BACK TO THE DRAWING BOARD?
I moved the front wheel forward about 5 inches which resulted in a 1.5" trail. A test ride confirmed that the smaller trail greatly reduced the wheel flop and the bike actually felt surprisingly stable, but still not as neutral as I would like. I seems that if I start a turn, the front end wants to continue turning. I think it's because of the excessive weight that is higher than the steering pivot (all the steel and intermediate gears and crap). You would think with my C of G being BELOW the pivot, that my weight would want to keep the steering neutral, but that's not quite the case. It feels almost like it's perfectly balanced, but when I initiate a turn, it will flop over into the turn. A safe geometry would be for the bike to resist a turning movement by returning to neutral on it's own.
The next thing I tried was tilting the seat UP a bit more (greater seat angle). It now matches the steering axis angle. This had the effect of further exaggerating the steering tendency - which I don't like. It doesn't take much steering input to keep the bike going straight, but once a turn has been initiated, it takes quite a bit of effort to straighten the bike back out - again, this is not a desirable feature. I am happy with the turning radius though.
The other aspect I am unhappy about is the weight shift on a turn. As you can see from the photo shot from above, when I turn right, my seat pivots to the left and the weight shift is to the OUTSIDE of the turn. That's not good because that weight shift will contribute to centrifugal forces that want to flip the bike over on a tight turn. Ideally, the weight shift should be right when turning right. The only way to accomplish that is to place the pivot BELOW the seat, but that comes with a price. When I tried that previously, you don't get much of a turning radius first, and second, it rides like a two wheeler. You have to balance the bike on top of the pivot just like you would a two wheeled bike - this defeating the whole purpose of a trike.
The other thing I did NOT like, was a bit of a wobble while peddling. Because the steering axis angle has a lot of vertical component, each stroke tended to want to push the seat back in the opposing direction. A shallower steering axis would probably fix that - of course the penalty would be a greatly diminished steering radius.
I'm not really sure where to go from here. I suppose I need to define exactly the features I'm looking for with this trike design:
hmmm... It's amazing how you can clear the confusion by simply making a list. It would seem to me, that the only way to accomplish both neutral steering AND weight shift to the inside of the turn is to place the pivot BELOW the seat, and to use springs or struts to neutralize. The only draw back to this approach as far as I can see, is greatly diminished turning radius. When I move the pivot below the seat, the steering axis angle is very shallow in order to maintain a 2" trail. As a result, with steering input, there's plenty of lean, but not a whole lot of turn.
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