4x4 Mega 4x4 Diff Input Gear Mod

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Megasty

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After throwing a BLX 85 & 3800kv 3556 motor in my granite, it has snapped 2 rear diff plastic input gears on 3S & one on 2S. The truck is easy to work on but I'm getting tired of replacing this part. I already tried filling the hollow part with a 2-part plastic epoxy & that seemed to work for a bit but it just couldn't handle 3S. Then I snapped another on 2S :rage:

So after brainstorming a few ways to fix this mess, I decided to take the easy route.

I used the broken gear end of the input (the part was already filled with epoxy), a wheel axle (AR310784), & a 2x10mm harden steel pin.

I drilled a 5mm deep hole in the center of the back of the gear with a 1/4" bit

Then I drilled into the center of that hole with a 3/16" bit 10mm deep (The bit was 15mm deep into the gear)

parts.jpg

This allowed the axle to be inserted clean into the gear enough to cover the pin hole with a VERY small amount of wiggle room.

Last I drilled the hole for the pin with a 3/32" bit. This is the hardest part but the hole in the axle lines up dead center in the groove. If the axle is fully inserted in the gear then the hole will have to be drilled 1.5mm from the end of the gear & use the groove on the axle to line up the hole.

I used a caliper & did the mod in less than 10 minutes.

assembled.jpg

The wiggle room comes in handy when inserting the gear back in the slipper clutch plate. The 2nd bearing covers the pin perfect & the slipper clutch plate sits right where the black part of the finished gear ends.

I thrashed the truck on 3S today with 4 whole packs & the gear is holding up magnificently. Even if the gear decided to fail, just like the original part, there's no where for the pieces to go. I made another one for the front but it doesn't seem to need it.

These pictures were taken after the runs. There's no distinguishable wear on the drilled holes on the sides.
 
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Nosedive4130

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Thanks for sharing and keep us updated on how well the set-up works over time.

I've been looking for more durable options.
 
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Megasty

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I just ran another 5 packs, including my 2 8000mah 3S beasts. All Good. I also added some adjustable links made from some spare miscellaneous Arrma & Slash parts. It's super beefy now. My crappy driving tested them right away with the same tree that claimed the entire left side of the truck a week ago, all links & hubs were smashed. The same hit today just broke the front left hub, easy fix. A few aluminum upgrades will make this thing bulletproof :smuggrin:

0108180938 (Copy).jpg
 

TooTall

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What shocks are you running? I have broken some perches and am looking to alternative shocks.

TT


I just ran another 5 packs, including my 2 8000mah 3S beasts. All Good. I also added some adjustable links made from some spare miscellaneous Arrma & Slash parts. It's super beefy now. My crappy driving tested them right away with the same tree that claimed the entire left side of the truck a week ago, all links & hubs were smashed. The same hit today just broke the front left hub, easy fix. A few aluminum upgrades will make this thing bulletproof :smuggrin:

View attachment 14633
 
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Megasty

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What shocks are you running? I have broken some perches and am looking to alternative shocks.

TT
https://www.ebay.com/itm/ALUMINUM-FRONT-REAR-BIG-BORE-OIL-SHOCK-DAMPER-B-TEAM-LOSI-1-8-8IGHT-2-0-BUGGY/332187998554?_trkparms=aid=888007&algo=DISC.MBE&ao=1&asc=41451&meid=f5b4dbc95c29419295cc53a130233adf&pid=100009&rk=1&rkt=2&sd=331781979446&_trksid=p2047675.c100009.m1982

They are slightly larger than the stockers, but that's good since they won't over-extend. I also had to use a servo mount spacer in the top holes. The stock shocks for the Typhon are a perfect fit, but the price is a bit steep & I needed them for another project.
 

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https://www.ebay.com/itm/ALUMINUM-FRONT-REAR-BIG-BORE-OIL-SHOCK-DAMPER-B-TEAM-LOSI-1-8-8IGHT-2-0-BUGGY/332187998554?_trkparms=aid=888007&algo=DISC.MBE&ao=1&asc=41451&meid=f5b4dbc95c29419295cc53a130233adf&pid=100009&rk=1&rkt=2&sd=331781979446&_trksid=p2047675.c100009.m1982

They are slightly larger than the stockers, but that's good since they won't over-extend. I also had to use a servo mount spacer in the top holes. The stock shocks for the Typhon are a perfect fit, but the price is a bit steep & I needed them for another project.
Thanks for the info, I will keep those in mind.
TT
 

Hounigan

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Is it just the torque on the brushless systems that is breaking the diffs? Just wondering so maybe I can set my punch lower when my system comes.
 
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Megasty

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I had my punch on level 1 from the start. I still don't trust the plastic drivetrain, shafts, diffs, etc to go higher than that. Level 1 is more than enough for this car as it is. I'm wondering how Arrma will upgrade this thing to handle the BLX 85 system.
 
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Hounigan

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Ok ya I'm wondering as well. I ordered a hobbywing sc8 and a reedy 540-sl4 3300kv motor. I'll set my timing to 0 and punch on 1 haha.
 

Nosedive4130

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Yup but I haven't got my brushless system yet just all stock so far.
Sorry, that question was meant for Megasty.

On another note does the Hobbywing Sc8 have really fine adjustments as far as punch are concerned? And how does timing work? I would just be worried about the torque that would be produced with that low of a kv motor...

Forgive me as I'm learning a lot here and really appreciate everybody's feedback.
 
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Megasty

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Sorry, that question was meant for Megasty.

On another note does the Hobbywing Sc8 have really fine adjustments as far as punch are concerned? And how does timing work? I would just be worried about the torque that would be produced with that low of a kv motor...

Forgive me as I'm learning a lot here and really appreciate everybody's feedback.
All stock so far, but . . .

I'll leave this as a teaser

0109180401b (Copy).jpg

0109180402 (Copy).jpg

0109180403a (Copy).jpg

32 pitch gearing :eek: Tons of next-level work, a 60t HR Axial Yeti spur & 17t pinion. In a limited test run it was super scary on 2S. Stupid speed, maximum tire ballooning, & standing backflips - & I still didn't break it :p

As much as I'm impressed by the car being able to handle this gearing for a 10 minute bash from hell, I'm more annoyed that I didn't break it, considering how many times I had to carry it back to the house with the 48 pitch gears.
 

Nosedive4130

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Nice! Do I see a pinned setup? As in no slipper? That's sick.

Btw, I just fell in that rabbit hole. Ordered a blx85 and a 3800kv...

I'm already slipping the rear diff gear on 2s brushed stock motor so I'll need to figure out a workaround quickly...
 
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Megasty

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Nope, all of the stock components are intact except for that black ring piece that holds the bearing & seats the spring. The slipper is also completely functional. Keeping all that junk working is what took over 3 hours to figure out :rolleyes:

I had to do one fairly hard thing to get this to work so I don't know about writing this one up. But I do think that Arrma should put 32 pitch gears in the brushless model. This thing has just as much power as my Pro-MT 4x4 on 3S.
 
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Nosedive4130

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Also, it kind of makes sense that it would survive with a 60/17 setup because you almost doubled your speed output and cut the torque in half. If I'm thinking correctly.

Either way, that's amazing. Vids or it didn't happen. Lol
 
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Moving up in gear size doesn't work in the same way as decreasing tooth count. You're actually increasing torque by the same factor as you're increasing the size of the tooth. The easiest way to see it is gear ratio, you're going from 6.5 to 3.5~, which is a 46% reduction - the half inverted speed output. Then you have the the increased torque which is the gear size. 48 pitch teeth has a surface area of 25~mm^2. 32 pitch is 50~mm^2, which is twice as much. So you're doubling the torque, but no system has 100% efficiency so let say it's somewhere around 80-90% more torque with 80-90% more speed.

This is why I'm wondering why this rig isn't in pieces now :p

As for a vid, I'll see what I can do. I might try my damndest to break it though.
 
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TooTall

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@Megasty
I can't seem to find what is inside the diffs for fluid/grease? Have you had yours apart yet? It doesn't appear they need to be opened up to change the input gear, but I want to have whatever on hand just in case.
 
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Megasty

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I had to open the diff to replace a broken outdrive. Yeah, I hit a tree & the outdrive actually broke. The plastic driveshaft was strong enough to rip it right off. Maybe I should heat treat them. Anyway, there's just a bit of bearing grease in them, nothing special.
 

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ok, thanks for the info.
 

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