First Day of Frame Building class: Design.

This is the first of an ongoing series of posts on a frame building course I took through master builder Doug Fattic. To see the whole series, click here.

Monday August 3, 2015

For the first day of class we went over the basic history of frame building, especially in America. Doug Fattic and a number of other young men swam to England in the 70's and apprenticed with the famous frame builders over there. Doug cut his teeth at Ellis-Briggs, but explored a lot of the other builders too. When the Americans came back home they set to work refining the process, sharing information, and building custom bikes. Some of them seemed to do quite well for themselves too, which isn’t hard to believe since they were the only game in the states. Doug says a master-built frame sells for around $3000, and an artisan-level master frame sells for up to $7000. Doug's assistant Herbie has gotten that much for his Helm brand bikes. 

After the history lesson Doug put me on a fitting bike and we figured out my ideal position. It’s not too far off from what I’ve been riding for years, except with a taller head tube to ease the strain on my aging back.

Once we figured out the basic measurements we set them to a fixture so we could see what the frame would look like in real space. BikeCadPro, a computer program, does the same thing, but Doug’s method lets us see how it will actually work, plus I got to try on different lugs to see if they’d fit my angles. I went with the Henry James lugs because they’re simple and fit the job.

Aside from the basics of geometry and fit, like finding the ideal top tube length and head tube height, you also have to consider bottom bracket drop. This is the distance between the center of the bottom bracket, and an invisible line drawn between the two hubs. Most production bikes have a drop of 7cm. This is because they have to assume the bike will be ridden by somebody who pedals through corners with 175mm cranks and platform pedals. The higher BB reduces the likelihood of pedal-strike. I, on the other hand, use shorter cranks and clip-in pedals and I rarely pedal through corners, but I do hop curbs. Although Doug suggested a 8cm drop, I went for 7.6.

The way to find the BB drop is by subtracting the radius of the tire (circumfrence divided by Pi, divided by two) from the desired BB height from the ground. For my 28mm tires I had a radius of 340mm and a desired height of 265mm, so my drop was 75mm.

My headtube angle is about 73, which is normal for a sport-tourer. Headtube angles go from 70-75 degrees, the steepest being for track bikes and the shallower is for Dutch bikes.

One interesting thing I learned is that there is a crucial position the bottom of the headtube must be in, relative to the ground, for the frame to work right. To find this position, take the radius of the rim, add that to the extra room for brake clearance, add the fork crown thickness (brake hole to crown race) and the lower headset’s stack height. The sum tells you how far from the ground the bottom of the headtube should be. I've replaced forks on bikes before and noticed that they didn't ride quite right, but now I know why. 

Here we are messing with the fixture. We pulled the seatpost straight out of the fitting bike, with the saddle on it, so that we could get an idea of where it would go in relation to the handlebars with a 100mm stem, which again, is symbolized by just a regular piece of tubing set in the spot where the headtube would be. The refinement comes later. 

The headtube angle is also important. Although headtube angle affects the handling of the bike, it is also important for determining the ideal weight distribution. The shortcut is that when you have just enough toe clearance, you probably have good weight distribution.