Part of the Test Mule challenge was doing things cheaply, such as using wood instead of metal, recycling old bike parts, and scavenging from my own hardware bins. Some of my wilder ideas, such as using hiking shoe soles as friction brakes, just didn't work out but that was part of the fun. How clever can you be, what can you conjure up from existing materials without having to make a trip to the store?
The steering geometry research gave me an hands-on learning experience that can't be found any other way. From now on, every time I read a motorcycle article about how a bike's handling has been "improved" by altering the rake and trail, I won't just know what the words mean, I'll know exactly how the change really felt.
In the original design of ProjectVF, I used a very shallow rake, much like a chopper. I want the front of the bike as low as possible and laying the steering angle back helps to achieve it. During the early kinetic tests of the Test Mule, it looked as though the chopper-styled geometry would work, but it turned out to be a false lead. When weights were added to the front of the Test Mule, it became more difficult to recover from a sharp turn at very low speeds. This effect is commonly referred to as chopper flop, but once past 5 mph it seems to disappear. As can be seen on the U-turn videos, a shallow steering angle by itself does not cause heavy steering or lead to a lack of control.
Subsequent kinetic testing revealed that a steeper rake seemed to work better. Having failed at measuring the effort involved in turning while simultaneously controlling a moving vehicle, I can only base my decision on my subjective interpretation of the situations I encountered. With that said, I'm going to start with a setting of 25° rake and 3.5" trail, which is slightly steeper and sharper than the original Interceptor setup. I already built the control arms to match these settings, so if I don't like the results I'll make new ones, but this is what I'm starting with.
Not being able to measure the effort of steering during the kinetic tests was disappointing. I think with more time and practice I might be able to accomplish that balancing act, or better yet, come up with another way to measure it, but I think I've taken this research as far as I need. Time spent on further research could be time spent building different control arms for the real thing, and so the Test Mule is retired, destined to be recycled for other projects. Like so many other things in life, it was fun while it lasted.
Update for 2009
After building the above suspension, I went back to the Test Mule and did more experiments with chopper-type rakes, along with more weight underneath to simulated an engine. Based on these test, I'm going to make a second suspension with a 40° rake and see how it works on the full-size prototype. Part of this is curiosity but most is aesthetics because I like how a chopper looks.