Same "car" at Port Townsend, WA. This is the mud pit where we shattered the differential on an other four-seater.
Note cleats strapped to tires, like snow chains. Yes, we got thru. Yes, I'm steering from the back seat.
About the 10-seater the Captain saw up there, I know of two that fit this description.
One lives in Eureka, CA, and is a Ford pickup chassis on Hoosier auto racing tires like I use. The weird thing about it is that the riders face outward, five to each side. It usually has artwork on top.
The other is in Corvallis, OR, and is based on two front axles from a small 4x4 (it steers at both ends!) -- I cannot for the life of me think of the name now. Tiny 4x4. Wheels and tires are farm tractor type. Here the riders face inward, and they push "treadles" rather than bicycle pedals. It has a deck on top, for the accordion band.
Both were on Playa in 2010, with Kinetic Cab Company. Yes, I had something to do with that.
As for extending a motorcycle crankshaft to use as input to the tranny, I have one like that in my barn, safely discarded. (Actually, no extension was needed.) It came with a vehicle I "inherited" because it did not work. Trouble with that particular item is that there is a huge down-gearing between the crankshaft and the tranny. On a different unit I counted 1:60. Humans cannot pedal 4,200 RPM! So on the ones I used, I went directly to the tranny input, where the clutch used to be. On the BMWs, I had to find used clutch discs and weld sprockets to the disc hubs.
This idea of running all the riders onto one shaft.... I don't quite follow that. (But I call that a collector shaft.) But I have a vehicle here (same as the one just mentioned) with exactly that, where the power from all four riders goes into one shaft, and then one chain from there to the one drive axle -- with a transmission in between. On this, we tore the #40 final drive chain in half coming up a boat ramp, and later tore the mounting ears off a cast iron flange bearing in the transmission. I'm in the process of splitting that drivetrain in two.
We have used diffs, and whole transaxles from lawn tractors and such. Trouble with those transaxles, again, it that we would need two or three in series for enough ratio spread.
One type of tranny that works well, is more of a method than a tranny. There is an input shaft and an output shaft, with their respective sprockets. On the "operational" side we have two-sprockets-and-a-chain, which can be quickly slid off their shafts and replaced with a different set. This is what I've been using on this vehicle lately. (I'll find a picture of it.) I had three of four (at different races) sets of "gears", and I had it down to 2 minutes to make the swap, with the others handing me parts and tools. It can work great, except that you have to stop a while to do it.
Yes, this is GREAT fun!