Friday, 29 April 2016

Mod: FK-180PH-3250 Rapidstrike Pusher Motor

Another motor experiment, this one of the -3250.

I wanted to get a 180 pusher motor that would get a slightly higher ROF than a -3050 on 3S (which gets 10-11dps), because I like high ROF and I find 10dps to be a tiny bit slow. Don't get me wrong, the -3050 is an excellent 10dps pusher on 3S, and in my opinion is the best ~10dps pusher of all the motors I've tried (only major one I haven't tried is the XP180). I just wanted a slightly higher ROF because I wanted something special and a little nuts.

The -3250 had no axle issues like the -3050 did, so was a simple drop in and resolder. Unfortunately, it spins just a little bit too fast, and so with a standard Rapidstrike wiring, tends to do trailing shots (ie you pull the trigger for 2 shots, then the pusher overruns and fires a 3rd). I went and changed the wiring from my current layout ("live centre") to a slightly different, more suitable one ("dead centre"). Torukmakto04 has an excellent wiring diagram that shows the difference:
Source: The Dart Zone
Live centre is a standard 3-switch build. It is how a Rapidstrike behaves in stock form (ignoring all locks and other unnecessary parts of course), and is how I like standard ~10dps Rapidstrikes. Regardless of when the trigger is released, the pusher will always complete its stroke and return to rest position.
The one difference between live and dead centre is that dead centre does not have the green wire. With live centre, if the pusher is not in rest position and the trigger is released, the pusher will be driven until it hits the switch and brakes. The problem is that if the pusher is travelling too fast or does not have enough braking torque, it will not brake completely and will travel (and drive) for another full cycle.
Dead centre solves this by removing pusher self driving, meaning that when the trigger is released, the pusher will coast, and if it hits rest position, will brake. This means that the pusher is far, far less likely to overrun, and cannot go "runaway", but also means that the pusher may not always stop in rest position, and may coast to a stop halfway through a cycle. For the most part, the combination of coasting and then braking will stop the pusher in rest position anyway, but in a small fraction of times (say 5-10% without significant practice), the pusher will stop while still out. This presents an issue for reloading as the pusher arm is still out and will get hit by the top dart in the new clip, but especially with practice and with loaded darts providing extra friction, this issue does not occur frequently.
Live centre is the preferred wiring schematic for anything that fires slower than ~12dps, since below that speed any good pusher should be able to brake to a stop with no problem. Faster than ~12dps, dead centre is necessary as just about any motor on live centre will just go runaway and won't stop itself.

ROF is around 12dps on a fully charged 3S, which was the target ROF. I was hoping for a pusher that would be able to stop itself reliably on live centre at 12dps, but unfortunately it overruns quite a fair bit. A diode or two to drop the voltage a little would probably put it at a perfectly controllable ~11dps on live centre, but I decided to stay with 12dps on a dead centre circuit.
Honestly I would not recommend the -3250 as an ordinary pusher motor. It's in an odd spot - it's just a little too fast to be reliable on live centre, but is substantially slower than the especially fast pusher motors that are often used.
If you want standard controllable ~10dps with live centre, I'd recommend the -3050 on 3S, -3240 on 2S or perhaps MTB's upcoming 180 on 3S if it goes into production. I don't recommend using a 130 for the pusher (at least for live centre), I used a Falcon pusher for a time and it overran a fair bit despite only being around 9dps and having quite a lot of torque for a 130.
If you want high speed ~15dps+, consider the MTB Honey Badger or -3240, both on 3S, and they'll need to be on dead centre, or MTB's 2 switch setup. They are guaranteed to go runaway on live centre, even the excellent 3240 does not have enough torque to brake to a stop at that speed.
Nevertheless, I quite enjoy having a higher than usual but still controllable ROF of about 12dps from the -3250.


  1. Are you considering trying XP180's anytime soon?

    1. As a flywheel motor, no. 17AUD each is far too much for me when I can get MTB motors for 5AUD or 8AUD each, and still have a few 6AUD Blade 180s/-3240s left over.
      I actually have one as a pusher for my Bullpup RS, there is a more recent post on that.

  2. I am looking to take a Rapidstrike and make it semi auto for use in a larp with rules about one trigger pull one dart. You seem to know what you're doing. Would you mind if I pick your brain? My minimal electronics knowledge is 20+ years old but I am pretty sure I can follow a circuit diagram.

    1. If you're looking for semi-auto, you would be much better served by just starting with one of the already semi-auto blasters available, e.g. Stryfe, Rapid Red, Modulus, Demolisher. Turning the Rapidstrike into a semi-auto would likely require an intricate timing mechanism or a microcontroller.