Monday, September 15, 2008

primer on bike handling, geometry
Center steering

Between classical front-wheel steering, and strictly rear-wheel steering, is a class of bikes with a pivot point somewhere between these two extremes and referred to as 'center-steering'. These design allow for simple front-wheel drive and appear to be quite stable, even ridable no-hands, as many photographs attest.[24][25]
These designs usually have very lax head angles (40° to 65°) and positive or even negative trail. The builder of a bike with negative trail states that steering the bike from straight ahead forces the seat (and thus the rider) to rise slightly and this offsets the destabilizing effect of the negative trail.[26] (pic)

D: compare to the disastrous handling of the original Flevobike.


No one who was present that day would consent to demonstrating the machine, but several people volunteered that they had seen a rider earlier in the day who displayed extended no-hands riding! I couldn’t believe it. I didn’t think it was possible to ride a recumbent without hands. At least my experience to that point hadn’t convinced me that it was possible.

So I spent half the day trying to learn to ride it, and it certainly exhibited some strange behaviors. To begin with, it just wouldn’t stay upright. The front half wants to fall over, and fall over fast. It isn’t the kind of bike that you can push along gracefully while holding the back of the seat. In fact, holding the back of the seat is a sure way to let the front half fall over and of course, that happens rather quickly. And to make matters worse, just like more conventional underseat-steering recumbents, the handlebars are a little low to be comfortably used to direct the bike when standing to the side. But in the case of the Chinkara, the handlebars don’t even provide enough leverage to counteract the front half of the bike, which seems to have a mind of its own. Such behavior made it hard to handle the bike. It wouldn’t stand up on its own. It seemed impossible to control, and this was before even trying to get on...

D: of course, there are lots of online vids of folks scuffing their hands on the Python too.

I thought something like the Triton Trike for steering would be desirable.
I'd be curious about a steering system with some gears to amplify the effect near the far range of motion. By that I mean:
- handles normally for minor movements
- when turned, say 45 degrees, amplifies the turn on front wheel by 1.5x
- at much more, by 2x.
I hear under-seat-steering has problems turning the wheel severely.

In a bike very sensitive to steering, maybe even reducing the wheel-turning below 1:1 in the middle would be desirable.
Steering which is speed-sensitive would be nice, but the mechanical details would be a nightmare.

D: latest Cruzbike thought.
I think an even more severe oval that exists commercially *might* be desirable.
I do wish the company website had warned against the combo of short wheelbase AND high seat height.
Like I said, I wouldn't trust the stability of mine at faster than 30kph.
I suppose that is fine for a commuter.
I finally figured out the aluminum sleeve included is to serve as a gap-filler on a stem adapter.
The spare parts for use in other kits were confusing.

Aside: I found a 135 degree angle for seat/pedal combo. It looks ideal for future models, with the pedals below seat height and a modest seat back angle.
I am also intrigued by laying in all electric lo/hi power wiring and shifter/brake cables internally on the internal-chain version.

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