Big Rock The Big Rock 4s conversion thread

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Arizona
Arrma RC's
  1. BigRock 4x4
  2. Kraton EXB
First and foremost, apologies if this is a duplicate thread. Searched the web and the forum and found fragmented information.

I plan on taking my BRCC to 4s in the future and thought it would be handy to consolidate information to one thread. This way all of the parts, tips, tricks, etc., are in one place.

To the vets, and those who have already planned or completed this upgrade…let’s get the parts list and advice rolling 👍🏻.

I’m just get started with my research. So far, I’ve read about the esc, motor, motor plate, and diffs needing upgraded.
 
You got a Big Rock. You put her on 4s period. You then see what gives then you replace it lol Seriously, no planning needed I have the 3200kv motor on mine and 6s esc and let it rip, wow is it fun. I never get to do full throttle but if I did I would go to 8s lol. Put a 4s battery on the B and send it baby!!!
 
You got a Big Rock. You put her on 4s period. You then see what gives then you replace it lol Seriously, no planning needed I have the 3200kv motor on mine and 6s esc and let it rip, wow is it fun. I never get to do full throttle but if I did I would go to 8s lol. Put a 4s battery on the B and send it baby!!!
Love it. I’m such an over thinker lol. Did you burn up the original esc or preemptively replace?
 
Overly simplified, but if you're patient, the "waiting to see what breaks" approach isn't a wrong one, it just takes the most time.

To make a 3s rig "4s ready", you'll want to consider updating the drivetrain with the following items:

4s metal gear f/r diff
1.35M Metal Input Bevel Gear
Spur & Slipper Set 0.8 Mod <-- currently out of stock at Jennys

The last suggestion I would make is a 4s capable ESC like Spektrum 120a or the Spektrum 130a and a lower kv motor like the Spektrum 2400kv motor. While people have run the 3200kv motor on 4s, it's not really made for more than 3s reliably.
 
Love it. I’m such an over thinker lol. Did you burn up the original esc or preemptively replace?
No but no I threw in my blx esc in it. I may have been a little dramatic. The most important thing is the ESC. I will take a step back here lol...try it on 4s and be nice to it. See how she likes it. I burned up like 3 junk china ESC when first trying her on 4s, she runs great. I mean not that I think about it I don't want you to burn anything. My low price recommendation is the HW 120A ESC. It works pretty well on 4s. Another good way to go would be to get the 6s esc and motor from Jenny's you will pay about the same on Amazon for a good ESC and motor. Don't buy anything from Amazon/China not worth it.
 
Do you want a 4s capable big rock or an actual 4s conversion big rock? I turned mine into a 4s beastie using 4s, v2 parts on a big rock’s long wheel base chassis. It’s my favorite car pretty much.

B5B6B0A3-64C1-45E9-A4CB-96CD3886A519.jpeg


79EFB769-FE89-46C2-8DAF-FA7EC989F56F.jpeg
 
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Overly simplified, but if you're patient, the "waiting to see what breaks" approach isn't a wrong one, it just takes the most time.

To make a 3s rig "4s ready", you'll want to consider updating the drivetrain with the following items:

4s metal gear f/r diff
1.35M Metal Input Bevel Gear
Spur & Slipper Set 0.8 Mod <-- currently out of stock at Jennys

The last suggestion I would make is a 4s capable ESC like Spektrum 120a or the Spektrum 130a and a lower kv motor like the Spektrum 2400kv motor. While people have run the 3200kv motor on 4s, it's not really made for more than 3s reliably.

Great post.... I did all that with using a Firma 130A ESC, 3050kv motor to stop running it at 4S and back to 3S, enjoying it more. A 6S rig filled that void and was properly suited.

That is a great list provided. I would add with all that power, and you will need Arrma CVDs.

I spent so much money doing that project with no 6S rig at the time in my collection...wish I had bought a Kraton, etc instead. Come to think of it I did in the 3S project!!!... all went into that damn 3S at the time. :LOL:

@HooknBook enjoy the ride and keep us updated (y)
 
Overly simplified, but if you're patient, the "waiting to see what breaks" approach isn't a wrong one, it just takes the most time.

To make a 3s rig "4s ready", you'll want to consider updating the drivetrain with the following items:

4s metal gear f/r diff
1.35M Metal Input Bevel Gear
Spur & Slipper Set 0.8 Mod <-- currently out of stock at Jennys

The last suggestion I would make is a 4s capable ESC like Spektrum 120a or the Spektrum 130a and a lower kv motor like the Spektrum 2400kv motor. While people have run the 3200kv motor on 4s, it's not really made for more than 3s reliably.
Awesome, thanks for the recommendations!
Do you want a 4s capable big rock or an actual 4s conversion big rock? I turned mine into a 4s beast using 4s, v2 parts on a big rock’s long wheel base chassis. It’s my favorite car pretty much.

View attachment 269345

View attachment 269346
Sweet ride. In all honesty, hadn’t thought about it that way. Was thinking 4s capable with upgrades to the suspension, RPM arms, 17mm conversion, etc.
No but no I threw in my blx esc in it. I may have been a little dramatic. The most important thing is the ESC. I will take a step back here lol...try it on 4s and be nice to it. See how she likes it. I burned up like 3 junk china ESC when first trying her on 4s, she runs great. I mean not that I think about it I don't want you to burn anything. My low price recommendation is the HW 120A ESC. It works pretty well on 4s. Another good way to go would be to get the 6s esc and motor from Jenny's you will pay about the same on Amazon for a good ESC and motor. Don't buy anything from Amazon/China not worth it.
So you just did the electronics and are running stock suspension, etc.?
 
Starting placing some orders for this journey.

Grabbed a Spektrum 150 esc from Jenny’s and found a brand new 3668-2400kv motor unit with fan from an OC 4s.

Will be looking into the other bits soon but will likely give the truck a shakedown run once the new esc/motor are installed.
 
and found a brand new 3668-2400kv motor unit with fan from an OC 4s.
That’s what I have in mine and it’s okay.
I decided to try something different and put a 4068, 2650kv rocket in my 4s vorteks Thing build and love it. It’s a bit high KV but I run it mostly on 3s. I picked up another one that’s 2050kv to put in old Yeller eventually. I run that one on 4s mostly.

What are you thinking about for the motor mount? I have the M2C mount and upper chassis brace in both mine. It’s expensive and worth it. I’ve had both in several different builds over the last couple years. The new 6mm stock mouth is good also. I’m running one in my granite jumper for months now and considering how many bad nose landings I have, the fact that it hasn’t bent is impressive.

One of the things I really like about the M2C mount though is that it seems to block a lot of the crud from getting underneath the spur case. By combining a little bit of soft Velcro underneath, it keeps almost all out.
 
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Do you want a 4s capable big rock or an actual 4s conversion big rock? I turned mine into a 4s beastie using 4s, v2 parts on a big rock’s long wheel base chassis. It’s my favorite car pretty much.

View attachment 269345

View attachment 269346
Able to breakdown what 4s parts you used? May end up throwing some of these into the mix as things move forward
That’s what I have in mine and it’s okay.
I decided to try something different and put a 4068, 2650kv rocket in my 4s vorteks Thing build and love it. It’s a bit high KV but I run it mostly on 3s. I picked up another one that’s 2050kv to put in old Yeller eventually. I run that one on 4s mostly.
What are you thinking about for the motor mount? I have the M2C mount and upper chassis brace in both mine. It’s expensive and worth it. I’ve had both in several different builds over the last couple years. The new 6mm stock mouth is good also. I’m running one in my granite jumper for months now and considering how many bad nose landings I have, the fact that it hasn’t bent is impressive. One of the things I really like about the M2C mount though is that it seems to block a lot of the crud from getting underneath the spur case. By combining a little bit of soft Velcro underneath, it keeps almost all out.
Right there with you on M2C for the motor mount.

Can’t decide what I want to do for a chassis brace. Arrma center brace of if something a bit heftier is needed.
 
Able to breakdown what 4s parts you used? May end up throwing some of these into the mix as things move forward

Right there with you on M2C for the motor mount.

Can’t decide what I want to do for a chassis brace. Arrma center brace of if something a bit heftier is needed.
Here my build thread on the mostly all v2 parts. Most of the info you want is the first few posts I think.
Thread '4s Velo scratch build, LWB chassis (Old Yeller)'
https://www.arrmaforum.com/threads/4s-velo-scratch-build-lwb-chassis-old-yeller.56168/

Here’s the all mixed up parts 4s build.
Thread '4s hybrid Vorteks…Thing (another Velo build)'
https://www.arrmaforum.com/threads/4s-hybrid-vorteks…thing-another-velo-build.58380/
 
Here my build thread on the mostly all v2 parts. Most of the info you want is the first few posts I think.
Thread '4s Velo scratch build, LWB chassis (Old Yeller)'
https://www.arrmaforum.com/threads/4s-velo-scratch-build-lwb-chassis-old-yeller.56168/

Here’s the all mixed up parts 4s build.
Thread '4s hybrid Vorteks…Thing (another Velo build)'
https://www.arrmaforum.com/threads/4s-hybrid-vorteks…thing-another-velo-build.58380/
Great, thank you for the links. Should have done more digging on my own.
 
Nah you’re good. Anytime I can help, I am happy to.
Just read both threads. Awesome work, each build is different and turned out awesome.

I may have missed it but what fan mount is that in the Vorteks build?

Love the wide stance but I’m not sure I’ll go that direction. Will definitely be using the same diffs. Your sealing solution seemed to work well. Which sealant did you use?
 
My list with loose dirt racing in mind and jumps as secondary:

1. Shocks. Either get the 6s Mojave/kraton shocks with lighter springs or the 4s v2 outcast/kraton shocks. I noticed the biggest difference in this upgrade.

2. Steering. Get a 20-35 kg steering servo, 25t servo saver, aluminum bell crank set, and either the metal ball kit for the turnbuckle links or full replacement (decide now if you want a wider stance with the 4s v2 arms or you’ll have to get longer links again later). I also very much recommend getting the hot racing hub extensions to 17mm. You’ll have to get new tires, but those completely removed every hub issue/inconvenience I was having with the addition of the wider stance.

3. Spur gear. If you can’t get the metal gear only from Jenny’s, the hot racing option is very good. You can wait til you burn up the plastic stock one of course, but mine went pretty early.

4. 4s v2 diffs and inputs. With the spur upgraded, your inputs and crown gears are the next link (a plastic link) to wear out or fail. The metal ones are much better.

5. Bash bar. I did this much earlier than 5 after breaking 2 chassis’s. Mine is home made for maybe $15, but there are a few fancy options out there. Replacing the entire chassis is pretty cheap to absorb, but it is a lot of wrenching every time and every adjustment has to be re-evaluated.

6. Esc and motor. At this point you’ve probably had some hours in, and if you’re still saying, “man, this 50mph stuff just isn’t quite doing it for me,” then add the power. My rig has been plenty fast for me to lose control for a half second and destroy it. I’ve also never lost a race because I just couldn’t go fast enough. So it really depends what you want to do. If you’re wanting a speedrun big rock, then you can disregard most of what I listed. A stunt truck would still want pretty much all of that, but you really don’t need 4s for that either.

-just my very limited opinion FWIW.
Oh yeah, ps:
The motor mount stuff is also very good. I have temporary support for the end of the motor can to reduce the chance of bending the plate, but will probably upgrade the plate as well.
 
I may have missed it but what fan mount is that in the Vorteks build?
That is the ‘Deet special’, a dude that was on here a while back, but disappeared because of health reasons I believe. He laser cut these out of stainless steel and bent them. It’s perfect for this car. I got one of the last ones he sent out before he vanished. Now I use power hobby fan mounts because I do not like heatsinks.
Your sealing solution seemed to work well. Which sealant did you use?
Do you mean for keeping crud from getting underneath the spurcase? I have the M2C motor mounts and upper chassis brace, which keep a lot of it out and then soft gravel crew on the bottom of the spurcase surrounding the hole and it keeps a lot of the remaining stuff out. If you meant something else, let me know so I can clarify.
Love the wide stance
It’s not even that wide really, I used 4s, V1 arms on this build and they’re a good inch and a half shorter than the V2, 4s arms I used on old yeller. I tell you, though, both of these cars rarely flip over on their lids, especially after I got the suspension dialed in. Having learned how to drive the regular 3S cars without flipping them often, it’s almost too easy with these 4S ones to keep them upright.
 
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Do you mean for keeping crud from getting underneath the spurcase? I have the M2C motor mounts and upper chassis brace, which keep a lot of it out and then soft gravel crew on the bottom of the spurcase surrounding the hole and it keeps a lot of the remaining stuff out. If you meant something else, let me know so I can clarify.
Definitely looking to get the M2C piece for the stability and to help keep the debris out of the housing.

I was referring to the liquid you used to seal the diffs. The ‘blue goo’. Sorry if you that info, I missed it if it was there.

Jenny’s has all the upgraded pieces in stock so I’ll have them en route soon to do my rebuild.

Thanks!
 
I was referring to the liquid you used to seal the diffs. The ‘blue goo’. Sorry if you that info, I missed it if it was there.
I’m still experimenting with that. Quite sure if it makes a difference or not yet.
 
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