Old 01-01-13, 02:49 PM
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TickTockToe
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Using Paterek method to find seat tube length: How to choose a BB height?

I am designing a fixed road bike to be my first self-built custom. It will be a traditional horizontal top tube steel frame, lugged or fillet brazed. I downloaded the old paterek manual, and I'm in the process of hashing out the frame geometry right now. The manual has me starting with the seat tube length. His method for determining ST is to find inseam length and subtract from that the BB height and TT-crotch clearance, then do the whole pythagorean thing with the ST angle to find out the actual tube length. Pretty self explanatory. I know my inseam is 85.7 cm, and I want a ST angle of 74 degrees. Paterek recommends a crotch clearance of 6 cm for racing. The only thing that Paterek doesn't explain well is how to choose a BB height.

I know that I want my BB placed so that it is as low as possible, without ever getting pedal scrape on tight corners. Currently I ride a Cannondale geared road bike that almost fits me perfectly, so i've been using that as my reference. The Cannondale has 265.25 mm of BB height with my current 700x23C tires (planning on using the same tire size on the new bike), and I've only scraped pedals once so far. I like that level of BB height, and it's probably a little bit more clearance than I really need.

I AM planning on increasing from a 170mm crank (cannondale) to a 175mm (new fixie) crank length. I'm assuming I can take that original BB height (265.25), add 5 mm onto it (270.25) and that will accommodate for the longer crank. Since I probably could go lower I'm gonna turn that into a nice round 270 mm, and use that for my BB height.

Any suggestions, critiques, etc...?

Any better ways of picking out geometry? I know the old paterek manual is a little outdated (still recommends FOPS, cubit-based TT length, and a couple other relics), but I haven't found any better way of creating geometries from the ground up.
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