Mid Motor Rustler Build

[therigliveson]

Looks good, nothing like a good home brew project!

[rc10nick]

Well it's been a while since I've posted but I've got some fun updates!

At some point I plan on building a bandit to go with my rustler so I wanted to get a bandit body to try out on my chassis. I wanted one of the original style bandit bodies because I like them over the newer ones but it turns out they're really hard to find. It took a while but I did luck out and snag a brand new bandit body on ebay. When it showed up I couldn't resist getting it on the car and snapping a picture:



Unfortunately my design needs a bit of a tweak - the rear shock tower is about 1/4" too far back on the car so the body doesn't fit quite right on the chassis. Regardless, I'm pleased to have such a clean example of an original bandit body! Since it was so hard to find this one I want to experiment with vacuum forming to see if I can make my own replica bodies to run.

I also decided the car (and my personal risk tolerance in the era of coronavirus) was ready for a race day. Anyway, long story short but the car ran about as well as I expected it would given it's a dusty clay track and the car has a gear diff and not much rear toe. The gear diff and less than ideal rear toe meant the car was very loose, but it still felt pretty good to drive. I didn't have to fight it to do anything and it survived the night without breaking anything so as far as that goes the night was a major success despite finishing 5th out of 7.

So there's definitely room for improvement to get the back end planted. First and foremost is a ball diff which I've already bought and put in the truck. Just that makes a huge difference in how planted it feels even driving on the street. The next step is to swap out the nitro rustler rear arm mounts with the rear arm mounts from the RPM transmission. RPM has a set of 0° and 3° mounts which is perfect for what I want - the 3° set will work on the rustler because rustler arms have no built-in to and the 0° set will work on the slash and bandit because they've got toe built into the arms. The RPM mounts are also neat in that the 3° mounts are offset to the rear to make up for the reduction in wheel base you'd get from angling the arms at the chassis instead of the hubs.

[Mr. ED]

Was the body pre-cut? If you want to make a mold it's much easier to start from an uncut body

[rc10nick]

Unfortunately yeah it was pre-cut. If you've got some tricks for how to deal with that I'm all ears.

[rc10nick]

I'm back, baby! I recently bought myself an Ender 3 V2 3d printer and this weekend was my first time using it. These things are so freaking cool, why did I sleep on them for so long?

Anyway, I've been kicking around the idea of designing an entire RC car and that pushed me to buy a 3d printer. I figured I should start with what I have, my Rustler, and gradually design and iterate new parts until there's nothing left of the old Rustler geometry.

My first steps are the front end - the steering block interferes with the caster block and limits the overall steering angle in both directions. I've also been intrigued by what Tekno is doing with their 1/8 platform - their 2.0 cars are designed to maximize suspension arm length and the front end is also designed to achieve 0 scrub radius. I want to implement those ideas in a 2wd platform. Here's what I've designed and printed so far to achieve those goals:



I'm just so excited and happy with how great these prints came out! I never would have been able to do this with my CNC. It opens a whole new world of possibilities to me.

Anyway, I've got some losi wheels on the way that I plan to use. They've got narrow offsets that will let me push the length of the arm even deeper into the wheel without making the track width any wider, and that should also help me get as close as I can to my other goal of 0 scrub radius.

[Mr. ED]

careful: you will start rubbing the ribs on the inside of your rims

[rc10nick]

careful: you will start rubbing the ribs on the inside of your rims

Definitely a good point. Going into this I knew that was one of the design challenges I'd have to figure out. The arm in the previous picture was basically just a placeholder I used since I didn't have the TLR wheels in hand yet to be able to measure/model/design a solution. I knew the placeholder arms were going to rub with the TLR wheels but I mainly wanted to get some initial prototypes printed and in my hands so I could evaluate other aspects of my design. The wheels arrived on Monday so I have been able to update the arm design to address the issue. My next goal is to have a rough final concept printed and on the truck this weekend for some very gentle testing on the street.

[rc10nick]

This morning I started printing out my latest design and it was finished by lunch time. I assembled everything to test how it all fits together and unfortunately there are still some minor issues I need to fix.

First, here's a picture of the latest design (front) along with the delrin arms I previous made and finally the stock parts. You can see just how much longer my new arms are and also how I went about avoiding interference between the ST wheels and the arm. You can also see the partially threaded rods I found on McMaster Carr that are perfect for use as hinge pins.



There doesn't seem to be any issues related to the inboard edge of the wheel rubbing the arm so I think that part of the design is good... well, other than the main question of whether an arm that is reduced down to a single 8mm x 8mm cross section will be durable. That remains to be seen. My gut says an impact resistant plastic should be tough enough but I'm not 100% on that.

As I mentioned, though, there is a minor issue that does need to be fixed. The webbing/supports on the inside of the wheel rub against the edge of the arm right around the outer hinge pins. The only way I can solve this is by moving the king pin outboard a few mm on the caster block. That will also require a corresponding reduction in length to my arm which is counter to one of my main goals, but I've already gained enough length on my arm that I'd be okay with sacrificing a mm or two in order to solve this problem while also keeping the scrub radius minimized.

So while not exactly the result I had hoped for, I'm still pretty excited with how the design is progressing and I'm even more motivated to keep going!

[rc10nick]

Oh man, I'm so close this time...

New parts printed and installed on the car


Minor snafu with the camber link - the link and the caster block hit and prevent full suspension compression. gonna have to slightly rethink this...


And now the main problem - the steering link hits the inside of the wheel before hitting full lock. I'm going to have to redesign the steering block so the steering link sits outside of the wheel.


So there's definitely progress from last time, but there's also new problems to fix. Either way its exciting to get parts on the car and I've got a second set printing right now for the other side. I'll at least be able to drive it around with some SCT wheels since those have tons of clearance.

[Dadio]

If you make it vertically thicker in the 8x8 section downwards and radius all the 90° and sharp corners to prevent stress risers then it may work in nylon , testing is all part of fun .The delrin one looks good !

[Coelacanth]

This area you pointed out looks like a weak point. There's too much weight and stress that will happen on opposite sides of that area, IMHO. I could be wrong but I think it's bound to fail and break, unless it only sits on a shelf or gets driven very gingerly.

[rc10nick]

Yeah, this whole project really hinges on if that part will be strong enough. It's really the only way to design an arm that meets the design goals I have in mind so it is what it is. Although I'm not too worried - I'm pretty sure I've seen some pillow ball suspension arms that are just as narrow right where the ball screws into the arm. And in my hand it does feel more substantial than what you might get from the pictures. But I guess I'll find out eventually! I have some 1.5mm aluminum plate sitting around too. I'm thinking it might help if I could machine some plates to skin the top and bottom of the arm in that area to give it some extra strength.

[GoMachV]

When designing parts for printing I like to use big radius for strength. If you have room, you might just add some radius to that area and a little bit will go a long way in durability. Just my .02

[rc10nick]

So the printer finished the parts for the other side of the car and after dinner tonight I wasted no time getting them on the car.



I was aiming for 12.75" wheel to wheel. Nailed it.



Hopefully that last 1/4" is enough width to accommodate any bulge from the tires and anything else that might affect total width like camber or toe.

I managed to get a few minutes driving it around the street on some SCT wheels and tires. The car drove okay for the most part. Nothing too exciting to report, and with so many issues in these parts it wasn't going to tell me much to anyway. But it was just fun to drive it with these new parts. Until I lightly clipped a curb.



It wasn't the part of the arm we thought would break! lol

Anyway, I've got some more changes I need to make to the design. And since you guys sound like you know better than I do, I'll be rounding off all those edges at the thin point on the arm. Thanks for the advice on that!

[Dadio]

Curbs always have been the natural enemy of RC cars , I'm guessing that's PLA or PETG and its fine for prototypes but just not really strong enough for off road , if you can you should switch to CF Nylon as without getting super exotic its the best material for this type of thing .