Bob Brown Cycles : services

Framebuilding Process

This page is an explanation of the process I go through building a typical lugged frame. This one happens to be a lugged 29" mtb, but the process is the same for most lugged frames. This page may take little longer to load than my other pages as it's picture intensive.

The whole thing starts with a customer placing an order. Once that happens we talk quite a bit about their intended use and fit. From there, I design the frame on an Excel spreadsheet. The spreadsheet is a modified version of Mark Bugier's frame design spreadsheet. I've changed it quite a bit to provide me with measurements specific to my jig system and my tube mitering system. All the information I need is contained in this spreadsheet, no drawings are produced.

Once the design is complete, I specify the lugs and tubing. I need to know the joint angles before I can decide on lugs and I need to know actual tube lengths before I can spec the right tube for the application.

Lugs are the first thing I usually tackle. All lugs need prep work, these got quite a bit of carving done as well. This bike used Richard Sach's Newvex lugs. On the left is a picture (borrowed from Richard's site) of the lugs as cast. On the right are the lugs after I carve them up.

Once the lugs are all carved up, sanded and ready to go, I can start mitering tubes. I use the machine pictured on the right to cut my miters, I can set the angle of cut on the large rotary table and then just feed the tube right into the cutter. This mill set-up is very rigid and the cuts come out spot-on every time.

The spreadsheet shown above gives me all the miter sizes and tube lengths that I need to cut the whole frame right now

Once the main tubes are mitered, I file the ends of each tube to de-burr them, then begin test-fitting them into the jig. I always start with the BB shell in my jig, then fit the seat-tube. Next I fit the top-tube to the seat-tube, then the headtube, and finally the downtube.

Once I confirm that the geometry is correct and the tubes fit properly, I take it all apart and sand all ends of the tubes inside and out and then flux them up.

Here's the front triangle all set up in the jig, ready to tack. Next I'll braze one small part of each lug to tack the frame together, then I'll remove it from the jig and fully braze it in a workstand. Once it cools from brazing, I soak the whole thing in water to remove the flux.

At left is what a lug looks like just after soaking. This picture doesn't show any excess silver since I fed the lug from the other side, but next I'll clean up any silver "flash" left from brazing with an abrasive blaster.

Now it's time to work on the rear triangle. On this frame I had to start by bending the chainstays to the appropriate shape for tire clearance. I don't need to do this on most frames but 29'ers often need more tire clearance than I can get with stock parts.

Once the stays have the right shape, I slot the ends for the dropouts and braze in the dropouts. I do this with brass as it fills the space between the dropout tab and the chainstay better. Once this brazing is done, they cool and the flux is soaked off.

Next I have to make the scallops and shape the dropouts. This is one of those small details that really can make a frame look nice. the scallops must be just right with no undercutting into the dropout. I do this with a Dynafile abrasive tool, which cuts very fast, so extreme care must be taken to remove only what needs to be removed.

Once I form each scallop, I shape the top and bottom of the stay-dropout to the desired shape. On this one you can see I filed in a small scallop on the underside of the dropout tab.

Now that the dropout to chainstay joint is all shaped, I like to clean up the dropouts themselves. Most forged dropouts have marks on them where the forging dies meet, so I hand file these to nice square edges. Then I file the window out so it's nice and clean.

There's a lot of detail work that goes into making a nice classic looking dropout. This area is probably the most time-consuming part of my framebuilding process, but if you want it to look good with paint, it needs to look good bare.

The seatstay treatment is next. This frame used side-tacked, scalloped seatstays, which is my most common method of doing it. I make my scallops from scratch and don't use plugs here. It's another nice touch and makes the frames lighter.

I start off by filing the end of the seat-stay to a very shallow miter. They're sized to fit the scrap tubing sections I've cut up to make the scallops. I use tubing from 19mm to 25mm to make the scallops depending on the seat-stay diameter.

Once the tubing if fitted to the stay, I braze it in place with brass. No jigs needed, just flux it up and hold the stay in a vice, gravity will hold the tube scrap in place.

Once it's brazed, I grind to to a rough shape and finish file it down to the shape you see on the right. This one happens to be a 19mm stay with 25mm tubing for the scallop. On smaller stays I use smaller diameter tubing to keep things looking proportional.

Now it's time to join those seat-stays to the rest of the frame. I cut them to the proper length, then file slots in the chainstay end of the stay. Then I file a small groove on each side of the seat-lug where the stay will contact it. This provides more surface area for the braze and makes the joint much stronger.

Once all that is done, I mount the stays and braze them in place. The dropout end is done exactly as described above with the chainstays. The seat-lug end is just silver brazed right to the side of the lug as shown below:

The main portion of the frame is done now, all that's left is to clean up the joints (abrasive blasting) and to install the braze-on's. I usually install all the braze-on's next : bottle bosses, cable stops, etc... then soak the whole frame one last time, then blast it all.

Once that's done, I build the fork, and in this case the matching stem in the same fashion (shown left).

Once all the fabrication is done, I do an acid etch on the frame and begin the painting process.

This frame had a very unique paint job. The "chrome" on the lugs is actually paint and the blue main color is a candy that is applied to the clearcoat.

After primer and sealer, I had to paint the whole frame silver, then I clearcoated the whole frame with the candy blue. The frame was then baked overnight to cure the clear. Next the clear on the lugs had to be polished out to a perfectly smooth finish and then all the tubing around the lugs was masked off, leaving just the lugs exposed. The downtube logo was done in the same manner.

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Once the hours of masking were complete, I was able to spray the "chrome" finish on the lugs and bake that paint overnight to allow proper off-gassing. Once it was baked, I polished it out and resprayed any areas that did not look perfect and baked again.

Finally once I was satisfied with the appearance I removed all the masking and sprayed the final clear.

The final clear is un-tinted and protects the chrome finish as it would any other paint. It also brings out the deep luster in the metallic.

And finally, it's a completed bike: