Mid Motor Rustler - V2

[RogueIV]

I took the Bandit out racing again this past Friday and this time nothing broke! I was able to run the car the entire night in the mod 2wd class and it drove really well. I managed to record the main by strapping a phone to my head - I'm sure I looked silly but the video came out great!

Car looks great out there.

Also, are you the same Nick from RCTech making tires?

[rc10nick]

I took the Bandit out racing again this past Friday and this time nothing broke! I was able to run the car the entire night in the mod 2wd class and it drove really well. I managed to record the main by strapping a phone to my head - I'm sure I looked silly but the video came out great!

Car looks great out there.

Also, are you the same Nick from RCTech making tires?

Thanks! And yeah, that's me! Taking a break on that because it simply hasn't been a very fruitful endeavor. The world of rubber compounds is vast and complex - who'd have thought? lol

[rc10nick]

Small update for today - I tweaked the design of the transmission case a bit. The main thing I did was beefing up where the shock tower mounts. I angled the back of the mount so its thicker towards the base. I also printed a new case in a PC blend filament which should be much more impact resistant than the PLA I was using mainly for prototyping. I did some torture testing on the car - I dropped it several times directly onto the rear shock tower from varying heights and even gave it a few good smacks with a hammer. It held up just fine so that gives me a fair bit of confidence.



I didn't get a chance to race this past weekend, but I'll be able to get out again on Friday for some more track time at the same carpet track. I've got the gear diff in it now so I'll test it with carpet tires just to see how it does.

[rc10nick]

Been a few weeks since I've updated, but that's because I've been racing this car!

I raced it on carpet two more times since the last post and even had a chance to take it out for a practice day on dirt! I made it out to Pit Stop Hobbies for the premiere of this year's track and while I couldn't stay all day, it was fun getting this car on dirt for the first time.

Even with a full dirt setup on this car, it can still hang on carpet with the right tires. My best hot lap was only 0.5s off of the race winner, and that's actually a bit closer than I can usually get even with my 22. Carpet season is over now, but come next winter I'll have this tuned to be a carpet machine.

The practice day at Pit Stop wasn't the best day for learning anything - it was a cold morning, the track was pretty wet, and it was covered in saw dust to try to help dry it up. Nobody had traction, even the ebuggies struggled. But it was still fun and I feel like my car did pretty well despite the traction. And speaking of low traction, that's the perfect segue into my next topic - I've been printing out test pieces to check my design for the standup trans configuration. I want to have the standup version of the car ready to go in case I feel like the laydown doesn't have enough grip in the dirt.

I've had the design for these parts finished for a long time now, I just never bothered to print them out until now. With one exception, everything printed out perfectly and bolted right up to the car without any issues! It's almost like I know what I'm doing at this point. The rear wing looks a bit goofy since I had to raise the mount a bunch so it would still fit. I'm leaning towards designing a new mount to get the wing back where I want it, but I'm also not sure if it's important enough.

Anyway, here's the pics!

[rc10nick]

Standup trans complete and ready to hit the track tonight!



The standup config definitely feels more planted running it around my basement track so I think I made the right call putting it on. Like I said, grip was really low when I went to the open practice at Pit Stop a few weeks back so I think this was a necessary change.

I also want to test my super secret xl5 gear diff setup. I've been investigating the differences between a ball and gear diff and I think I figured something out. My gear diff feels pretty good on my basement track, way better than any other gear diff setup I've ever tried in any car, so I'm excited to see how it does on dirt and how it compares to the ball diff.

Either way it should be pretty fun, and I even plan on strapping my phone to my head like a goofball to get some good running shots of it.

[rc10nick]

Race day report!

For the most part, this car did great on dirt! At least as good as anything I've driven on this track, which includes a very modern TLR 22 5.0, so already a huge success. And nothing (major) broke. My waterfall brace did break after a particularly bad crash, but it was a clean separation across a layer line and I was able to super glue it back together. It held up perfectly fine the rest of the night. Otherwise everything held up amazingly well and considering all the prints are just basic PLA I'm super excited with how durable it is.

I did test out my super secret gear diff setup and while it was good for a gear diff, the ball diff still easily beat it. I think with some more refinement and tweaking I can get the gear diff to be even better, but I didn't want to spend anymore time futzing with it at the track so that will have to wait until next time.

The nitro rustler rear arm mounts are great with this car. I used the +/- 1° mount to try 4° and it was perfect - plenty of rear grip and stability without making the car hard to turn.

In practice I found the rear roll center to be a bit too high causing some instability at high speeds so I raised the rear inner link position to help fix that. I think I went overboard raising it, though, because it made the rear end of the car too free and while it wasn't bad in the qualifiers, it made the car much too slidey in the main when grip disappeared.

My best run of the night was Q2 - the car felt the best in that run and I drove my best. I ran so well that I qualified 3rd overall. 2wd buggy was the biggest class that night with some pretty amazing drivers so I was stoked to qualify so well going into the main. Unfortunately like I said the car didn't do well in the main and I ended up last. I could still put in a decent lap, but I was wrecking so much due to the car sliding around too easily that I just didn't have a chance.

I'm confident the problem was the rear roll center being too low - my car did the exact same thing on my basement track the day after. It just slid around way too much. Dropping the inner rear camber link height fixed the problem completely on my basement track so now I know the next time I'm at the track my car will be even better.

Also, I completely forgot the last time I was out at the carpet track with this car I put in some really heavy shock oil - 45wt all around. On dirt I'm usually somewhere around 30wt so that definitely wasn't ideal. I've already got that fixed for next time, and when I was changing my shock oil I noticed both of my rear arms were bound up pretty bad. So I fixed that, too. Looking back I'm surprised the car drove as well as it did, and I suspect these issues may have caused some of the instability issues I noticed in practice. Now that I've got these problems sorted out I can't wait to try it again because I know it will be even better next time.

I strapped my phone to my head to get some good running video of the car. Here's the full Q2 run so you can see how well the car did. Like I said, this was my best run of the night, but reviewing it I can see where the rear end does seem to slide around a bit too easily coming out of turns and it looks like I'm really waiting for the car to settle before getting back on the gas. Also, just a warning there's a particularly foul mouthed driver up on the stand so be careful with volume and who's around when you're watching.

[tmaxx51]

That's pretty awesome that it's so competitive. I will probably get attacked for this, but I think most of what makes a car good is the weight distribution. That's assuming that you can adjust your suspension to suit your needs.

When are you going to share your super secret diff recipe?

[rc10nick]

Yeah, one thing I'm learning from this build is as much as buggy design has changed over the last 30 years, there really are just a few basics that if you get right, you'll have a pretty competitive buggy.

As for the gear diff, my super secret setup is actually incredibly dumb in how simple it is. The secret is a thin cardboard shim in the middle of the output gears that causes the diff to bind up when I tighten the case screws. I can adjust how tight the diff is by adjusting the case screws. I also have a thin coat of black grease on the gears.



I tried this because I noticed ball diffs actually have a slight bit of lock before they start spinning, and once they start spinning they've got almost no resistance even under heavy load. The problem with most gear diffs is even with a really heavy silicone diff fluid, they don't replicate that "stuck" feeling before they start spinning. Try this experiment to see what I mean:



Hold the diff by the main gear and watch what the outdrives do. A ball diff will support the weight of the hex driver bit without spinning. A gear diff, even with heavy diff fluid, will start spinning immediately. The viscosity of the diff fluid only changes how quickly the outdrives rotate. The cardboard makes the gear diff behave like the ball diff in this experiment. The black grease on the gears helps the gear diff stay free feeling under heavy acceleration, but it's still not as free as a ball diff. It's probably the best feeling gear diff I've driven, but the ball diff still beats it.

[Chewbacca]

Cool Diff tweak !

Maybe slipper clutch pad material might work better than cardboard.

Do you have the Traxxas Pro ball Diff ?

[rc10nick]

I had thought about slipper pad material, but I don't have any pads I can cut up and there's not a whole lot of room. I first used a 0.5mm metal washer and even that was too thick. The cardboard is perfect because it's thin and it squishes a bit like a spring. I tried a shock o-ring and that tore up right away, then I even tried casting my own custom sized o-rings and that didn't work either.

And yeah, it's the traxxas 2520 diff in there. I got lucky a few years back and scored one at my LHS for only $30.

[rc10nick]

5/3/2024 Race Update

Last night I had this car out at Pit Stop hobbies for another Friday night race program. The car was much faster this week with the changes I had made. I beat my fast lap by 0.7s and I ran a 15/8:15 in the main compared to the 14/8:33 last week. Significant improvement. There were only 3 drivers in 2wd buggy last night and I got 2nd to the guy who won last week, but the time I ran easily would have put me in the top 3 in last week's main.

I didn't do any more testing with the gear diff. I just didn't feel like ripping apart the rear end multiple times at the track to do it.

I think there's still more I can get out of this car. Corner speed isn't where I think it could be - it feels a bit pushy and it still wants to slide out a bit too much mid/late corner. There's a few things I might want to try to fix it, one of which is actually trying the laydown configuration since their big advantage is carrying more corner speed.