The drag arm connected to the forks must be parallel to the control arms and the same length. If the drag arm isn't the same length, the handlebars will turn side to side when the suspension moves up and down.
Imagine the handlebars fighting back: that's called bump steering. Not all designers understood the geometry and as a result a lot of old cars suffered from bump steering. The Model-T had plenty of it.
Ideally the drag arm pivot would be aligned with the control arm pivots, but it doesn't have to, as shown in the next photo.
As long as the drag arm pivots are the same length apart as the control arms pivot, and parallel to the control arms, everything will move without binding or bump steering. When the piece on the right side is moved up or down, it remains exactly parallel with the piece on the left side.
Once again a cheap cardboard experiment proves an idea works.
For the ProjectVF steering I turned to small bench-top simulations and CAD versions of the designs to figure out what worked, and just as importantly, what didn't. Getting the steering to work with so many constraints was the toughest puzzle of all. I'm still not completely sure it'll work but after the research shown below, it better.
Here's a CAD drawing of the latest steering.
The basic design is two pairs of levers which are all the same length. A centerline (yellow) is drawn from the center of one pivot, such as the handlebars, to the center of the next pivot.
Two short lines (red) are drawn at 90 degrees to the yellow centerline and located at each end. The short lines represent the levers. The ends of the levers establish where the drag arm (also red) connects.
The length of the drag arm equals the distance between the two pivots. In this drawing, the red drag arm is the same length as the yellow centerline.
If the levers and drag arms aren't designed this way, one drag arm will move a shorter or longer distance, creating steering that doesn't match the way the handlebars are turned.
The vertical levers were intended to push/pull the drag arms but the design failed. However, for the purpose of this video they provide a reference point. Watch the tops of the vertical levers and see how little they move as the forks go up and down and side to side. Good geometry is how bump steering is eliminated.
This worked very well. There was no bump steering and the forks turned the same amount as the handlebars just like a standard motorcyle. Now I need to fine-tune the design, move it inboard for asthetics and make it much more heavy-duty.