Arrma Boost metal diff implosion

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ShankAaron

ShankAaron

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Arrma RC's
  1. BigRock 4x4
  2. Senton 3s
Ahead of our jump to the 4S world, I bought the Boost Box metal diffs for the BRCC and got them installed. We took them out to a place here that used to be a WW2 US Army airfield (Miller Field) which is largely unchanged from its original design from back then. Just a massive, wide open grass field that's constantly maintained and mowed by the city. Figured it was the best place to go for a shake down run with the new diffs, plus, my boy and I just felt like letting the puppies run, you know?

Keep in mind that the BRCC is still a stock 3S truck, and I dropped both diffs in right out of the box without any changes. The upgrades/changes to the truck are in my build thread, but the important bits for this issue include metal A-Arms, steel RCAWD driveshafts, and a set of Trenchers. I let Bubba drive the truck around and the first thing he says to me is "Dad, this thing isn't running right. It's constantly pulling to the right". I didn't hear anything abnormal, and thought it might have been a stuck servo which we've been having off and on issues with over the past couple weeks. I had Bub's run it a little closer to me so I could listen for anything and as he's getting closer the truck snaps hard over to the right and starts cartwheeling. I also noticed he didn't turn the wheels when this happened. I get closer to the truck and notice the RR hinge pin pulled out of the chassis and the chassis is broken. I got it to the shop and found out the diff wouldn't turn at all, then found all this mess when I opened it up. Every tooth in the outer outdrive gear was broken off (not stripped, broken), the small center bearing was separated with no guts, and both end shafts had broken splines from the shaft grub screw mounting. The planetary gears don't look all that great, but don't seem to be damaged. You guys ever seen anything like this from Arrma metal diffs before?

20240402_043058.jpg
 
Wow, that’s crazy. Definitely a new one on me. Normally this kind of carnage is reserved for those madmen that put crazy 6s systems in them. It did this on a stock 3s truck on its first outing? It must have been a defective or poorly assembled diff from the factory. Many smart RC’ers check every diff before running. Sometimes I do, but mostly I don’t. I usually run a pack or two through them to allow them to run in, then pull them apart to check fluids and shim. I’d see if Horizon would warranty it. And then check the replacement.
 
I see that where the driveshafts connects to the diff, on both side the outgoing shafts of the diff are broken.
I can imagine when this fails under full load, you get such an enormous torque peak on the gear of the diff, that the gears fail.
What baffles me, is that you have the driveshaft-diff connection damage on both sides. And the torque in that connection should be transferred via the splines and not the hex bolt.

What kind of drive shafts do you use? Arrma brand or from ebay/Ali? EDIT: I see you use RCAWD driveshafts, and they also use 4x4mm set screws, like Arrma on the 4s line. Forget my next sentence.
I would think the hexagon head bolt that goes in there is too big and is hitting the splines, while it shouldn't.
 
Last edited:
Neelis said:
I see that where the driveshafts connects to the diff, on both side the outgoing shafts of the diff are broken.
I can imagine when this fails under full load, you get such an enormous torque peak on the gear of the diff, that the gears fail.
What baffles me, is that you have the driveshaft-diff connection damage on both sides. And the torque in that connection should be transferred via the splines and not the hex bolt.

What kind of drive shafts do you use? Arrma brand or from ebay/Ali? EDIT: I see you use RCAWD driveshafts, and they also use 4x4mm set screws, like Arrma on the 4s line. Forget my next sentence.
I would think the hexagon head bolt that goes in there is too big and is hitting the splines, while it shouldn't.
Click to expand...

Ok, so after reading this I double checked the RCAWD CVD shafts I installed the day I got the diffs, and the drive cups are smoothbore and the only connection to the output is that single grub screw. I'll definitely be changing these out asap, not sure with what, but the cups will definitely have female splines.. I don't know if this is the direct cause, but I'm confident that it was a contributing factor in the failure. Still, I'm still having a hard time working out how every tooth on the outer output gear snapped off one by one.

RCAWD BRCC CVD.jpg
RCAWD CVD.jpg
 
Don't underestimate the power that the drivetrain creates and the forces these small teeth have to deal with. If one of the splines of the output gear fails at full throttle and then the set screw says hello to the next spline, I am not surprised something had to to give. Parts accelerate and 1/2mv^2 starts to its magic.

At least you found the issue.

I am now working on Mojave 4s arms and driveshafts on a Typhon 3s. If you are interested you can check my build thread.
 
Neelis said:
How often and what kind of drive shafts do you use? How tight is your slipper clutch?
Click to expand...
About once a month, Stock driveshafts (swapped from rcawd), and i would say my slipper clutch is not too tight.
 
Well I could of explained better but I strip the teeth on stock driveshafts, I strip both on the rcawd’s.
 
Neelis said:
Don't underestimate the power that the drivetrain creates and the forces these small teeth have to deal with. If one of the splines of the output gear fails at full throttle and then the set screw says hello to the next spline, I am not surprised something had to to give. Parts accelerate and 1/2mv^2 starts to its magic.

At least you found the issue.

I am now working on Mojave 4s arms and driveshafts on a Typhon 3s. If you are interested you can check my build thread.
Click to expand...

Yeah, this is just one of those eff around and find out moments. I contacted Arrma about it, but waiting on a response. There is a silver lining to this shitshow. My boy gets to work on and learn more about diffs, transmissions, and drivetrain.
 

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