Sunday, August 30, 2015

Nerf Rival Zeus - Lightning Link, RS390-4735 Swap, Stock Motor Info

Last time I posted about my Zeus, it was still semi-auto. Shortly after, I decided to change that, because I am strongly dissatisfied with the stock trigger's heavy pull and unreliability at higher ROF. IMO, something else needs to be done or developed aside from the standard Zeus if you want to use Rival automatics as a general-purpose (semi-auto or up to 15rps full auto with precise controllability, in my book) primary.

So my Zeus is now converted to uncontrolled feed. While it isn't the most practical with 25+ rps on a 12 round mag and awkward reload, the trigger is nice, and it is awesome.

There are 4 changes to make in order to get this functionality:

* Remove the ram lever and its torsion spring.

* Remove the rear gate and its shaft and torsion spring.

* Grind out the detent on the right feed housing half. Visible at front inside the feed path

* Grind out the detent on the front gate (the thin area above the semicircular curve). You want this gate to match the feed path seamlessly when fully open.

After doing so you will have screaming fast full auto - take a loaded mag, open the detent by hand, see the balls shoot out? That fast, except going downrange at a hundred odd fps. It's a great special weapon. Unfortunately, I can't think up a good selective fire or 2-stage trigger strategy using the stock parts.

Motor Upgrades

EDIT: I guess I forgot to update this, but my advice is to avoid straight motor swaps to anything higher torque than the stock 360 on these flywheels. They will NOT hold up to any more torque with the stock hubs. This setup hammered out one of the shaft bores in the flywheel about a month later. If you want to replace, fix or upgrade motors for these stock flywheels, look into brushless outrunners.

The search for good Zeus motors has been ongoing, and thanks to /u/farmcoffee on Reddit (member of the Draugr team) I received a free pair of this 0.47mm/35 turn 390 motor which was my first stab at it.

Specs on this are sparse as with most Chinese motors, but what is specified is 21000RPM@11.1V giving a kv of 1892 - fairly low for a 35 turn wind, but such is the expectation for a long armature with this sort of aspect ratio like the 390 "stretch" motor. You can also see this with 130 and 180 motors and their turn count versus kv.

I did encounter a snag. I was sent two motors obviously of different batches. One had stronger magnets and different color leads, and definitely revved slower as is expected from the magnets. Not exactly promising for prospective buyers of these for Zeus installs. These motors, however, have external flux rings - a steel sleeve on the outside of the can, to provide an additional magnetic path. These are removable and optional, and removing them is equivalent to weakening the magnets - adding speed, reducing stall torque slightly, and reshaping the torque curve as expected from these changes; so I ended up removing the ring from the slower motor, cutting it in half and putting it back on and bam, perfect speed match. Even with the flywheel load and the imbalance of the stock Zeus flies, they match about as well as any .50 cal gun ever would - Not shabby at all.

Stock Motors

This is also the first time I (or anyone?) has pulled the stock motors from a Zeus:

Once again confirming 360. Appears to be a Johnson clone. I don't think Hasbro ever uses genuine motors, it's all Chinese generics except those SMCs in the barricade. Also we have 24000rpm@9.6V printed on the can, and this gives a kv of 2500.

The 3160 wind is rather weird to me because it seems to be deliberately suboptimal copper fill. Note that a common AEG motor (Stampede, Swarmfire, Vulcan) is a Mabuchi clone RS360PH-3560 meaning the standard 300 series armature stack can take at least 0.35mm wire at 60 turns. In any case it is unfortunate that this motor is wound this way, although it works fine it could have had more guts with 0.35mm wire. Was this something to do with current and the C cells perhaps?

The shaft is 2.3mm (of course), 8mm protrusion from the bearing and a total stickout of 11mm from the mounting surface. It has the typical rolled splines for extra grip that a lot of 300 series shafts have. Doesn't seem that critical, the flywheels install super tight. This however is a shorter shaft than I expected. That is good, makes finding suitable motors easier as virtually ALL of the 300 series motors in existence will mount this flywheel properly.

Mod wise on the gun:

* The Zeus has rubber sleeve style mounts. Those familar with Stampede gearboxes will remember them. I had to trim about 1/4" off the top edge of mine to clear the flux rings on the 390s which needed to stay since I matched my motors by modding them.

* The stock hold-down brackets are used with longer screws. Be careful not to overtorque and break/bend the bracket since it doesn't seat against anything now. FYI, mounting the motors with screws into the can is NOT possible with the stock flywheels because the flywheel web runs SUPER close to the mount socket surface. Unless you countersunk the screw perfectly (which might make it weak) you need to clamp your motors into the socket from the outside. Similarly, you need either the stock rubber or a shim between the can and the bottom of the mount socket. The bearing snout hole in the socket will align the motors without the rubber, but the bearing snout will stick out too far into the inside of the cage and hit the flywheel.

* Normal clearance hole in the left shell.

* Shafts zipped to length after installation. Necessary to clear some stuff, BTW.

As an update, here is what I have done endbell cover wise:

And a little fix for the nasty sharp edge underneath as well. No paint yet.

Some other stuff:

* Flywheel installation depth is with the vertex of the V-grooved flywheel surface set just below the edge of the cage.

* You can probably remove these flywheels with a proper puller, but the easiest method for most is to suspend the cage with the motor hanging free and the flywheel sitting against the mount socket, and drive the motor shaft through with a punch smaller than 2.3mm (I used an old 130 armature).

* I found some evidence of contact of the flywheels with the cage: OD of mount socket (due to mold flash inside the fly that I removed) and face of mount socket due to install depth. This was all factory. If something seems to have friction or make a funny noise in a zeus, investigate it, there is very little clearance in these things and rubbing flywheels is a possibility.

* Pinion removal on larger motors (if you receive or scavenge a motor with a pinion on it) - use proper puller or lock pinion in vise and drive shaft through with <2.3mm punch. Don't pry against the can. You can do that with little motor stuff, not these.


As I expected, these motors wanted 4S, so I am using the same 4 cell pack I was thrashing my stock motors on. By the numbers that is 28000RPM unloaded - compare to stock motors on 3S at 27750RPM unloaded (and smaller in comparison to the static load). Chrono data incoming but it shoots nominally.

Chrono session:

Units of feet per second. Mostly dead battery!

Handling of high ROF is definitely improved, I haven't seen a visible shootdown problem yet. It definitely doesn't get droopy and inaccurate on the end of a mag dump like the stocks did.

Response is improved as well.

Speed match is improved substantially.

Stall torque is now sufficient to not be jammable by feeding a ball into stopped flywheels. The 390 motors have enough grunt to force through even the densest Rival ball from a standstill, and if you are familiar with how much crush the Zeus cage has and how damn hard it is to manually push the balls through the flywheels that is seriously impressive. The brushes aren't even broken in yet and the battery is half dead! The stock 360s didn't have a chance of recovering from that.

It is nice to be able to pin numbers on things for sure now that we have stock motor wind and kv data. Aim for 28000+ rpm and as much torque as possible.

I think next I have to explore the RS380SH-4535 and -4045 and similar winds. It is a much more common motor, and genuine Mabuchis are readily available (a good thing, so you don't end up like me hacking around with flux rings or getting lemons of sketchy Chinese motors); and also a bit shorter than the 390, simplifying the mount and cover mods.

In the end though, I think the stock motors are absolutely fine. If you are only shooting semi-auto, there is not one reason to replace them. Just wire properly and run 3S and you are good to go. Couldn't be easier to understand, and it's a nice change from the little fiddly 130s and shitty metal brushes that you have to confront in a stock .50 cal flywheeler.


  1. What 3s battery would you recommend for a novice lipo user? Links are helpful. Thanks in advance.

  2. Have you stumbled upon any other motors of equal or greater spec but equal in size? i'm looking to accomplish higher velocities without having to modify the shell. Any suggestions?

    1. You really won't get much beyond the stock 3160s. You could try some 3560s out of Stampedes, but I don't think they would be worth the effort to pull flywheels to swap and if they don't have splined shafts it's a no go.

      Also, I have since done bigger and better things with this Zeus that I can't wait to post about and test, but it is currently down because the flywheels started turning on the shafts and wallowed the bores out. I can't say these big high torque motors are advisable long term without some sort of flywheel modification. The strength of ABS is not sufficient to form the shaft bore and the press fit just can't hold the torque. Time for metal hubs with set screws and shaft flats.

      Velocity? If that's what you want, you can get almost what these get with stock motors and 3S or 4S. These are all about ROF support.

      You will not find much that isn't longer and yet has substantially more torque than the 3160 oem ones. 360 is not a common size for any kind of aftermarket or hobby specific motor, all the RC stuff goes straight to 380. Most 360 motors are the stock Mabuchi and Johnson variety.

  3. Any further info on this? Missing reading up on the exploits of this.

    1. It's still waiting on me to draw up those flywheel hubs and get some machine work done. I've been busy and unwilling to spend much. It will get done eventually.