Micro Racer's SuperMiniKosh

[micro racer]

What happens when you don't think your 6x6 build is crazy enough, but don't have the shelf space for another 29 inch long truck?

Well, you build an 8x8 Oshkosh HEMTT out of 1/16 Revo parts! Yeah, I do realize this still isn't going to be a particularly small truck, even using the mini parts, but I'm just gonna ignore that.



(490mm from the extents of the front and rear diff housings. Chassis will be longer)

Instead of breaking out the JB weld, Dremel, and a pile of stock bulkheads, I decided to go about this the new-fashioned way. That is: use CAD and 3D print a chassis. I've started by drawing up a set of differential housings, then I'll draw a skeletonized support structure between them, with a pair of 1/2" aluminum channel rails to finish off the chassis. Considering the infamous lack of durability of mini Revo diffs, I'm using a larger inner bearing on the pinion, and increasing the width between the diff bearings to allow for extra shimming. Also, since Traxxas hinge pins generally suck, I'm designing the mounts around Axial's AX31497 2x20mm pins from the Yeti Jr. They're still not amazing, but they're of a more useful length (20mm vs 14mm) so I can support them better.

While I slowly chip away at the chassis design (and to provide necessary dimensions), I picked up four differential gear sets (actually six, because the pinion isn't sold separate from the ring gear), all the seals and internals, two sets of rear A-arms, and all four pairs of knuckles. I actually plan to run mini Summit driveshafts on the front four wheels because I will be using 4 wheel steering, since the Summit's CVDs should work better that the plastic universals of the Revo.



I got tired drawing so I threw together all the diffs and both sets of rear arms.



Unless I start breaking a bunch of arms, I don't think RPM arms will be necessary for this truck. At the hubs, I specifically chose the stock parts so I could use the upgraded Traxxas aluminum pillow-ball cups.

That's all I have for now! Hopefully my Amain shipment will show up tomorrow- *checks clock: 2AM*- ahh, shows up later today.

[rockhugger]

Subscribed!!!!

[Robotboy]

Interesting indeed! Looking forward to seeing the progress

[micro racer]

CAD progress is slow progress, but doing it right the first time means I won't have to reprint things...



I'm using a variation of the stock 1/16 Traxxas steering crank, modified to handle, well, an 8x8, and a standard size servo instead of the minis.



This truck will have 1.5 axle steering; the front axle steers like normal and the second axle has about 1/2 throw (will need to be tuned upon assembly).



Once I received all my differential and driveline parts, I was able to lay out and reconfirm approximate wheelbases for my drawing.





I needed to space out the front axles a little more to accommodate steering clearance, but this now confirms the general size of the truck (which I forgot to pull off the model for this post, but ehh it didn't really change much from the first pic).

Without the body or anything, this truck is like 60% tires lol



Speaking of which...

Wheels!



Mmmm *anticipation building*



Lookin' pretty shiny...



Ahh beans, that's a lot of screws (and I didn't even order these with rings, so I'll have to do MORE SCREWS at some point in the near future).



Since I don't have rings yet, I haven't mounted any tires yet (duh), but I did get my five pairs (4 running, 1 pair for directional spares) of RC4WD Flashpoint tires from the hobby shop a couple weeks ago. They're super soft and super sticky. Should work well.

I'm starting with a stock transmission housing and motor plates, but I just needed the stock gear set for a couple pins and a shaft.



Swapping stock gears for Hot Racing hardened gears. I'm leaving the spur plastic for now, mostly to see how long it will last, and I can always swap in an HR steel spur later. However, HR does not make a steel 50 tooth spur, only smaller, so I would like to try and make the plastic spur last; I know this truck will both *have* a lot of power and *need* a lot of power, but I plan to drive it sensibly and will be running a Crawlmaster 550 to keep the punchiness down but keep overall torque.



Little bit of Loctite on this slip-fit gear to lock it tighter to the shaft.



Taa-Daa:



That's all I've got for today! I want to finish the general front end mechanics and get some basic hardpoints fixed in so I can break up the technical work with some body design. I like the original HEMTT overhanging cab design, but I also am quite fond of the MAN KAT1 cabover style as well. If I make the cab large enough, I can probably fit design elements/cues from both, if I really wanted to.

Those are the only two military 8x8's I'm generally familiar with, besides the old rounded Soviet trucks, but post up some multi-axle cab pics if y'all have them...

[JatoTheRipper]

Impressive project! Nice design work. Awesome wheels! I'm looking forward to seeing this progress.

[meatmonkey]

This seems like quite the undertaking. I assume your going to print the body as well? What's your final drive ratio look like? I have a 1/16 traxxas rally car and it is surprisingly fast with only 2s and small tires.

It's kinda crazy how much scratch building has changed since this site started. Instead of hours cutting, drilling, filing, its hours of measuring, designing, printing.

That 550 should be plenty for those diffs. Good idea on upsizing the pinion bearing. Wish someone offered knuckles with oversized bearings cause I'm sick of replacing the kitty bitty 4x8's every few packs.

[micro racer]

Originally Posted by meatmonkey

This seems like quite the undertaking. I assume your going to print the body as well? What's your final drive ratio look like? I have a 1/16 traxxas rally car and it is surprisingly fast with only 2s and small tires.

It's kinda crazy how much scratch building has changed since this site started. Instead of hours cutting, drilling, filing, its hours of measuring, designing, printing.

That 550 should be plenty for those diffs. Good idea on upsizing the pinion bearing. Wish someone offered knuckles with oversized bearings cause I'm sick of replacing the kitty bitty 4x8's every few packs.

Yes, the body will be printed as well. I was gong to just screw it down with a floor plate, but I think I may save some space and material with a flip-forward type of mount. Still not sure, and it's not like I'm chained to using any pre-existing parts lol

I built the transmission with the underdrive gear placement, so I've got 3.71:1 at the trans, compared to 1.94:1 for a stock MERV. Total internal ratio is 9.72:1, and a 55t spur gives a range of 59.4-41.12:1 across 9-13t pinions. Though this truck will inevitably have a lot of driveline drag, I think I'll start with 55/11, for a 48.6:1 FDR.

I've rebuilt quite a few MERV diffs while working at the hobby shop, but I can't tell if the problem is tolerances, materials or simply size. Regardless, I figured the big pinion bearings couldn't hurt, and I made the diff housings 0.5mm wider to allow room for more aggressive shimming.

I've tried the cutting/welding/fabricating thing, and I *can* do it, my dad's an aircraft mechanic and is infinitely helpful, but CAD is so much cleaner and more precise and, if you know what you're doing, is extremely adjustable and allows for future updates or easy repairs to broken parts. Also, mid- to high-end 3D printing (like the SLS printing Shapeways provides; desktop FDM printing still sucks), is constantly reducing costs and is extremely durable (again, with the correct designs). It's still hours upon hours of work, but it's definitely a case of "work smarter not harder." (I know that's not what the saying actually means, but whatever).

Besides, I've been playing around with CAD since 4th grade, so the hardest part is coming up with the design, not drawing it up.

I hadn't seen anything about the 4x8's being a weak link, but then again I don't actually *have* a 1/16 Traxxas, I just thought their parts would be an appropriate size to use. However, I did see a lot of people complaining about ripping pillow balls out of the back of the knuckles, which is one of the two reasons I did not go with RPM knuckles. If either of those become a real problem for me, I guess I'll just draw up some new knuckles *shrugs*.

[micro racer]

Originally Posted by JatoTheRipper

Impressive project! Nice design work. Awesome wheels! I'm looking forward to seeing this progress.

Thanks Jato!

I really wanted black wheels, because it's a military truck, but I can't bring myself to rattle-can these. I'm just going to paint the cab and stuff with an arctic camo (or similar), even though it's kinda out of place here in sunny So-Cal...

[meatmonkey]

Hows this mini-monstosity turning out?