I can’t tell you how many times over the years I’ve seen webbernet ‘experts’ say with conviction that the tube to tube miters on a lug bike don’t need to be tight because the lug covers them up. I’ve even read that lugged builders choose lugged construction because it allows them to cover up their shoddy mitering work with the lugs. Considering that most of these ‘experts’ have never made anything more complicated than a fist it can be frustrating at best to see just how sure they are of themselves – when in reality they don’t even know how lugged construction works.
So, for those that are interested in learning how this actually works I’ll give you the Cliffs notes version.
A frame lug is a cast piece that the tubes slide into that gives a large surface area for the tubes to be stuck to one another. The lug does provide a small amount of support for the joint but it’s main job is to give a large surface area for the filler (usually silver but can be brass) to bond to. This is what keeps the two tubes stuck together. The tubes have a very close fit into the lugs and it’s this close fit that allows for the molten silver to flow between the two parts with capillary action. If, unlike me, you paid attention in high school physics class you will remember that a liquid will flow between two surfaces if they fit closely together. Picture two panes of glass and how water will flow even uphill between the two panes and you get the idea.
It’s this capillary action that allows the builder to flow the silver into the joint, but remember that this capillary action requires a very tight fit because if the silver is flowing through the joint and hits a gap where the fit isn’t tight it will stop dead in it’s tracks. This is where the tight and accurate miters come into play. When the builder brazes a lugged joint he will heat the lug and tubes up to the point where they are warm enough so that the silver will melt and run between them like the water between the panes of glass. Let’s consider the top tube/head tube joint – in this case the builder will apply the heat and then add the silver rod to the edge of the lug where the top tube enters. He will then use the heat to get the silver to flow between the top tube and the lug until it meets the head tube and then he will flow that silver out to the edge of the lug where the head tube inserts. So he is flowing the silver in one direction, straight through the lug. This assures that the entire space inside the lug is filled with silver and that the area where the top tube contacts the head tube hidden inside the lug is fully brazed.
If the fit between the top tube and head tube isn’t perfectly tight and consistent the capillary action will stop and the silver will not flow all the way through leaving the joint with voids. This will result in a very weak and incomplete joint. This is why a very tight meter is needed. Some will say that you need a very tight miter to TIG weld a joint but that since the lug covers the joint on a lugged bike that this isn’t needed. Those people are talking out their butts and know much less than they think they do. In fact I would say that a skilled tig welder will have no trouble filling a small gap in the miter if needed while a lugged builder just won’t get this to happen inside the lugs without the capillary action.
You can see in the photos just how tight the miters should be for a proper lugged joint. The two tubes are fit together without the lug first to be 100% sure the fit is just so and then the tubes are loaded into the lug and brazed. I dry fit the entire front triangle first into the jig to check all the miters before the lugs get anywhere near the bike. Once the tube fit is right then the fit of the lugs is checked and then once that is done everything is cleaned, fluxed and brazed. You can see a few photos of the upper head tube lug with the top tube inserted. You are looking at the mitered edge of the top tube where it will meet the head tube. This will give you an idea of the fit between the two parts and what the miter itself looks like. You’ll also see how the mitered joint looks from the outside sans lug and what the whole thing looks like with the lug slipped into place.
I did the mitering and lug fit for the pictured frame yesterday and today I will clean it all and braze the front triangle together. Since the miters and lug fit is so tight it will be easy to braze and I look forward to it. I’ll take a few photos today and post them later.
Lesson’s over. Get to work!
Thanks for reading,