e focus on radial normal force. There are a lot of mild-concave profiles out there.
This is actually me CADoodling some large-diameter ideas prior to that spark, and what it is, is an excellent example of a mild-concave flywheel profile. You have probably heard me use that terminology, and specifically what I mean by "mild" is that the groove radius is greater than the root-to-root clearance when installed in a cage. As a result, the generating shape for the flywheel surface, which is also the shape of the resulting gap between them, is an intersection of two arcs that looks like an American football. Usually the endpoints of the "football" are off in the distance to the side of the cage and already get wiped out by the rim width of the flywheel. With SSS family parts based on compatibility with the old Hasbro 43.5mm center distance, there is further severe truncation imposed by the clearance in the cage or the blaster it is supposed to go inside. But you get the drift - with Artifact and Hooligan and especially BlasterParts and CC flywheels, you're forcing a round dart through a profoundly football-shaped hole. Seems right. Right? Isn't that what you want to do to a dart when there are two flywheels? We're applying a deformation in one direction. Right?
I'm gonna take a big leap and say that this is very wrong. We're not (necessarily) dealing with a deformation or a normal force in one axis. The normal force we are worried about is normal to what, exactly? Not some arbitrary axis going horizontally through the dart, ortho to it and the flywheel webs. No no no. Normal to the contact surface. Which can be and usually is curved.
So there's key element one, we shouldn't be throwing away the horizontal (parallel to the motor shaft) components of dart deformation and resulting force against the flywheel surface. Dealing only with the vertical component of the normal force is a relic of the cylindrical era, when the horizontal component was always zero.
Key element two is fairly obvious in my book. The whole reason to optimize profiles is to optimize the normal force distribution to achieve the greatest area under that curve with the minimum peak deformation. Deformation has consequences namely dart incompatibilities (n.b. people have had catastrophic results and lockups when a super high crush SSS setup is accidentally fed a FVJ dart) and higher shock loads due to the dart tip. If we didn't need to worry about that, we could just keep cranking old cylindrical flywheels, which have that deformation and force curve I mentioned with the giant peak in the center, as tight as we need to get the total normal force and thus grip we want, but clearly that is not going to end well. If we want to minimize the max deformation at any point and apply a given normal force over a given contact area, then, the best case is a perfectly flat force (and deformation) curve. We need to be crushing the entire dart surface uniformly.
(Note that DrSnikkas seemed to have the very same logic with flywheels that are also quite high envelopment, though I haven't had access to loose uninstalled ones to measure and know what the groove radius actually is.)
Key element three is about envelopment. We want to contact as much of the dart surface as possible with the flywheels. Not because "contact area" is itself worth anything, but rather because once again, it reduces the deformation needed to achieve a given area under that force curve. Or, maximizes the area achievable given a minimum flywheel gap.
And here is where all this logic is going.
This is a model of a high-envelopment large format flywheel showing a preliminary groove profile. It is 42mm root diameter, 9.0mm groove radius, and 49.5mm OD, intended for a 51mm center distance cage.
The design principle is (somewhat erroneously?) describable as hydrostatic compression at the system centerline, i.e. the projectile is subjected to a uniform radial deformation. Instead of forcing a round dart through a rectangular or football-shaped hole, we force a round dart through a smaller round hole. Why a round 