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I know that there been testing that has shown that past a certain amount of rpm FPs is lost as the flywheels can't gain purchase on the darts as they are spinning so fast.

What is that RPM ceiling again?

... And do we know if after market cages and concave flywheels can allow more RPM used efficiently?

I think the RPM limit was theorized in the 35k to 40k mark, i'm sure one of the more technical mind will be along shortly to correct me.

I'm not sure if the aftermarket cages/flywheels will increase the rpm limit, their machining removes toy quality manufacturing tolerances, reduces vibration, and increases dart squeeze, so the flywheels spend more time in contact with the dart.

My experience so far, via Daredevil and Nagnori tri-hellcat rapid pistols with aftermarket cages and flywheels are up at the 140 to 150fps mark and theres no obvious dart burn going on, though a fair few deheadings...

For standard flywheels/cages it's closer to 22-25kRPM (at 22,000RPM the outer surface of a 33mm diameter flywheel is travelling at the equivalent of 38m/s or 125fps - at 25,000RPM that becomes 43m/s or 142fps). The reason why you need to use motors that spin at 30k+RPM is because you need to stay above that critical speed under load. A Rhino, for instance, only spins at 33kRPM at no-load - under load (and especially at high RoF) that can easily fall below 30kRPM, possibly lower. Obviously if the flywheel is spinning faster than that then it will never stop "slipping" on the dart (this is why "dart slippage" simply isn't a thing - you never want it to gain traction and enter static friction if you want consistent high velocity regardless of RoF). The no-load speed required to stay above critical will vary based on the motor (you can tend to get away with a lower no-load speed with a 180 motor since it produces more torque and therefore slow down as much as an equivalent 130 motor under a given load).

The reason why an aftermarket flywheel/cage can push this "glass ceiling" higher is because they increase the amount of friction between the dart and the flywheels. Friction (both dynamic and static) is proportional to the normal reaction force applied to them (F=μR) - the harder you press down on something the greater the reaction force and so the greater the friction between the two surfaces that are being forced together. If you make the gap between the flywheels smaller (either with larger flywheels, a flywheel cage that moves the motors closer together or a combination of both) then the friction between the surface of the flywheel and the surface of the dart increases as the dart is squeezed through them. You can also change the material/surface finish used to change the coefficient of friction "μ". Surface area largely shouldn't matter (other than for thermal/wear reasons that could alter the coefficient of friction) so flywheels that are concave likely increase the frictional force between the surfaces by preventing the dart from flattening out thereby working to increase the normal reaction force "R".

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