Mid Motor Rustler Build

[rc10nick]

very impressed with your driving on that tiny track.

Thanks you! Ever since I built the track I've been mostly using it to improve my driving and I'm amazed at the progress I've been able to make.

Anyway my mini-b is broken and I'm waiting on supplies for another project so I turned my focus back onto the mid motor rustler. Now that I have a 3D printer I want to start cost-reducing out some of the more expensive components in my design. I started with the lowest hanging fruit - the battery hold downs. The little thumb nuts I used were $1.60 each. That's not crazy expensive, but it's a lot just for some hardware. I designed and printed a thumb nut that uses an inset m3 locknut as a replacement and they turned out fantastic. A drop of CA glue holds the nut in place, they're easy to use, and I personally think the splash of color they add to the car makes it look way better. On top of all that, they're cheap! Between the PLA and the locknut they're no more than $0.10. That's a drastic reduction.



The aluminum threaded standoffs I use for the battery brace are the next to go. They're about $5/piece and again, it won't cost more than $0.10/piece to replace them with a print.

[rc10nick]

Done! That's $16 worth of parts from McMaster Carr replaced with less than $1 of prints. Looks better, too.



Now that I've got some experience with mold making I want to try to try replacing some of the parts with casts next. The battery brace is the obvious first choice because of its simple design, cast side pods would be good to eliminate the delrin and composite side stiffeners, but the ultimate goal for casting is the transmission case. I get them SLS printed and they're $15/each with a min quantity of 3 when I need to order them. Casting them from a rigid polyurethane would make them much cheaper with the major benefit of being more durable.

[rc10nick]

I love these new side braces! They look so much more... factory? Like not as obviously DIY'd. The car is just as stiff with them and I think the 3D prints will be tough enough to survive racing. So far they've survived my basement track and street bashing.

[TRX-1-3]

That's dope.

[rc10nick]

The PLA parts all survived a race night! Check it out, someone even recorded videos of the races. Here's the 2nd qualifier I ran. The truck and the track felt the best in this run. I even managed to work myself up to 1st in the heat until the announcer called it and I promptly yeeted myself off the track.



Anyway, the truck survived, but it just didn't feel very planted or stable. I think it might have something to do with how little the truck weighs (I'm right at the minimum legal weight) and the fact that I'm using closed cell inserts that weren't broken in at all. I'm going to play around with weights and switch to a heavier battery as well as acetone off the tires so I can work the foams a bit. The guys at the track gave me some good tips for modifying the foams to make them work better.

[rc10nick]

I had the truck out for another race this past weekend. Once again it survived without any breakages AND it drove much better this time.

Since the last race I went up to a full size shorty which added about 50g then added in another 42g of stick on weights to chassis between the battery and the motor. I wanted the weight to be as central as possible but just slightly towards the rear.

Once I got to the track I played with my rear camber links to see if that could help and that made the biggest difference. Raising the inner link position drastically improved rear stability and made the truck so much easier to drive. I think making the link longer would help, too, but I'll need to CNC a new inner camber link plate for that.

Stadium truck was the biggest class of the day with 11 racers showing up and they ran us all as one big 11 truck main. I had some rough qualifiers and started 9th, but after a bit of a rough start in the main I managed to settle myself down and started ticking off clean lap after clean lap after clean lap. I worked myself up to a 2nd place finish which was way better than I had expected going into the main. I was super pleased with myself and my truck!

Here's a video of the main:


I still think I have some work to get the back end of the truck settled down even more. Watching that video I can see how despite being much more planted than last time, the back end still wants to step out quite a lot.

[Chewbacca]

Nice race!

Since it is an outdoor dirt track I am curious how the same car with a rear motor setup would handle. Since they always say rear motor is better for low grip conditions.

[FlyingTigerDad]

Congratulations on the strong podium finish. You've made a lot of progress with this truck. You are correct about the rear camber link - a long, low and level link (parallel to the arm) will provide a very low roll center & help lock in the rear on a smooth track. But for a loose/dusty surface like that with a low roll center, you'll just want to make sure you have enough rear negative camber (1.5° should be good to start, but you might need a little more) and you'll also need to make sure the rear spring/shock oil setup is fairly light to allow enough weight transfer for good traction through the turns. A low roll center can provide for a lot of initial chassis roll to a point, but then will stiffen up quite a bit, so those other adjustments are needed to compensate.

[rc10nick]

Nice race!

Since it is an outdoor dirt track I am curious how the same car with a rear motor setup would handle. Since they always say rear motor is better for low grip conditions.

There are actually guys at that track who still run rear motor cars. Two of the trucks in the main were rear motor 22T 2.0's. One of them actually won the race, the other placed 6th. How much motor configuration played into it is anybody's guess. The guy who won is an amazing driver who'd win no matter what he drove and the guy in 6th is a newer racer who's still developing his driving chops.

I was racing pretty regularly at that track around the time the switch from rear to mid motor happened. I remember at the time all the fast guys switched to mid motor and they weren't losing time to people running rear motor cars. I also remember running my dex210 in mid motor at that track and it drove really well. Personally I think a lot of the recent clamor about bringing back rear motor cars is driven more by nostalgia than performance.

Congratulations on the strong podium finish. You've made a lot of progress with this truck. You are correct about the rear camber link - a long, low and level link (parallel to the arm) will provide a very low roll center & help lock in the rear on a smooth track. But for a loose/dusty surface like that with a low roll center, you'll just want to make sure you have enough rear negative camber (1.5° should be good to start, but you might need a little more) and you'll also need to make sure the rear spring/shock oil setup is fairly light to allow enough weight transfer for good traction through the turns. A low roll center can provide for a lot of initial chassis roll to a point, but then will stiffen up quite a bit, so those other adjustments are needed to compensate.

Thanks! I think my issue with the camber links was just not giving them any attention after having the truck set up to run on turf. I had a pretty high roll center setup which really helped it corner faster, but is definitely less than optimal on a loose dirt track.

[rc10nick]

I'm going racing tomorrow night so I spent the night getting the car ready. I wanted to have a solution making that rear camber link longer so I added some extra holes to my rear camber link plate in the 3D model and CNC'd out a pair of them from some scrap carbon fiber I had sitting around. This was the first time in month's I'd ran my CNC and I forgot how much that machine puts the fear of god in me while I run it. lol.

The last race trashed my ball diff so I rebuilt that using some ceramic nitride balls I bought cheap off of Amazon. They were $13 for a pack of 100 and they feel really good in the diff - way better than the stock balls for sure. The stock balls felt gritty brand new out of the package and only got worse.

And here's some pics! First is the new and old rear camber link plates, second pic is the new plate on the car with the links lengthened out and the ride hide lowered down to 20mm.




Hopefully all the changes make the car faster round the track. I'll found out tomorrow!

[Chewbacca]

REALLY NICE!!

Looks promising and good luck for the race!

[Mr. ED]

nice 'n looooow

[rc10nick]

Thanks guys!

The truck drove great last night, probably the best its ever driven at that track. The longer camber links really helped to stabilize the rear end of the car. In practice I was able to run a 30.6 which is about a second faster than my hot lap from the last time out at the track. Unfortunately I wasn't able to repeat that when it mattered, but I was still easily on pace with the rest of the drivers. The main didn't quite go my way this time. I started 7th this time and quickly managed to work my way up to 4th, but unfortunately I stuffed it in a pipe where there wasn't a marshal close by and lost 10 seconds. That was it for me, unfortunately. I was only able to work my way back up to 6th but it was still a really good race because even that deep in the field I really had to work and battle with other talented drivers to maintain that position. I wish I could have finished better but it was still a good race and the car was the bests its ever been so overall I'm really happy.

At this point I think I'm ready to declare the build, and the project, finished. It drives really well, it's durable, and I've raced it and done well on every surface you could race on. I'd say the project has been an overwhelming success.

I know what project I want to work on next - a 2wd buggy - but what I haven't decided on is what to do with this car and the design. I'm actually leaning towards open sourcing the design and posting all of my files on thingiverse. I had thoughts of making kits to sell but honestly that's more work than I have time for and I'm not sure the demand would be there anyway.

[rc10nick]

I made a minor improvement to my car today. I never really had a solution I liked for mounting the shocks on the CF towers. I've had cars in the past with shock bushings that had captured lock nuts and I always loved that design, so I made a set for my car. For something so small and simple, these little bushings bring me so much joy. Now I just need slightly shorter screws. That's going to bug me.

[rc10nick]

I'm going to eat my words about declaring this build done and finished. I never did convert this truck to a buggy like I planned. Instead I bought a TLR 22 5.0 that popped up locally for really cheap so the Rustler gets to stick around for a while longer.

Before I bought the 22 5.0 I was designing and prototyping a modern 2wd buggy using the Traxxas parts from my Rustler but now that I've got a buggy that project is shelved for the time being. However, one upside to all the design work I did is I can easily transfer it over to the Rustler! Now that its outdoor season again I'm looking to implement one of the features I designed for the buggy that I think will help the Rustler the most - a standup transmission. I never truly felt the laydown in the Rustler was the optimum choice for the outdoor dirt track I was racing at which is why I designed a standup transmission for the buggy I was working on. Here's a prototype of that design on the buggy:



And here's a 3D model of the standup transmission adapted to the Rustler chassis:



This design moves the motor and battery as far back on the chassis as I can get them in order to maximize the amount of weight on the rear tires. Compared to the laydown transmission, the battery is 34mm further back and the motor is about 28mm further back. Something unique about my design that you don't see on any other car is how I brace the transmission case to the chassis in a way that lets me put the battery right up against the motor instead of having a 1/4" gap I'd need for a typical waterfall brace. It's exactly what I did to my dex210 years ago. That design gave it the same weight split as a rear motor B4 and I remember that car being absolutely planted. I'm hoping the same is true with this design on the Rustler.