Kraton Front diff main gear stripped. What happened and how to avoid in the future?

[BattleShrink]

Hey guys, I just ran my first two packs of 6s through my kraton that I had been using 4s on (maybe 12 packs), and it was a blast, did a ton of backflips onto grass. About halfway through the first pack it started making horrible crunchy noises on acceleration. It still made it through the second pack though.

Got home, front wheels were crunchy when turning manually, took out the diff and found this.

The front output gear looks ok. Ordering a new main gear now. What the heck happened and how do I prevent it in the future?

I was using backflips tires. Diff fluid was 60k front, 100k center, 30k rear. 6s 5200mah 75c batteries. Stock motor/esc, punch on 9. Transmitter exponential throttle on -40%.

 

[Crash]

See you only have one shim there. Try to add a few more to force the diff over a bit to get a better bite on the gears. Personly i wouldn't only replace the one gear. I would do both but everyone will have their own ideas on this.

 

[BattleShrink]

Thanks, will do! Ordered shims and an output gear. Gonna have to go through my other 3 diffs now haha, I didn't mess with those shims at all.

 

[Zaiden15]

I would get some bearings from FE or somewhere and replace them while I had it torn down too.

 

[olds97_lss]

I just had the same thing in my 2018 outcast after 15/20 runs. I run 4S only, so I was surprised to have this issue, at least this early. I ran mine 2-3 packs on stock diffs/oils, then my shims and oils showed up and I tore all 3 down to reshim and replace the oils with heavier. On the ring gear, I was able to fit 2 shims in there. Then 2 weeks or so later, my diff did what yours did.

When taking mine apart, two screws on the diff cover on the front diff didn't feel like they were biting into much, so I'm assuming I striped the threads, or nearly stripped them when I had them out before. Then with the force of brakes/backflips caused them to strip fully.

When I put in the new diff, I ordered a fully assembled diff/case off ebay. I redid the oil/shims inside the cup and I was able to jam 3 shims on the ring gear side. However, it did have a high spot with a lightly noticeable amount of binding. I ran it easy for the first pack and then it seemed fine.

 

[Crash]

When you're using plastic differential cups the screws have a tendency to back out this has been my experience. If all four screws are not tightened through the gear to the cup evenly the gear will wobble creating issues when it comes to gear mesh in your front rear and Center differential. What I chose to do is to get aluminum diff cups from hot racing because at that point you can use blue Loctite for the screws and they won't back out. When you're putting it together with aluminum diff cups you can feel the screws seat all the way so you know that they're all screwed in evenly and I tend to usually Titan in a cross pattern. My differentials don't leak anymore either. But this is not to say that others don't have this issue it's just the route I chose to take.

 

[Spar69!]

I double shim all of my diffs and haven't had any issues since.... I used to strip front diff gears like crazy...I only run 6S...

 

[Toddys]

Hey guys, I just ran my first two packs of 6s through my kraton that I had been using 4s on (maybe 12 packs), and it was a blast, did a ton of backflips onto grass. About halfway through the first pack it started making horrible l made it through the second pack though.

Got home, front wheels were crunchy when turning manually, took out the diff and found this.

The front output gear looks ok. Ordering a new main gear now. What the heck happened and how do I prevent it in the future?

I was using backflips tires. Diff fluid was 60k front, 100k center, 30k rear. 6s 5200mah 75c batteries. Stock motor/esc, punch on 9. Transmitter exponential throttle on -40%.

Hey guys, I just ran my first two packs of 6s through my kraton that I had been using 4s on (maybe 12 packs), and it was a blast, did a ton of backflips onto grass. About halfway through the first pack it started making horrible crunchy noises on acceleration. It still made it through the second pack though.

Got home, front wheels were crunchy when turning manually, took out the diff and found this.

The front output gear looks ok. Ordering a new main gear now. What the heck happened and how do I prevent it in the future?

I was using backflips tires. Diff fluid was 60k front, 100k center, 30k rear. 6s 5200mah 75c batteries. Stock motor/esc, punch on 9. Transmitter exponential throttle on -40%.

 

[Toddys]

My spiral gear looks like that as well. After further investigation the cause for me was twofold. First, the small bearing in the diff housing near the input gear cup had broken apart causing play. Secondly, the larger bearing became out of round as a result of the play. So even after I replaced the small bearing it happened again. While shimming the diff is important, it should not require more than one of those shims. My diff housing causes me to struggle to get two in there without bending and ruining a few shims in the process. I believe what's more important is to ensure that your bearings are both in good shape and that your input gear after the cup has been attached has no back-and-forth play to it. To prove the point about the one shim, try moving your diff left to right after installing it all the way and you will find that there's no play in it with that one shim. A big take away from reading this thread for me was the diff Cup itself and the disadvantages of using the stock plastic. It makes total sense to me that if you don't snug up the four screws holding the spiral gear on in an even fashion that it would be wobbly, which again would cause things to go awry. I don't love the idea of going with aluminum due to the added cost, but if this alleviates that problem then perhaps it's justified. Ultimately a better designed input and spiral gear with larger teeth (allowing for more flex while maintain mesh) would be ideal. As it is the most minute separation causes teeth to miss and eventually parts needing replaced.

 

[olds97_lss]

My spiral gear looks like that as well. After further investigation the cause for me was twofold. First, the small bearing in the diff housing near the input gear cup had broken apart causing play. Secondly, the larger bearing became out of round as a result of the play. So even after I replaced the small bearing it happened again. While shimming the diff is important, it should not require more than one of those shims. My diff housing causes me to struggle to get two in there without bending and ruining a few shims in the process. I believe what's more important is to ensure that your bearings are both in good shape and that your input gear after the cup has been attached has no back-and-forth play to it. To prove the point about the one shim, try moving your diff left to right after installing it all the way and you will find that there's no play in it with that one shim. A big take away from reading this thread for me was the diff Cup itself and the disadvantages of using the stock plastic. It makes total sense to me that if you don't snug up the four screws holding the spiral gear on in an even fashion that it would be wobbly, which again would cause things to go awry. I don't love the idea of going with aluminum due to the added cost, but if this alleviates that problem then perhaps it's justified. Ultimately a better designed input and spiral gear with larger teeth (allowing for more flex while maintain mesh) would be ideal. As it is the most minute separation causes teeth to miss and eventually parts needing replaced.

It depends on the diff housing. I was able to get 3 shims in there without much fuss on the one I got off ebay. On the original, I got 2 shims in. Granted, I didn't take the entire original off the truck, so getting shims in the new housing was easier without the arms/tower in the way.

 

[Slogrits]

My rear input gear loves to chew the main diff gear. Shined with three Tekno shims. Now it is just part of maintenance.