My first self-designed bike was intended to be a gravel bike, but I quickly realized that the dynamics of two-wheeled vehicles are complex, even more so for off-road conditions and someone without the right engineering background.
Nevertheless, 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.
Gravel bikes are a relatively new concept, and their geometries continue to evolve. I decided to study how road bikes have changed over time, focusing on the Cannondale Supersix and Specialized Tarmac series as examples.
credit: GeometryGeeks.bike
credit: GeometryGeeks.bike
During the past decade, 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.
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)
So, what do I think would make a better size 52 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.
Inspired by these ideas, I set out to create a road bike design.
