Bipolar

For 20" wheels, double-pivot folding mechanism is used for max compactness. The seat is split into 2 pieces, with the backrest mounted to the rear pivot to control the rear wheel, which makes this bike all wheel steering (AWS), with rear wheel turning only when the body leans into a turn. In theory, the rear wheel has positive trail and seat rise effect (60deg pivot) to stabilize side motion.

Build

The rear pivot is somewhat redundant in terms of steering, and auxiliary at most (only during turns). However, it's primary purpose is to enable 2 folds for compactness, yet without any folding locking mechanism and seat removal like the commercial Grasshopper or Cobra or the μPython. I was prepared to make a pivot lock in case it's not ridable, but after some testing, it seems that I can skip the lock and consider the extra instability as a compromise / feature.

Over the next few months, I will be comparing its mobility / utilization vs. the μPython, going different places. The μPython will probably win the popularity contest, with the tiny motorcycle tires alone.

Modification - Nov 2011

The two-piece compact seat was also too uncomfortable, especially at lower back where the 2 pieces meet, so it's replaced by a full seat which brings back upper body control, and is welded to the frame leaving the rear pivot completely free to rotate.

Walking was horrible (embarrassing among pedestrians), as both wheels want to collapse/flop, plus the 2-piece seat was not helping either. Together with the new seat, a locking mechanism made from seat post / clamp couples the wheels so they can roll together when folded. Now walking the bike is quite pleasant, kind of like a Brompton but without the roller wheels. (video)

Test

The folding is really quick, and surprise people who watch it un/folds...

4/10/11 First ride, turning feels tighter and smoother than regular python; going straight is not too bad, but instability seems more sudden / unpredictable...

There is a steering linkage between the wheels, an organic one (the rider). I was surprised that the weight of my back is able to keep rear pivot / wheel pretty steady going straight without any correction effort, not much different than regular python. Some coroplast friction between the backrest and seat bottom helps, at least walking the bike. Turning is more different, almost a new experience, very smooth like butter and without moving the shoulder outward. Perhaps, this is its best riding characteristic, which I would recommend people to try.

4/13/11 I did a quick sprint test at lunch in the parking lot ~10-13mph (16-20km/h), faster than the μPython for sure, and couldn't feel much movements transferred from the rear wheel to my back. However, I do get nervous at higher speed as I am still in training. Any upper-body movement would transfer directly to the rear wheel, so this is a rider's issue, not the bike. At most, I can say that the bike is not very tolerant to random body movements (inputs), and therefore may be more difficult to ride at higher speed. I vaguely recall similar description for RWS bikes.

4/14/11 completed a 5-mile trail loop, and fell several times on the gravel surface, going a little too fast ~10mph. The BMX tires probably played a part. Mountain bike tires should help, elbow pads and old jeans also. However, slowing down is probably the more sensible thing to do... Dirt surface was fine, and asphalt also. Otherwise, the 20" wheels are more stable and efficient than 16" Mini Python and 10" μPython.

7/10/11 When disassembled, it managed to fit inside a medium luggage (largest that I have), where the micro python would need another inch or so. Thus it got to go traveling first. Fortunately, the total weight is under 50lbs, so no extra luggage charge ($100?). See travel photos on Panoramio

Climbing is rather poor, as front weight distribution is ~40%. Walking is tough, and barely with a stick at the front. Carrying the 25lb folded bike uphill is no fun either, so a upright folding bike is better here, especially in the city...

Downhill is not too scary anymore with some braking. I can go faster than going uphill now :) On the flat, I am comfortable enough to share trail / pathway with pedestrians now (when legal).

The 2-piece seat is marginal so I had to pick up a mesh back support on the trip, not bad for $1. A hotel towel would also helps. Because the seat rest is coupled to the rear wheel, starting is a little tougher even with sticks, and care is required to ensure that the back seat / rear wheel is straight.

Luckily, while experimenting with uncontrolled rear wheel, I found that the bike is actually more compliant. The rear wheel is now mostly free from the seat back, other than some pressure / friction, and straightens out nicely under weight, and seems to move slightly with the front wheel due to the pendulum effect, and probably vice versa (another invisible link like the bango steering ).

There also seems to be some sort of self-stabilizing effect (PSI reduction?) between the two wheels that deserves more analysis, beyond quick all-wheel steering. Since the camera is mounted to the rear frame, so it's not very obvious that the rear wheel rotates unless you pay attention to the view through the wheel. See the PSI test at 1:06-1:15 on youtube. The bike feels quite stable as going straight. I will repeat the test on regular pythons...

11/25/11 The Bipolar has evolved. Initially, rear was controlled / locked by the seat back, then loosen, and now completely free. The free rear wheel auxiliary steering is a little different, but can be learned in minutes. AWS steering was not the original goal, but now that the rear pivot is not occupied by the seat post, the rear wheel can be controlled by a handle bar for a RWS study, should be fun...

Other examples

-KrankiKobra by Spinner Guy Feb/2013

Terry was really brave to learn python with an unlocked rear pivot, but he was able to ride it in a relatively short time...

-Bipolar02 - 26" wheels built Apr/2013

Mashup between FP02 and Bipolar, Rear pivot locked by seat post clamps, fixed handlebar for shifter & brake, must be disassembled to fit inside MINI boot, no room left for seat :)

Pivot Angle [°]

Trail [cm]

Seat Height [cm]

BB-Seat Diff. [cm]

Ground Clearance [cm]

Wheelbase [cm]

Weight [kg]

BB-FWA [cm]

BB-EOS [cm]

Turning Circle [m]

Front Tubing [mm]

Wheel Size [inch]