Thursday, May 14, 2015

Demolisher Flywheel Cage: Interesting.

In my Tacmod 3.1 build I have come to the point where I need a flywheel cage before I can go much farther. The Tacmod cage is modified from the standard configuration and the bodywork is built around it.

Given that I have a Demolisher currently waiting to be turned into something experimental, I decided to check out what sort of parts were in there.

A few observations going in:

Cage geometry (see - the flywheel definitely looked deeper than a regular Nerf flywheel, and the cage wider. It is also noted that the gap has been decreased, and the flywheel diameter is not why, that's all in the cage.

Performance: All the following are data from FDS using the same chrono, procedure, etc. Demolisher, FK180SH-3240, 2S lipo; RapidPistol, FK180SH-3240, 2S lipo; Stryfe, FK180SH-3240, 2S lipo.

Now consider this, the Demolisher has the longest inner barrel of any of those. Yet it shot the hottest, with identical motor speed and torque curves.

So something's up. Whether it is the gap, the coefficient of friction (see below), the inertia (see below), something's not so ordinary about our nerfy Little Friend in construction orange.

So here is what I found:

Demo on the left, RS on the right! Note the two are aligned on the barrel axis!

The Demo flywheel is MUCH deeper. It does NOT align on the bore axis, it sits to the side and the dart contact is close to the non-hub edge in the stock installation.

The motor mount socket is deeper and the motor sits farther inboard.

The flywheel is a different material. It seems to be ABS or a similar styrenic - not the familiar Delrin.

Here you can see the two cages lined up snout to snout, showing the mounting ears aligned. Note that the motor side is nearly identical in terms of where the wall of the cage is, but the cover side is pushed out quite a bit to accept the taller flywheels on the Demo cage.

Compatibility observation: the RS cage in stock configuration has more crap sticking out farther on the cover side than the Demmo cage, regardless. So there should not be a clearance problem with installs of these into the Stryfe/RS.

Here are the cages from the feed side:

Demo (motor side left)
RS (motor side also left)

The stock flywheel installation leaves a LOT of room for the fly to be seated farther on the motor shaft. This is a good thing for FK series Mabuchi users, because the shaft length is not ideal, and this cage allows moving this flywheel closer to the motor for more shaft engagement without affecting what is seen by the dart at all nor having the flywheel dangerously close to anything.

This position is with the entire hub bore engaged on the FA130 shaft. A regular fly in a regular cage would be about centered when installed similarly.

The Demo cage bolts up to the front mount bosses on a RS barrel shroud. The alignment all works.

The lower rear receiver boss also bolts up, but the upper one doesn't and would need to be modified and some webbing trimmed.

But here's the problem:

The Demo flywheels I have fit WAY TOO LOOSE on typical motor shafts. No way I would trust them to stay on.

I have considered sending a flywheel job to a machine shop, get them copied (with slightly larger OD and no taper) in aluminum, and kiss the balance and shaft fit problems goodbye forever, but if I am going to do that, there is no point in using the Demmo cage at all. I can get the same result as a Demmo cage in the Stryfe cage if custom flywheels become involved, and the Stryfe cage bolts up directly without modifying the receiver of my RS.

Since the flywheels aren't Delrin, I could use an old trick, and use some superglue down the bore to tighten them up. Hell, I could even sand my shafts and glue the flies the hell on. But do I trust that? It gives me the heebie jeebies to be running around with that sort of hack thing done in HvZ, with XP180s that will have shocking torque trying to bust apart anything in that bore area. Delrin and proper fit would give me peace of mind that a few FPS are not worth. Plus I want to try the Stryfe feed guides still.

So back to the hunt for a cheap Stryfe to perfect the TM3.1.

I figured I would post this anyway as it may be useful information.

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