Hairba11's mk48/17

[hairba11]

Hairba11's mk48/17 (4x4 tractor + 4x4 trailer = 8x8)

So I picked up a HEMTT body set a few months ago and started collecting parts for this build.
The Mk48 differs from the HEMTT in a couple of ways. Although it looks almost the same, it's a 4x4tractor/4x4 trailer arrangement instead of an 8x8.
The Mk48 tractor can be attached to a handful of different trailers. The 14 is a flatbed, the 15 is a wrecker, the 17 is a cargo, etc.

This is the tractor without a trailer. I've driven one once, about 200 feet from one trailer to the next, and it's one of the most unique vehicles I've driven. Like being able to steer a roller coaster. I think my seat moved over a foot vertically between gas and brake.
Because the tractor is self contained the fuel tanks are on top of the fenders, the spare is on top of the hitch, and I have a lot of work ahead of me.

Suspension should be pretty easy. Walking beam with leafs and multi link setup. When the tractor is maneuvered without a trailer, little blocks fold down between the frame and rear axle to lock out the suspension.

The pivot at the back of the tractor will be a challenge.

The rest of the hitch is a fairly simple box. The actual connection to the trailer is at the front of the trailer frame. Sixteen 3/4" bolts hold the hitch plate to the trailer. Add in a couple of electrical a couple of air, and a couple of hydraulic quick connects, and eight 3/8 bolts for the drive shaft flange and a couple of hours later you have a new trailer.

And don't forget about the big hydraulic rams. The trailer hitch has forced articulation tied in with the steering


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[hairba11]

So I know the 1:1 has a straight frame with suspension trunnions and I knew my ability to knock out true, straight, level trunnions conflicts with my patience. So I cheated. I found aluminum shock towers for the rear of the scx10/generic scale chassis for about $15/pair on eBay. Lots of adjustability with a bunch of threaded holes.

Add a set of leaf springs and mount it to the chassis upside down.

It's really hard to find military looking wheels. The militants would be perfect, but 2.2 is a little large, and the price is steep for 8 wheels.
I ended up finding gmade 1.9 steel wheels.

These things were awesome to put together. They included longer screws to pull the wheels together. They are finished with hex head bolts and include a tiny socket which fits on the end of the 2.5mm wrench

For tires it was tough finding the height and width needed in 1.9, so I used some 2.2 Rock Lox and cut-n-shut them.

So making so shorty upper links, and hooking the axles together with long lower links and locating the lower with the leaf pack gives me a decent walking beam.

 

[hairba11]

So here it is.
Now to work on the hitch


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[calcagno45]

This should be interesting. Lots of details there. Good start so far!

 

[Jeepy84]

That suspension is super trick!

Are you thinking actual rams for the trailer hitch/steer?

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[hairba11]

Thanks everyone.
I'm planning on using some 1/4 scale servos to run the articulation. Im thinking of running the links through some shock bodies to mimic rams


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[calcagno45]

Did you check into the linear actuators?

 

[hairba11]

I thought about it, but I already own 1/4 scale servos. 433 ozin each should move it


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[hairba11]

So I started mocking up some metal tabs for hinge points

Then welded up a hitch shaped box.

The holes on the hitch side are 13/64, the holes on the tractor side are 11/64 tapped 10-32 so I don't have hinge nuts interference with the drive shafts

Mocked in place

And trailer bolted to hitch like the 1:1, with my custom long driveshaft

There it is. 9" wide 10"tall, 39.5" long. I was worried about making my hitch too long. It was too short. The actual length of a mk48/17 would be 41" in this roughly 1:11 scale. The masking tape roll is almost spare tire size

The front axle steers and the trailer articulates. Should be a decent trail rig.


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[calcagno45]

Very cool

 

[hairba11]

The body work still needs to be done. The everything between the cab an hitch gets reworked.

This morning I got the motor/trans and articulation servos mounted.

Links and Tamiya's shock bodies make decent fake rams

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[hairba11]

So this morning I brought a radio to the shop, got my articu servos set up and made link mounts on the hitch. I also brought a speed control along and got it moving around a little

Flexing it out on the forklift!

And here's the 1:1 my friend has.
As pictured from the crane hook.
I may have been ratchet strapped to a desk chair for this.

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[BigSki]

Very cool build ! I remember these rigs from The Marine Corps, that crane version and a tanker version. I witnessed someone trying to use the crane to recover a stuck LAV, it didn't end well for the "Dragon Wagon" "thumbsup"

 

[hairba11]

Thanks! I think they're really cool. When you have Uncle Sam's budget and labor pool keeping them on the road.

I got a steering test video
https://www.youtube.com/watch?v=ccbeN9V8fKo&feature=youtu.be


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[Szczerba]

Oh boy! This is such a rad build!

 

[Jeepy84]

That steering is awesome!

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[hairba11]

Thanks guy's.
It's a runner!!

Started with some soldering
This was the steering harness

So I cut a couple of feet of wire out of it

That should help the voltage get where it needs to go.

The axial esc and 20 turn stinger motor gave me a slightly above walking pace and had no grunt.
So I swapped in a Novak Super Duty and JP Warrior 45 or 55 turn handwound and added another battery. This gives decent crawl and gets me to a quick walking speed.

I'm going to make a home under the bed for the batteries and add a castle bec.

That doesn't mean I can resist going for a spin though

Gotta love a walking beam

And I'm going to need a steel front bumper for sure.


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[hairba11]

https://youtu.be/sDXH8M8C3C8

https://youtu.be/Kfa7pIF9rwA

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[hairba11]

Ok. So now that the chassis is mostly there, it's time for the cab changes.

A big paper cutter makes short work of a lot of straight lines.

And I now have a bed full of little rectangles

My last bottle of plastistrux is long dry from disuse, so I did some searching and decided to buy straight MEK. $22/gallon beats $9 for 2oz.

And the first joint looks good.


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[calcagno45]

Where did you pick up the body originally?