Question All about 2DOF pivot points...

So my understanding that for seat movers, the pivot point is more or less centered under your butt with some users favoring slightly fore or aft of the ol' bunghole. Point being that the bullseye for where it should go is reasonably fixed.

For a fixed frame scenario I am now a firm believer of 3-points of contact vs 4-points of contact because three-points of contact is the best way to define a plane. I read an excellent comment with youtube videos proving their point about the fidelity of motion being superior with a 3-actuator full frame vs 4-actuator full frame. I'd like to link the source but I cannot post external links yet.

Now that being said you can accomplish three-points of contact with two actuators and the third point-of-contact being a pivot point.

Let's assume that you could source two actuators with enough force/speed to move a full-frame. Where should the pivot point go? Instinctively one would want to put the pivot under the center of mass so you are largely balanced upon it. This puts the least amount of load on the actuators allowing them to use their speed to improve responsiveness.

You can do this gimbal style with two nesting planes pivoting perpendicular to each other, as I have seen some users do roughly like this...



Now the issue strikes me that in a vehicle, the pivot point for pitch/roll isn't underneath the driver! The pivot point for roll is down the centerline of the car... beside the driver. The pivot point for pitch would be just behind the driver's seat.



So for an accurate 2DoF full frame experience of pitch/roll would involve an offset position for the pivot point. The clear x's would be actuators/motors and the brown x would be the pivot. I suppose if the pivot was a third actuator that would give you heave as well, but the purposes of this discussion I'm mostly interested in pitch/roll.

Now excluding concerns about the footprint of the this type of setup, would it actually create more realistic pitch/roll forces upon the driver? Or am I just unnecessarily complicating something that won't necessarily translate into a more immersive user experience?

Disclaimer: I have zero sim rig experience beyond the most basic of FFB wheels. This is all strictly theory I playing with.

This is my thoughts too, in an F1 you straddle the center of the car, but not in a GT or Rally car. I like your tripod idea it would help keep the footprint of the rig manageable.

Interesting idea. You should be able to do everything a corner motor rig can do with this approach.

I think in addition to the foot print the single point behind the driver is going to be supporting most of the load most of the time. Seems like it will really want to tip. So that would tend to put a lot forces on the back piece.

There are some pay systems out there that claim correct cog. They are really expensive and they are concerned about moving that point up as well. Sorry no link it’s just a memory.

I am building a 2 dof rig with the weight mostly resting on the U-joint. If it doesn’t make me hurl I might upgrade. Also having experience in a more traditional setup would help you appreciate what the limitations really are.

Sorry for being so late in the conversation but let me write down my point of view:
I have been thinking about this because I was involved in a sim some years ago (startup, went down before first prototipe)
My idea was that this point (in Spain we would call it CIR or Instantaneous Rotation Center) can be moved to whenever you want for a little period of a rotation. Thats what you can achieve with a couple of rods.
So you can point it over the driver and in a point where, for example while braking, you have a rearwards movement of the seat while rotating to the front, so you have the transient movement right, for that start of the braking, but while you're braking, you still facing forward to feel the frontal Gs.
I cant explain further before I present this to my boss at my new job in the field, but let me left here this:
https://en.wikipedia.org/wiki/Instant_centre_of_rotation

I've spent the last few days thinking about that and I believe for a 2DOF it doesn't really matter. Roll and pitch are the same no matter where you sit.

The easiest thing to imagine this would probably be right seater vs left seater going over a curb on the right hand side of the car. No matter if you sit left or right the pitch will always be the same.

Things go differently when you add heave as an additional degree of freedom. Then in a left seater you probably don't experience much of a heave effect, you'll mostly just roll to the left. In a right seater however you'll get to experience massive amount of heave when running on a stiff suspension.

There is absolutely nothing you can do about this when in a 2DoF platform the pivot point is under your butt. When the pivot is under your feet and the 2 actuators under your but you could fake it to some degree but you'd introduce roll like you're on a catapult. For a 3DoF platform the software imho needs to account for the position of the driver in the car.

I'd be curious if any of the currently available (and affordable!) motion softwares account for this?