Lugs - Noob Question
 

I'm just starting to learn about framebuilding so please excuse the noob nature of my question.

I Understand that the top tube and seat tube lugs are going to set the seat tube angle, the head tube angle and the top tube angle. What i don't understand is how you get the correct angle on a lug for the downtube.

For instance, If one were to use these Pacenti Artisan Lugs

http://www.bikeforums.net/attachment...1&d=1216236845

The Top tube and Seat tube lugs measure as follows:
TT: 28.6 x 31.8 x 73.0* x 45mm Pitch - Horizontal TT
ST: 28.6 x 28.6 x 73.0*

Which I understand to mean the seat tube is at a 73 degree angle, and the headtube is also 73 degrees, and the top tube will be horizontal (straight across)

the downtube lug on that set measures
DT: 31.8 x 31.8 x 58.5* x 39mm Pitch

Now, if I recall my highschool geometry correctly, this means that the top tube length has to maintain the ratio to the seat tube length or the angle of the downtube lug will be different.

If thats the case, how does one go about building a frame with a different downtube angle? Just hunt until you find a Down Tube lug thats correct? What about finding a Bottom Bracket with the right angles? Is it all just a big hunt for the lugs with the right angles when it comes to building lugged frames?

Do you just muscle it and bend em to fit?

As you surmised Deez, this lugset is matched to one particular ratio. The set comes with a frameset drawing of the angles that correspond to the ratio. For the size frame you intend to build with this set then, it is your job to determine a matching scale if the frame is other than the standard 56cm sketch the lugset comes with. You then set your enlarger/reducer feature on your photocopier to hit the scale that matches any size larger or smaller than 56. And then you calibrate what size rims, tires, tubes and spokes you need to have custom manufactured.

You learn to TIG weld or fillet braze so you're not limited by the angles of the available lugs...

:D I knew you were gonna tell me to learn to TIG weld.

I just want that cool lugged look and i've got a frame geometry in mind with a long top tube.

this might be another pretty dumb one but, are lugs cast or could I create my own using some thin gauge copper tube and a ton of patience?

I believe most lugs are cast, but I don't build lugged frames so I don't know for sure...

Normally one would make them out of chromo tube, copper is not an option. But you would then need to assemble that with TIG or brazing. Some lugs have one lug missing to accomdate odd geo. And example would be a BB lug missing the downtube or star lugs to allow brazing. Henry James makes some.

Lugs weren't always cast they can also be formed out of sheet metal. Not sure the exact process.

Lugs typically have a little slop, or you can open up the sockets slightly to accommodate an extra degree or so. Bending lugs is also common (I’ve done this several times) to accommodate an even greater variation from the original angles.

Regarding the lower head tube lug angle, most modern lugs are using 58 – 59 degrees which works for the typical frame with tall fork (like a carbon fork). Bottom bracket angles need to be judged as well but most will work as long as you don’t go too low with the drop.

I suggest making a full size drawing of the main triangle, putting the tubes where you want them. After that, measure the angles you need. Third step is finding the lugs you need or modifying the drawing in an intelligent way to accommodate your lugs of choice. Again, the lugs can be bent fairly easily (up to about 2 degrees or so) but there is more work involved since you need to fabricate some bending bars and clamps to hold the lugs while bending.

Hope this helps.

very much, yes.

Thanks for all the info folks. :thumb:

So far, I have found that if you are trying to build a track frame that is going to have a fork with little clearance, you are going to need a different DT angle because the lug is probably made for a frame that will have a fork fitted for a brake. A friend of mine just decided to do the HT/DT joint fillet brazed and it came out looking pretty good.

Hmm, I have bought a lot of lugs from Kirk and I don't ever recall getting a 56cm scaled drawing with mine. Mine are just wrapped in newspaper and dropped in a box. Not sure what the benefit would be in a scaled drawing.

While it is true that as you extend or shorten the top tube the lower head angle changes, you can bring the lower head angle back into line by making other changes in your frame design. For example, a 73/73 road frame with a 56cm top tube produces a lower head angle of 59 degrees, assuming you have 75mm of BB drop, a 365mm a-c fork, and 7mm of lower head lug below the down tube. But, if you wanted to build a frame that was 54.5cm top tube, your lower head lug is now about 57.5 degrees. Too much difference to muscle the lug into place, in my opinion. But, you can reduce the a-c length of your fork by 3mm and reduce your BB drop by 3mm to bring your lower head lug back into about 58.5 degrees. The last half degree can be worked out of the lug with elbow grease and leverage.

This, of course, assumes you are building your own fork. If you aren't building your own fork, you would design the frame around the a-c length that gives you your required lower head angle and then accept the fact that frame angles, trail, BB height, and stand over height will be slightly altered from your original design.

Am I making sense or just rambling?

makes sense to me!

In my mind its just a big triangle and a matter of adjusting the length of the sides and the angles to get what i'm after :thumb:

Its really interesting to me because just like arithmetic Geometry, there's more than one way to change things to get close to what you want....and I'm a math nerd.


and I did consider that the downtube might just be better off fillet brazed, but only as a last option...I like lugs :)