Kraton EXB Rear Diff Discovery ---UPDATES!!

[Engineer]

Welcome to the Arrma Fam

I’m from NW Ohio, Toledo

And of course, now the parts for the “fix” are unavailable ANYWHERE ??‍??‍???

Yep - I ordered some diff plates from Omni on eBay this weekend and just got the “out of stock, we’re sorry but not sorry” email and a refund.

 

[Adawg]

?‍ tune the shims if you think it will solve the problem (I'm positive it won't!) Your pictures are helpful because it confirms that the shims don't cover the surface area of the gear, like the LS plates do

There's a reason why HH are sending out LSD diffs as replacements, instead of "correctly" shimmed open diffs!

Bro the clutch pack in a limited slip diff floats and then it start to slip it limits. If they were so concerned about covering the entire part of the gear they wouldn’t use two smaller diameter shims they would make all 3 larger diameter. All that matters is that the shim thickens pushes the sun gear into the planet gears. I’ll just give you one last example. Look at how they put shims on the outdrives or anywhere else there is a bearing. They use a small shim That only sits on the inner race and it’s not like the bearing flex’s over. All that matters is that your moving the object over.

 

[SrC]

Thanks dude! Killer suggestion! I didn't think of that. I'll do that until they figure out the EXB BS. I emailed HH. I saw on YT, some people that got ahold of HH with the same problem are being sent an entire new LSD diff to replace. Hopefully, that will be my case, as HH seems to be going that route for the fix now. If the LSD fixes the problem with the rear diff, that will be awesome! My only question is, is there any downside to running full LSD? Handling? If that's key, why didn't they come that way to begin with?

Why not just use a standard non EXB open diff at the rear? Would need the standard diff bulkhead.
Is that also an option? At least until the EXB Rear positioned open diff is sorted out? IDK
edit : silly question....

 

[Mike-NLD]

@jym73

Mine are allready here..

Ordered a set of internal gears aswell.. After i checked the diffs ill be running it stock.. If it breaks.. It breaks.. New internals on hand.. With that rebuild ill put in the LSD plates..

 

[bicketybam]

If you want to run the LSD but tone down the effect, you can do this:

I definitely don't want aggressive LSD in tbe rear with my power setup.

 

[Yas]

Bro the clutch pack in a limited slip diff floats and then it start to slip it limits. If they were so concerned about covering the entire part of the gear they wouldn’t use two smaller diameter shims they would make all 3 larger diameter. All that matters is that the shim thickens pushes the sun gear into the planet gears. I’ll just give you one last example. Look at how they put shims on the outdrives or anywhere else there is a bearing. They use a small shim That only sits on the inner race and it’s not like the bearing flex’s over. All that matters is that your moving the object over.

It's also about stabilizing the gear too. Yes you need the correct thickness of shims, but you also need the correct diameter, or else it will torque twist the gear. Can you see what I mean?

Example: A standard sun gear from a RTR 6S has a 5mm shaft bore and an OD of 17mm. Which would work better: 5x17 shims which cover the gear's surface area, or 5x6 shims which hardly cover the gear's surface area at all?

If you want to run the LSD but tone down the effect, you can do this:

View attachment 92723

I definitely don't want aggressive LSD in tbe rear with my power setup.

That's a great diagram! Is this in the manual?

 

[jym73]

@jym73

Mine are allready here..

View attachment 92717

Ordered a set of internal gears aswell.. After i checked the diffs ill be running it stock.. If it breaks.. It breaks.. New internals on hand.. With that rebuild ill put in the LSD plates..

I won't even try the open diff. and put the LSD plates directly.

If you want to run the LSD but tone down the effect, you can do this:

View attachment 92723

I definitely don't want aggressive LSD in tbe rear with my power setup.

Great info, thks for sharing.

 

[Adawg]

Yas I think you logic does make sense. I fully understand what your saying and why you would think that. I guess the question is does this actually do that. We just have a different opinion and that’s ok. If you hold a sun gear it’s too thick to flex. It just would crack. That’s my only difference is I don’t think it’s gonna flex and I don’t think another couple mm larger shim would make a difference. I could be wrong. You could also find a shim with the same inner diameter but a larger outer if it was something you think Is a weak spot

 

[Yas]

Yas I think you logic does make sense. I fully understand what your saying and why you would think that. I guess the question is does this actually do that. We just have a different opinion and that’s ok. If you hold a sun gear it’s too thick to flex. It just would crack. That’s my only difference is I don’t think it’s gonna flex and I don’t think another couple mm larger shim would make a difference. I could be wrong. You could also find a shim with the same inner diameter but a larger outer if it was something you think Is a weak spot

I don't think it will flex, rather I think the gear becomes unsettled whilst under torque. Thanks any how ? nothing wrong with different thought processes when problem solving ? the more the better!

 

[Adawg]

Absolutely, it’s the debate that brings up other points of views that solve things. I think if it was a single input gear then it would make more sense like your saying. For this particular situation you have 4 planet gears pressing on The face of the sun gear some what evenly. Honestly I think I’m the end it’s gonna take some experimentation to see what works long term

 

[bicketybam]

Well I figured I'd pull my rear diff and check it out. I have it out of the truck now and when I turn the outdrives in opposite directions it is smooth as silk. I've only run the truck once when I made my first run video but I put a lot of power through it and I definitely wasn't easy on it. Once I get it apart and cleaned I will report on what I find. It's probably worth noting that I do not have an external shim on either side.

 

[SrC]

 

[TooTall]

In my experience form feel and sound with no fluid in the rear diff, the shims definitely make a difference, now not sure about @Yas point about it unloading under situations, but I do know that when you tighten the screws it is definitely pushing the whole thing together, so I don't think there is space to tilt at all.

I really think the part number swap is a contributing factor in this, I work in vehicle assembly plant and bill of materials are followed strictly if those part numbers being swapped made it to the assembly area, it could have made a 0.2mm difference total for the whole diff.

I understand Arrma's thing about 0.7 or 0.8mm depending on diff, but the diagrams don't show that, and every Arrma diff I have opened has always been built the same, they might have been hoping that 0.7 would be good enough, but I have yet to see anyone post anywhere that they had 0.8mm in their diff, everything I have seen has been 2 large and one small diameters, which adds up to 0.7, but diagrams all show 1 large and 2 small diameter which would be 0.8mm.

Just my 2 cents.

TT

 

[SrC]

I feel those 29mm diffs were originally designed as LSD's. For some reason they made sure the rear diff was open.
Was it to save $$.
Was it because an open diff at the rear made it drive much better?
Perhaps converting the LSD diff to an open design for the rear opened up a can of worms by removing the plates and shimming became a problem.

 

[bicketybam]

Ok. Here is my report. First, both sun gears had a total of 3 shims under them. In addition, all of the internal gears looked brand new.

The wider shims measured 0.2 mm. The smaller diameter shims measured 0.32 mm. When stacked they measured 0.69 mm. I know the math doesn't work but I blame my cheap calipers.

Next I dry fit the diff together to see how it felt. It felt smooth but you could feel the gear. I wouldn't call it notchy like some of my previous Arrma diffs. And again, no fluid so I was expecting it to feel this way. Lastly I refilled with 30k and it felt like I would want a 30k diff to feel like.

I've read the theories surrounding the sun gears and I'm leaning towards the notion that the cut out shouldn't be an issue. With the 3 shims on each side the two sun gears are meshed right to the planetary (smaller gears). I think what is causing the issue is the incorrect number of shims in certain units. Fortunately mine has the correct number. Another possibility could be a bad run of sun gears and/or shims. I do not think it is a design issue.

 

[Buckwhea7]

Thanks for checking yours out Bam! That is helpful. My rear diffs also have 3 shims on each side and they didn't last 15min. I checked them before I ever ran the truck and filled it with 20k fluid. So weird?‍

 

[glooba]

This is my diff, never run, delivered from Horizon Hobby. I have 3 shims on either side. I think the reason why we're seeing rear diff breakages is not only because of some diffs shipping with only 2 shims, but even those with 3 shims the thickness is inadequate. I'm sure we can agree that .25mm undersized on either side for a total of .5mm off at least would very likely cause teeth to break off.

 

[bicketybam]

Thanks for checking yours out Bam! That is helpful. My rear diffs also have 3 shims on each side and they didn't last 15min. I checked them before I ever ran the truck and filled it with 20k fluid. So weird?‍

That is really odd. What exactly broke on the inside? The teeth on the sun gear? If you still have the shims I'd measure them stacked and see what you come up with.

View attachment 92762

This is my diff, never run, delivered from Horizon Hobby. I have 3 shims on either side. I think the reason why we're seeing rear diff breakages is not only because of some diffs shipping with only 2 shims, but even those with 3 shims the thickness is inadequate. I'm sure we can agree that .25mm undersized on either side for a total of .5mm off at least would very likely cause teeth to break off.

Is that 0.54 the 3 shims stacked? It must be. What does each individual shim measure? They must be way off.

 

[jym73]

Your caliper should be very good quality and calibrated when measuring thing kind of thin thickness, otherwise it is useless to try to verify the thickness.

 

[bicketybam]

Your caliper should be very good quality and calibrated when measuring thing kind of thin thickness, otherwise it is useless to try to verify the thickness.

It's what they sell in home depot and it has measured my screws to with in a few hundreds of a millimeter. I don't think are that far off that a 0.2 mm shim would read 0.3.