[python] Re: Jaculus riding experience



I am curious to what the deciding factor is for stability at speed. Is
the pivot angle or the trail the deciding factor?
I know that my trike (http://jetrike.com) is nothing like the python in that is has positive trail, but I too have wrestled with stability problems, and only now am I getting up to speeds above 50kph. Being FWD with a moving bottom bracket, I figured that some of my observations might share some commonality with the python.
My trike is a bit bi-polar in its behavior, in that if I lock my elbows against my seat rails, and hold the handle bars rock solid, because my trike maintains balance with its self centering effect, I can cheat gravity. In this manor I can sprint and climb hills, but it fundamentally changes the trikes behavior, and switching back to riding in a relaxed mode causes a momentary fish-tail action and I just have to stay relaxed to get out of it.
That being said, I prefer to spend most of my time riding around in the relaxed mode, where my feet really do the steering, and my hands rest on the handle bars. In that mode I have found that both cadence and crank torque have a big impact on stability. If the cadence is too high, it creates sympathetic harmonics in the drive train that cause shimmy. If the gearing is too high, causing the crank torque to be high, it limits the ability of the steering to self correct, and the trike will fail to make appropriate course corrections, and so tends to wonder off course. So there is a sweet spot somewhere in between, and I have to regularly change gears to stay in it.
I have 150mm cranks, with a 44t and 60t chainring double on the front with a 20" drive wheel. The 60t has improved down hill stability and speed, because I no longer top out, I can keep a lower cadence and still apply plenty of force on the pedals. Nevertheless, I also had to reduce the wheel base to 1100mm, move my center of gravity back (its now about 50:50 weight distribution), raise the seat angle to 40°, and lower the seat height to 250mm -- all of this to get it to work properly. I originally had a seat angle of about 25°, but it was just terrible. There was too much weight bias on the front wheel. I also used to have a pivot angle of 55°, its now 65°, and the trail was reduced from +100mm to about +40mm. Some of these changes were necessary because of my trikes self-center geometry, but others were to improve the stability of the FWD moving BB drivetrain.
The nice thing about steel is you can change your mind. The way I see it, if your bike isn't handling well enough to ride then you have nothing to loose cutting bits apart and making fundamental changes. Only your once sleek frame will start to look Frankenstein-ish. These are experimental designs, so you have to expect they will need a bit of fine tuning. But its amazing what a difference a few changes can make. My trike has gone from having fairly poor handling to actually working well (within limits), it may not be perfect, but I ride it to work every day and its a blast, and now that I am getting fitter -- it just gets faster and faster. 

-h
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