frame welding advise

[smootz]

The idea of building a bicycle (particularly a recumbent) intrigues me. I own a small machine / fab shop so I should have all the equipment I need. In fact, my recent addiction to biking began while machining some small bike parts for a friend.

Most of the bike operations I read about involve "brazing" components. I am proficient with mig, tig, and oxy/act. welding. Is there an advantage of brazing vs. these other methods?

Is shrinkage, warping, or constriction a major concern when welding or brazing to a head tube or bottom bracket?

Thanks for any advise. I am just an old fart machinist looking for another hobby.

SCOTTIE

 

[alienator]

There is a frame builders forum somewhere out there in the internet tubes, but I can't remember where that is? Anybody remember where the frame builders forum is?

You might try googling such a thing.

 

[alfeng]

There is a frame builders forum somewhere out there in the internet tubes, but I can't remember where that is? Anybody remember where the frame builders forum is?

UBI used to have a frame builder's forum ... there must be others.

 

[alfeng]

The idea of building a bicycle (particularly a recumbent) intrigues me. I own a small machine / fab shop so I should have all the equipment I need. In fact, my recent addiction to biking began while machining some small bike parts for a friend.

Most of the bike operations I read about involve "brazing" components. I am proficient with mig, tig, and oxy/act. welding. Is there an advantage of brazing vs. these other methods?

Is shrinkage, warping, or constriction a major concern when welding or brazing to a head tube or bottom bracket?

Thanks for any advise. I am just an old fart machinist looking for another hobby.

MOST frames are welded, now.

Brazing means that your joints don't have to be as precise as they would be if you are a meticulous welder because the 'lug' is often used to bond the adjacent tubes. Of course, the joints on the "best" hand-made bikes are close to perfect & the lugs are minimal compared to the lugs on a mass produced bike.

Because brazing occurs at a lower temperature, it is probably better for attaching 'braze-ons' (e.g., cable guides) onto thinwall tubing.

The jig you make & the tubing you use will affect the amount of warping, if any ... I think that warping may be more of a 'problem' with brazed frames because when a frame is brazed you really want to try to work on adjacent areas (e.g., the head tube) one-after-the-other and some cooling & deformation occurs on the portion which was just worked on relative to the part which is being heated.

Consequently, the best steel frames are often 'corrected' (i.e., aligned) to some extent after they are brazed-or-welded ... pinning goes a long way to limiting alignment problems ...

If you fabricate the frame in well thought out modules you probably will need to effect very little after-the-fact tweaking.

BTW. In the distant past, the presumption was that the FIRST frame you built would be un-rideable. So, you may want to make your prototype with CHEAP tubing.

The FORK is the most difficult part to make properly, and an off-the-peg fork is probably something you should plan to use ... so, choose your fork & front wheel size before you work out the rest of the frame's particulars.

THIS suggestion will probably offend all the 'bent riders, but if I were in your immediate situation then I would probably get the least expensive BMX bike from Goodwill & use that as the foundation for the first prototype.

 

[smootz]

Thanks for the advise so far. I am not a stranger to intricate assemblies or even tubing work. I just need some guidance (or a good book) to help me avoid problems others have already solved.

In my machinist mind I would have thought that head tube and bottom brackets would receive a finish bore or reaming after final fit and weld.

My first project will be a Lightning P38 clone. I have a friend with an original I can use for a model. There are a few changes I want to make to allow more seat angle, etc. The frame on that bike appears to be pretty simple, relatively speaking.

I have looked at some "off the shelf" bike fittings. Can custom bottom brackets and headsets per my own design be purchased? Obviously a recumbent is going to require some radical parts as compared to a standard bicycle. I want this to be something special and recycling old bike frames is not really the direction I want to go.

SCOTTIE

 

[alfeng]

Thanks for the advise so far. I am not a stranger to intricate assemblies or even tubing work. I just need some guidance (or a good book) to help me avoid problems others have already solved.

In my machinist mind I would have thought that head tube and bottom brackets would receive a finish bore or reaming after final fit and weld.

My first project will be a Lightning P38 clone. I have a friend with an original I can use for a model. There are a few changes I want to make to allow more seat angle, etc. The frame on that bike appears to be pretty simple, relatively speaking.

I have looked at some "off the shelf" bike fittings. Can custom bottom brackets and headsets per my own design be purchased? Obviously a recumbent is going to require some radical parts as compared to a standard bicycle. I want this to be something special and recycling old bike frames is not really the direction I want to go.

I don't know who you would contact for a custom bottom bracket shell OR head tube ...

You really need to address this on one of the frame building forums.

BTW. Presuming the P38 has a 73º head tube angle, you may want to change the head tube angle to something slacker (e.g., 72º) to make it less twitchy ... if the head tube is 72º, then I would build it with a 71º head tube angle, etc. A longer wheelbase might compensate for the harsh ride that the miniscule front wheel will give you ... there is a "suspension" front hub, but I don't recall who makes it ... a 24" wheel will be better than either the 16" or 20" wheel designs if comfort is something you plan to design into your "clone."

 

[dabac]

The idea of building a bicycle (particularly a recumbent) intrigues me. I own a small machine / fab shop so I should have all the equipment I need. In fact, my recent addiction to biking began while machining some small bike parts for a friend.

Most of the bike operations I read about involve "brazing" components. I am proficient with mig, tig, and oxy/act. welding. Is there an advantage of brazing vs. these other methods?

Is shrinkage, warping, or constriction a major concern when welding or brazing to a head tube or bottom bracket?

Thanks for any advise. I am just an old fart machinist looking for another hobby.

SCOTTIE

You might want to take a look at Henry James investment cast steel lugs. He sells frame parts like drop outs, tubing and such.
Don't know how much traffic it's seeing but I believe there's a frame builders sectio over at Bike Forums

 

[Rabbitbunny]

You don't actually need any special parts, just remember old derailler pullies make great idlers, and that you can use a derailler as your jack chain tensioner.

I built my tadpole trike out of two kids 20" bikes, a wheel chair, and a mens' 24" MTB. I used only mig welding and I have a little over 1500miles on it. I even used the cheapy one-piece crank by welding a section of 1/2" rod to the cogwheel on the lefthand side. It contacts the crank arm and transmits torque.