Building a Bike Frame

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Last year, I enrolled in the frame building class offered by United Bicycle Institute (UBI).   I registered for the lugged/fillet brazing class (of course) but they also offer other courses in TIG welding and titanium frame building.

As it turned out I was the oldest member of the class and the only woman in the class (including the two instructors).  That wasn’t so daunting as was the quick pace of the class – and it caught all of us off guard!

UBI shop facilities in Portland, Oregon

I wanted to build a frame to my own specifications and size because I haven’t ever ridden a frame that is exactly to my liking.  Many shorter riders probably have never experienced the joy of riding a properly designed frame.  I have seen smaller riders with their arms straight out, riding frames with too long top tubes, inappropriate 700c wheels, and very bad front end geometry.

Having ridden hundreds of bikes over the last 10 years, I had a strong feeling about how to design a bike frame to fit my 5’4″ height and riding style. I wanted a frame built for 650b wheels, with steeper angles, plenty of fork rake to reduce wheel flop and trail to an acceptable level, long enough chain stays for good sized rear bags, and enough front/center distance to eliminate toe overlap – all in a frame small enough so that I could stand over it reasonably well.

I have often ridden bikes that were slightly tall for me, so I have never worried a great deal about standover height. The most important frame measurement is actually the top tube length.  And, I have come to learn that I like steep angles so that I can get more of my body weight on top of the cranks and closer to the front end of the bike.

Much is mysterious when it comes to bike frame geometry, and much is disputed, even among the experts.  My own personal experience tells me that, for the type of riding I do mostly (commuting in Portland, Oregon and longer weekend rides), I needed a frame with very stable slow speed handling, but decent cornering at high speeds.  This translates into a bike with low wheel flop and fairly mid range trail.  My frame geometry, noted at the bottom of this post, yields a wheel flop factor of 11 mm and trail of 39 mm.  Just about perfect.

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After spending the first day learning flame control and doing practice brazes with silver, we  began by brazing the head tube to the top tube.  Silver is used for the lugs because it can be brazed at a lower temperature so there is less risk of overheating the main tubes and weakening them.  In the midst of that we needed to begin our full sized drawings so that we could properly select, cut and miter our tubes.

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Doing the full sized drawing came very naturally to me, but was difficult for some of our fellow students.  Harder for me was the flame control and brazing process.  It took awhile to believe that not only was the shop not going to explode when I ignited my flame each morning, but that my fellow students were NOT going to burn the place down, either.  I was a bad and slow brazer initially, and it took quite a while to get the hang of it without destroying my hands with flux and lug filing (still, my hands were a mess at the end of the class).

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My favorite day was “fork bending” day – a process which proves that frame building is as much art as it is science (with a little luck thrown in).  Forks are bent on a mandrel, and mandrels can come in different shapes and sizes.  There was only one mandrel at our class, so our fork blades would only vary by the amount of rake we selected.  There is no gauge or  measurement to insure that you get the right fork rake when you bend it (or “wang” it, as I am fond of saying).  Fortunately, I managed to “wang” my fork blade to the exact amount of rake I was looking for – 60 mm – on the first try.  Whew!

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My fork dropouts are a mess – this was our first brass braze and the process on the dropouts is slightly different and with higher heat.  Fortunately, my skills improved on the chainstay dropouts, although I did get the tubes a bit hot.

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I’ve got the my chain stays in and I decided to use these “plugs” for my seat stays rather than hand making a seat stay attachment, as some of my fellow students did.  I was behind schedule, so had to proceed full steam ahead.

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Here are the plugs, which I have brazed to the seat lug and bent inward to wrap slightly around the tube.  Then, the brake bridges need to be measured for the proper wheel size, mitered and then brazed.

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Here is the completed frame.  It has several mistakes that need to be corrected – I brazed the downtube shifter bosses askew and the seat stays are not perfectly aligned.  The former can be corrected by re-heating the bosses and re brazing them, the latter is a small enough variance that I can fix it by doing some creative filing.  Then, all the joints and brazes need to be filed and cleaned up before the frame can be painted.

My frame varied only slightly from my original drawing:  my seat tube angle ended up slightly slacker than 74 degrees. Not a bad result for a first time effort!    Here are the specs (all measurements center to center):

ST 50 cm, TT 53 cm, BB drop 71 mm, ST degrees 73.5, HT degrees 73, Fork rake 60 mm, Fork length, 367 mm,  chainstays 441 mm, wheel size 650b.  Standard diameter tubes – Kaisei 4160 Cro-Mo double butted.

If you are interested in taking this class, and if you haven’t brazed before or used shop equipment, you might want to find a way to get some background first before enrolling.  While we all managed to complete our frames, we didn’t get to complete the final process of learning how to file, sand and prep our frames for painting because the class, as a whole, was too far behind.  The class proceeds at a very fast pace, so it’s best to be rested and have nothing else going on in your life while attending – you’ll be exhausted each day – but energized by the new knowledge and skills you are gaining.

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