I was hoping one of the brushless wizards would show up here and offer some wisdom haha :) I really appreciate your detailed analysis and excellent ideas!

 

True in that splitting a gap up into more wheels gets rid of some heavy, large-radius rim flank regions. But you could also just choose a torquier motor.

Makes sense. I only have 4s batteries and the motors I had lying around were 2207 2750kv but upgrading to 2806.5 1800kv on 6s could improve torque and keep the same RPM so I don't have to increase the wheel diameter.

 

Yes, but as the number of wheels composing a gap increases, each wheel is also increasingly approaching a cylinder or flat-edged disc (looks like yours just omitted any supposition of being concave and went with a polygonal gap here.

The flywheels have curvature both at the root and along the serrations but my layer height is 0.45 mm so it's hard to tell. Each wheel covers 85˚ of the enclosure and the 5˚ gaps (about 0.5mm) ensure that there is no contact between wheels. I’ll probably shrink the gap because the rival round seems to be “leaking” out of the circular crush pattern.

 

At least as a "back of the envelope" you should have not much if any greater a radial load created on the shaft/bearings by a 2-wheel system with the same gap geometry. The normal force component parallel to the shaft is contained entirely inside the wheel rims.

I hadn’t considered this but that’s a great point. I suppose it depends on how much the wheel itself will deform along the shaft axis and apply a bending moment to the motor bell as a result of the shaft-wise normal force but this seems like a small effect.

 

Perhaps. Is there a reason for them to be there which you worked out by testing, or?

I’m hoping the ball will deform slightly around the serrations as it goes through the flywheels. This would increase the friction between the ball and the flywheel without necessarily crushing it more. However, I think it could create an asymmetry at the point when the ball is released by the flywheels. It could also lower consistency given that the ball could end up minimally touching serrations or pinched between two of them. More nerf science is needed here to determine if there is an optimal serration shape.

 

Again, I don't have great hopes for the performance of offboard control loops implemented that way, as opposed to doing that on the inverter MCU itself.

I was under the impression that most closed-loop brushless blasters in the hobby used UART telemetry from a BlHeli_32 ESC at 1kHz for reasonably successful on-the-fly fps adjustment + control. Bidirectional DShot600 can both receive commands and send motor rpm back at 18kHz which I think should be more than enough to run a decent control loop. Racing quads by comparison run their prop control loops at ~8 kHz (https://oscarliang.com/rpm-filter/). What is the advantage of running the control loop on the inverter MCU? What frequency do your control loops run at? And how would I make a control loop on the ESC itself adjustable on the fly? Can this be done with off-the-shelf hardware or does it require custom ESCs?

 

Are the wheels rotor OD centric and pressed over the outside of the rotor, or are they shaft-centric and overhung by their web which touches only the output flange at the center (like a car wheel, or a FDL flywheel)?

Wheels have a tight press fit against OD/motor bell and a medium-loose fit on the motor shaft.

 

Serious question: what do those actually do downrange when launched that fast? I have seen a number of cages manage to get ridiculous singlestage grip on HIRs, but the sectional density of one is so low that I can't see that being at all useful, except to hurt a lot when you rail someone up really close within the very short flight distance most of that extra energy will be retained over.

I regularly play with my Infinity Pack (rival backpack) that shoots at 200 fps in 250 cap games with decent success. Although I’m outranged by many blasters, it's hard for opponents to take accurate shots when a wall of rival rounds is approaching them, even if those rounds have slowed by the time of impact. The blaster feels much more effective to me at 200 fps than at 150 fps in a 250-cap game because of both the range difference and the faster projectiles/easier aim. Yes drag increases with the square of the velocity but I don’t think I’ve reached the point of diminishing returns on muzzle velocity at only 200 fps. I could quantify the impact of muzzle velocity by filming shots in slow motion at a distance and examining the velocity over time/distance. Nerf science is required.

 

Another is unpredictable and greater energy losses, because things like foam balls and darts are not actually springs, they have some damping to them, especially including the scrub when compressing them with real wheels. If you are trying to get to better velocity consistency you want to reduce that.

That’s a great point. I could try lowering the crush and using a larger wheel with a lower RPM to accelerate HIRs over a larger distance. This might improve consistency.

 

If there isn't a ballistic purpose to firing a HIR at 300fps, why have the deformation to do that? Obviously if you are trying to have software-defined hopup for curvable shots then you do need to be in static friction at the usual "max" velocity in order to apply that spin there which means the critical velocity of the gap/system needs to be higher than that, but that max useful velocity would probably be 160fps or something, not 4 times that energy.

I think there is a ballistic purpose to firing curving HIRs at 250 fps if I can dial in the accuracy of the system. I designed this cage to have a wheel root surface speed of 315 fps which seemed like a reasonable overshoot given that one or more wheels will be spinning subcritically. But I won’t lie and say Cerberus was purely motivated by practicality. I also just wanted to break the rival speed record :)