cro-mo and atleast an inch would need to come off the TT length and height to have it fit just right. To do this, the TT, the HT, and the DT would need to be cut off and relocated with the DT welded at a steeper angle to keep the HT at the proper height. Problem is the butting.

I've read and seen pics wherein folks have had a frame modified or repaired, so cutting out tubes and/or relocating them appears okay to do if the circumstances will allow for it. But, in reusing the existing tubes, I have concerns about how much of the tubes are butted. IOW's, if I were to cut off a half inch or so from either end of the TT (for example), would there be enough butting left for a solid structure? I gather I couldn't really know unless I could see the insides of the tubing. Even then, how can you tell? Also, which tubes are actually butted? I read usually only the DT and ST are butted with most production frames. Would that be true? This a mid-end Giant "designed" in the US......but, probably actually made in Taiwan or some place.

If the ST is butted, the TT, being move down an inch, would be rejoined just opposite/a little lower than level with the center point of where the chain stays are joined at the back of the ST. Right now, the TT is joined above that location.

Here's a very close example of how it would wind up looking at the TT/seat stay/ST area:

https://www.pbase.com/canyonlands/image/56986562

Although, my end product frame would retain a horizontal TT and not sloped like that one. This would mean I'd be left with a couple of inches of ST sticking up where the seatpost is inserted, but I figure if the ST is butted there, and the butting atleast extends lower than where the seat stays are joined, cutting it down and slotting it for post binding purposes is probably okay. This would give me about 4 inches of exposed seat post in the end.

Anyway, I'm not intending to weld it back together myself. I met a professional structural welder who would do the TIG stuff. I'd only be doing the cutting and mitering on my jig so as to have the mod'd frame set up to take it to him for the welding.

Maybe this is a bad idea...lol! And, I hope I'm not frustrating the hell out of you guys who are tired of answering questions like this. The welder seems to think it's a fine idea because the welding points would be fresh compared to if I were wanting things rewelded in the original locations (save where the DT connects at the BB). This from a metallurgic stand point. The HT could be flipped around to weld to the front side, for example. It just depends on the butting issue. I realize new tubes don't cost that much....relatively, but if the original tubes can be reused, why not try? If it's a failure, I'm only out the $20 or $30 I could have gotten for selling the frame. Not a big deal.

So, what do you guys think? If there's any details you'd need to know, ask. And, whatever the consensus opinions, I'm not gonna argue them....promise......jus' so ya know.

Thanks

unterhausen

this only works if you find a guy that's never done the work before. As framebuilders get more experienced, they realize how much fussing around this takes. It's far easier for me to start from scratch than to modify something. I have heard of this modification being done though.

As TIG welding became more common in bike frames, the butts on tubes got very short. So you may not be able to do what you want anyway. There are lots of pictures of butt length measuring tools out there.

Craig Ryan 

Unterhausen is right. I don't think you can save money by building or modifying a frame over just going out and getting a new one.

thook

Well, maybe the welder already has that tool. I just never thought to ask him. I will, though. He's never done a bicycle frame. Just lots of other things similar in nature with tubing, complex joints, and stuff.

So, if you don't mind my asking, just because I'm curious, what makes modification harder than starting from scratch? I mean, it'd be on me to do the hard part of cutting, mitering, and aligning....and, I wouldn't mind that. I'm a shop gnome by nature, so it'd be fun. I know I'm totally ignorant of the processes you guys deal with, so maybe that's something too hard to explain over the web. Just seems to me once everything's cut from the main triangle, it'd be like starting over, basically. Except, in a case like this, one would already have some starting angles to go by with the rear triangle and seat tube. Then, of course, set up the other tubes to their new angles.

So, right.....the butts maybe too short, anyway. Pffth.......oh well. I don't suppose that type of tool can measure butting with the frame still being intact, eh? That'd be nice to know before hacking it up just to see...lol!

thook

I don't know. Provided it could be done well, if the welding/modification cost me only $50 and a comparable frame might cost $100, seems I'd save some money. But, building one from scratch? Yeah, fork that. I've looked into it.

unterhausen

Sounds like a fun experiment. All I have to say about riding someone's first frame is keep it clean so you can see any cracks develop

here is one example of a butt measuring tool

It's unlikely that a welder has ever worked with butted tubing. Most people don't care about the weight enough to deal with the hassles. Straight gage tubing is good enough for almost everybody.

Mark Kelly 

Here's the most useful butt measuring tool of the lot:

https://www.ndtsystems.com/Thickness_...00_series.html

I use the older 800 series instrument which I bought for peanuts on Ebay. It can also be used to determine whether you've achieved full braze penetration in silver joints.

unterhausen

I was going to mention ultrasonics since he could measure before he took the bike apart, but I figured he wouldn't want to invest that much money in the whole project..


I work with ultrasonics a lot, I was scouring ebay for a while but never found one that I could afford and went thin enough. I guess I could make my own delay line

Mark Kelly 

Unterhausen

The echo/echo units from Nova work extremely well on very thin tubing. I think I paid $350 for mine.

Next thing I'm going to do is to get the appropriate cable to attach it to my oscilloscope. I'm thinking a visual indication will give me more information. CRO is currently down with PSU issues so I have to fix it first.

thook

Thanks for the replies, guys. I appreciate it.

The welder doesn't have one of those ultrasonics, and I'm certainly not gonna buy one....or any tool, really. There may be some machine shop or something around here that has a tool. I've got a friend that works at one. I'll ask him. If it all becomes too much hassle, though, I'm just gonna sell the frame. No biggie!