Hi Taku,
After some crazy days I finally decided to sit down and answer your questions, only to find out that a lot of this stuff I don’t really want to disclose. No point in telling other bike companies how to make a road bike that DOES work with a moderately forward seat position. But I’ll gived some general observations.
First of, I don’t think it is rocket science. It all comes down to common sense, unfortunately there isn’t enough of that in the bike industry. I remember that ten years ago a magazine had a few pages full of drawings, and each drawing showed a bike with one change compared to the previous bike, and how that change affected the properties of that bike. So it would show a longer chainstay, and it would note that this increases the weight on the front wheel and reduces the bb and torsional stiffness. And it would do that with all sorts of variations. Unfortunately some of the conclusions were wrong but I believe this is the type of understanding that a bike designer needs to have. Yet if you were to walk around at Interbike and ask people what effect a longer chainstay has, very, very few people would be able to tell you.
And since you cannot rely on anybody to get this right, as a designer you basically have to figure out all these possibilities for yourself. Not terribly hard, it’s more an approach to thinking you have to get used to.
So to use that knowledge to the Soloist, the starting point was that we wanted this bike to be a 100% road bike. We have our ideas on what that means to weight distribution, and handling quickness, but we also know we can achieve these goals several different ways. For example, Andrea Tafi likes the Colnago C-40 but also the Cervelo Soloist, even though both bikes have completely different geometries. But they are the same where it matters to him.
The thing is, the C-40 wouldn’t be worth anything with a forward seatpost, yet the Soloist rides very well in that position. That is because out of all the options we had, we chose the road geometry that achieved all our road handling goals and was the least susceptible to problems when the center of gravity of the rider is shifted forward.
It’s not perfect, it can’t be, the Soloist will always be 100% road bike and 50% tri bike, but that’s better than the 100% road bike 20% tri bike that most other road bikes are. It’s just a matter of finding the proper geometry to accomplish that without losing any of its road capabilities. Luckily our philosophies for road geometry already incorporated many details that made them less susceptible to weight shifts, it’s just that before the Soloist this feature was not that important while now it is.
I have a feeling I’m straying from the issue, but maybe this gives you some insight. If you have some more focussed questions I’ll gladly try to answer them, and I won’t take as long.