Kronos Stripping Diffs

This site may earn a commission from merchant affiliate
links, including eBay, Amazon, and others.
Stormwind

Stormwind

Well-known member
Messages
85
Reaction score
95
Location
UK
Corally RC's
  1. Kronos
As some of you may be aware, I've had several issues with my Kronos over the past few months and have been getting particularly frustrated at the money I have kept pouring it to try and resolve all the issues I have been seeing.

I had decided around Christmas time that I would just replace the whole rig with an Arrma Kraton 6S. By the time I got around to ordering it, it is completely out of stock all the way throughout Europe with model stores having no ideas when it will be back in stock and has been this way for a couple of months now.

That being said, I have now decided to spend that money instead on a smaller, cheaper rig (perhaps a cheap drifter or on-road car) and use the rest of the money to fix up the Kronos once and for all.
I am going to fix my ESC issues discussed in another thread by replacing the Torox with a Max8 G2 (keeping the Kuron motor).

The other main frustration I am having with the Kronos is the fact that it keeps stripping diffs (main bevel gear in the front/rear diffs).
I think I have replaced the diff gears a total of 3 or 4 times in a matter of months including a full diff replacement.
The usual scenario is that I notice the grinding sound, find the stripped gear, replace it. I then run it happily on 4S and everything sounds nice and happy. Run a couple of packs through it and then try the 6S. 1/4 of the way through the pack, crunch crunch.
Possibly the first time it happened was general wear and tear, after replacing it the first time, I had no real clue about the purpose of the shim and didn't position it correctly so stripped it again very soon afterwards.
The next time I replaced it, I made very sure that the shim was positioned correctly and the diffs spur and pinion gears were well meshed.
However, it has still stripped again under 6S power.

I would like to try and resolve this issue once and for all and so will likely buy a whole bunch of replacement parts. I'm fairly sure that both diff housings are fine so I may leave those as is but will likely replace all diff related bearings, the shims and the stripped spur again.
Is there anything else that I can easily replace or look at changing? I will of course be checking all pinions whilst I am in there.
I did watch a video before about what shims before where the guy added an additional shim / changed to thicker shims in his Arrma since the "gears can move as they bed in". Is this a general practice?
 
Stormwind said:
As some of you may be aware, I've had several issues with my Kronos over the past few months and have been getting particularly frustrated at the money I have kept pouring it to try and resolve all the issues I have been seeing.

I had decided around Christmas time that I would just replace the whole rig with an Arrma Kraton 6S. By the time I got around to ordering it, it is completely out of stock all the way throughout Europe with model stores having no ideas when it will be back in stock and has been this way for a couple of months now.

That being said, I have now decided to spend that money instead on a smaller, cheaper rig (perhaps a cheap drifter or on-road car) and use the rest of the money to fix up the Kronos once and for all.
I am going to fix my ESC issues discussed in another thread by replacing the Torox with a Max8 G2 (keeping the Kuron motor).

The other main frustration I am having with the Kronos is the fact that it keeps stripping diffs (main bevel gear in the front/rear diffs).
I think I have replaced the diff gears a total of 3 or 4 times in a matter of months including a full diff replacement.
The usual scenario is that I notice the grinding sound, find the stripped gear, replace it. I then run it happily on 4S and everything sounds nice and happy. Run a couple of packs through it and then try the 6S. 1/4 of the way through the pack, crunch crunch.
Possibly the first time it happened was general wear and tear, after replacing it the first time, I had no real clue about the purpose of the shim and didn't position it correctly so stripped it again very soon afterwards.
The next time I replaced it, I made very sure that the shim was positioned correctly and the diffs spur and pinion gears were well meshed.
However, it has still stripped again under 6S power.

I would like to try and resolve this issue once and for all and so will likely buy a whole bunch of replacement parts. I'm fairly sure that both diff housings are fine so I may leave those as is but will likely replace all diff related bearings, the shims and the stripped spur again.
Is there anything else that I can easily replace or look at changing? I will of course be checking all pinions whilst I am in there.
I did watch a video before about what shims before where the guy added an additional shim / changed to thicker shims in his Arrma since the "gears can move as they bed in". Is this a general practice?
Click to expand...
The Kronos isn't known to have weak diffs. If anything, the stock Arrma diffs are known to be it's weak point!

If you're stripping the crown gear, the mesh between it and the input gear can't be right. There mustn't be any backlash on these gears. They shouldn't be tight, either. The right number of shims will depend on how yours mates up.

I've had mine since November 21 and had zero diff failures. I don't smash mine about, but I do rip it around at high speed on 6S. Setting the mesh on the input crown gears is more critical in some ways, because it's not readily adjustable like the pinion/spur mesh.

What I will say is that there's less accuracy with moulded parts, so the transmission cases are never going to be exactly the same. This is why some may need more or less shims to get the mesh right than others.

The Kraton 6S EXB looks great and the alloy transmission cases look nice. 😎

Setting the mesh right is definitely easier than building drones! 👍💪🫡
 
And yes - as the gears wear, you may need to add shims. Putting the shim/s on one side will will move the crown away from the input gear, where as on the opposite side, it will pull the crown towards the pinion. Some models will also provide shims for the input.

Corally don't specify shim requirements on the front and rear diffs on my 21 version.

They do offer these though:

1000016104.png


Shimming is quite clear in my Xray XT8 17 manual:

See the dimensions of the diff and note the acceptable tolerance:

1000016106.png

If the diffs are larger than the specified size - there's no room for any adjustments with the shims. Even the difference within specification is 2 shims thickness.
 
Another area that you may want to inspect are the drive cups and bearings and the center diff mounts. Check the cups and the bearings for excessive play; that could be one of the culprits for your grinding. Check the diff mounts as well to make sure that the diff is level and not canted fore or aft, or canted port or star.

You'd want to do the same for your F/R diffs; make sure the cups don't have excessive play and that you're shimmed on the drive cup for proper mesh. Make sure the F/R diff boxes are tightened down sufficiently as well.

Just some areas to look at, off the top of my head.
 
VaporTrail said:
Another area that you may want to inspect are the drive cups and bearings and the center diff mounts. Check the cups and the bearings for excessive play; that could be one of the culprits for your grinding. Check the diff mounts as well to make sure that the diff is level and not canted fore or aft, or canted port or star.

You'd want to do the same for your F/R diffs; make sure the cups don't have excessive play and that you're shimmed on the drive cup for proper mesh. Make sure the F/R diff boxes are tightened down sufficiently as well.

Just some areas to look at, off the top of my head.
Click to expand...

Thanks for this input, I originally started looking at the centre diffs at the very start of my issues and had that on and off many times. This could be one of the causes of my issues. I'll certainly replace the bearings there and possibly also the housing too if it's cheap enough. I'm sure I upgraded the entire central cups to be the 7075 ones fairly recently so sure they won't be too worn or anything.

that you're shimmed on the drive cup for proper mesh
Click to expand...
Do you just mean the shims that sit over the cups that connect to the wheels or are you saying I should potentially be adding a shim elsewhere?

Does anyone know what size the shims actually are? I can't really see anything about them in the manual at all.
I can't say I particularly want to spend £40 on a big shim pack alone when I will only want a couple.
This is pretty much all the Corally shims available at my model shop:
1708771338061.png
 
Stormwind said:
Does anyone know what size the shims actually are? I can't really see anything about them in the manual at all.
I can't say I particularly want to spend £40 on a big shim pack alone when I will only want a couple.
This is pretty much all the Corally shims available at my model shop:
View attachment 5471
Click to expand...
The diff shims would likely be 13x16x0,2mm (or maybe 0,1mm), so none of the ones pictured would work.
The outer diameter would be the same as the bearing diameter.
 
The bearings are 8x16x5.

So shims/spacers need to be >8x16x0.1(or 0.2) - thinner ones being easier to adjust (by adding or removing).

FG 8x16x0.1mm shims (2pcs) for viscous differential. Part # FG 8600/12
£3.99

Or

HUSQVARNA KTM SHIM 8X16X0,1​


£1.53

These will all fit, but

Edit to add:

!!!Note: It's easier to fit 13x16x0.1 shims over your outdrives!!!

This is what I'll be ordering for my T2E and Kronos.

Kyosho Shims 13X16X0.15mm Sus.​


MB Models - Diff Shims

Cheaper at Modelsport!

Modelsport Diff Shims
 
Last edited:
Jeffwazz said:
That link should have been pointing towards a 16mm OD, 10mm ID, which would be functional, but not ideal.
Click to expand...
Oh, right. 👍

£5 for the Kyosho ones. Most diffs are supported by 16mm bearings these days, so they should fit numerous different models. The only question is what diameter are the outdrives. 13mm should clear fine.
 
Stormwind said:
Thanks for this input, I originally started looking at the centre diffs at the very start of my issues and had that on and off many times. This could be one of the causes of my issues. I'll certainly replace the bearings there and possibly also the housing too if it's cheap enough. I'm sure I upgraded the entire central cups to be the 7075 ones fairly recently so sure they won't be too worn or anything.


Do you just mean the shims that sit over the cups that connect to the wheels or are you saying I should potentially be adding a shim elsewhere?

Does anyone know what size the shims actually are? I can't really see anything about them in the manual at all.
I can't say I particularly want to spend £40 on a big shim pack alone when I will only want a couple.
This is pretty much all the Corally shims available at my model shop:
View attachment 5471
Click to expand...


I believe Horatio and Jeff were able to sort out the shim sizes. They would be the same OD as the bearings, with the ID being big enough to fit the diff cups. Knock on wood mine have been good thus far. The stub axle bearings are going to need replacement soon, though.
 
I've managed to swap out my dodgy Torox for a Max 8 which seems to have resolved my electrical issues.
That leaves me with two major issues still: the stripping gears and a huge imbalance that makes the damn thing look like it's trying to do a crap impression as it drives down the road - super wobbly.

After some initial research, the main culprit of a wobble seems to be bad bearings so I'd bought a whole bunch of replacements in my last order. I spent some time this afternoon swapping out both sets of bearing on each wheel hub, some of those were indeed completely ****ed (I've obviously not dried them well enough after a wash...).
I gave it a little shakedown again to see if it had made much difference. Nada. The thing is still wobbly AF.

I've isolated the issue to being one of the two at the back by holding on the rig with the front/rear off the ground meaning the centre diff is only spinning up those in the air. When the front was in the air it was pretty stable, when the back was up, the thing is going crazy.
I then came to the realisation that perhaps my two issues are actually related, if the back end is wobbling all over the place, could the diff issues (seems to be the back) be caused by this?
This has now become my priority to fix this in the hope that the diff issues will go away once this is sorted.

In order to resolve this, I only really have one idea remaining now and that is potentially one of the tyres is way off balance. I will test this theory when I next get it out by swapping all the wheels around and testing in the same manner as above. Before I go buying a tyre balancing station and putty etc, are there any other thoughts on what to check for this? From what I could tell when I was swapping the bearings around, the rear hubs all looked damage free, there was no obvious distortion in the rear axles.
Could it possibly be that rear diff housing is somehow misaligned which is causing the imbalance and thus the diff damage too?
 
Stormwind said:
I've managed to swap out my dodgy Torox for a Max 8 which seems to have resolved my electrical issues.
That leaves me with two major issues still: the stripping gears and a huge imbalance that makes the damn thing look like it's trying to do a crap impression as it drives down the road - super wobbly.

After some initial research, the main culprit of a wobble seems to be bad bearings so I'd bought a whole bunch of replacements in my last order. I spent some time this afternoon swapping out both sets of bearing on each wheel hub, some of those were indeed completely ****ed (I've obviously not dried them well enough after a wash...).
I gave it a little shakedown again to see if it had made much difference. Nada. The thing is still wobbly AF.

I've isolated the issue to being one of the two at the back by holding on the rig with the front/rear off the ground meaning the centre diff is only spinning up those in the air. When the front was in the air it was pretty stable, when the back was up, the thing is going crazy.
I then came to the realisation that perhaps my two issues are actually related, if the back end is wobbling all over the place, could the diff issues (seems to be the back) be caused by this?
This has now become my priority to fix this in the hope that the diff issues will go away once this is sorted.

In order to resolve this, I only really have one idea remaining now and that is potentially one of the tyres is way off balance. I will test this theory when I next get it out by swapping all the wheels around and testing in the same manner as above. Before I go buying a tyre balancing station and putty etc, are there any other thoughts on what to check for this? From what I could tell when I was swapping the bearings around, the rear hubs all looked damage free, there was no obvious distortion in the rear axles.
Could it possibly be that rear diff housing is somehow misaligned which is causing the imbalance and thus the diff damage too?
Click to expand...
Check if there is water inside the wheel. One of the inserts could be soaked.

For what it's worth, it's well worth the effort balancing the wheels. You can use putty. I like XTR bicycle wheel weights.

Screenshot_20230604_215413_com.ebay.mobile_edit_243503313458676.jpg
 
Horatio said:
Check if there is water inside the wheel. One of the inserts could be soaked.
Click to expand...

So I originally thought that this couldn't be the case since I had vented the wheels.
Just took it out again to try and diagnose again and found that I had only vented two of the wheels....
Possibly I did all 4 and then replaced 2 with new ones and forgot to vent them or possibly wanted to see how venting would work without doing all four, can't really remember.
This was definitely the case though. I vented the remaining two and ran up the wheels in the air to drain them. You were correct, absolutely soaked. After draining them, it now seems way more stable on the road now.

I'm gonna charge some packs and hopefully give it a proper run at the weekend to see if the diff issues have gone away due to this change now. Thanks very much for the reminder!
 
Stormwind said:
So I originally thought that this couldn't be the case since I had vented the wheels.
Just took it out again to try and diagnose again and found that I had only vented two of the wheels....
Possibly I did all 4 and then replaced 2 with new ones and forgot to vent them or possibly wanted to see how venting would work without doing all four, can't really remember.
This was definitely the case though. I vented the remaining two and ran up the wheels in the air to drain them. You were correct, absolutely soaked. After draining them, it now seems way more stable on the road now.

I'm gonna charge some packs and hopefully give it a proper run at the weekend to see if the diff issues have gone away due to this change now. Thanks very much for the reminder!
Click to expand...
Glad you're getting it sorted. 👍
Edit to add that venting wheels let's water in. Especially if you use a hose to wash down your rig.

You may have to add a small hole in the tyre to let the water out when you spin them.
 
Update time!

I have just changed the shims in the rear diff over to the Kyosho ones mentioned above, 2x0.15mm instead of of the 2x0.1mm ones from the factory. I did also notice that I had obviously put waaaay to much grease in there the last time I tried to fix this issue.
I cleaned off all the grease and put the diff in dry just to rule out any slippage being caused by the grease.

The damn thing is still slipping. I hold the front tyres up and lightly step on the rear of the rig so those back tyres are going nowhere and I can still see the rear axle hub slip occasionally along with that god awful noise of metal on metal that is not supposed to be there.

I am running out of things to try here other than just chucking a whole bunch of money at it replacing it everything: diff housing, diff spur, new drive pinion (again.... both are relatively new and don't really look worn), possibly even the rear->center drive shaft. First up I'm gonna see if I can dump a third shim in it perhaps.
 
Back
Top