Designing My Road Bike: Why did I bother? (Pt. 1)

Many people, including myself, learned to ride a bike during childhood. As I grew older, staying fit became more challenging and shifted from a natural activity to a constant battle to prevent injuries. However, I eventually reconnected with my love for cycling. Unfortunately, I was struck by a car during one of my rides.
Previously, I owned road bikes, but the experience of building a gravel bike revolutionized my approach to cycling and transformed my relationship with the sport.
My cycling mileage increased, and I started road riding again to cover even more distance. I soon realized that my gravel bike suited me better than my road race bike. I began dreaming of participating in events, both local ones and major ones like Unbound 200. To prepare, I trained harder and eventually got a Wahoo Kickr bike. When the Covid-19 pandemic hit, I shifted to using Zwift to improve my fitness and MyVeloFit to monitor my position to maximize my chances of enduring the rigors of a 200 mile bike race.
My bike underwent several changes, including specific modifications for Unbound.
The resulting bike fit saw me through thousands of miles of training and the 200 mile race; but the measurements were slightly unconventional vs. the "numbers" bike fitters typically consider normal.
Over time, I started doing more road group rides again and I began to realized that my gravel bike wasn't ideal, particularly for technical descents and I began to look for a for road bike. 
Having a background and education in engineering, I took a technical approach to trying to understand why I didn't love descending on my gravel bike and what I should be looking for to give me the feel and performance I wanted. 
I delved into research papers and PhD theses to learn as much as I could. I discovered that the primary variables governing bike handling are head tube angle, wheelbase, height and position of the center of mass relative to the axles. Trail seemed to be a secondary variable.
While there were clear observations about the effect of negative trail and near-zero trail, this seemed to be more of a secondary variable. One paper suggested that headtube angle is the critical variable stating: 
"The head tube angle results are interesting because the weave speed can be decreased with a steep head tube angle without adversely affecting the capsize critical speed, thus simultaneously increasing the stable speed range and decreasing the weave speed. This is ideal if it is assumed that a low weave critical speed is beneficial for take off and a broad stable speed range is beneficial for cruising with little control input. Trail is typically of particular interest, with many bicycle designers claiming that it is the most important parameter affecting handling qualities. Tim Paterek, an expert frame builder, claims that the comfort zone for trail falls between 5 cm and 6.5 cm for most bicycles (Paterek 2004). No correlation can be drawn from Figure 8 and Paterek’s claims." Moore, Jason & Hubbard, Mont. (2008). Parametric Study of Bicycle Stability. 10.1007/978-2-287-09413-2_39. 
Studying how road bikes have changed over time, focusing on the Cannondale Supersix and Specialized Tarmac series as examples I saw that as equipment and riding has changed, bike geometries have evolved.
Noting that the Tarmac SL7 geometry doesn't include the spacer required for internal routing, we can see that the geometries have evolved to be shorter and higher at this size. Meanwhile, several key developments have impacted our cycling experience. Tires have widened from 23mm to 28mm or even 30mm on race bikes, while tire pressure has decreased. Aerodynamics have become more important, leading to narrower handlebars. Wheelsets are now deeper and heavier, with disc brakes and thicker tires.
Although some purists argue that these changes can make bikes slower, I believe they have improved my performance. Wider tires let me maintain an aero position for longer, and disc brakes help me descend faster and brake later. However, these changes have also introduced new compromises for smaller riders.
Specific to me the rising stack height and decreased length means I need a longer stem, but the rise of integration means that stem choices are limited. Those choices once again mean that a longer stem increases stack height. The compound result is that getting in the position I want on a road bike is limited to one or two production models. Further, the experience of a rider on a size 52 frame now differs significantly from that of a rider on a size 56 frame, where designs are more consistent across key handling variables. (note that I added my gravel bike as well as the "Custom Ti" design that follows to these charts)
After considerable modeling I ended up with the following principles to follow when designing my own road bike:
1. Steepen the head angle to 73 degrees.
2. Lower the rider's center of mass to decrease stability, in line with the steeper head angle.
3. Optimize weight distribution for better handling.
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