How to Build a 1/87 Scale RC truck a.k.a.TTTT

[Overlander]

This is a long atricle. Please keep a copy of it for your reference when you are ready to build a 1/87 truck.

If you are serious about wanting to build a 1/87 TTTT, please read on......

For direction on my first TTTT project, I mostly used pictures from the German websites. Since I can't read German, I tried to use Bablefish as much as I could to translate, but that is only about 85% effective. Bablefish would give you results in English like, "make nice still depronplatten ergattert and a small carrier tinkered". Huh?! So allot was just guess work and interpolation. I did make email contact with a German guy who writes a little English, and he gave me some pointers.

At first I thought I had to somehow buy all German parts and electronics, but a little research showed I could get almost all of it here in the U.S. The only German parts I wish I had are some of the gearboxes from microantrieb.com. Their webpage is impossible to translate because there is no actual text. It is all in bitmaps. Go figure... I ended up using a U.S. gearbox made for model railroad applications and the ground clearance is not that great.

I would really like to see more interest here in the U.S. about TTTT (or whatever U.S. name we decide to give it) so I will share as much about my experience as I can, and eventually us non-German connected folks will have enough experience to help each other out without having to use bablefish. The problem is, that most young hobbyists in the U.S. are NOT MODELERS.

Our collective brains have evolved smaller and smaller until no one wants to build anything any more. Most kids just want their RC toys RTR (ready to run) right out of the box, and so no one is learning the skills of model engineering. This is evident just looking at a typical hobby shop inventory. Try to ask a typical hobby shop employee about anything other than a RTR T-Max, and all you get is a drooling "Rain Man" impersonation for an answer. But I digress. My point is that this hobby is not likely to take off and be popular because it is ALL custom fabrication and not likely ever be available in a kit form (though maybe RTR?).



HOW I DID IT...

Basically, the size of even the smallest off-the-shelf electronics prohibit building anything other than a truck with a covered cargo area to fit it all in. I have a special interest in Unimogs so I decided to use the Roco UNIMOG body.

Lesson learned: for a first attempt, the MAN 5t would be better, because it has more room than the UNIMOG.

I decided to build a frame out of brass, but it could have been done from a flat deck made of fiberglass or other composite. The problem with composite frames is that they are too light. This is one form of micro modeling where you want things to be heavy. I bent the frame rails exactly the same shape as the scale Roco UNIMOG frame rails were.

Lesson learned: No need to do that. A flat rectangular frame would have been sufficient and less work. Also the scale frame interfered with my servo placement and I hade to modify it later (see below).

The pieces of frame were soldered together using basic electrical solder. Silver solder is stronger but not necessary and requires too much heat for these little parts. All of your joints would melt as you tried to solder a new one.

The best way to design any custom RC model is to lay out the design of the chassis in the following order:

First, the motor and drive train. This is the most important system, of course.
Second, the steering geometry. (I blew that one when I made the scale frame. I am sharing this so others wont make the same mistake on their first truck)

Everything else has to be designed to fit around those two systems (especially the frame). The motor and gearbox are relatively tall and needed to stick up through the cargo bed of the UNIMOG. The steering servo needed to be mounted in the cargo bed. The Roco plastic is soft and easy to work with but after you cut all the holes in the floor of the cargo bed it becomes very flimsy and delicate. This is significant because you will be holding and handling this piece of the truck most of all during construction.

You will have to find a way to mount the bed to the brass frame. CA glue will be used when all the geometry is aligned but that is almost the last step of construction. I ended up tapping two 1.5mm holes in the rear corners of the frame so I could screw the bed down. The bed has to be attached and un-attached to the frame about a hundred times while you are working out all of the geometry, so gluing is not an option.

The only way to fit all 4 batteries in a UNIMOG is with 2 of them stuffed in the cab. Since the idea of TTTT is to be scale, you will need to carefully dissect the cab, bumper and fenders to be attached back in place on the brass frame later. I glued the fenders, bumpers and cab together, then screwed that assembly into the front of the frame with 1.5mm screws through the front bumper.

Wheelbase/Truck Height
I matched the wheelbase from the scale UNIMOG and made marks on the frame where the axels should be. This an important part of the overall geometry design. Also, the height or "lift" of the whole truck needs to be considered. I made my body 2mm higher than the stock. This allows for the
front axel to articulate without hitting the fenders.

Front Axel
The most time consuming part of the process is the design and creation of the front axel. I managed to find on one of the German websites a PDF plan for a front axel. It was not exactly what I needed for the UNIMOG, because it has a smaller width, but I used it as a basis. I did everything in millimeters and found it really easy to do. I used a Sherline mill to make the main axel housing but if you don't have access to a mini mill, you can
fab all of this stuff using razor saws and files. The knuckles are just 3x3mm brass blocks with 1/16" holes drilled vertically for pivot pints and 2mm holes drilled horizontally tapped to accept the 2x5mm axle screws.

Wheels
I used 2x6mm bearings for the front wheels. The Roco tires fit right over them and a 2mm screw into the mini steering knuckle worked as axels. Really simple and very effective!

Suspension
I decided to make only the front axel articulate for my first project, and this is pretty common on most of the German trucks I have seen pictures of. Keep it simple. I just used a single pivot point in the middle of the axle, sandwiched between a pair of brass angles suspended from the frame. For the pivot, I used a 00-80 screw and nut, so I could remove the axle as it was
constructed.

Steering
At the beginning of the design, I knew where the servo had to be, but I was not sure what the linkage geometry would look like. You almost have to have 90% of the rest of the design done, including how the body will be mounted, before you can tackle the linkage. I had dreamed up all kinds of complex control horns pushing and pulling etc., but when it came down to it, I found that the UNIMOG was so short, I could just link the servo control horn directly to the knuckle. Simple is always better. I used 1/16" brass wire as control rods. For the control points on the knuckles and soldered pieces of bent brass with 1/16" holes for the rods to the knuckles. I had to angle the control points on the knuckles in about 15 degrees to avoid hitting the inside of the tires.

Receiver
I am a criminal. I am forced to use the aircraft 72MHZ frequency for the radio because no company makes a 75MHZ or 27MHZ reciver small enough. If you know of one, LET ME KNOW! I cut off all of the pins where the servos connect and soldered all contacts. It barely fits around my gearbox.

Speed Control
I tore apart a HiTec HS-55 micro servo, and used that as a speed control. I removed the pot and replaced it with a mini adjustable 5k ohm resister. This makes it easier to find the neutral point and is much smaller. I mounted the resistor facing down through a hole in the bottom of the cargo bed so I could adjust it from the bottom of the truck. Note: If you have never used a servo as a micro speed control, email me and I will tell you all about it.

The rest is just assembly and fitting and re-fitting as you go along. If you don't have a blueprint or manual to build from, you have to try to design as much as you can at the beginning, then play it by ear the rest of the way. It's like playing chess alone; you have to consider if placing a piece in a certain location is going to effect another critical piece further towards the end of construction.

If your eyesight and nerves can hold out, you CAN build one of these little things! My German penpal says a little red wine sometimes helps.

Parts List
All prices are approximate.....

Servo - Cirrus CS-10 BB $26
Reciver - GWS GWR-4P $24
Rx Crystal - GWS micro $14
Speed Control - converted HiTec HS-55 micro servo $25
Batteries - four 120mAh NimHs $24
Transmitter - JR 4 channel $130 (another freakin' radio set with a receiver I didn't even use on this project!)
Gearbox - NWSL 172-6 50:1 w/ 60 tooth brass flywheel on input shaft $30
Motor - some little motor out of a micro servo I found in one of my junk parts drawers w/ 9 tooth pinion. $??

Total gear reduction - I dunno, you do the math. I got 50:1 to start with, then I add the 60:9 teeth ratio for the motor and flywheel. Whatever that is. I should have stayed in school and learned more math............ If someone can figure this out, please let me know.

Other materials:
Two 2x6mm bearings for front wheels $8
micro adjustable resistor
1/16" square brass $1
1/8 x 1/8 brass angle $1
some more brass $2?
nerves of steel $priceless