What is going on with my diffs?

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hpicelery

hpicelery

Well-Known Member
Messages
222
I was removing a bent dogbone when I discovered an alarming amount of play in the diff outdrives. Rather than try to describe it I took a video:


This can't be right. What did I do wrong to cause this? What can I do right to prevent it in the future?
 
It looks like the bearing could be a bit loose, but the cup is moving around way more than the rest of the diff. I haven't had the chance to pull the diff yet, but it seems like the outdrive shaft is wobbling around inside the diff cup.
 
X25 said:
Sounds like a good time for a different mod
Click to expand...
Maybe so, but that's a bit disappointing. I just built these diffs from all new parts except for the outdrives and pins. They can't have more than ten packs on them, and I only run 4S. Why would they get wobbly so quickly? What kind of diff mod would you suggest?
 
So there's no way to keep this from happening to the BP Diffs? Did I waste all that time and money installing brand new diffs so recently?

I'm running a Mamba Monster 2 with a Neu 2200kV motor, and I only ever run 4S. It feels wrong to me that the brand new "bulletproof" diffs could be so wobbly already. It's worth noting that the diffs themselves are still pretty snug in their bearings, and the slop that is seen here is from the outdrive shaft wiggling around in the hole in the diff cup.

If I must spend another hundred dollars on my diffs, is this what I need? It's all I can find on eBay.
http://www.ebay.com/itm/CEN-Racing-...673503?hash=item5d4c37a45f:g:uFAAAOSwMTZWRDbo
 
hpicelery said:
So there's no way to keep this from happening to the BP Diffs? Did I waste all that time and money installing brand new diffs so recently?
Click to expand...

The only thing you can't bulletproof are the bearings. They are a very weak link in the diff.
No matter how much money you spend.. The $1 or more bearing will still break..

Best thing is to open them up and then see what exactly is broken. If it is just the bearings i wouldn't spend more than necessary to replace them.
 
Caretaker109 said:
The only thing you can't bulletproof are the bearings.
Click to expand...

I don't doubt that, but the issue here isn't the bearings. The pinion and crown gear mesh is still good and there is very little play in the diff itself. The crown gear and diff cup barely wiggle. The issue is the outdrive shaft is very wobbly in the diff cup. There is no bearing at this junction, just a shaft through a hole with a rubber O-ring. I should be able to get a better picture of what's going on tonight when I pull the diff out of the truck.
 
I'll find out for sure when I tear the diff apart tonight, but I don't think the part is broken. The truck was driving fine when it last ran, and spinning one rear wheel spins the other one backwards just like you'd expect with a working diff. It seems like the hole for the outdrive is too big for the outdrive shaft.

Is it possible that this is because the new "super heavy duty" outdrives on the newer Savages have a slightly different diameter than the old ones? All of the diff parts are new except for those outdrives, which are original Savage 25 equipment.
 
Probably so. Unfortunately, I don't have any of the new outdrives on hand, and I just ordered a new set of HD dogbones to replace the bent one that started all this. If I have to swap out the outdrives, then I have to change the dogbones and axles too...
 
I hate to say it but your having an issue I never had. When I had diff issues it was the ring and pinion. I always trashed them lol. Can't wait to see better pics. This has peaked my interest
 
It looks like both @Jam Racing 1 and I were right.

I'm afraid I didn't take any pictures, but here's what I discovered when I tore down both ends of the truck (I sure wish I had the newer X style bulkheads. This would have been so much easier).

All four diff cups have a surprising amount of play and can be wiggled at least a couple of degrees from the axis of the diff bearings. On the one in my video, however, the bearing supporting that end of the diff is also gone. Those two sources of slop combined to create the extreme wobble in the video.

The fix for the bad bearing is obvious, and I've already got a full set of new diff bearings on the way from Fast Eddy.

The loose diff cups are more perplexing, and my only clue is that both the cups themselves and the rubber O-rings (HPI #6811) are original equipment from 2003.

While searching eBay for a set of super heavy duty drive cups to test with, I found a brand new assembled diff including said cups. I'll be using my caliper to compare the diameters of the old and the new shafts as well as the holes in the diff cups and gears. This should tell me if the newer drive cups have thicker shafts or if my diff cups were somehow widened. I'll post my findings once my stuff comes in.
 
They do wear out over time. Plastic, aluminum or steel, everything wears down eventually. Some diff cups had a steel insert for the output shaft so it was stronger and last longer. I don't recall if the savage ever had that.

Mine have the alloy cups now with the BP machined diff gears.
 
@olds97_lss That's exactly what I am running as well. It surprised me that the parts would be so loose with those brand new parts.

I am glad I found the roasted diff cup bearing, but I wish I had never discovered the play in the drive cups... On a whim I tried to wiggle the drive cups on my relatively new (<20 packs) Savage XS, and THEY WIGGLE TOO. I'm starting to suspect that this is just a normal amount of play for all of these Savage diffs, and I may be validated when that new assembled diff comes in. If it wiggles too, then there's nothing I can do about it.
 
Too bad they didn't put a steel insert in the alloy cups as you have steel against aluminum on the cup side vs steel against steel on the spur side. I guess it's just one of those things.

What's a pack's run time for you? 10-15 minutes? If so, that's around a 1 gallon mark for nitro. 20*160=3200ml, 1 gallon = 3785ml. Guessing perhaps the torque of electric might be harder on diffs, not just the gears, than nitro.
 
It's not uncommon for a pack to last me 30 minutes or more. I run 4S, 7500mAh, and I don't spend much time above 50% throttle because of how powerful that motor is and my poor track record with high speed driving.

I'm still waiting for that new diff to show up. I'll be able to tell immediately if the cups have the kind of play that mine have. I suppose it's possible that the tolerances of these parts make it normal.

I'm not ready to pull the trigger on a CEN diff swap, but if I were to do so, is this the kind of thing I'd need? It's pretty much all I can find...
http://www.ebay.com/itm/CEN-Racing-...rvr_id=1008034313040&;toolid=10001&rmvSB=true
 
So, your getting 2-3 tanks out of one pack. Which is around 2.5-3 gallons in a comparable nitro. Considering the torque, that's a lot of wear time.
 
I never looked at it that way. Still, I don't know how much play is in a truly brand-new diff, so there may not be that much wear after all.
 
From what I recall, there is a bit of play. I notice it a bit more on my revo than my savage. You said you reused your output shafts/cups, right? I know I did that too at first, then I got the Super HD bones/cups/axles and put the HD cups back in. The old SD cups I had were pretty wore and I did notice quite a bit of slop. They had to have 5+ gallons on them. I probably only have 2-3 gallons on the ones in it now. If that. Mine currently have a little bit of slop. Maybe 1-2mm if I rock the cup back and forth with a screwdriver. I checked both the front/rear diff on the same side. One would be the ring gear, the other would be the cup.
 
Yeah, that's what all of mine do too, except of course for the one with the bad bearing. When I have some brand new parts to compare to I may be able to actually measure the wear. New Super HD drive cups vs. original Savage 25 ones, and new alloy diff cup and crown gear vs. post-electric-conversion ones. I may even dig up the old micrometer if the caliper isn't precise enough.
 
Warning, lots of text incoming!

Well, after carefully comparing the new diff to the one from the truck it's obvious that the outdrive cups do develop more play as they wear, though it's not as much as I thought. The brand-new diff had more play in it than I thought it would, maybe 2-3 degrees each way. The worn diff can wiggle maybe 3-4mm. Most of the wear appears to be in the diff cup itself, because the original Savage 25 outdrives and the brand new X ones measured pretty much exactly the same diameter, and sticking the old outdrives in the new diff cup gave the same amount of play as all new parts. I was surprised to find that even the machined, hardened crown gear had worn more than the shaft running through it.

I believe it's safe to say that this is just normal wear for these diffs. My main concern going forward is that it might be harder and harder to keep diff fluid from leaking out of the diffs as the amount of play increases.

Speaking of diff fluid, I previously never used it and just greased the gears in the diffs. I wonder if the resistance from actual diff fluid will reduce the amount of spinning the outdrives do relative to the diff cup and therefore lead to slower wear of the surface between the shafts and the cup and crown gear?

Folks who run diff fluid: How long can you usually run before you have to refill the diffs? Does thicker fluid stay put longer?
 
I filled mine with 50K fluid when I built them. Not sure how many gallons I've run through it since then, would guess 3 or 4. That was 3 years or so ago. I got the X used at the end of 2012. Shelled the stock diffs pretty early on after ditching the engine and throwing in an LRP28. Got about half way through break-in and blew the rear diff, so I went to the BP diffs right away, cleaned them out and threw in OFNA 50K I had on hand.
 
hpicelery said:
It looks like the bearing could be a bit loose, but the cup is moving around way more than the rest of the diff. I haven't had the chance to pull the diff yet, but it seems like the outdrive shaft is wobbling around inside the diff cup.
Click to expand...


Check the diff case and bearing.. Good chance. both are shot.. Also.. Check upper and lower A arms.. and bearings there. Since things got wobbly, and you just noticing it at diff case.. The problem also went on to the bearing in the knuckles.. Having said that.. I would check both sides, as well as the internals.. That kinda wobble/wear puts alot of strain and shock on that entire differential.. I would even drain the oils in the shocks.. and rebuild as necessary.. I'm currently building a shelf queen too bash.. to kick off the shelf queens out there!.. :)
 
also good chance.. the clearances of the internals were never checked.. and created a "Dominio effect" on your ride.
 
hpicelery said:
It's not uncommon for a pack to last me 30 minutes or more. I run 4S, 7500mAh, and I don't spend much time above 50% throttle because of how powerful that motor is and my poor track record with high speed driving.

I'm still waiting for that new diff to show up. I'll be able to tell immediately if the cups have the kind of play that mine have. I suppose it's possible that the tolerances of these parts make it normal.

I'm not ready to pull the trigger on a CEN diff swap, but if I were to do so, is this the kind of thing I'd need? It's pretty much all I can find...
http://www.ebay.com/itm/CEN-Racing-...rvr_id=1008034313040&;toolid=10001&rmvSB=true
Click to expand...
I have heard good things about the diff swap.. from Cen Gen.. I personally have never done it.. But .. as of today.. I'm not sure. if us old guys who have the orignal savage 4.6ss nitro, are the parts still avail. But every thread I read so far. It makes the diff bullet proof.. But.. Doing wheelies alot.. Really kills the diffs from the shock of the from landing.. The speed difference between all the running gear creates alot of torsional shock to the entire drive train..
 
Hello Relax1023. I've already completely torn down the diffs and compared to brand new parts. The diff cup is slightly worn, but even new diffs have some play in them. The internal gears are all nice and snug. I had a blown bearing on the diff cup side, and that has been replaced along with all the other bearings supporting the diffs and axles. The truck is back up and running at 100%.
 
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