Originally Posted by
PromptCritical
I've had my road bike fitted, which was a huge help. However, I had to use a long stem extender, so it is a bit of a frankenbike. Making a frame that will fit me will involve some geometry changes, and I want to make sure I know what they will be before I build the frame. Besides, I'm a recovering engineer, so I need to know everything!
The problem with your request is that if your primary concern and controlling factor in frame design will be based on a comfortable fit and that will involve some compromises to a frame design. Those compromises will result in using variables that have to be adjusted from perfect. In fact unlike a production frame, you can't test ride a custom design until it is actually finished. For example you might ask what is the best head angle/fork rake combination to use on a bike I'm riding on average at 15mph for maybe 25 miles at a time that is using 700 X 32 tires. The answer you get from what might be the best combination for that situation is not what will fit best into a frame that matches your more upright riding position. All their variables interrelate with each other. They are not stand alone dimensions. But the adjustments from what is best if you were 25 again will probably be still okay just not optimal. The solution of course is to find someone with experience making the kind of frame you are wanting to build and trust their opinions on frame design choices.
I'll repeat what I've suggested before. Use a lower BB based on your actual crank length and pedaling through corners style. Production frames will be higher to avoid hitting a pedal when going through a corner. Make the frame size be as large as you can comfortably straddle to the millimeter. Production frames have 2 cm steps between sizes. Slope the top tube either a bit if using lugs or a lot if fillet brazing or tig welding. Both of these factors raises the top of the head tube so there isn't an ugly amount of spacers or quail extension above the head tube. Additionally add mms to the head tube length to reduce spacers or quill extension. Use a lighter weight tubing based on your actual weight and pedaling strength. Production frames have to assume you are super fat and powerful and have to use thicker/heavier/bigger ø steel to prevent breakage. It isn't the weight that is a problem but rather the ride quality of too much steel. I vastly prefer the ride quality of lighter/thinner steel tubes. Again it is not about extra weight.