Way to quickly and cheaply achieve a custom sized frame for myself
 

Hi,

I posted a question in the fitting forum and thought I would post this portion of it here for comment and insight. Context: I have stubby legs and a long torso. I need a bike frame that doesn't have a tall standover height, but does need a long top tube. Quote follows.

"I think I may have an idea.

If I take a smaller frame, and remove the bottom tube, and cut the top tube in half,
And then add 3 or 4 inches to the top tube with a slip-fit tube that fits tightly around the original top tube ends,
And then put in a new bottom tube from crank to headtube,

It would only be 4 welds. The top 2 welds would be simple to do well; If I use an elliptical tube for the bottom, the whole mess would be even stronger than the original because the welds from bottom tube to bottom bracket, and bottom tube to headpiece, would be longer. I would MIG weld it.

I wouldn't have to fabricate chainstays or mainstays or mess with geometry or fork angle or any of that hard stuff. I'd just be lengthening the top tube and replacing the bottom tube which would also be longer and have a different angle, defined by wherever I decided to put the head tube. No fancy jigs, either; get the top tube lengthened/welded so the seat tube and head tube are in the same plane, and I've got it.

Voila! Custom frame. Just need a lightweight, well made cromoly donor.

The search is on. Let's see how cheap I can do this. If I find the right donor, I won't even need to buy components like wheels and such."

And here's the crappy diagram I made up.

What do y'all think? Will this work or is it a bad idea for some reason?

I also have the torso of a much taller person and stubby legs. Fashion is conspiring against us, everyone is making bikes with short top tubes relative to their seat tubes.

As far as the modification idea goes, it's a lot of work for not much payoff. The front triangle coming apart means that most of the hard work has to be done.

I didn't read your thread in the fitting forum. The question I have is how much stack height are you going for? A lot of people are going with massive stack height, and for them a larger frame with a radically sloping top tube will work.

"If I take a smaller frame, and remove the bottom tube, and cut the top tube in half,
And then add 3 or 4 inches to the top tube with a slip-fit tube that fits tightly around the original top tube ends,"

I sometimes comes as a surprise to people that there are not nice little tubes available all designed to slip fit to each other. But there aren't Tubes are defined by outside diameters and wall thickness, this means that there are rarely good ferule like fits. They can be reamed, but you probably just blew your budget in that word. Pipes are defined by internal diameter, and you would not need to use 4130 for this kind of job, but a 1.125 pipe will not fit over a 1.125 tube, without reaming. Of course if you treaded this as a demountable, you could slit the tube, braze on fittings and crank the thing together with screws, and you might even get away without reaming. Your splice can be glued on, by the way. You can glue tube butt joints, but you can glue a sleeve.

"And then put in a new bottom tube from crank to headtube,"

Essentially this tube can be a cable, so at some level of theory you cut the tube out and replace it with a lot less than a tube. Since a lot of your problems with jigging and alignment come from taking this tube out, I would consider something like a demount system in this area designed so as to leave the original tubes in place, look good, and have some function. Putting in a new downtube is non-trivial.

"It would only be 4 welds. The top 2 welds would be simple to do well; If I use an elliptical tube for the bottom, the whole mess would be even stronger than the original because the welds from bottom tube to bottom bracket, and bottom tube to headpiece, would be longer. I would MIG weld it."

You were good right up until the MIG weld comment. I have done a lot of work, perhaps as much as anyone, on non-standard welding. Both because I wanted to figure it out so folks could build cheaper, and also for the old roadside touring welding thing. MIG is possible, but you should look up the many pages of stuff largely directing oneself away from it. Any system that doesn't allow one to control the puddle and the filler independant of one another, in real time (not just some presets), is asking for trouble. It should be said that almost anything you can imagine where welding is concerned is either being done, or was done for some material economy reason during one of the world wars, or something even kludgier was made to work... But getting from that to your first build is a whole other thing.

The thread is here. http://www.bikeforums.net/showthread...oad-bike!-Help

What is 'stack height'?

Thanks for the detailed reply. I knew creating the slip-fit was going to be a challenge (or was figuring that out by looking at available tubing). I was leaning towards slitting the splice tube down its length and then welding the seam.

I understand your reservations about MIG. I've blown holes in tubing before using it. My main reason for using it is, I have the gear and a lot more experience with it than other processes. I have dozens of little projects under my belt with MIG. TIG, I have done maybe 3 sloppy seams in a class.

Maybe this is the project where I finally launch into using TIG... that would be another justification for moving ahead. "Well, it turned out like crap and I had to throw the frame away... but at least I got some TIG experience...." :lol:

I would stay away from the MIG and go all in with TIG. I'm thinking it may be cheaper to just build from scratch rather than picking up 2 bikes and hacking them up. Take a peak at THIS thread, maybe it will give you some positive motivation to dive in. Good luck!

What sort of ST and TT length would be ideal? If you only need a few cm on the TT I would keep it simple and try a long stem with traditional drop bars that have long reach.

Perhaps there is some quality hybrid-type bike somewhere. If it is designed for flat bars it will have a long TT.

hybrid conversion to drop bars is a good idea

Already have a bike you like , but wish the top tube was longer?
determine how much you want, then cut the top tube longer

If the Top tube is still to be horizontal, miter angles would reflect seat and head tube angles ,

the miter of the downtube angles at BB and bottom of the headtube would change..


I built a 58 seat tube , 60 top tube bike in the 70's , the front wheel does feel lighter ,

Because it's further ahead of the CofG of me in the saddle..

As someone that worked as a certified code welder for almost ten years using GMAW (mig) and GTAW (TIG) on metals from less than .035" up to 1/2" I have to say that you "could use mig" if you are not using a "hobby welding machine". You will need thin diameter wire that is compatible\complimentary to the composition of your base metal. You will need some pretty fine control on your settings, and your choice of gas will affect the properties of your arc - which in turn affect penetration and finished appearance of the bead. Way more than I would want to deal with when a better option is available - TIG..... If you are talking modifying a welded frame, it may be easier to replace the top and down tube entirely. You WILL need a fixture to hold it all in place as alignment is critical - yeah I know I'm stating the obvious....

And then there is the relationship between your head tube angle, your rake\trail of the fork, and how the change in weight distribution figures in....

A lot to consider for a DIY project that could easily end up a pile of scrap metal.... Are you in school for welding, working in a fab shop, or just like burning wire? Why not get a tube set and build one up from scratch?

Not in school, not in a fab shop, just an eccentric dude who builds things. Sometimes the things end up a pile of scrap, and sometimes I invent an improvement on John Harrison's Grasshopper escapement. I never know until I start working.

Well I wish you luck in your endeavor. I really think it would be just as easy or easier to replace the tubes with the desired length and weld\braze them where they're supposed to be joined anyway.

I notice the professional builders are avoiding this thread and I probably should as well, but I've built a few frames and you are seriously underestimating the project and the risk to your safety from riding it.

1. There is a good reason why MIG is not used in bicycle frame construction and it's not just because you will burn holes in the tubing. It is easy to do a cold weld with MIG, where the bead looks good, but there is little to no penetration and it will fail.

2. Once you cut the top tube and down tube out, you will most certainly need to use some type of jig to hold the headtube in plane with the seat tube. Not just parallel with the seat tube, but exactly in the same plane so the front wheel will be aligned with the rear wheel.

3. You will "need to mess with geometry", because your head tube angle will be critical to retaining the proper trail with the existing fork and, after cutting out the top and down tubes, there is nothing to hold it at the original angle.

What you want to do can be done by fillet brazing and using new tubes, but I still wouldn't recommend it. New top and down tubes from Nova are less than $50.

A preferable option would be to just build yourself a lugged frame by silver brazing. The tubeset and lugs to accomplish this can be had for less than $200 and I built my first frame this way by aligning and brazing each joint one at a time.

All good input but I think you're overstating your case some.

Anyways I've modified my original idea, I am going to start with the largest frame I can find, and just relocate the ball-busting top tube down. Lots easier than stretching a shorter frame. The downtube won't be touched at all. The geometry of the bike will be what it came from the factory with. The only change will be clearance to the boys.

I may use lugs and silver brazing, or I may TIG it up.

I'll post pics when I get underway.

cheers,

when I built my frame in 1975, I already knew how to Braze.. Bene-sugg: I'd gain that skill first

I got the Tube set from a small builder
and got to use the pedal powered tube Miter cutting . machine he made.

And had friends and the Community-College to call on for tools I lacked..

My certification is LTRF ( Long Time Redneck Fabricator)

First rule of redneck fabrication is never to ask for advice on how to do it. If you have to ask you are chicken yellow.

Second rule is that all testing is prefaced by the phrase " Hey Hold my beer a minute"

Third rule is all testing must be videotaped, If the testing fails and you hurt yourself in an amusing way the video goes on Youtube. If the test is a success no one really gives a sheot so don't bother posting it.


About your project. I did this project in 1990. My cousin Roger is proportioned like the missing link, his pants are like a 28 inseam and his shirt sleeves are 35" He wanted a MTB frame that didn't need a 200 mm stem to fit him. So for 2 doobies and a 6 of Dos Equis I took an old 22" Muddy Fox (Tange #2 )frame and cut it top down to a 17" frame.

First I cut 5" from the seat tube, then I cut the head tube down 3" (I couldn't go lower without buying a new fork). I then separated the seat stays from the rear drop-outs. Next I carefully cut the top tube away from the stubs of seat tube and head tube. By sloping the top tube it makes the tube shorter so I had enough length in the old top tube tube to hand file the miters. Next I separated the seat stay( mono stay) from the seat tube stub and filed it down to fit. With all the fitting and filing done, I used my trusty Benzomatic MAPP torch and some Harris Stay-Silv 15 rods to braze everything together. Roger still has that bike. I have a picture of it, if I can find it tomorrow I'll post it.

The new plan sounds much safer and easier. Good luck and I look forward to seeing the pics.

The problem with moving the top tube down is what to do with the seat stays? Honestly, this entire project sounds only slightly easier than simply building a frame from scratch.

Contact Tsunami. I think they are in AZ. Very reasonably priced custom Al frames.

Why ruin a good frame by modifying it? I built mine for the same reasons. My changes were pretty minimal, but it was satisfying to build my own & in knowing that it fit me the way I wanted it to.
The other point I wanted to make is that a perfect fit doesn't always need to be in the frame. Buy a size smaller & add a little stem. None of my production frames are a "perfect fit". I ride them often w\o issue & quite enjoy them. Part of the reason many MFG's offer less sizing options these days.

Have you tried fitting a compact style frame like Giant TCR. You may be able to find a frame with even more standover. Get a larger size than normal, ie for TT length. You may have a high head tube but its worth investigating.
Who makes the most compact of compact style frames?

Get a nice quality steel hardtale MTB frame from the 90's and convert it into a road bike by removing the cantilever bosses, adding a brake bridge suitable for sidepull calipers. MTB frames have a very long TT and low headtube in relation to the standover height.