questions about building a lugged frame
 

Can a whole range (52cm-62cm, for example) of frame sizes be built using the same set of lugs?

ISTM that increasing the front triangle size will require one of two "solutions" since the front triangle's tube angles are fixed by the lugs. For example, if I built up a 56x56cm (seat tube x top tube length) with a set of lugs that gave me a bike with 73 degree seat and head tubes when using a fork with 400mm axle to crown height, then used the same set of lugs to build a 62x62cm frame and used the same 400mm for, the 62cm frame would have a lower bottom bracket and steeper seat and head tube angles due to the front triangle being relatively "tilted" forward.

Am I making sense here? Is this just the way things work when building frames with lugs? Or am I missing some nugget of framebuilding whereby the front triangle's tubes relative angles are not "set in stone" by the lugs?

Basically, the same set of lugs can be used since the angles similar for most frames. Also, you can bend the lugs a degree or two to accommodate differences.

I had to adjust the chainstay ports on my BB to accommodate "track" angles, from what I understand this is very common. However, theres no "tandem" lugs per se and no "pursuit" lugs (funny bike) so lots of those are fillet brazed. Or TIG welded nowadays.

the lugs that would have a different angle going from a small to big frame would be the down tube lugs, going from the bottom of the steerer tube to the front of the bottom bracket, if your lugs are tight, they may be hard to adjust at these areas. Older bonded frames used to fail due to the frames being under strain from the set angled lugs (alan, graftek)

Exactly.

So how does one solve this problem so that larger frames don't end up with steeper seat/head tube angles? Is bending the DT/BB and DT/HT lugs the standard way of solving it? If so, is this done with cold-setting or is it done while the lugs are warm?

Also, if I'm asking a question that is answered in a good framebuilding book/reference, I'd love to know where to look for more information on this bit of frame construction.

cheers!
b

I don't know if this will answer your questions, but standard lugs do come in different angles, and the frame designer choses the appropriate lug angles closest to the angles needed. Lugs can be tweaked a degree or two (or even more if the lugs don't have a lot of meat).

Henry James investment cast lugs can be ordered in 1 or 2 degree increments and can be altered a degree or two either way. Each lug can be ordered separately to make up a set of three with any desired angles. Seat lugs are available in 73, 74, and 76 degree angles. The lower head lug comes in 57, 59, 60, and 62 degree angles (this is the angle between the head tube and the down tube). The upper head lug comes in 72, 73, and 74 degree angles.

Looking at your hypothetical 56 x 56 frame with 73° HTA and STA, and the down tube angle is 47.1° (all angles are relative to a baseline drawn through the centers of the front and rear dropouts). That makes the angle between the head tube and the down tube 59.9°, so your three lug set would consist of a 73° seat lug, a 73° upper head lug, and a 60° lower head lug (you'll have to tweak the lower head lug 0.1°).

http://i32.photobucket.com/albums/d7.../56Squared.jpg

Looking at your hypothetical 62 x 62 frame with 73° HTA and STA, and the down tube angle is 43.2° (all angles are relative to a baseline drawn through the centers of the front and rear dropouts). That makes the angle between the head tube and the down tube 63.8°, so your three lug set would consist of a 73° seat lug, a 73° upper head lug, and a 62° lower head lug (you'll have to tweak this one 1.8°).

http://i32.photobucket.com/albums/d7.../62Squared.jpg

For consistency, the BB drop in both cases is 70mm.

Thanks for that. I did a few quick (not nearly as nice as the ones you provided!) calculations and found that +/- 2 degrees of "adjustment" would allow a full range of sizes to be built with a given set of lugs.

On that note, do you have any suggested reading about how to adjust the lug angles? I've run across references to the Paterek Manual. Would this be a good start in my remedial framebuilding studies?

Many thanks for all the help here, folks!

cheers!
b

I used a steel bar milled to size of the ports, in my case 22.2 mm. For oval ports i would have rigged section of chainstay filled with melted zinc. it's easiest in my case. For the larger lug ports I plan on making a selection of expanding bucks. probably out of hardwood.

The Paterek Manual is an excellent resource, and the lug adjustment procedure is explained on page 5-26 of the new 3rd edition. As white folks suggests, the angles are cold-set using cheater bars. Cheater bars that have the same O.D. as the I.D. of the tubes are inserted into the tubes and pressure applied to adjust the lug angle.

Tim Paterek also has DVDs that take the viewer by the hand and show, step-by-step, how to build a frame. It's like having a personal tutor, and at $89.95 is a helluva bargain.

Here's a photo from the Paterek Manual showing how the head tube/down tube lug angle is adjusted:

http://i32.photobucket.com/albums/d7...DTLugAngle.jpg

The lugs video is good. I've seen it twice, though I don't remember a lot on bending lugs, of course I probably wasn't really looking for that info either. I bend and then I hammer form to get the piece tight to the new angle.

http://www.prodigalchild.net/DMbicycles2.jpg

Lugs can be “pulled” to any angle as the frame is assembled. I always brazed with a lightweight (4 oz.) hammer in my right hand along with the brazing torch. (Picture above.) If there was a gap between the lug and tube due to adjusting it to a different angle, I would simply switch the torch to my left hand, continue to apply the heat, and close the gap by tapping the lug down with the hammer. Then switch the torch back to the right hand and continue brazing. It was a continuous non-stop process.

I never brazed a lugged joint without the hammer there at the ready, even if I didn’t use it. Early framebuilders were blacksmiths, and I always saw framebuilding as refined blacksmithing.

Thanks, Dave. Great info, as always!

Dave, are you right or left handed? I'm real new at this (right handed) and have been using the torch with my left hand and feeding filler with my left ...thanks..

I am right handed; therefore, I normally held the torch in that hand except when I transferred it to use the hammer. When brazing it is the torch that needs controlling, more than the filler rod. The temperature of the metal is controlled by constantly moving the torch away if the metal gets too hot, and back again if it cools. The actual motion of the torch tip was in small circles, with an occasional flip away and back.

The goal should be to reach the melting temperature of the filler material, then maintain that temperature constantly until the joint is finished.

In the welding comunity it is considered unsafe to switch hands with the torch. Not a word of criticism, just a note to those who may be playing in home shops with a combustible floor or surroundings.

Thanks Dave, i'll keep that info in mind next time around, i too had a little ball peen handy, the chainstay ports needed some "massaging" so to speak:) I don't know why I did it lefty since I hold a TIG stinger in my right hand and feed rod with my left...unless I'm going the opposite direction for some reason...

also, when i was in josh ahearnes' shop i saw his torch hoses run from the tanks to a little brass dome then to his torch, which hung from a wire rod poking for the brass dome thingy. It had a small pilot light on the other side...i was too embarrassed to ask what it was, I at first though it was some sort of gas-flux apparatus but now i think it was some sort of hanger-activated pilot light. anyone know for sure?

A good safety point. It may also be against regulations in many places to even have welding gasses in the home. In my shop in California I had to build a fire proof wall with a double thickness of sheet-rock both sides between the framebuilding area and the paint area.

That's exactly what it is. Hang the torch up, and it cuts the gas supply off; pick up the torch and re-light it from the pilot light. This would be a luxury for the hobbyist, but it does save a lot of time, and is a must for any kind of production work.

"It may also be against regulations in many places to even have welding gasses in the home."

So true, that is one reason why I use propane. I am not really sure it changes much, in the sense that I have a torch attached to two bottles in my detatched garage. But I basically have oxygen which is used medically in the home, and propane in the same bottle every BBQ owner has. So at the very least I hope to escape a lynch mob. I never weld in the cold season, so I am always open to the air. Regretably, around here that is about 50% of the year that I miss out on!

Another reason to use propane in regular torches when (rarely) possible.

I almost had a fire the first time I welded a water bottle rack. It's an 1/8" thick which means 125 amps, except there isn't anything like the amount of material there to require that, so I chose some number, and even that got it so hot a molten section dropped to the floor where it started a fire. In the welding hood, ancilary fires aren't always obvious. It can be a good idea to have another set of eyes in the shop shielded from the main flash, but able to see any developing hazards, the first time one tries something completely new.