Though lugs are the traditional way to build bicycle frames they are somewhat out of fashion. It is certainly quicker to build using a tube to tube method, and a lot more flexible for custom designs, but lugged frames have their own advantages and traditional beauty when you consider the contrast between the lugs and the rest of the frame, polished stainless steel being my personal favourite. Traditional chromium plating is no longer done on new build frames as far as I can tell, due to the cost, complexity and possibly toxicity of the necessary process. It is still done for restorations. Lugs also produce a strong frame, and I would contend more forgiving in fabrication.
The main problem with lugs to my mind is the restriction on frame angles. With modern techniques such as 3D printing, this should no longer be the case but dare I say it is still a long way from practicality for the average builder.
At present most builders still seem to be using cast lugs from a limited number of suppliers. Pressed / stamped lugs can still be found but are very uncommonly used. I am only aware of Llewellyn bikes producing oversized lugs for purchase and so the range of tube sizes one can use is also limited. These, however, are also available in sloping top tube designs, previously not possible with traditional lugs.
The main limitation to lugged design is the fixed angles, and there are a relatively few of them. The first frame building book I ever read suggested you are stuck with that and you have to adhere to the angle of whatever lug you are using. That is not strictly true. I understand it used to be common practice to adjust the angles of lugs by heating them up and inserting long steel bars into them and simply bending them to the desired angle. I am not sure what the limits of this would be. I would love to have a go, but when I considered it I was put off by the cost of a selection of long steel bars!
Tim Paterek in his frame building manual states he has reduced the down tube / bottom bracket angle (The one most likely to be needed) by as much as 5 deg. This involves using a die grinder (like a Dremmel) to alter the shape of the top of the down tube port then tapping the underside projection of the bottom bracket to close the gap created. You do loose some of the tang on the top. I have done this (once) but would not have been confident with more than 2 degrees. I also found it easier to tap the underside of the bottom bracket into place during brazing when the metal was hot.
A newer idea to simulate lugs is commonly known as the ‘Bi-laminate’ technique. Whilst I have tooled up for this I have never tried it in its pure form. The method involves brazing on a sleeve over the ends main tubes (lug shaped) and then mitering and joining the tubes by fillet brazing. This effectively means there is no restriction to the frame angles. The disadvantage for me is if you use stainless steel sleeves and wish to produce a polished lug effect you will always be able to see the braze line.
Semi-lugged designs
These can give you a lugged aesthetic without too much restriction. I have used head tube and seat tube lugs together to enhance designs. This enable you to fillet braze the bottom bracket. You can also still obtain bottom brackets with chain stay ports only, enabling you to fillet braze the seat and down tubes to the bottom bracket at any angle. combining that with a couple of bi-laminate sleeves and no one would know it wasn’t a fully lugged frame.
BikeCAD and lugged design
In the final analysis I try and design the frame around the actual lug angles, and this is where CAD programs are useful due to their ability to constantly adjust. I have only ever use BikeCAD, probably the most popular and one of the very few bike design specific software programs, as it is fairly intuitive for computer dummies like myself. The problem with BikeCAD is it DOES NOT assist you in designing lugged frames. You have to design your frame and make sure the angles are within the parameters you set. I have always tried to make sure the frame angles are within 0.3 deg, which seems to give no fit problems, the lugs fitting together as per plan. However on reviewing some of my old designs I note that I have managed with up to 1 deg of discrepancy at one lug, provided the other 3 angles are within 0.3 deg. I do not know if you can get away with anything greater discrepancies without having to modify the lugs themselves. I would love to hear from anyone who has. Having completed your design the program allows you to overlay a lug which is purely aesthetic (see header drawing).
Building a lugged frame
My sequence of building a lugged frame is as follows:
1. Create or draw your design, starting as usual with the forks, whose dimensions will determine the frame dimensions, along with personal preferences for angles, trail, wheelbase, mudguard clearance etc.
2. Fit the lugs and bottom bracket to the tubes .i.e. make sure the lugs are a sliding fit on the relevant tubes. It is best in any case to get the mating surface down to bare metal. Make sure the lugs are clean and I would file and sand off any significant casting marks, and, if aiming for polished stainless steel finish, file and sand smooth down to 400 grit emery cloth.
3. Measure, cut and mitre the main triangle EXCEPT the top tube, though you could mitre the head tube end initially. Mitering is up to personal preference. Paper templates are a great help, and a few computer programs are available to generate them. BikeCAD generates a full set from your design. You can machine mitre on a lathe or milling machine, with or without paper templates. You can mitre by hand, with or without paper templates, the latter being the most skilful. Another alternative for lugs is to mitre to a line drawn around the inside shape of the lugs, confirming with trial fitting.
There is no obvious need to mitre the seat and down tubes as they enter the bottom bracket, they can be allowed to poke in a way. However I do mitre them, still allowing them to poke in, just to make any final grinding down easier. Some would rely on the final thread chasing as sufficient as the cutters would usually cut through any prominent tubing. In the end I like the inside of my bottom brackets to look tidy.
4. Measure, cut and mitre the chain stays. These are easily mitred by marking inside the bottom bracket as, like the seat and down tube the appearance in the bottom bracket is not critical. Most time is spent on making the fit in the sockets as good as you can so it is best to work on the bottom bracket end before cutting down the dropout end to fit. You can now go on to cut and mitre the seat stays, deciding beforehand what type of seat lug attachment to create and making sure both ends are a good fit.
5. Dry assemble all the tubes to check it is all correct. This assumes you are using a whole frame fixture (Jig). Then dry assemble with the lugs added to make sure it all feels right. This is where you find out if your mitre angles were good.
6. Drill appropriate vent holes after working out where the air can escape when brazing, Lugged frames have an advantage here that the bottom bracket is open. I have got away with not putting a vent hole in the bottom head lug but invariably need one in the top head lug.
7. At this point I set up bottle cages with straps and mark the holes for bottle cage bosses.Bottle cage boss spacing is standard, but the relation of the cages to the boss is not! Drilling bottle boss holes before assembly is strongly recommended and they act as additional vent holes.
8. Prior to brazing ensure all components are clean and mating surfaces should have been cleaned and abraded. I wipe down with surgical spirits, which is cheap and available. There also needs to be no grit left on the surfaces. Acetone is another traditional cleaner. For silver brazing cleanliness is critical. It is alleged it is less important for brass brazing.
9. My brazing order is 1. Seat tube to bottom bracket, 2. Head tube to down tube (important because the orientation of the down tube against the head tube is critical whereas through the bottom bracket slight rotation will not be a problem. 3. Down tube to Bottom bracket 4. Chain stays to bottom bracket. 5. Rear dropout to chain stays.
10. Then cut and mitre the top tube, or if you have already done the front, do the seat tube end to fit. Tap it into place with the lugs to check fit. The orientation of the top tube is most critical as it has mitres at both ends. It is important to mark an accurate centre line for orientation of your mitres.
My reasoning for delaying the top tube cutting is that unless you are brazing the whole frame in one go (see below*), there is likely to be fractional movement in the joints leading to a slight lengthening of the top tube measurement. Even a tiny amount can affect the position of the top tube. It also would save having to mess about trying to cold set the frame before fitting the ‘already cut’ top tube.
11. Braze the top tube.
12. Braze the seat stays, usually dropout end first.
13. Cut and mitre the brake bridge and braze in, similarly a chain stay bridge.
14. Lastly braze on brake and gear cable bosses and gear lever bosses if required together with bottle cage bosses in the pre drilled holes.
Can I braze the frame all in one go? *
Well yes you can. Though I have never brazed a lugged frame in this way, the technique is apparently go around the fully assembled frame (including top tube) brazing the lug and bottom bracket tangs. This fixes everything in place just like the initial tacking of a fillet brazed frame. Following which you can fully braze up the frame in the same order as above.
Non frame jig methods.
I am disappointed to say I have never not used a full frame fixture to build a bicycle frame. It is possible to build in sections, using your preferred method to fix each section for brazing. I imagine I would use the same order of doing things. The full table method (a traditional one, and advocated by the Paterek manual) involves brazing with the frame sections lying on a table suitably supported by different height spacers and building up the frame that way. In fact flat surface tables with fixtures for fixing the frame down were once popular and can be home made.
Another traditional way is to pin the frame together using small square nails through holes drilled in the lugs, and then braze it of course!. Once widely used in the UK but little used now. Frame nails are still available from suppliers.
