Fillet show bike.

I think it was about 1985 or so when I got my first fillet brazed frame. I’d been racing BMX for years and was moving into mountain bike racing and ordered a custom Fisher. I didn’t know at the time that Fisher had contracted with the great Albert Eisentraut to build their custom bikes. I drew up the design (my first ever) and sent it off and some time much later the bike arrived. I’d never seen fillets like them before — so smooth and organic — and I fell for them. I knew little about how this kind of thing was done but I knew I wanted to be able to do it.

Flash forward a bunch of years and I now build my own fillet bikes and I’m proud to say that I’ve won the ‘Best Fillet’ prize twice now at the annual NAHBS event. I’ll be bringing a fillet frame to NAHBS again this year and once again it will be shown in the raw — sans paint — so people can see what they are really like in the flesh.

I thought it might be fun to document the building of the frame in this space and this is the first installment.

Before the actual construction of the frame starts all the tubes need to be laid out and inspected. I check to be sure that all the tubes are straight and that they weight what they are supposed to weigh (if underweight I know that the wall thickness is off). I also check for simple things like dents or defects at this time.

The building of a fillet frame starts just like the building of most any other frame with the setting of the jig and the mitering of the tubes to fit. I use a software program called BikeCad to produce dimensioned drawings that are used to set the jig as well as giving the lengths and angles the tubes need to be cut to. I feel strongly that it’s very important to get very tight and accurate miters of the tubes if one expects to get a straight frame. If the miters are not tight then it leaves room for the joint to ‘pull’ when the brass is added and cools. If care isn’t taken the frame can end up anything but straight and it can take some serious cold-setting to try to man handle it into alignment. I aim to do no cold setting so the miters need to be tight.

Once the jig is set and the tubes are mitered and set in place I’m ready to start hooking stuff together. Before I can braze anything together I need to clean everything well, both inside and out, and rough it all up using 80 grit emery cloth. This will ensure that the brass will ‘wet out’ as it should and that I get a lifetime bond. With everything cleaned and etched I set up the seat tube and BB shell in the jig and ‘tin’ them. Tinning is the first part of the fillet process and it’s just a simple brazing operation where I flow brass into the joint and space between the tubes. In this case I braze the seat tube to the BB shell and the pinch barrel to the top of the seat tube (I’ll be attaching the fastback seat stays to it later).

While this is cooling I prep the rest of the main tubes and get them ready to add to the seat tube. The tinning process is just the same as it was with the seat tube to the BB shell. The whiteish paste covering the joints is brazing flux that is an acid that cleans the tubes as they are heated and allows the brass to bond to the steel as it should. The rest of the tubes are loaded into the jig and I completely tin each of the joints one at a time. Once done, they take just a few minutes to cool to the touch.

Once cool the front triangle is removed from the jig and placed into a Park repair stand that allows me to spin and position the bike as I’d like to do the brazing. It gets a fresh coat of flux and then I start brazing. The actual laying of the fillets takes about 35 minutes or so. It always seems like it takes no time at all as I end up so focused while it’s happening. The laying of the fillets becomes one of those autopilot kind of things where I run on instinct and little conscious thought occurs. I really enjoy it.

You can see in the photos what the fresh and still hot joints look like. They are covered in a glassy coat of melted flux that makes it a bit harder to see what is going on. Once cool the joints are soaked in a very hot water/corrosion inhibitor solution to soak the flux off. This takes awhile — maybe 20 minutes per joint. This is a good time for an email check and bathroom break before getting back to it.

Next come the time consuming part — the finish work. I do it in stages starting with the most aggressive and getting milder as the process goes on. You can see the stages in the photos starting with the raw joint and ending with the fully shaped and polished joint. The first stage is roughing the shape with a hand file. If I’ve done my brazing job well there is little to do here and this goes very quickly. Next is ‘file backed emery’ which is just like what it sounds like. I use an old worn out file to support 80 grit emery cloth to further refine the shape of the fillet. Next comes the use of the ‘3rd thumb’ tool. Yes it’s a high tech device consisting of some rubber tubing shoved over and old round file. It’s soft and allows for a smooth contouring of the fillet. This is now more a polishing act and less of a shaping one. The next to last step is to use narrow strips of emery cloth to further polish and blend the joint and the final step is to use a rotary brush in a drill to give the joint some shine. Frankly I don’t really care about it being shiny but the shine makes any defects really stand out so I can go back and work them some more. The various joints take different amounts of time to finish. A simple head tube joint takes about 12 minutes from start to finish. There is lots of room tom work on these and easy access so it goes really fast if the brazing is clean. The bottom bracket can take a sold 40 minutes or so because some of the areas are harder to get at and are concave in nature.

You can see in the photos what the finished fillets look like. It’s pretty damn hard to take good photos of the finished and polished fillets as there is nothing for the camera to focus on but you’ll get the idea.

You no doubt noticed that I’m building just the front end at this point and that it has no braze-ons. This is for purely pragmatic reasons. I work on the front end first and then add the rear because it makes it much easier to work on. It’s just a much smaller and easier to handle thing to deal with in the vice. I’ll leave the braze-ons off until the end for safety reasons. When I am doing the work with the strips of emery cloth it is being pulled hard and fast over the joints and it sometimes breaks. I can tell you from personal experience that if it breaks and I rake the back of my hand over a split cable guide things get really messy, really fast. So I leave them off until all the fillets are finished.

The next step is to prep the tubes to be used in the rear end of the frame and to add them to the front. That will be our next installment.

For what it’s worth — this frame is being built just fort the show and does not yet have a home. I’ve included the BikeCad image so you can see the sizing. If you think it might work for you and are interested in purchasing it please contact me. Once the show is over it will go to Joe Bell’s for the owner’s choice of paint.

Thanks for reading. I’ll be back with more later.


This entry was posted in Bike, Process.  

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4 responses to “Fillet show bike.”

  1. Dwight Kellams says:

    Since you and Carl Strong have such a good working relationship, how about Carl building a titanium frame with Terraplane seat stays? Is that possible? Would it enhance the ride?

  2. kirks says:

    Hey Dwight,

    Carl and I are indeed good friends and I think he knows that if he wanted to use the Terraplane design all he would have to do it ask. That said I doubt he ever would. Carl builds very nice bikes of his own design and my guess is he feels, like I think I might, that it’s best to use your own designs.

    Are you in line for a Strong Ti?


  3. John says:

    It is always a treat to visit your site and see the detailed things you share.

    Being a decent fabricator, and a decent photographer myself, I appreciate the art you display.

    Keep it up!

  4. Dwight Kellams says:

    No, I was reviewing your website and starting to leave and go to Carl’s website when I got the idea for a Strong titanium with Terraplane.

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