Designing a bike is all about balancing trade-offs. Anyone who claims "no limits, no compromises engineering" is not being truthful. Good design involves optimizing based on acceptable trade-offs. Although my engineering background isn't specialized in this area, with help from knowledgeable experts, I refined my design parameters to create a race bike that offers a descending advantage for smaller riders while maintaining control on >60mm deep aero wheels.
I focused on designing the steering for descending in the drops with a 38mm wide bar at the levers and 32mm spaced hands in the aero hoods position. I learned that trail is crucial for how a turn feels. So, I aimed to reduce the mechanical trail provided by the frame to maintain similar steering leverage with narrower bars. This approach also counteracts the greater precession caused by heavier disc brake wheel setups, reduces pull when an aero wheel stalls, and accounts for the increase in pneumatic trail due to wider tires at lower pressures.
DESIGN TARGETS:
1. Head angle: ~73 degrees.
2. Weight distribution: 57% rear, 43% front (similar to Porsche race cars, including the driver).
3. Center of mass height: Lower than modern race bikes, with an even lower bottom bracket to compensate for optimization around 28mm-30mm tires versus 25mm tires.
4. Trail: 51mm-52mm when using 28mm-30mm tires, to account for narrower bars and increased total wheel weight (60-70mm wheels + brake disks + larger tires).
5. Fit: Roughly a size 52.
ADDITIONAL CONSTRAINTS (via SRAM and Shimano):
1. Chain stays had to be at least 405mm long.
2. Angle between the chain stay and the seat tube needed to be at least 63 degrees to avoid limitations in cassette or chainring compatibility.
3. Carbon forks were available in 43mm, 47mm, 50mm, and 55mm.
After many iterations on BikeCad.ca, I arrived at a design that is very long and low.
However, when considering the headset and spacers, it begins to resemble a Specialized Tarmac SL4 tuned for descending: lower and with less trail, or perhaps a slightly sharper and slammed SL6: lowered with a steeper head angle. Compared to a modern bike its almost like many brand's size 52 geometry in terms of vertical measurements, but like the size 54 in terms of horizontal measurements.
The trade-off for this design is that you need to run a 165mm or shorter crank to minimize the chances of pedal strikes when pedaling through corners.