Custom Solid Axle Monster Build

[RustyHunter 42]

Hello everyone, this is the project I've been working on for quite some time. It's built around the older SCX10 axles, a Chinese 3-gear transmission, Proline Badlands MX 2.8 tires, and a custom chassis I've designed. The TVPs are 2mm 6061 aluminum that I had cut out by SendCutSend while all of the red parts are 3d printed PETG. For now, electronics are XL-5 ESC, Reedy Radon 2 15t motor, EcoPower WP110S, and Traxxas 2.4GHz radio.

I'm not looking to race or anything, I've just thought solid axle monsters are cool and wanted to build one for quite a while. I got to do some limited testing before it got snowy and cold, and it seems about perfect for my small backyard.

However, I'd like some help in setting up the suspension on this thing. Right now, it's very stiff and the shocks don't seem to do any damping. Dropping it from a foot or two, the car basically just bounces off. I know solid axles are a little bouncy and not as smooth as independent suspension cars, but it seems like I'd be better off with some friction shocks!

My first thought was to try softer springs to see if that allows some damping, then look into different oil weights. I talked to a guy at my LHS, and he recommended going from 30wt oil up to ~50wt while leaving the springs alone.

I'd appreciate any pointers on suspension tuning, and thanks for checking out my build!

[SupraChrgd82 1425]

Good choice for axles. The older AX10 axles are very robust and smooth.

Regarding the suspension, I suggest attempting to move the upper damper mounts closer to the center of the chassis or locatingsofter springs.  
I don’t know what damper pistons are inside or how heavy the battery is. Those are additional factors that will effect how the suspension compresses.  
Generally monster trucks have supple compliant suspension.  For this reason I doubt that thicker damper oil is the direction you would want to go.  Damper oil would best be sorted after softening the ride.

[Saito2 6686]

Awesome project. I really dig it. My suggestions mirror those of SupraChrgd82's for the suspension. Perhaps @87lc2 can offer some suggestions with his knowledge/experience in MT racing. I know having the shocks mounted to the axles vs the lower links, like the SMT10, can be trickier to set up but Losi seems to have nailed it with their LMT. Besides the shocks look so much better/realistic attached to the axles.

[smirk-racing 253]

Wow this is incredible!

Are you doing your bounce tests with your battery installed? That's a big component in terms of weight, obviously.

I have a mod-clod that a guy built for me and he used 20wt oil in the shocks which surprised me. I'd suggest going *softer* for all shock components until you get some real damping. So - lighter oil (why not just go straight to the limit and try 10wt), softer springs, more holes in the pistons, and shocks arranged to give more leverage to the axles (usually moving the lower shock mount away from the axle by an inch or so, and then moving the upper mounting location even further towards the center of the truck for proper angles). This is less scale-looking than some other designs but might be easier to tune for now.

Once you feel like you're getting some actual damping action, you can see about tuning rebound and other elements.

The tires obviously play a very large part in these kinds of trucks. The badlands you have installed are potentially harder/stiffer than the clod-style tires people often use. You might look into softer tires once you feel like the shocks are actually involved in the dampening. Then you'll need to evaluate not only tire carcass stiffness but also the foams.

It will take some effort but given the *beautiful* truck you've designed and built, I am sure you'll succeed in very short order. Amazing work!!!!!!!

I am also keen to hear how the PETG holds up for you.

 

[RustyHunter 42]

Thanks everyone for the kind words and suggestions on tuning! So it looks like the general consensus is to go for softer springs and lighter oils. I do have some softer springs I can experiment with, but I'll have to pick up some lighter oils after the holidays. I'll also have to see what I can do about tilting the shocks slightly more and think about how I want to work these suggestions in.

11 hours ago, SupraChrgd82 said:

Good choice for axles. The older AX10 axles are very robust and smooth.

I've been very happy with them so far, although I wish they were just a touch wider.

7 hours ago, smirk-racing said:

The tires obviously play a very large part in these kinds of trucks. The badlands you have installed are potentially harder/stiffer than the clod-style tires people often use. You might look into softer tires once you feel like the shocks are actually involved in the dampening. Then you'll need to evaluate not only tire carcass stiffness but also the foams.

That's a good point and something I hadn't thought of! I'm going to assume these are harder than clod-style tires, but I like the size and tread pattern of them. I'm hoping I can keep the tires I've got, so I'll focus on the shocks for now.

7 hours ago, smirk-racing said:

I am also keen to hear how the PETG holds up for you.

I'll be curious to see as well, so I'll keep you posted. I'm not expecting to run this very hard, but I think the obvious weak spots are the 4-links. I have the metal to cut my own links, I just haven't gotten around to cutting them, and I think I want to see how PETG does at this point. I've been pretty impressed with how strong my other parts have felt, so here's hoping it all holds up!

[ad456 331]

Awesome build, looks class and can’t wait to see the progress. 
Tyre softness is a massive part of the suspension on a solid axle truck, I have vented the tyres my Mod Clod to get the damping right on it. I use 10wt in my shocks and the tyres take the initial shock and the shocks smooth out the rest. Those tyres you have, while looking good, might not have enough tyre to soak up the hit. I can drop mine from 6ft and it doesn’t bounce, just plants down.

I’d second a shout for @87lc2 to lend experience on this. 

[Nikko85 3453]

Awesome!

[87lc2 2666]

Very cool truck!  I agree that the AX10 axles are a great choice, and those chassis plates look awesome, nice job. 

As for the suspension, whatever you do don't take the hobby shops advice   Kidding, but you defintiely don't want 30-50wt oil on a solid axle truck with the shocks mounted on the axles.  First off, I see that those are Traxxas shocks, correct?  If so they are notoriously oversprung due to being used in a link or arm mounted setup from the factory.  Fantastic shocks but the springs just won't work in any capacity on that truck.  Get some softer springs and then see where you sit, and when I say soft I mean the softest you can find.  

Getting axle-mounted shocks to work properly is much more difficult than a link mounted setup, they need to be much, much softer.  You'll want to angle the front a bit as well if you can, the rear angle looks OK.  For the shocks themselves, start with 4-hole pistons and 20wt oil with a soft spring and go from there.  I also like to add a small bump-stop (fuel tubing or such) to the bottom of the shock shaft.  This keep the piston from going all the way to the top of the shock bore and allows you to underfill the shock a bit without creating too much of an issue with air.  The last thing you want a solid axle shock to do is rebound without a spring.  You want that shock setup soft enough that when you put the truck down on the table it sags 1/2" or more all on its own.  

Once you get the shocks soft enough and get the bouncing under control you'll most likely  need a sway bar of some sort to compensate, but that's a whole other topic.  

If you're looking to replace those shocks altogether, these are readily available on Amazon and quite cheap.  Just fill them with 20wt and go, they would work great on that truck.  I use them on axle-mounted SMT-based setups and they work great:

https://www.amazon.com/KYX-Racing-Suspension-Absorber-Crawler/dp/B08L7BYCYD/ref=sr_1_1?crid=N0WQ3XEQP6P2&keywords=kyx+90mm&qid=1672179433&sprefix=kyx+90mm%2Caps%2C94&sr=8-1

 

 

[RustyHunter 42]

16 hours ago, 87lc2 said:

As for the suspension, whatever you do don't take the hobby shops advice

Haha, it felt wrong to me at the time, but in his defense I didn't bring my truck in with me, just told him what I was thinking.

16 hours ago, 87lc2 said:

First off, I see that those are Traxxas shocks, correct?  If so they are notoriously oversprung due to being used in a link or arm mounted setup from the factory.  Fantastic shocks but the springs just won't work in any capacity on that truck.  Get some softer springs and then see where you sit, and when I say soft I mean the softest you can find.  

These are Traxxas Ultra shocks. I had thought they were a good choice since they're cheap and fairly reliable, but it looks like soft springs are a bit difficult to find. I found a slightly softer set, but they're not the best.

16 hours ago, 87lc2 said:

You'll want to angle the front a bit as well if you can, the rear angle looks OK.

Main issue I'm running into with this is that if it's angled, the front wheels will run on the shocks when steering. I might be able to find a few mm thicker hexes to get clearance, but I'll have to test that some more.

16 hours ago, 87lc2 said:

If you're looking to replace those shocks altogether, these are readily available on Amazon and quite cheap.  Just fill them with 20wt and go, they would work great on that truck.  I use them on axle-mounted SMT-based setups and they work great:

Those look very promising, and it might be more cost effective to get shocks I know will work rather than mess around with springs and oils that might or might not work. Did you have to rebuild these shocks with green slime to prevent leaking, or were they pretty much good to go?

 

16 hours ago, 87lc2 said:

Once you get the shocks soft enough and get the bouncing under control you'll most likely  need a sway bar of some sort to compensate, but that's a whole other topic.  

Hopefully we can cross that bridge when I get there!

 

Thanks for all the pointers, they really help!

[87lc2 2666]

Glad it was helpful.  For those KYX shocks I sometimes add slime to the lower rings/shafts, and sometimes don't but always a good idea.  I haven't had any major issues with leaking that I know of and I have them on a few trucks.  I'll get some moisture from time to time, but I use them on fairly heavy race trucks that take a beating so always cleaning/refilling shocks anyway.   The good thing about those shocks is the springs are super soft, about as soft as you're going to find.  Another option are original TXT-2 springs, they are super soft as well and I use them on a lot of trucks but unfortunately they're a bit hard to come by these days. 

Traxxas shocks are fantastic and I use them a lot, but typically only on trucks with link-mounted shocks.  The springs are just too heavy to use on an axle-mounted shock setup unfortunately.  I have had luck with TXT springs on TRX shocks and I'm sure there are others out there that will work.  Take a shock off and bring it to the shop, maybe you can find something soft that will work properly.  

[RustyHunter 42]

I ended up finding some springs that are much softer and will fit the Ultra shocks. I went with Axial 14x70mm springs - 1.04lbs/in for the front and 1.43lbs/in for the rear. The part numbers are AX30223 and AX30224 if interested. The Ultra shocks have a 13mm diameter body, so these will fit over it without any issue. The stock springs are 15mm ID and these are 14mm, so I had to print a new piece to hold the spring on the top as well as an insert to hold it on the bottom. Hopefully these pictures make it a little clearer what I'm talking about.

The suspension isn't perfect, but it now settles a bit when I drop it on the ground, and I actually see torque twist. With the slightly narrower springs, I'm going to see if I can angle the front shocks in slightly, and I'm also going to pick up some lighter oil - probably 20wt - this weekend. I won't be able to take it outside for quite a while (we just got another foot of snow), but it's headed in the right direction!

[87lc2 2666]

Nice, glad you were able to find a spring that works.  20wt oil and 4-hole pistons should help even more like you said.  The torque twist can be controlled by sway bars once you get the suspension set how you like it.  

[RustyHunter 42]

Surprisingly, torque twist isn't too bad, at least when I accelerate hard in my basement. It does pick up the front right tire, but it's not as bad as some trucks I've seen online. Do people usually use piano wire to create sway bars?

[Saito2 6686]

6 hours ago, RustyHunter said:

Do people usually use piano wire to create sway bars

I've used rod available at the hardware store in heavier applications like the TXT, piano wire, and in some cases even a clothes hanger, straightened and cut up.

[87lc2 2666]

As long as its not too bad no need to worry about it.  On "fun" trucks that I don't build for racing I rarely worry about sway bars since a little torque twist is not really an issue.  You can use piano wire, spring steel, or pretty much anything to make a sway bar if you decide to.  Ideally the bar will tie the chassis to the axle, but tying it to the lower links works as well.  As a last resort tying the lower links together (like the stock TXT setup) sort of works, but doesn't really do much.  

Here's what an ideal sway bar setup would look like if you're interested.  It would definitely be possible on your chassis - either use an existing hole or just drill a new one.  With a setup like this the truck will be very stiff, so if you're looking to use the truck for any crawling or light trail duty this would not be ideal.  

[RC4x4 142]

RustyHunter, great looking truck. The chassis looks sweet

[RustyHunter 42]

Thanks for the feedback @Saito2 and @87lc2! That gives me plenty to think about, but I might end up waiting and seeing how it does outside. I haven't ever had a crawler, so I could see myself goofing around with it as a light crawler just to see what it's all about. The torque twist is also slightly worse than my last post since I figured out I had some stiction going on with my shocks. I just had to slightly loosen the lower caps and now things are moving freely.

That's also an awesome looking truck you posted! Where'd you get the chassis from?

Thanks @RC4x4, I appreciate the praise! It was surprisingly easy to design, I just had to do a bunch of 3D printed prototypes before I felt ready to order it for real. Also helped with mocking up the links a lot.

[RustyHunter 42]

It's been a while since I've updated, but the weather finally warmed up and allowed me to finish up the body. I've gotten a few batteries through the truck, and it seems like it's working pretty well now! The wheelie bar wasn't essential since it only wheelies on pavement, but I think it's still nice to help protect the body.

 

 

I might have to play with the gearing a little, but the only other change I'd like to make is getting the stance a bit wider. I was thinking these https://www.towerhobbies.com/product/mini-12mm-wheel-widener-4/RC4ZS0407.html might be a good option, but does anyone have experience with these or alternatives?

[Saito2 6686]

On 5/26/2023 at 10:14 PM, RustyHunter said:

I'd like to make is getting the stance a bit wider. I was thinking these https://www.towerhobbies.com/product/mini-12mm-wheel-widener-4/RC4ZS0407.html might be a good option, but does anyone have experience with these or alternatives?

I have those exact wheel wideners. I used them on a fairly overpowered Twin Detonator. They've worked fine for me so far with no issues.

[RustyHunter 42]

On 5/29/2023 at 5:01 AM, Saito2 said:

I have those exact wheel wideners. I used them on a fairly overpowered Twin Detonator. They've worked fine for me so far with no issues.

That's great to hear! I haven't gotten them yet, waiting for them to get in stock in about a week or so, but where did you use all the washers at?

[Saito2 6686]

2 hours ago, RustyHunter said:

but where did you use all the washers at?

I only needed to use one under each nut. They basically take up any slack if the nut winds up bottoming out before the wheel is drawn up fully tight.

[RustyHunter 42]

It's been a while, but the truck has evolved quite a bit over the summer. The hex extenders really helped with the stability, but the brushed motor consistently ran very hot. It's partly my fault for mainly running this truck on the grass in my backyard, but I decided to go brushless with it. I ended up going with the Hobbywing 10BL120 and a 4000kV 3652 motor. With a 17t pinion and 56t spur, this motor almost never gets over 130F after a full battery in the grass and has plenty of speed and torque for the chassis.

The new motor had so much torque though, it was twisting apart the Traxxas 2wd driveshafts I was originally using. I ended up going with Junfac steel driveshafts which I've been happy with overall. The set screws have fallen out on me a few times, but otherwise they seem to be holding up well. I might look into doing some TXT style swaybars this winter, but I think this truck works really well for the space I have, and I'm quite happy with how the build came out. Thanks to everyone for their feedback and tips, they've helped quite a bit!

 

Almost forgot to mention, the 3D printed parts have held up very well! The only parts I've had break on me are the wheelie bar side plates and the old body posts I printed. The body posts kept breaking because they were too tall and bent every time the car tipped over. The new design is a lot more stiff and has held up well so far, knock on wood.