Build Thread Arrma GROM speed run build!

[picodroid]

I've been working on building an ARRMA Typhon GROM BLX into a a speed run car. I am new to RC and haven't done this before, so I'm still learning plenty about how to setup the car and make the runs. My goal is to get as fast as I can with the stock motor and ESC.

I've made a fair amount of changes to the Typhon, but have not pushed it for any high speed runs yet. I have mostly been waiting for some cooler weather (I'm in the desert!), but that waiting has given me time to keep modifying the car to (hopefully) improve it's speed run capabilities.

You can view the history of the build (started 2 weeks ago) at this link https://www.arrmaforum.com/threads/typhon-grom-blx-speed-run-build.79071/

I'll start this thread with explaining the car in it's current state, mainly the mods as of tonight!
- Losi NASCAR chassis/front splitter/rear diffuser
- TLR Aluminum 203.5mm driveshaft
- Extotek steering bellcrank
- HR steering links
- BSR Nitro 30mm foam wheels (38 shore)
- Arrma 31T pinion gear

I need to give it a new shakedown run, which I will likely do again on 2S only and hopefully get its full speed on 2S in this configuration. My last attempt in the Typhon chassis only hit 41mph but I ran out of road so it should have gone a little faster. I expect this to reach at least 70mph on 3S.
If my 3S tests with this setup go well, I am probably going to try out an LRG RC 44T spur gear out which I think could get this up to 100mph if aero and electronics don't kill it.

 

[picodroid]

I mistakenly forgot to strap the SkyRC GPS when I went out today, and I was too lazy to walk back home to do it lol. I took my "Typhon" GROM out for a shakedown run since I swapped everything into a Losi NASCAR chassis.

Anyway, I measured the speed of the car from frames in the video and it goes around 52mph on 2S. I was expecting between 50-55mph so this falls right in line.
If all goes well, I expect to hit 70-75mph on 3S. And if that goes well, I may go with a smaller spur for even more speed.

 

[picodroid]

I tried fitting the Typhon body on with magnets - that didn't go well! So, I opted to forcing the original Typhon mount onto the NASCAR top deck part. I also added a tab in the rear which sits snuggly in a foam retainer, so this body is as easy to remove as the original car. No pics of the inside right now though, but it's nothing special.

I've started work on an under tray, and decided to go with tall vertical edges. This is all hand-cut, so no curvy shapes! I think it could help air flow under and around the sides, but I'm a novice to this stuff. I know aero can be quite fickle. I intend to keep it removable in case it just makes things worse.

I will do more work to make the side fins more rigid, then paint it all black. I also want to do something with the front splitter, make it more of a wing with a nose - but that may take a while.

 

[Doom!]

That looks great! If it performs as good as it looks, it's going to be killer! I love the Grom series.

 

[picodroid]

The undertray has tested the limits of my patience, with my limited tool and skill set. I wish things were a little more clean! That said, I am happy with the outcome and it should serve its functional purpose well.

I am not sure how I will mount this. I can't find a lot of options for screw holes since the flat portion of the chassis is only about 44mm across. I am considering using something like E6000 to fix it to the chassis semi-permanently.

With this completed, I think it is read for its full 3S speed run. Unfortunately, it's still hot out here so I may be stuck waiting another couple weeks before the pavement isn't heat soaked. I want these stock electronics to survive at least one run lol. I might do a quick 2S shakedown once this is in place, and if so I should finally get a 2S stop speed recorded.

 

[Doom!]

I really like the look, would velcro hold it in place?

 

[picodroid]

Thanks! Velcro could probably hold it in place, especially if I used all the available flat space under the chassis.
But it would add thickness which is why I avoided double-sided tape too.
I have less than 10mm clearance now that it's lowered, and with the under tray. Worried about getting too low since I won't have perfect roads to test on.

 

[Doom!]

If you have good contact between the chassis and your add on. You can use the same 2 sided tape they use for replacing screens like cell phones. It is an acrylic two sided tape that can tolerate a decent amount of heat. But you have to have good contact no gap between. it is about the same thickness as scotch tape.

 

[WickedFog]

If you have good contact between the chassis and your add on. You can use the same 2 sided tape they use for replacing screens like cell phones. It is an acrylic two sided tape that can tolerate a decent amount of heat. But you have to have good contact no gap between. it is about the same thickness as scotch tape.

If that is anything like the OEM phone sticky goo... yeah that would be some good stuff. But you lose any vibration dampening, which might be fine for a speedrun car since everything gets desyroyed in a crash anyway

 

[Tunedfrog]

Looks pretty cool.

 

[picodroid]

I took the car out for some successful runs on 2S. Unfortunately, it only reaches 43mph. So now I expect it to reach only somewhere in the 60mph range on 3S. It seems that downsizing to the rigid foams have sapped more mph than I expected, and potentially more drag.

Anyway, it was fun to finally see it going. I left the button head screws in to help hold the under tray and they spark over bumps

That said, this road is definitely a bit too rough for speed runs with this low car. I might still try 3s there, but I should drive to an alternate spot that will be clean and smooth.