Mid Motor Rustler Build

[rc10nick]

Click here for the latest mid motor rustler design
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Hi everybody! Over the past two years or so I've been having some fun with the 2wd Traxxas platform. It all started when I got burned out on racing. I took a break from racing for a while and when I got back into it I wanted to do it different - instead of going out and buying the latest and greatest of everything and stressing out about what to buy I wanted to see what I could do with a Traxxas. I set a challenge for myself to make a Traxxas competitive.

Since all the new cars are mid motor I knew I had to make it mid motor to keep up. I also really liked the look of the vintage Traxxas racers like the SRT, TRX3, etc and I drew inspiration from those as well as other cars of the era. There's something really cool about the simplicity of a dual deck fiberglass or carbon fiber chassis.

Anyway, I started out with a slash. I designed my own mid motor conversion in Fusion 360. Here's the CAD:


And here's it is in real life:


It's got a G10/FR4 chassis I cut out on my homebrew CNC router, but the real centerpiece of the car is the 3d printed laydown transmission case:


Here it is on its first outing. The track was only running on-road that weekend but it was still really fun. I TQ'd and won.


And here it is on another race day where they had it set up for off-road. Another TQ and win for the Slash.


So that was two winters ago. This past winter I wanted to fix some issues with the design and convert it into a rustler since stadium trucks were my first love (my first real RC was a T3). Here it is in its current state. The biggest change is in the back end. I moved the transmission case back towards the rear of the car. I had to move it forward from the stock position on the slash so I could keep the stock shock tower, but this time I just designed my own shock tower and camber plate and redesigned the transmission case to mount them on it. Doing all that allowed me to shorten the chassis up for the rustler/bandit wheelbase and still fit all the electronics.


After I turned it into a Rustler I needed a new body for it. I went with the most vintage and coolest looking option available for the Rustler, the J-Concepts F150. I couldn't think of a more perfect body for it.


What I've been working on now is finalizing the materials. G10 isn't the stiffest material out there and my design has a good bit of chassis flex in the rear. I was going to buy some carbon fiber to cut out new parts, but I realized that's taking the easy way out. I made the crazy decision to MAKE my own carbon fiber because why not?

Here's one of the first panels I made, and what I made out of it:




Anyway, making carbon fiber that looks good is turning out to be much harder than I thought so that's kind of where I am at the moment. Futzing around with epoxies, vacuum pumps, fabrics, etc until I figure it out.

Anyway, that's all for now. This post ended up way bigger than I expected. I hope you guys enjoy this as much as I enjoyed reading all the build threads I stumbled on here while researching my build!

[Dadio]

That's very impressive especially making your own CF

[Mr. ED]

Super! Well done throughout. Maybe you can add a carbon skin to the existing g10 parts? Bit like the first losi chassis etc

[rc10nick]

Thanks for the kind words Dadio and Mr. ED!

Mr. ED - I never knew Losi did that. Was it just for looks?

Over the Memorial Day weekend I tried making two more panels. The first was a 6" x 6" x 2.5mm panel to use to replace the motor mount, battery brace, and transmission brace. This panel was a near disaster. I had some issues with older epoxy and ended up with lots of surface pitting/holes on the panel and about a 1" x 4" section of the panel that didn't get any epoxy in it at all. I'll still make parts out of it, but they'll be kinda ugly.

I followed that up with a smaller test piece after thinking about what went wrong with that last piece. Thankfully this one came out really good! It has a perfectly glossy finish with no pinholes at all.


I'll update the thread later this week when I get around to cutting out the parts from the 2.5mm panel. Even though the panel is pretty ugly, I'm still excited to get more of the G10 replaced with CF.

[Dadio]

The home made CF really needs a thread of its own to do it justice !

[rc10nick]

The home made CF really needs a thread of its own to do it justice !

Thanks Dadio! I never actually thought about that. I just see it as another step in the process. If I ever finish this project maybe I'll make a post about the CF!

I got a couple more things done on the car throughout the week. I took that ugly 2.5mm panel I made and cut some parts out of it and installed them on the car.

Here's that ugly 2.5mm panel I made. You can see the portion that didn't get any resin in it and you can easily see all the surface pitting. Yuck


Here it is ready to be machined on my CNC:


Here are the parts I cut out and how they compare to the G10 parts they are replacing. The black G10 parts are 1/8" so it's not exactly a fair comparison with the thinner 2.5mm CF parts.





And here they are all installed on the car:



It's pretty exciting to get more CF on the car even if it is a little ugly. It's still good where it counts - strength, stiffness, and weight!

[TRX-1-3]

Ugly? The last mid-motored ruster/bandit platform I saw was a factory chassis Dremel and epoxy nightmare. This thing is sweet. Schwing factor of 10!

Very thoughtful and well executed.

[rc10nick]

Oh no, I didn't mean the car itself is ugly. Sometimes I'll just sit and stare at it while I admire my work. It was a tough decision taking it out on the track the first time.

I just meant the three CF parts I put on this week were a little on the ugly side due to my manufacturing defects. Still, that messed up CF panel looks better than the G10 it replaced and from a performance standpoint is unquestionably better.

I've seen plenty of those dremel job mid-motor conversions you're talking about. In fact they're part of what motivated me to do this - I still haven't seen anyone else out there making a proper mid motor Traxxas conversion. Even the good ones still take the entire transmission and flip it around 180°. I've never thought that was a great solution because doing that creates so many other problems. I wanted to be the first to make a good, no compromise mid motor conversion.

[Marshky]

The easiest and most original way to do a comparable conversion would be to take the transmission of the nitro hawk or nitro buggy and create a custom motorplate to accommodate the electric motor.

- I still haven't seen anyone else out there making a proper mid motor Traxxas conversion. Even the good ones still take the entire transmission and flip it around 180°. I've never thought that was a great solution because doing that creates so many other problems. I wanted to be the first to make a good, no compromise mid motor conversion.

[rc10nick]

The easiest and most original way to do a comparable conversion would be to take the transmission of the nitro hawk or nitro buggy and create a custom motorplate to accommodate the electric motor.

Wow. I can't believe I've spent all the time designing a transmission case I could have just bought. wow...

I looked at the current production nitro cars to see if I could use one of those transmissions (I didn't like how they move the shocks behind the rear arms) but didn't think of checking the older models. At any rate it was a good learning experience. It was my first time using CAD to model something other than a flat sheet part.

[Marshky]

Did this already . Maybe interesting option for a second version. If you are interested I could send you the 2d cad file for this motorplate.

[rc10nick]

Nice motor plate! That looks really well machined.

I really appreciate the offer, but I've already completed all the design work with what I have so I may as well stick with it. Although the price of my 3D printed transmission compared to the price of the Traxxas part may be a good reason to switch over later. But then again I've also thought about trying urethane casting my own if I find I'm breaking a lot of them. Speaking of, how well does that shock tower mount hold up? Seems like it would have some wiggle/slop in it.

[Marshky]

The mount for the shock tower does wobble a bit. I managed to decrease this by drilling a hole to connect it to the gearbox with a screw. Other option would be to design a new mount that mounts on the same points, but also rests on the front and rear of the gearbox. This would eliminate all the wobble. But this is just an idea. I never worked it out.

[Chewbacca]

That’s a really nice build and cool project!!

Is that transmission case 3 or 4 gear?

I flipped a transmission case around on a Radicator Expert (Graupner labeled Rad 2 with the blue eagle chassis). Except for closing (epoxy) and re drilling the transmission mounting holes it was not a Biggy at all. The other options where the rear arm mounts are not attached to the transmission case would be The RPM case, since that comes with the aluminum motor plate.

I also have some questions, are the bearings a direct fit into the printed case, or did you have to ream them to size?

Thanks, Tony

[rc10nick]

The mount for the motor plate does wobble a bit. I managed to decrease this by drilling a hole to connect it to the gearbox with a screw. Other option would be to design a new mount that mounts on the same points, but also rests on the front and rear of the gearbox. This would eliminate all the wobble. But this is just an idea. I never worked it out.

Maybe I didn't waste my time. I'm not sure I could live with it wobbling. That would drive me crazy.

That’s a really nice build and cool project!!

Is that transmission case 3 or 4 gear?

I flipped a transmission case around on a Radicator Expert (Graupner labeled Rad 2 with the blue eagle chassis). Except for closing (epoxy) and re drilling the transmission mounting holes it was not a Biggy at all. The other options where the rear arm mounts are not attached to the transmission case would be The RPM case, since that comes with the aluminum motor plate.

I also have some questions, are the bearings a direct fit into the printed case, or did you have to ream them to size?

Thanks, Tony

Thanks! It's a 3 gear transmission case and for the most part the bearings are a direct fit - I didn't really consider tolerances when I designed the part so the fit ranges from tight to really tight. I sent my part of to an SLS printer. Those have much better resolution than your typical hobby style FDM machines you see all over the place. The transmission case in the pictures is direct from the printer with no post-processing.

Like I said, the main thing I don't like about flipping the stock transmission is how it moves the shock tower from in front of the arms to behind the arms. Not a big deal on the bandit (though technically it does increase your turning MOI), but on a rustler that means you can't use the stock body. I guess you could use a nitro rustler body, but I don't think there's as many options out there for it. Also, my transmission is more like a laydown. It moves the motor and the battery further inboard than the stock transmission would. That was another thing I wanted since theres a few carpet/turf tracks around me and I like racing on those kinds of surfaces.