First Arrma build Big Rock 6s

[DesertRC928]

I’m just getting back into this hobby. Recently bought my first arrma at my local hobby shop. Decided to go with the 1/7 Big Rock 6s. Very happy with it so far. To me planning, upgrading, and wrenching on these things is just as fun as running them. I still have a lot to learn about this platform so I’m hoping for some guidance as I build this thing up.

Goal of this build

Build a bulletproof desert basher! I live in Arizona and my backyard is literally the desert that is outside of city limits. So I want it to be setup to handle in the sand, not overheat in the hot climate, and be able to take a beating without constantly breaking.

Current upgrades

• Savox monster sb2290sg servo
• Castle bec (8.4 volt output for servo)

Incoming parts ordered.

• Treal aluminum front/rear hubs (these have 3 bearings per hub 2 small and 1 “oversized” inner) My big rock eats stock inner rear bearings. I’ve killed about 4 with about 15 packs through the RC.
• Treal aluminum front lower suspension arm mounts.
• Arrma EXB aluminum front upper suspension mount

Tuning and setup

• hobbywing LED program card (just more convenient for messing with esc settings)
• Different diff oil weights: 30,000, 80,000, 500,000 cst.

I’m going to play around with different diff oil weights to figure out what I like. I think I’ll start with

Center diff: 500,000

Front diff: 80,000 (maybe eventually run a limited slip front diff?)

Rear diff: 30,000

Open to any and all diff tuning suggestions.

Current charger and battery setup. Bought come cheap lipos from amazon to try out. I’ll probably eventually grab some CHNL lipos after reading some on this forum.

Future mods??? (Will update as we go)

Honestly I’ll probably end up eventually upgrading everything haha! I’m open to suggestions from you guys. First I want to upgrade all the most common weak points, stock up on the most commonly broken spare/wear item parts, then upgrade the fun go fast stuff.

Questions for you guys.

Aside from the parts I have incoming what are the most commonly broken parts and things to upgrade first? I’m assuming maybe shock towers next?

What spare parts would you recommend keeping on hand? The stuff that inevitably breaks.

I live in a very hot climate. What motor and esc fans would be best to keep it from overheating?

Is there any benefit to running a limited slip front diff? I want it to track pretty straight under power in the sand and still turn tight. I know there is a give and take balance to this. Just looking for a best place to start.

 

[DesertRC928]

Update. Got some new parts in and installed.

Really like EXB upper hinge pin mount. The ones that come in the big rock would constantly back out. With the way the EXB pins are locked in this should no longer be an issue

Installed both lower suspension mounts in the front and rear.

While I had the front apart I replaced the bushings that came in the steering components with Fast Eddy bearings.

Also installed the Treal aluminum rear hubs. Still running stock fronts as I haven’t had any issues with those yet. My stock rear hubs would eat bearing constantly. The inner bearings seem to be under sized and/or not sealed very well (I run it in the sand).
You can see from the pic that the rear inner bearing seized and spun inside the plastic hub melting it lol.
The Treal hubs use 3 bearings total. 2 smaller bearings and one “oversized” inner. In the pic you can see the stock inner size bearing vs the Treal inner. I also picked up a 10 pack of the oversized inners that are “double rubber sealed” maybe that will help in the sand. Guess we’ll find out.

I also broke my rear diff and stripped the pinion. I’m sure the first occurrence of many lol. Happened on a hard landing. Only thing hurt inside the diff was both outdrive pins where broke.

New input gear and diff pins and it’s back together. The rear diff only had two shims from the factory and that was too loose. I added another shim and the gear mesh seams more appropriately in the rear now. Hopefully it lasts a little longer this time around. Probably had 25 packs through it before the failure which doesn’t seem too bad considering the gear mess was too loose out of the box.

I also decided to try an LSD front diff.

Diff oil weights I’m trying out for now

Front (LSD) 10,000 cst
Center 100,000 cst
Rear 30,000 cst

Will report back when I upgrade something else or when it breaks again haha.

 

[DesertRC928]

Update. Did a few mods and upgrades. I had a front driveshaft pop out on an uneven landing. You can see from the pic it chewed up the lower suspension arm and diff outdrive a little bit.

Also note the cracked front diff cover. (More on that later lol)

In an effort to stop the front driveshafts from popping out again I removed the shims on the top pivot ball and I drilled out the metal cap on the lower arm. This allows the entire hub to sit a few mills closer not allowing the driveshaft to pop out under full compression.

One issue I ran into after this mod was the pivot ball binding up the steering at full droop. I added two shims to the droop screws and that fixed the issue. I have been running this thing hard since and have yet to pop a front driveshaft out again

Now back to the cracked front diff housing. Unfortunately I didn’t notice that and put it all back together to test. I ran it hard like I always do lol and the front diff started clicking The cracked diff cover allowed the diff to move enough to chew up the ring gear on my brand new LSD diff.

Picked up a new diff main gear and upgraded to an EXB aluminum diff housing. I used a brand new pinion gear too when I put it all back together. This aluminum housing ended up needing 2 shims behind the pinion gear and 2 shims for the diff to get the gear mesh set properly. I think the front should be good to go now

Next up I think I’m going to upgrade to aluminum diff cups. I’ve noticed the diff screws keep coming loose everytime I pull a diff housing cover to check things out. I tighten them as much as I feel I can without stripping the plastic threads. With aluminum diff cups I’d be able to use thread locker. I might try some cheap nail polish on the diff screws in the plastic cups while I wait for the aluminum diff cups to arrive. I know regular thread locker is a no no with plastic threads.

I think I’m going to give a Treal aluminum diff housing a go for the rear. It seems like the plastic housing keeps getting looser the more I run it. I’m still on the same diff main gear in the rear, but it seems like it needs more shims to keep the gear mess properly spaced everytime I pull it apart. I think I’m up to 4 shims in the rear now. I think upgrading to all aluminum will allow more hard runs with less tear down and maintenance.
I’m curious what kind of life are you 6s guys getting out of your diffs? Any tips and tricks to get more life out of your diffs other than what I’ve mentioned?

 

[Neelis]

I think if you look at the new Kraton 6s EXB and all the upgrades it got compared to the regular Kraton 6s, most of your questions will be answered in terms of reliability.

Somebody on the forum wrote that if you drive your RC hard and make big jumps, the new Kraton 6s EXB is worth it, otherwise the normal Kraton will do. I think you need EXB alllll theeee waaaayyy