you’ll probably steer only with fron wheels, there will be torsion movement in the deck - front side will twist a bit, the rear not 
@Esk8_Spirit runs something similar on his daily 4wd - very loose front, very stiff back (albeit, he’s standing on it )
12s28p 50Q. got incredibly lucky to get my hands on them . atm I’m waiting for alu enclosure to be built, after that some 3dpriting, and then this 3years-old-WIP buid will be finally ridden
What even is voltage sag?
True. What about 0deg baseplate angle on the rear then?
never tried, never seen, but makes sence.
on the contrary, I’ve ridden very steep baseplate angle (caliber baseplate on skateboard deck), and it was hell. not much movement with huge force produced some turning angle, and when you went over something, it easily affected my stability - with 240mm truck the lever advantage from the wheel was huge.
the paragraph higher is probably not important for you though 
so tl;dr with the 0° angle baseplate - no clue, but at least do it for the science. I’d try 3dprinting prototype before making something more permanent
0 degree rears are pretty common in LDP setups to get the most out of each pump since the wheels scrub more rather than turn in the direction of the pump. However they also tend to have much higher angles at the front, so a 0 degree rear might hinder turning more than it’s worth on a regular board setup.
also, TKPs. interesting
Okay, so. Sit-down test setup
Even made this little grip for the end of the front bar.
Result. Speed wobbles already at around 10km/h speed, albeit low frequency, but very pronounced in amplitude for the given speed. I think I rode about 100 meters in total and just concluded that this won’t work with this setup and will not be the solution for the ride comfort. The wobble only stopped after giving a very distinct body lean input to left or right, which then overpowers the straight line instability, but it was pretty impressive how easily it started.
I believe to “fix it”, it would be to lock the back truck in place, so it doesn’t provide any turning input and have all the steering done through the front, as couple people mentioned above. But with the Trampa trucks, locking the rear truck and having only the front one to steer with + not being able to get more weight on the front compared to standing. Would result in a very limited steering, so therefore I don’t think with the current trucks, this sitting setup is not feasible. Would have to almost be a some kind of rack and pinion setup with a steering wheel and the wheels would turn and not the whole truck itself. Essentially a car/gokart-like steering setup.
I will be protoing some 3d printed concave addons for the deck to improve foot ergonomics, but due to having not ridden a concave deck for many years I don’t really have a good idea what to start with profile wise. If someone can recommend or have a recommendation for the profile of the concave I’ll be interested in hearing it.
I think I’ll try to have the surface of the print done in a way that provides enough grip for my shoes, so I don’t need to add griptape on top of it.
Just dewedge the rear to zero or move your seat forward. Seat so far back is your main problem. Having built street luges back in the day I know the pain of bad weight distribution.
Or dewedge and use a motorcycle steering damper, those still allow movement, but adjustable damping at max makes for a rock solid ride.
What you could do for a sit down setup is to use the extra bar to add a footstop far ahead of the board, and have the seat as close to the front truck as possible, just like a street luge. That way you remain stable due to the weight distribution on the front truck rather than the back, and turning doesn’t get impacted that much. Might need some guards so you don’t hit the wheels though.
This
Yeah I say do this and then try and get the absolute softest bushings you can buy
The initial position with the feet so close to the wheels would have been sketchy. Move a foot 5cm on a bump and you’re toast. Seat in the middle of the board, where a battery box would be, and have a footstop protruding 30-40cm at the front would be better. Your knees would be then pretty much over the front axle. Also makes it easier to switch from seated to standing position. Not sure if this helps enough to reduce wobbles though.
Any progress on this project?
