Silver Case

The bike broke because the bike was poorly built — the weird webbing of metal the tried to link up the head tube, top tube, and down-tube achieves nothing more than making a massive stress riser on the downtube, which is exactly where it broke.

As Uncle Sheldon describes:

Stress Riser

A stress riser is a notch, crack or other irregularity in the surface of a part which creates a starting part for a crack or tear. A familiar household example of stress risers is cellophane: It is fairly difficult to start a tear in a straight edge of a piece of cellophane, but once a tear has started it is almost impossible to stop.

A similar effect occurs with other materials, including those used to build bicycle frames. Good design avoids placing stress risers in heavily loaded areas of the frame.

Stuff like this isn’t good, isn’t helpful. Look at fine lugged-bikes. The builders file the tips of the lugs down as smoothly as possible to avoid making stress risers. Contrast JP Weigle with this monstrosity.

Uploaded with plasq‘s Skitch!

Straits Dispatch held its annual Bring Your Child to the Sweatshop day today. It was a comparatively cool day, I only sweated through three tee-shirts. If I ever can do this permanently full time, I must move into a fully-airconditioned workshop.

But Luke enjoyed himself. He kept wanting to do things around the floor. He was quite keen to do sanding of the bike frame I’m working on. I was pleased to let him, as I had spent the previous three hours polishing the brazes.

I checked frame alignment of the 9Velo today. Not too bad. Maybe a millimeter twist in the headtube, but acceptable. Now that I know how to develop good fillets, I can tack the next frame more deliberately and go for a totally straight frame. But at any rate, this one is straighter than Le Chacal, and I never noticed any shimmy in Le Chacal.

Spent the rest of the day (which wasn’t that long) fussing with the chain stays. Fortunately Michel came in and said that they wanted derailleur dropouts, not the nasty horizontal track dropouts I was planning to use. That saved me from some ugly re-work.

I refilled the welding gases and then at 6:30 we had the first class of our eight evening Aerospace TIG Welding class. The “Aerospace” in “Aerospace TIG Welding” maybe gives this class a sexiness it doesn’t actually have. Aerospace in ITE’s diction, seems to simply mean that we’re going to do a lot of simplistic plate aluminum welding. Not exactly what I want to learn but at least a start. Welding butt and lap joints of aluminum plate is not the same as welding the intersections of 0.6mm-thick steel. Hopefully some training from Brew will help out on that matter.

Flammable Gas transport vehicle

Freshly-brazed triangle

Today I brazed the front triangle of Ryan’s Mini-Velo. Unlike Le Chacal, where I brazed and mitered piece by piece, on this one, I mitered and fit the whole frame and then brazed at one go.

“At one go” is an optimistic way to describe it. It took the better part of the day. I varied my technique a bit — I ‘tinned’ the joints first and then added the fillets.

Sulaiman convinced me to make my fillets thicker after I first brazed the frame. That added some time to the process, but it was good practice. I learned some tricks to build much nicer fillet mounds. Fluxing the rod itself enables me to feed it much more smoothly into the fillet mound. And I also can identify the correct color of red that indicates a fillet at the right temperature for building.

I wrapped the joints in wet rags overnight so I can remove the flux tomorrow and then measure the alignment. I am hoping for the a nice, straight frame. I added to the fillets after taking it off the jig — hope that didn’t warp things.

Small change due to gardening leave

Ryan’s Mini-Velo main triangle

I finished mitering the main triangle of Mini-Velo #1 today. I’ll braise it tomorrow morning. Using laminated mitre templates printed from BikeCadPro makes this go way faster. I’ve got four to build in total, so I need to work quickly.
In a stretch, tomorrow I’ll also do the chainstays. I measured out tire clearance today to ensure the frame will work on some large Schwalbe Marathons that are roughly 45mm.

Lots of teeth

I’m hoping these guys will want to use these sexy CNC’d single-speed aluminum big chain rings. I think they should be perfect for gearing and look great.

The Bike Cad Pro miter templates worked perfect to do the interference miter of the Seattube onto the Downtube at the Bottom Bracket. It also was fine for me to enlarge the TT/ST miter instead of building a spindle for the power sander. The miters look quite sharp. Now we just need to put everything on the jig and braze it together. Three hours work?

I’ve recently spent three monastic evenings in the Straits Dispatch workshop building the front triangle of a new bike. Our current project is a lady’s size Le Chacal tuned for mountain bike use. This has been a far more precise and efficient operation than my first project.

I learned during the first bike that Measurement and Marking is an absolute prerequisite to any hope of accuracy and control.

So this time, we modeled it in Bike Cad Pro, a parametric CAD program designed for bicycles. From this I can get accurate dimensions and angles on any part of the frame.

I put more care into scribing the lines too, using a carbide scriber (basically a steel pencil) and making the lines as thin as possible. When I’m trying for sub-millimeter accuracy, it doesn’t help to have a scribe line 0.8mm wide drawn in pencil that rubs off.

I didn’t measure twice, cut once. I measured many, many times and cross-checked measurements using different techniques. I caught a few errors this way.

Cutting miter with belt sander. (The blue lines are NOT cut markings!)

Mitering a single joint now takes less than 15 minutes. That’s big savings versus hand-mitering, which can take 30-60 minutes. Of course the belt sander is more precise now that we’ve figured out tricks of clamping and feeding the stock.

It’s pretty close to being done. Two more miters tomorrow and it’s ready to be stuck on the jig and brazed. In the meantime Sulaiman has been working on some tricky rear-triangle engineering.

On a side-note, I’m excited to be attended the 2010 North American Handmade Bike Show in Richmond Virginia in late February. The craftsmen displaying their projects there have stunning work to show off.