Kyosho SST transmission diagnosis

[stickboy007]

It seems like this topic has been covered a few times here already, but in all of the thread comments that I've read so far, I could not find a clear description of *why* this gearbox was such a problem. I have a few of these gearboxes that I'm trying to rebuild, and I'll point out some of my observations here on a Pro-XRT gearbox I've been working on. I've spent several nights just playing with the damn thing until almost 2am, and here is what I found so far.

1.) With everything together (both halves and all gears installed), there is significant binding in the gearbox.

2.) With both halves apart and the diff gear in either halves, everything seems to spin pretty freely, from the intput gear to the diff gear to the output gear. So something is happening here when the halves are pressed together.

3.) If you look closely, you'll see that there is very, very, very little clearance between the center diff gear and the metal output gears, which tells me this is probably where the binding is happening.

4.) The thrust bolt is quite long, perhaps too long, with a fair amount of thread protruding from the nut on the other end of the diff after tightening the diff down to an appropriate level. There is possibly some binding between the end of this bolt and the gearbox, but again, everything spins freely when each half is in isolation.

So it seems to me as if the center diff gear is too wide, and/or the metal output gears are too wide, and when the gearbox halves come together, there is simply not enough room. Maybe it's just small enough that putting the halves together adds some pressure between the side of the center diff gear and the sides of the output gears. If that were the case, though, the gearbox would simply feel tight, but still smooth. It feels like the gears themselves are meshing poorly, though, when both halves are put together.

I tried a few things just to experiment around and see if I can free things up a bit. First, I tried adding some washers between the gearbox casing and the bearings which house the diff. This pushes the diff away from the gearbox casing on each side of the gearbox. Granted, you cannot properly close the gearbox this way, but at least you can isolate the diff-to-casing contact/binding (if there is any). In this scenario, I still get binding, so it doesn't seem like a diff-to-casing friction problem.

I also tried removing the input gear and having just the diff and output gears in the gearbox. Less binding observed, but still some, especially on the left side of the gearbox.

I then tried taking the left side output gear and shaving the inside surface down with a dremel and a grinding bit. This will make the gear a bit narrower and provide some more clearance between that surface and the center diff gear. That did seem to help a little bit, but again, with everything assembled, still a lot of binding.

I'm still working through diagnosing the root cause/s, but so far, it seems like there are a variety of problems rather than just one. It is plausible, though, that each of these things could all be due to the gearbox casing itself. Maybe the mold did not have a very good design, and so the bearing housings on each half were not well-aligned? I can't say for sure.

I do, however, have a Rampage Pro that I've converted to electric, and the gearbox in that one is pretty smooth. Some very minor binding as you rotate the spur gear and notice a little tight spot, but it's not bad. That was a brand new gearbox that I built myself, though, with all brand new parts. We'll see how it holds up. The Rampage Pro gearbox is a different mold than the Pro-X gearbox, though, so maybe....**maybe** those Rampage Pro casings don't have as much of a problem? Anyway, I'm going to keep fiddling around with this. If I can figure out a good fix, I'll of course report my results, or at least make a YouTube video.

[highwayracer]

There was a casting defect in the gearbox halves. After kyosho realized this, they just included the corrected parts in the kit as an addition. So you had 2 sets of gearbox halves. Same can be said with the ball studs. The result here is that there are "new" gearbox halves floating around that are defective. For those that had assembled and ran the bad gearboxes, they pretty much wore out their transmission gears because of the binding.

To make it worse, I don't think there's a way to identify the new gearbox from the old.

I've had about 4-5 complete gearboxes and they are just pretty weak. The overall tolerances are just bad in general. In addition, the gear materials just aren't that good.

[stickboy007]

I see. I was a bit worried that may be the case. I had actually tried taking the output gears in one of them and filing the space between the teeth a little bit, hoping that might loosen up the mesh, but to no avail. Based on what you're saying here, it seems to me that the trick is to have many gearbox cases and make sure to use fresh gears, and then just try them all until you end up with one that doesn't suck.

[Lavigna]

Great info. I have been hunting parts to build a Pro XRT. I can't find a truck for sale anywhere, so I am slowly buying parts one at a time. Mine will just be a shelfer. Please let me know if you have anything extra.

I look forward to your progress. Please post pictures of your XRT. There is so little information on them. They are such great looking trucks.

[stickboy007]

I think I figured it out.

[jwscab]

I would be super careful about taking out the other bearing. If I had this assembly, I would stick a used bushing on that side, so it has a little bit of clearance, but when you put a bunch of power through it, when the gears spread, it will at least touch the bushing.

[stickboy007]

We will have to see how it does with power, for sure. That is why I added a bearing on the slipper side, though. So it's not just the standard 5x10 holding it in there, but that plus a 5x11. Granted, it's on one side, but if you use the "standard" setup with 5x10 bearings on both sides and wiggle the input shaft, it really doesn't feel that different. I have several of these, so I'm really not too worried about killing one in a test, if it comes down to that.

My take from this is that the bearing housing for the input shaft on the left half of the gearbox is where the mold went wrong. It seemed to push the input shaft too close to the diff gear, and that was the source of (most of) the binding. You have a fair argument that a worn bushing would at least offer some support on the left half. If I can find a 5x10 bushing in my collection, I'll bore out the middle a little bit and try that out. In the meantime, though, I am fairly convinced that adding the 5x11 on the right half is the way to go here.

[jwscab]

Cool, interested to see how it holds up!

[stickboy007]

Quick update:

As a safeguard, I installed a 6x10 bearing on the left half of the gearbox housing for the input shaft. Since the default is a 5x10 bearing, the 6x10 should still allow the left side of the input shaft to float a little bit without permitting too much deflection in the event that it does get jostled around somehow. With everything all closed up, it spins pretty freely and there isn't much flex.

So to recap:

1.) remove slipper hub drive pin from input shaft
2.) remove e-clip from input shaft (permanently. You will not be using this again)
3.) install 5x11 bearing on input shaft (on the long end)
4.) install 5x10 bearing on input shaft (on the long end. This will butt against the 5x11 bearing)
5.) reinstall slipper hub drive pin on input shaft
6.) install input shaft on right half of gearbox casing
7.) install 6x10 bearing on input shaft portion of left half of gearbox casing

[stickboy007]

I've been playing around with the SST nitro gearbox (Rampage Pro, Outlaw Rampage Pro), trying to resolve the same gear mesh issue. It's really a crap shoot with these things, as sometimes they're smooth and other times they're not. The floating input shaft (with a "guider" bearing on the other side) really helps a lot, though.

The thing with the Rampage Pro gearbox is that, in order to convert to electric, if you use the stock internals, is that you have to modify the slipper hub to move the spur gear close enough to the gearbox to mate with a pinion. Otherwise, the motor shaft is not long enough to reach a spur gear that's way out there. This is because the drive pin for the slipper hub on the SST35 part is further away from the gearbox than on the SST7 part (input shaft for Pro-X/XRT gearbox). Using the SST7 part on the nitro SST gearbox is possible, and avoids this slipper hub mod, but it's not a simple plug and play. Here is what I came up with.

1.) Swap SST35 (or SST35H) for SST7 (or SST7H).
2.) remove slipper hub drive pin from input shaft
3.) remove e-clip from input shaft (permanently. You will not be using this again)
4.) install 5x11 bearing on input shaft (on the long end)
5.) install 5x10 bearing on input shaft (on the long end. This will butt against the 5x11 bearing)
6.) reinstall slipper hub drive pin on input shaft
7.) install input shaft on right half of gearbox casing
8.) install 7x11x3 bearing on input shaft portion of left half of gearbox casing (you can use a 6x11x3 as well...I just had a 7x11x3 lying around)

You will note that step 8 is different from step 7 in my previous post (for the Pro-X/XRT gearbox). This is because SST7 is shorter than SST35 and so does not reach to the end of the other half of the nitro gearbox. So you have to use a 11mm bearing to basically sit a little further inward and act as a spacer to keep SST7 from being pushed into the gearbox too far. I have tried this on one SST nitro gearbox and it is very smooth and has very little wobble.

[K Rob]

Need to drag up an old topic here please.

stickboy007 - I've watched your YouTube vids with interest (very informative - thanks!) as I'm planning to convert a Rampage Pro to electric too, I hope you can help with some queries.
I'd be looking to out the Pro-X layshaft in ideally for better slipper positioning but I can't find out if the Pro X layshaft and the Rampage layshaft have the same amount of teeth on them.
Do you know if they are a direct swap?
Same for the diff gears. they have different part numbers but I can't tell why.
I know the lower output gears are a different size to cater for what was a truck ratio box.

[stickboy007]

It's almost a direct swap for the layshaft. My post right above yours describes the steps for doing this

I'm pretty sure the diff gears have a different number of teeth, as do the output gears. That's why they have a different part number. The reason the layshafts have a different part number is not because of the teeth count (which is identical), but rather because of other differences, such as the drive pin location, again which I point out in the post above.

[K Rob]

Okay, got some bits for the gearbox rebuild but I want to check on what you've done with the layshaft.
There is a considerable difference in diameter of the toothed section between the standard Rampage Pro layshaft compared to the Pro X version (SST7)
It does go in (once spaced correctly as you've mentioned) but there is considerably more backlash obviously due to the smaller diameter - to be honest I didn't think it would even drive.
I certainly wouldn't be confident driving with it. Have I misunderstood something?

[stickboy007]

Okay, got some bits for the gearbox rebuild but I want to check on what you've done with the layshaft.
There is a considerable difference in diameter of the toothed section between the standard Rampage Pro layshaft compared to the Pro X version (SST7)
It does go in (once spaced correctly as you've mentioned) but there is considerably more backlash obviously due to the smaller diameter - to be honest I didn't think it would even drive.
I certainly wouldn't be confident driving with it. Have I misunderstood something?

I think what you mean is the lengths (not diameters) of the toothed sections are different, and yes, they are. That is compensated with an additional bearing on the longer side (per my recollection and as stated above in the thread and video link), to pick up that extra slack.

[K Rob]

No, I don't mean the lengths.
I mean the diameter of the section of shaft with teeth on it.
Unfortunately I don't have the Rampage shaft to hand as it's with a chap making a motor plate for me to show a comparison, but it is bigger in diameter than the Pro X version.
So the Pro X version does not give a good connection to the diff gear when it's in the Rampage box.
There is too much backlash between the gears using the Pro X shaft.