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Lug Design Factors

Old 11-08-19, 06:37 AM
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mikeread
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Lug Design Factors

I am toying with the design of a bilaminate frame and am trying to get my head around what I can and cannot do with regard to the shape and size of the lugs/laminate/sleeves.

Looking at the lugs you can buy, they are all of very similar design and this must be for a reason. If I am going to make my own, I don't want to just copy what has gone before, but neither do I want to repeat mistakes of the past.

So anyone with a view on these things, I would be pleased to hear your opinions.

Things I am thinking about are:

The points of a lug are always on the top or underside of the tube, why is this? I assume this is the plane of maximum stress?

Why do the lugs have points at all? I assume it is to prevent a sudden change in cross section and distribute the stress, but if this is the case, why is the underside of the lug sometimes rounded?

A tig welded frame has nothing to distribute the stress so why does a lug need it? could a lug just be a short round collar - like a big tig weld?

If the points are for looks only, why is there not more variation? I accept there are plenty of variations of ornate lugs but the basic shape is very similar on most I have seen.

Thanks

Mike
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Old 11-08-19, 07:34 AM
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Originally Posted by mikeread View Post
A tig welded frame has nothing to distribute the stress so why does a lug need it? could a lug just be a short round collar - like a big tig weld?
I think it's just as you stated with there being higher stress on the top and bottom of the tubes, where the points are. This is mostly just theoretical though and doesn't matter in the real world. I also think lugs are the shape they are because of tradition. A "short round collar" would function just fine for the application.

Personally, I like lugs that have a fair bit of surface area on both tubes being joined because they are more fault tolerant of a poor fill job and/or a gap between the tubes that gets hidden/covered up by the lug. The shorter the points (or collar as it were) the more exacting the brazing and fitment needs to be.

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Old 11-08-19, 09:44 AM
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Many head lugs are rounded on their underside, instead of being pointed, to minimize the can opening effect that this part of a lug can do when a severe front end impact happens. As the fork/head tube are bent back, hinging on the upper head lug, the lug's point can tear open the top tube at the lug point. Rounding this off reduces this, although one is left with a still messed up frame...

Given the real world experience I agree that lug shorelines have less to do with structure then aesthetics. Shorelines are one way to brand a bike, remember all those Peugeots with their black and white stepped lug edges? Bruce Gordon made a series of little holes his "look". Andy
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Old 11-08-19, 02:13 PM
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There are frames out there being made with "bikini" lugs that simply have no points on them, but shadow the butt of the tubes being joined. The most extreme I have seen is 10mm of lug for surface area on the top and down tubes. Not something that I would personally ride, but I am sure if done right it will be fine for most riders.
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Old 11-08-19, 04:09 PM
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Originally Posted by TiHabanero View Post
There are frames out there being made with "bikini" lugs that simply have no points on them, but shadow the butt of the tubes being joined. The most extreme I have seen is 10mm of lug for surface area on the top and down tubes. Not something that I would personally ride, but I am sure if done right it will be fine for most riders.
Logically what is the difference between 10mm of lug and a 10mm fillet?

It is a funny thing, a 10mm fillet would look fine to me but I would be wary of a 10mm lug. It must be what we are conditioned to expect.
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Old 11-08-19, 07:35 PM
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Serotta is an example of there being points at 3 and 9 o'clock on the DT and scoops at 6 and 12 o'clock. Notice it's also a nice place to put a visible 'S' so Serotta's design could also be marketing related. I agree with Andrew that having the points at 6 and 12 o'clock is probably more style than structure. Maybe this is how they achieve vertical compliance while maintaining lateral stiffness.




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Old 11-08-19, 07:35 PM
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A few thoughts and overheard guidelines.

Is the size of a fillet the length along a tube's surface or from the tubes' corner to the concave point closest (this "thickness" of a fillet called the "root" size by some)?

I have read of a 3X dimension for a weld fillet to be as strong/still as the tubing is, as in 3 times the tube wall. But is this also for brass fillets? Is this a "root" or a tube surface dimension?

For bonded joints I have read of figuring out the interface surface which will be bonded and do the math with the bonding agent's tensile strength. Compare to the tubing strength.

I have always considered a lug to be of massive over design when it refers to a socket depth/tube strength relationship. While the long and sexy lines of a well done long point can be intoxicating, add cuts outs and watch out, the brief skimp of a bikini line can be breath taking. Andy
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Old 11-08-19, 07:56 PM
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Originally Posted by Andrew R Stewart View Post
Is the size of a fillet the length along a tube's surface or from the tubes' corner to the concave point closest (this "thickness" of a fillet called the "root" size by some)?

I have read of a 3X dimension for a weld fillet to be as strong/still as the tubing is, as in 3 times the tube wall. But is this also for brass fillets? Is this a "root" or a tube surface dimension?
Andy
I can't find it right now but, Bontrager did a study of fillet size and joint strength. What most of us consider a 'correct sized' fillet is much larger than what he found to be optimal. My understanding was that the greater mass of a big fillet cools slower and expands the HAZ as well as lets the steel cool slower causing changes to the grain. There are certainly a lot of bikes out there that have many miles under their big flowing fillets. It might be more theoretical than practical. I have been aiming for slightly smaller fillets than when I first started.
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Old 11-09-19, 09:36 AM
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that's really interesting, maybe he was aiming at Ritchey with his huge fillets.

Those Serotta bb shells are pretty weird. Just thinking about the failures I have seen on seat tubes, I might make the lug go up the side of the seat tube. I don't recall ever having seen a down tube failure, but I think seat tube/bb failures are the single most common failure that I have seen.

I think that a straight shoreline on a top tube would probably okay, on a down tube, I think that would be a bad idea because all the cracks I have seen on the down tube just go straight around.
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Old 11-09-19, 01:35 PM
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Here are my suggestions for creating the design on your bilaminate. If you email me I can send you a lot more detailed instructions on how to design and cut out a blank lug that will mostly transfer over what you are wanting to do. It is intended to be an aid for those that buy my blank Nikko lugs but the principles also apply to cutting out bilaminate tubes or even head badges. I find that doing the drawing at 200% size makes it much easier to draw and correct. Furthermore when you reduce your drawing down to actual size it makes the lines more precise.

I like to use Vellum paper because it is somewhat translucent and is a lot stronger than regular paper and can be erased many times. The translucency helps when your design is folded in half to copy the same on other side. Start by using a miter program to print out the outline of the shape of how one tube will fit another at an angle when it is flattened out. These programs are popular with hobby builders that don’t have a mill or lathe and use hand files to miter their tubes. Wrapping the template around the tube serves as a filing guide. Bicycle builders say “miter” and other industries say “coping”. Nova Cycle Supply has a link to a miter program and there is another one here: Tube Coping Calculator

I use a variety of tools assist in drawing the design. They include circle and oval templates, a compass, multiple erasers and rulers of all types. I like the clear ones. I prefer using a mechanical pencil with templates. Once I think I’ve finished, I set my copier at 50% to print out the design actual size. I cut out and place it on the tube to see if anything needs to be changed. Once I have finalized my drawing, I print it out on sticky backed label paper I buy at office supply or art stores. I prefer if it is clear so I have an easier time placing the label paper on the center line I drew on the tube (or one that was created from the miter program). Of course just paper with the design can be glued onto the tube. Rubber cement is a little easier to use than contact cement because it can be moved a bit during placement but it doesn’t stick as well during cutting. If I am using glued on regular paper I like to put clear shipping tape over the design before I cut out the outline so it is less likely to get smudged while handling during cutting.

My main tools for cutting out the design is a jeweler’s saw and files. I drill out areas that are semi-hoIe shapes before cutting. I cut close but not exactly on line and then file to the line. Dremel has come out with a new slitting blade that can hog out blank areas or create paths to make saw blade cutting easier. They work a lot better and faster than the old brown disks that came packaged in tubes.
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Old 11-10-19, 06:27 AM
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Thanks Doug and I may well take you up on your instructions :-)

I am not having much joy sourcing the Cromoly Tubes, it seems nobody stocks the sizes I need and are not interested in ordering in small quantities. I will keep trying.
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Old 11-10-19, 12:35 PM
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Originally Posted by unterhausen View Post
that's really interesting, maybe he was aiming at Ritchey with his huge fillets..
HAHA, that's exactly what I was thinking - Bontrager was a TIG guy so, why is he studying fillet brazing? <wink>
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