Arrma 3/4S metal diff bound 1/4th turn. Losing my mind

[bukshypro]

So this is a full, £90 metal diff from Arrma. I wanted to change the yoke to the red metal ones, but I also had to refluid the diff because we know Arrma (it was about 20% filled and with only 10k).
After re-assembling, the sides of the crown gear are not the same, so as I am turning the diff around, its binding a bit more on one side. It's doing it with the plastic yoke too, but not as much. the metal diff came with 3 shims, if I take them off, the binding is not there, but then there's a very little play between the metal input gear and the crown gear (way less than how much we want on the spur/pinion for instance). iirc, we want this to be as tight as possible.

• my clues are: the shims are poop and maybe a bit bent? but shouldn't the whole diff and bearing just push them fully down?
• I might have overfilled (I actually did) the diff and so one side is bigger than the other? this clue because I tried to keep tightening the diff and so a bit of the play went away, but I cant keep tightening it more because the diff housing is just composite.

any clues people?
here are some vids where you can see the play a bit; like how I turn it around one side always goes a bit higher, and that's what the input gear then binds, cuz input gear stays cool.

vids:

Slow mo vids:

To add: I took it apart, cleaned the case and all planetary gears and everything and build it back together and refluided it. It still binds.

I put it in the car too to see and I can definitely feel the binding when I roll the car.

Any tips ?

 

[Grimbles]

Have you tried backing out the yoke screws a very small amount? 1/4 turn or so? I used to get this issue with the plastic diff yokes if I had overtightened.

 

[bukshypro]

Have you tried backing out the yoke screws a very small amount? 1/4 turn or so? I used to get this issue with the plastic diff yokes if I had overtightened.

hey, unfortunately the uneven turning is there even when the top yoke (the input gears yoke) is not put on, so I don't even necessarily get to the point where I have the yoke together. I can already see with only the diff being in the yoke that it turns uneven for 1/4th of the full turn. So for whatever reason, one side of the crown gear is a bit higher and so it creates the eneven-ness. If I remember well, I was able to change the uneven-ness by playing around with the 4 screws on the diff lid+crown gear, but there's not much wiggle room since that has to be tight so the silicon doesn't leak out :/

 

[Grimbles]

hey, unfortunately the uneven turning is there even when the top yoke (the input gears yoke) is not put on, so I don't even necessarily get to the point where I have the yoke together. I can already see with only the diff being in the yoke that it turns uneven for 1/4th of the full turn. So for whatever reason, one side of the crown gear is a bit higher and so it creates the eneven-ness. If I remember well, I was able to change the uneven-ness by playing around with the 4 screws on the diff lid+crown gear, but there's not much wiggle room since that has to be tight so the silicon doesn't leak out :/

Hmm... is the diff the right way in?

 

[bukshypro]

Hmm... is the diff the right way in?

Yes, it is. It wouldnt work the other way, because the little tab by the input gear side yoke would block the crown gear.
I suspect, its composite diff case, thats uneven or something

 

[Camaroboy383]

Unfortunately, it sounds like you didn't have one of the pins aligned when you reassembled the diff... One mistake commonly made on these...

When you pulled it apart to add fluid one of the pins most likely came out.. without cleaning the case out completely it's very difficult to get the pin back in the bore with fluid present...

If this is in fact the culprit the composite case is usually smoked after being tightened down.. wherever the pin lies, it will drive it into the case (typically right against the side of the bore)...

I would tear it all back down and check the bottom of the case.. it will be plenty apparent if the pin wasn't in the bore..

 

[bukshypro]

Unfortunately, it sounds like you didn't have one of the pins aligned when you reassembled the diff... One mistake commonly made on these...

When you pulled it apart to add fluid one of the pins most likely came out.. without cleaning the case out completely it's very difficult to get the pin back in the bore with fluid present...

If this is in fact the culprit the composite case is usually smoked after being tightened down.. wherever the pin lies, it will drive it into the case (typically right against the side of the bore)...

I would tear it all back down and check the bottom of the case.. it will be plenty apparent if the pin wasn't in the bore..

Hey,
First time I didn't even take the pins out, because I just kept the bottom(deeper end) of the case and filled it up w diff fluid. Second time, I took it all apart, cleaned all the gears, planetaries, diff case, both diff outdrives, holes with a broken toothpick (to clean the silicone out), and only then I filled it with silicone.

The diff also spinned freely, without problems, so the pins, planetaries and diff outdrives were in place.

But the tip with the pins is still a good idea to check, because its obvious that one side, so about 1/4th of the case/crown gear was a tiny bit higher hence the bound

 

[Camaroboy383]

You didn't have to pull the pins the first time.. when you pull it apart it will do so on its own via suction..

Take a picture close up of both the upper case bores and lower case bores..

Edit; also, just because the pin wasn't in the bore wouldn't prevent the internals from spinning, especially if the pin was leaned outward towards the outer edge of the case...

 

[Ollie_M]

does it bind to the point where it locks up or does it just go tight? if it goes tight i personally would just run it for a bit and it'll loosen up then check your shimming

 

[bukshypro]

You didn't have to pull the pins the first time.. when you pull it apart it will do so on its own via suction..

Take a picture close up of both the upper case bores and lower case bores..

Edit; also, just because the pin wasn't in the bore wouldn't prevent the internals from spinning, especially if the pin was leaned outward towards the outer edge of the case...

Thats an amazing tip, regardless of how it felt or how it worked. I will take it apart again and see the heights of the pins and all.

I also see some vids saying the shims go on the composite case side and some say it goes on the crown gear side. So where it goes then?

Its a bit ridiculous tho, that one buys a £90 original part that is 1/4th filled w silicone, has to be taken apart to fix and then prone to mistakes and malfunctioning.

If it doesn't work or it broke in the process, I ll send it back. A £90 item should be PERFECT without me having to work on it.

does it bind to the point where it locks up or does it just go tight? if it goes tight i personally would just run it for a bit and it'll loosen up then check your shimming

It just binds a bit, but only like on 25% or 1/4th of the full turn. When my new plastic spur gear did this, I left it because I knew it had to be broke in, but with metal parts I am worried that shouldnt be like that.

 

[Camaroboy383]

The shim goes where it is necessary to maintain a good mesh/ backlash... Only you can determine where and how thick the stack is, there is no one method or placement..

When you do pull it all back down, definitely take some pictures of the pin bores for us to see... Both of the upper cap and the lower case...

 

[bukshypro]

The shim goes where it is necessary to maintain a good mesh/ backlash... Only you can determine where and how thick the stack is, there is no one method or placement..

When you do pull it all back down, definitely take some pictures of the pin bores for us to see... Both of the upper cap and the lower case...

Family is going out soon so I ll take it all apart again, make pics, clean it, put it back together, make pics of how it looks and see how it goes, make more pics and post here.

Thanks for the tips

 

[Ollie_M]

Thats an amazing tip, regardless of how it felt or how it worked. I will take it apart again and see the heights of the pins and all.

I also see some vids saying the shims go on the composite case side and some say it goes on the crown gear side. So where it goes then?

Its a bit ridiculous tho, that one buys a £90 original part that is 1/4th filled w silicone, has to be taken apart to fix and then prone to mistakes and malfunctioning.

If it doesn't work or it broke in the process, I ll send it back. A £90 item should be PERFECT without me having to work on it.


It just binds a bit, but only like on 25% or 1/4th of the full turn. When my new plastic spur gear did this, I left it because I knew it had to be broke in, but with metal parts I am worried that shouldnt be like that.

I would just run it and get on with it

 

[bukshypro]

I would just run it and get on with it

You did read my mind then, I joined fam and will get the car through 2 packs. will disassemble after to see how it is

 

[bukshypro]

Test failed.

Reason: other.

 

[Velodromed]

the metal diff came with 3 shims, if I take them off, the binding is not there, but then there's a very little play between the metal input gear and the crown gear

That’s okay to have a minuscule amount of play in a metal diff. It’s how I set mine up.
Is the binding consistent all the way around or is it just in spots? You may need to Dremel this part below a little bit. It can catch on the raised screws on the face of the ring gear.

Popping a drive shaft typically means you spur is to tight. Especially with metal diffs I run my spur a touch on the loose side. Spur pads can be sanded a few times when they glaze over and are also cheaper than drive shafts. You have high traction large knobs tires it looks like also, which adds to the drive shaft stress especially when running in high traction surfaces like grass. Those big knobs just bite in, giving too much traction.

 

[bukshypro]

That’s okay to have a minuscule amount of play in a metal diff. It’s how I set mine up.
Is the binding consistent all the way around or is it just in spots? You may need to Dremel this part below a little bit. It can catch on the raised screws on the face of the ring gear.

Popping a drive shaft typically means you spur is to tight. Especially with metal diffs I run my spur a touch on the loose side. Spur pads can be sanded a few times when they glaze over and are also cheaper than drive shafts. You have high traction large knobs tires it looks like also, which adds to the drive shaft stress especially when running in high traction surfaces like grass. Those big knobs just bite in, giving too much traction.

View attachment 266581

View attachment 266583

Thanks for your answer!

My binding did not get me to actually place it inside the diff cover/holder. If you check the videos, it was rotating even. I suspect something is pushing one side if the crown gear/composide diff holder cap up a bit. It was consistently on one spot, where the crown gear would be a bit higher.

Btw I did run 12min on my car, gave it good pressure in the grass(plenty full throttles) and it sounds good.

The drive shafts already had tons of pressure and they were next to fail anyway. My spur gear is great. It was a bit tight when installed because its form from factory was a bit weird, but I ran it in and it has great mesh now(bit of clickiness on each teeth).

Because my rear diff kept going off and now the full metal diff is in, I expected the driveshafts to be next.

 

[Velodromed]

Thanks for your answer!

My binding did not get me to actually place it inside the diff cover/holder. If you check the videos, it was rotating even. I suspect something is pushing one side if the crown gear/composide diff holder cap up a bit. It was consistently on one spot, where the crown gear would be a bit higher.

Btw I did run 12min on my car, gave it good pressure in the grass(plenty full throttles) and it sounds good.

The drive shafts already had tons of pressure and they were next to fail anyway. My spur gear is great. It was a bit tight when installed because its form from factory was a bit weird, but I ran it in and it has great mesh now(bit of clickiness on each teeth).

Because my rear diff kept going off and now the full metal diff is in, I expected the driveshafts to be next.

They don’t have to be. My 4, 3s cars run stock driveshafts. Only my two 4s cars have 4s cvds. I tend to get around 40-50 packs per set of drive shafts. Well, the rears get a little bit less but the fronts last a long time. I just keep the slipper a bit looser than normal.

It works well, I hammer the throttle, love wheelies and run it hard through high traction/low traction surfaces, all the things you’re not supposed to do. When the slipper starts to scream a bit too much, I simply pull it apart, sand the slipper discs and swap the order putting it back together.

 

[bukshypro]

hey people.
I don't understand the physics of this whatsoever, but I took it apart again and just put all 3 shims on the opposite side and everything is perfect.

Lmao, by putting the shims on the top, I actually loosened the whole thing. Now Im unsure how to decide whether the mesh is tight enough or not.

Anyway, I got some pics, who can spot it?

 

[Velodromed]

hey people.
I don't understand the physics of this whatsoever, but I took it apart again and just put all 3 shims on the opposite side and everything is perfect.

Lmao, by putting the shims on the top, I actually loosened the whole thing. Now Im unsure how to decide whether the mesh is tight enough or not.

Anyway, I got some pics, who can spot it?

View attachment 266664

View attachment 266665

View attachment 266666

Are the bearings fully pressed onto each side? They should be flush up against the diff. I use a socket and the plastic handle of a screw drive to tap it tight. If all is good there…

Hold the input gear shaft and move the ring gear back and forth. A tiny bit of play is fine so if you hear a slight clicking sound you’re good. If there’s a lot of play, remove 1 of the shims and try again. Make sure you tighten the yoke screws well when testing it.