The design phase
Once the engine choice had been made, the detail design of the rest of the bike could be begin. Some of the main decisions that were now required included:
- Caster angle and front trail
- Fore/aft and height position of the engine
- Swinging arm length
- Swinging arm pivot position
- Rider position
Our work quickly provided confirmation of two basic, pretty much unavoidable concepts:
- Motorcycle design is a series of compromises.
- Packaging of all required components is the largest source of constraints.
Remembering our original goal of building 4-stroke near-equivalent Honda RS-250 bike, we let the following rules guide our design efforts:
- Make the castor angle as steep as we dare. The RS-250 uses 22.5 degrees.
- Move the engine as far forward as possible without compromising front wheel travel to result in the longest possible swinging arm given its wheelbase. The RS-250 has a wheelbase of 1350 mm.
Just about everything else followed based on those two self-imposed design rules. It bears repeating that Tony Foale’s book Motorcycle Handling and Chassis Design and his whole-bike design software were invaluable tools for completing and to a significant extent quantify the design process. It was, in fact, the reading of his very book that led us down the path of designing an entire motorcycle. It’s all his fault, really.
The design phase of the project took from November 2005 to February 2006, a very short amount of time all things considered.